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The southwestern corn borer (SWCB, Diatraea grandiosella Dyar) and sugarcane borer (SCB, Diatraea saccharalis Fabricius) are two related insect species that cause serious damage in maize production in subtropical and tropical regions of Central and Latin America. We analyzed quantitative trait loci (QTL) involved in resistance to the first generation of both borer species in two recombinant inbred line (RIL) populations from crosses CML131 (susceptible) x CML67 (resistant) and Ki3 (susceptible) x CML139 (resistant). Resistance was evaluated as leaf feeding damage (LFD) in replicated field trials across several environments under artificial infestation. Leaf protein concentration and leaf toughness were evaluated in one environment as putative components of resistance. The method of composite interval mapping was employed for QTL detection with RFLP linkage maps derived for each population of RIL. Estimates of the genotypic and genotype x environment interaction variances for SWCB LFD and SCB LFD were highly significant in both populations. Heritabilities ranged from 0.50 to 0.75. In Population CML131 x CML67, nine and eight mostly identical QTL were found for SWCB LFD and SCB LFD, respectively, explaining about 52% of the phenotypic variance (dr2) for each trait. In Population Ki3 x CML139, five QTL for SWCB LFD were detected, explaining 35.5% of ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]). Several of these QTL were found in regions containing QTL for leaf protein concentration or leaf toughness. A low number of QTL in common between the two RIL populations and between RIL and corresponding populations of [F.sub.2:3] indicated that the detection of QTL depended highly on the germplasm and population type. Consequently, chances of successful application of marker-based selection (MBS) for corn borer resistance are reduced when QTL are not identified in the germplasm in which the final selection will be carried out., The related insect species southwestern corn borer and sugarcane borer are among the most important lepidopteran pests affecting maize production in Central and South America. First generation larvae of both [...]

Future maize (Zea mays L.) productivity increases require breeding materials with high grain yield potential and multiple disease resistance. As part of an integrated program to develop breeding populations with high grain yield potential and multiple disease resistance, two maize synthetics were reciprocally recurrently selected for yield and mass selected for multiple disease resistance. The objective of this study was to determine selection response of two maize synthetics to six cycles of tandem selection for multiple leaf diseases (MLD) and multiple stalk rots (MSR). Plants were inoculated each cycle and evaluated for MLD including their causal agents; northern corn leaf blight, (NCLB) [Exserohilum turcicum (Pass.) Leonard and Suggs, Races 0 and 1], southern corn leaf blight (SCLB) [Bipolaris maydis (Nisik) Shoem.], northern corn leaf spot (NCLS) [Bipolaris zeicola (Stout) Shoem. Races 1, 2, and 31, anthracnose leaf blight [Colletrotirchum graminicola (CES) G. W. Wils.], and eyespot (Kabadella zeae Narita and Hiratsuka). Following anthesis, plants were inoculated and evaluated for resistance to MSR including their causal agents; diplodia stalk rot (DSR) [Stenocarpella maydis (BERK) Sutton = syn. Diploidia maydis (BERK)], anthracnose stalk rot (ASR) (Colletotrichum graminiscola), gibberella stalk rot (GSR) [Gibberella zeae (Shw.) Petch.], and fusarium stalk rot [Fusarium moniloforme, Shield). In 1993 and 1994, selection cycles 0, 2, 4, and 6 of synthetics RSSSC, RBS10, and their cycle crosses were evaluated. Selection response to MLD, NCLB, SCLB, NCLS, gray leaf spot (GLS; Cercospora zeae-maydis Tehon and Daniels), MSR, DSR, GSR, and ASR were measured in separate experiments. Decreases in leaf blight severity from Co to [C.sub.6] in RSSSC was 29% for MLD, 23% for NCLB, 33% for SCLB, 28% for NCLS, and 21% for GLS. Decreases for RBS10 were 34% for MLD, 33% for NCLB, 37% for SCLB, 49% for NCLS, and 16% for GLS. Cycle crosses were usually intermediate in values for leaf blight reductions. For stalk rots, the reduction in percentage internode area discolored from [C.sub.0] to [C.sub.6] for RSSSC was 44% for MSR, 42% for DSR, 39% for GSR, and 18% for ASR. Reductions for RBS10 were 63% for MSR, 67% for DSR, 64% for GSR, and 63% for ASR. Selection for multiple disease resistance along with a reciprocal recurrent program for yield resulted in significant improvement in resistance to multiple and individual diseases in RSSSC, RBS10, and their crosses., From 1950 TO 1980, genetic improvement of maize hybrids was estimated to be 92 kg [ha.sup.-1] [yr.sup.-1]. Yield gains were associated with consistent reductions in root and stalk lodging, reduction [...]

Genetic markers linked with quantitative trait loci (QTL) may enable indirect selection of complex disease resistance. Construction of separate linkage maps to identify QTL for each complex disease resistance trait of common bean (Phaseolus vulgaris L.) is unfeasible, however. We investigated whether selective mapping could be used to hasten identification of random amplified polymorphic DNA (RAPD) associated with QTL conditioning bean golden mosaic virus (BGMV) or common bacterial blight (CBB) resistance. The mapping population ('Dorado' x XAN 176) consisted of 79 [F.sub.5:7] recombinant inbred lines. A bulked segregant analysis (BSA) of as few as three individuals and selective genotyping was used. The 101 RAPDs identified between the parents were tested across resistant vs. susceptible bulks for BGMV reaction, combined greenhonse (leaf) and field reactions to CBB, and pod (greenhouse) reaction to CBB. Fourteen of 22 RAPDs selectively mapped because they cosegregated among lines within bulks, were linked with seven of the nine QTL conditioning resistance as identified by QTL mapping using all 101 RAPDs. The two QTL not identified by this approach had minor effects. BSA and selective genotyping required only about one-third the cost and labor of completely classifying the whole population with each marker and was similarly effective for identifying RAPD markers associated with major-effect QTL that condition disease resistance in common bean. Two-locus models ([R.sup.2]), for select environments, explained 60% of the phenotypic variation in BGMV reaction, and 65, 58, and 46% of the phenotypic variation in greenhouse-leaf, -pod, and field reactions to CBB. Repulsion linkages between QTL for BGMV and CBB may complicate the combination of resistance alleles for these two traits.

Screening of cowpea [Vigna unguiculata (L.) Walp.] germplasm for additional resistance to the root-knot nematodes, Meloidogyne incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood, revealed an accession (IT84S-2049) from Africa with resistance to diverse populations of both root-knot species. The resistance was effective against nematode isolates that are virulent to the resistance gene Rk present in commercial cowpea cultivars, in addition to isolates of gene Rk-avirulent populations. This study was conducted to elucidate the inheritance of a newly identified, highly effective source of resistance, and to determine its relationship to gene Rk in existing cultivars. Egg mass production on root systems of plants in growth pouches was used to assess susceptibility-resistance of [F.sub.1], [F.sub.2], [F.sub.3], [BC.sub.1][F.sub.1], [BC.sub.1][F.sub.2], and [TC.sub.1] populations generated from crosses of IT84S-2049 with California Blackeye no. 3 (CB3, susceptible), CB5, or CB46 (both possessing Rk). Segregation of resistance to gene Rk-virulent M. incognita in progenies from IT84S-2049 x CB3 showed that resistance in IT84S-2049 is governed by one dominant nuclear gene. No susceptible recombinants were found among 1206 [F.sub.2] and 1144 [TC.sub.1] progeny from IT84S-2049 x CB5 or CB46 and inoculated with Rk-avirulent M. incognita (used to detect both resistance phenotypes). Resistance in IT84S-2049 is conferred by an additional dominant allele of the Rk locus, or by another gene locus very tightly linked to Rk within 0.17 map units. We propose the symbol [Rk.sup.2] to designate this new resistance factor in IT84S-2049. Thus, Rk may be a complex nematode resistance locus, analogous to those reported for other plant pathogen-host combinations.

Black root rot, caused by Chalara elegans Nag. Raj and Kendrick (syn: Thielaviopsis basicola [Berk. and Broome] Ferr.), is a common soil-borne fungal disease in cultivated tobacco (Nicotiana tabacum L., 2n = SSTT = 48). Only polygenic partial resistance to this disease is available in the original tobacco genomes; however, Nicotiana debneyi Domin, a tetraploid relative of tobacco (2n = XXYY = 48) has complete resistance to a wide spectrum of black root rot biotypes. A fertile somatic hybrid between N. tabacum 'Delgold' and N. debneyi was previously produced and was used to transfer the N. debneyi black root rot resistance into tobacco. Tobacco addition lines carrying the N. debneyi chromosomes with the gene for black root rot resistance were produced after two generations of backcrossing to the recurrent tobacco parent followed by two generations of selfing. Random amplified polymorphic DNA (RAPD) analyses of the disomic (2n = 50) addition lines and 14 monosomic (2n = 49) addition lines derived there from provided strong evidence that the two N. debneyi chromosomes in the 2n = 50 addition lines were identical. However, the tobacco genetic background apparently prevented homologous pairing of these chromosomes or caused their desynapsis. Lagging chromosomes at anaphase I and micronuclei at telophase I were observed. Transmission rates of the N. debneyi chromosomes through the egg and pollen of the addition lines were 8.1 and 10.1%, respectively. The N. debneyi chromosomes in the addition lines occasionally paired with tobacco chromosomes, and therefore, translocation lines with black root rot resistance (2n = 24 II) may be obtained in the self-pollinated progeny by crossing-over. Seventeen RAPD markers were found to be associated with the N. debneyi chromosomes in the addition lines and these should be useful in monitoring further transfer of N. debneyi chromosome fragments into the tobacco genomes.

Bacterial streak caused by Xanthomonas campestris pv. translucens reduces yield of wheat [Triticum aestivum (L.) em Thell] worldwide. Attempts to control the disease with bactericides have been unsuccessful. Resistant cultivars offer the best protection against yield loss, but little is known about the inheritance of resistance. The objectives of this study were to estimate the heritability of resistance to bacterial streak in soft red winter wheat and to derive an effective strategy for selecting resistant genotypes. About 60 [F.sub.2:3] and [F.sub.2:4] lines from each of three populations (Pop1, Pop2, and Pop3) were tested for resistance to bacterial streak in three Louisiana environments with two to three replications per environment from 1991 to 1994. Variation due to genotype x environment (G x E) interaction occurred in each of the populations and was one-fourth to one-half the magnitude of error variance. Heritability of resistance to bacterial streak was relatively low and ranged from 0.12 to 0.70 (average 0.31) with parent-offspring regression and from 0.18 to 0.40 with variance components (average 0.36). Heritability increased by 0.22, 0.29, and 0.29 for Pop1, Pop2, and Pop3, respectively, with means of two replications and two environments. Non-normal distributions in two of the three populations may have inflated estimates of heritability. Across populations, four lines were resistant in all three environments and 12 lines were resistant in two of three environments. Because of GxE interaction, testing should be conducted with at least two replications in two environments under high disease pressure. Such a testing program should be effective in selecting wheat lines resistant to bacterial streak.

Smooth bromegrass (Bromus inermis Leysser) germplasm PL-BDR1 and two seed increase generations derived from it are highly resistant to brown leaf spot caused by Pyrenophora bromi (Died.) Drechs. [Drechslera bromi (Died.) Shoemaker, conidial state = Helminthosporium bromi Died.)]. Lesions are about one-half the width and one-tenth the area of spots on susceptible cultivars. Physiological or anatomical mechanisms of resistance have not been identified, but they could have a hearing on stability of resistance and on forage quality components. This study was conducted to determine whether certain resistance mechanisms identified in other leaf spot diseases of grasses operate in PL-BDR1 germplasm. The three resistant generations were compared with susceptible cultivars Belmont, Radisson, and Saratoga for fungal penetration, halo formation, and leaf vein architecture. Susceptible leaves supported higher numbers of penetration attempts and successful penetrations. No effect of plant line was found for appositionai wall formation (=papilla formation), initial colonization rate, distance between parallel veins, proportion of large vascular bundles, or girdering of vascular bundles. Resistant lines had wider vascular bundles than susceptible cultivars indicating a possible role for strength of the vascular framework in restricting lateral spread. Extensive cell necrosis and chlorotic halo formation in susceptible cultivars indicated possible involvement of a fungal phytotoxin. Resistant lines had few halos, and reduced sensitivity to toxins may be involved in resistance of PL-BDR1 to brown leaf spot.

Sunflower rust, incited by Puccinia helianthi Schw., is a significant disease problem in sunflower (Helianthus annuus L.) production areas of North America, Argentina, and Australia and has been identified on sunflower grown in all regions of the world. Five races of rust presently have been identified in the USA. Recently, a new rust isolate gave a previously unreported virulence reaction pattern and was designated North American Race 6. A genetic study was conducted to determine the inheritance of resistance to Race 6 found in MC29 (AUS), a germplasm line introduced from Australia. [F.sub.1], [F.sub.2], and testcross progeny were produced from two crosses following hand pollinations: HA89/MC29 (AUS) and MC29 (AUS)/MC69. A multiple race inoculation technique was utilized under greenhouse conditions to evaluate the resistance or susceptibility of each plant to North American Races 1, 3, and 6. Chi-square tests for goodness of fit and heterogeneity were utilized to test each [F.sub.2 and testcross family. A single, dominant gene in MC29 (AUS) was found to confer resistance to North American Races 1, 3, and 6. The gene was independent and nonallelic to the genes [R.sup.1] and [R.sup.2], and was designated [R.sup.10]. Identification of resistance to Race 6 has increased the genetic diversity of resistance to sunflower rust in North America.

The random amplified polymorphic DNA (RAPD) assay has recently provided a marker system that may be useful for indirect selection and pyramiding of monogenic pest resistance traits in crop species. Our objective was to investigate the efficiency of marker-assisted selection with RAPD markers linked in coupling and in repulsion with a single resistance allele. Common bean (Phaseolus vulgaris L.) near-isogenic lines differing for the recessive bean common mosaic virus (BCMV) resistance allele bc-3 were screened to identify linked RAPD markers. Two cosegregating RAPD markers were identified and scored across an [F.sub.2] population of 103 individuals segregating for the bc-3 allele. One RAPD marker was linked in coupling (1.9. [+ or -] 2.6 cM) with the bc-3 allele. Categorization of the bc-3 genotypes in the [F.sub.2] population revealed that selection against the repulsion-phase RAPD, as opposed to selection for the coupling-phase RAPD, provided a greater proportion of homozygous resistant (81.8 versus 26.3%) selections, and a lower proportion of both segregating (18.2 versus 72.5%) and homozygous susceptible (0.0 versus 1.2%) selections. Selection of individuals based on the phenotype of both RAPD markers (i.e., the plus form of the coupling-phase RAPD and simultaneous with the minus form of the repulsion-phase RAPD) was identical to selection based solely on the repulsion-phase RAPD alone. The selection results obtained in our case study were consistent with the theoretical expectations, and both demonstrated that repulsion-phase linkages provided greater selection efficiency than coupling-phase linkages, even when the former have greater linkage distances from the pest resistance allele. Because repulsion-phase RAPD markers are more useful in marker-assisted selection for monogenic pest resistance traits, researchers will want to design their screening experiments in ways that will optimize the discovery of these.

Knowledge of the genes for resistance to leaf rust caused by Puccinia recondita Roberge es Desmaz. f. sp. tritici present in bread wheat (Triticum aestivum L.) cultivars and breeding lines is useful in developing resistant cultivars, estimating potential disease loss, and determining pathogen over-wintering. The objective of this study was to postulate the known Lrgenes conferring low seedling reaction in 30 cultivars and 56 hard red winter wheat breeding lines from nine winter wheat breeding programs. Entries were inoculated with 17 isolates of P. r. f. sp. tritici previously characterized for avirulence/virulence to 16 Lr genes. The genes identified and the cultivars were as follows: Lr1: Karl, Mit, Newton, and Thunderbird; Lr3: Brule, Hail, Karl, Redland, and Pioneer 2180; Lr10; Arapahoe, Karl, Mit, Sandy, TAM 105, and TAM 106; Lr14a: Hail, Larned, Pioneer 2157, Pioneer 2163, Pioneer 2172, Pioneer 2180, Vona and Victory; Lr16: Arapahoe, Brule, and Redland; Lr24: Abilene, Arapahoe, Cimmaron, Collin, Siouxland, Thunderbird, and TAM 200; and Lr26: Siouxland. No genes for resistance were detected in 'Agseco 7846' and 'Agseco 7853', 'Chisholm', 'Eagle', and 'TAM 107'. Fourteen of the 16 genes were postulated as being present in the parental lines. Lr24, Lr3, Lr1, and Lr2a occurred with the greatest frequent, followed by Lr26, Lr10, Lr14a, Lr16, Lr9, Lr17, Lr3ka, Lr18, Lr30, and Lr11. In combination, only Lr9 and Lr24, and Lr9 and Lr16 of these known genes presently provide resistance to leaf rust in the seedling growth stage in North America. However, the additional incorporation of Lr3ka, Lr17, and Lr30 would provide added protection.

Soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) infests soils throughout the mid and lower Mississippi River Valley, the Ohio River alley, and the southeastern Coastal Plain in the USA, and can reduce soybean (Glycine max (L.) Merr.) yield significantly. This study compared the yields of susceptible ('Essex'), moderately susceptible ('Cordell'), and moderately resistant ('A5474' or 'A5403') Maturity Group V soybean at different levels of SCN (Race 14) infestation in a greenhouse. Level of initial infestation of SCN was 0, 200, and 600 cysts L to the -1 in the top 15 cm depth of greenhouse pots. Seed yield of all cultivars declined with increasing level of infestation in 1990, but in 1992, seed yield of all cultivars declined significantly only between the 0 and 200 cysts L to the -1 level. Yields of Essex and Cordell were essentially equal within all infestation levels, while yields of A5474 and A5403 were greater than those of Essex and Cordell in the infested treatments. Yield declines between the 0 and 200 cysts L to the -1 treatments averaged 55% for Essex, 50% for Cordell, and 20% for A5474 and A5403. Seed yield was significantly correlated with number of pods and number of seed in both years, but not with weight of seeds or seeds per pod. These results indicate that SCN will reduce yield of moderately resistant and moderately susceptible cultivars dissimilarly. These results also indicate that the classification of cultivars for resistance to SCN reproduction is not necessarily indicative of the yield potential of these cultivars in the presence of SCN Race 14.

The soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is a serious pest of soybean (Glycine max (L.) Merr.). Most genetic studies of resistance in soybean to SCN have used qualitative models, although continuous distributions of phenotypes from zero to many cysts have been observed. We investigated the mode of inheritance for resistance to Race 3 of SCN by weighted least squares regression analysis of generation means for genetic models having additive and dominance parameters. Reaction to SCN was determined by counting the white females present on plant roots 30 d after inoculation. Four crosses, including one reciprocal, were made between resistant and susceptible genotypes. The ancestry of the resistant genotypes ('A20', 'Jack', and 'Cordell') traces to different sources of SCN resistance. Six generation means (P(sub 1), P(sub 2), F(sub 1), F(sub 2), BC(sub P1), and BC(sub P2)) in three of the crosses, and four in one (P(sub 1), P(sub 2), F(sub 1), F(sub 2)), were used to estimate the genetic effects contained in the means. The data were transformed for analysis because they were not normally distributed. The results indicated that an additive genetic model was sufficient to explain most of the genetic variation for resistance to Race 3 in each cross. Analysis of the pooled data from all crosses, however, indicated the presence of dominance effects as well. Recovery of parental phenotypes in the F(sub 2) and backcross populations indicated that no more than four genes were involved in the inheritance of the trait. Estimates for broad sense heritabilities on single plant basis ranged from 0.48 to 0.81. The relatively high heritability and additive genetic control indicate that breeders should be able to select genotypes with an intermediate level of resistance.

Stem rust, caused by Puccinia graminis Pers.:Pers. subsp. graminicola Z. Urban, is a serious disease of tall fescue (Festuca arundinacea Schreb.). Reliance on field evaluation of resistance can be unreliable when climatic factors do not favor disease development. The objective of this study was to develop an effective controlled-environment screening procedure for stem rust resistance and to compare response to disease in controlled environments with disease responses in the field. Twenty cultivars of tall fescue were used to evaluate the infection responses to stem rust in 5- and 10-wk-old seedlings in controlled conditions and disease severity on the same plants grown in the field during three consecutive years. Although there was a significant difference (P = 0.001) among cultivars for both 5- and 10-wk-old plants for stem rust infection types, all cultivars were rated susceptible. Some cultivars (e.g., Mesa, KY31-F, and Arid) had smaller average infection types (AIT) and more stem rust resistant plants within a cultivar. Stem rust severity differed significantly among cultivars on each date of assessment in 1990, 1991, and 1992. Conditions were favorable for stem rust development in 1990 and 1992. In 1991, stem rust was slow to develop with 74% of the plants remaining rust free. When individual plant responses were compared at 5-wk, 10-wk, and on the final assessment date in the field (29 May 1992), 81.1% retained the same rating for resistant or susceptible as young plants and field plants, 15.8% were rated susceptible as young plants and resistant in the field, and 3.1% had mixed responses for resistance or susceptibility. A reliable glasshouse/field screening program was developed enabling rapid progress in producing stem rust resistant cultivars of tall fescue.

Host resistance would be a valuable defense against the sting nematode, Belonolaimus longicaudatus Rau, a pathogen of many crops, including turfgrass. Sting nematode damage and host suitability were compared among diploid and polyploid Stenotaphrum genotypes. A time-course experiment compared diploid (2n = 18) 'FX-313' and polyploid (2n = 30) 'FX-10' St. Augustinegrasses (Stenotaphrum secundatum (Walter) Kuntzel), planted in pots with 250 mL Margate fine sand (siliceous, hyperthermic Mollic Psammaquent). Nematode numbers were determined 42, 84, 126, 168, and 210 d after inoculating with 50 sting nematodes per pot. Root and shoot dry weights and transpiration rate were compared with uninoculated pots of each grass. Sting nematode numbers increased more rapidly on FX-313 than on FX-10, reaching a maximum 2580 nematodes (adults plus juveniles) per pot of FX-313, 84 d after inoculation, compared with 380 nematodes per pot of FX-10. Root damage by sting nematodes was severe in FX-313 but was minor in FX-10. Sting nematode numbers and root dry weight of inoculated FX-313 were essentially unchanging from 84 to 210 d after inoculation, which suggested possible nematode suppression due to nematode injury of roots. Transpiration rate, a nondestructive indicator of nematode effect, was reduced in inoculated FX-313, compared with uninoculated controls, throughout 84 to 168 d after inoculation, while FX-10 transpiration rate was not affected. While FX-313 was susceptible to the sting nematode, FX-10 was resistant through a mechanism of antibiosis. We subjected four polyploid and four diploid Stenotaphrum genotypes to the same procedure, but harvested 128 or 129 d after inoculation. Nematode numbers differed (P < 0.001) among genotypes, ranging from 980 per pot of FX-313 to 140 per pot of FX-10. Root dry weight of diploids was reduced 33% due to nematodes but polyploids showed no effect. Some good hosts such as pembagrass (S. dimidiatum (L.) Brongn) 'FL-2195' showed little or no damage, thus may represent resistance through a mechanism of tolerance.

Little is known about the consistency of the resistance reaction of peanut (Arachis hypogaea L.) genotypes to late leafspot (Mycosphaerella berkeleyi Jenk.) in varied environments. This study was conducted to determine resistance reactions of peanut genotypes in two disparate environments. Four peanut lines with known levels of resistance to late leafspot in Florida were used to create 12 cross combinations. Parents and segregating generations from these crosses were grown at Gainesville, FL, and Kasinthula and Chitala, Malawi and inoculated with late leafspot. Latent period (LP), lesion diameter (LD), and amount of sporulation (SP) were evaluated. Measurements of the components at Gainesville were significantly correlated with those at Malawi. The correlations between locations for each component were LP, r = 0.52; LD, r = 0.64; and SP, r = 0.64. In both locations LP was negatively correlated with LD (r = -0.55) and SP (r = -0.63); LD and SP were positively correlated (r = 0.44). Narrow-sense heritability estimates were similar at Gainesville and Malawi. Based on parent-offspring regression, heritabilities averaged 0.62 for LP, 0.55 for LD, and 0.42 for SP. Realized heritability estimates in Malawi were 0.69 for LP, 0.63 for LD, and 0.51 for SP.

CIMMYT germplasm derived wheat (Triticum aestivum L.) cultivars are grown on a large area worldwide. The status of leaf rust Puccinia recondita Roberge ex Desmaz. f. sp tritici resistance in various cultivars is unknown. In this study, designated Lr genes were postulated in 26 cultivars released in Mexico between 1950 and 1989 by inoculating their seedlings with 14 pathotypes of diverse avirulence-virulence combinations. The adult plat resistance was evaluated with two currently predominant pathotypes during 1991-1992 crop cycle of Ciudad Obregon in northwest Mexico. Additionally, the probable presence of Lr13 and Lr34 was identified by determining the presence of linked genes Ne2 and Ltn, respectively. Eight Lr genes were present either singly or in combination: Lrl ( in 12 cultivars), Lr3 (4), Lr13 (18), Lr16 (1), Lr17 (3), Lr23 (2), Lr26 (1), and Lr34 (13). Virulence existed for the above genes, except Lr34. The 13 cultivars with Lr34 displayed low to intermediate adult plant disease severities and reduced area under the disease progress curve (AUDPC). Variation in the adult plant disease responses of these cultivars indicated the presence of additional genes for resistance. Lr34 has provided durable leaf rust resistance since the release of Yaqui 50. Several other cultivars lacking Lr34 also displayed reduced ADUPC. A complete database on leaf rust resistance genes in Mexican wheat cultivars has use in increasing genetic diversity in cultivars currently grown. Furthermore, based on the present information additional useful genes can be incorporated in the new germplasm.

The barley yellow dwarf viruses (BYDVs) constitute an important group of plant viruses of cultivated cereal crops in which only tolerance and/or low levels of resistance are known. High levels of resistance, which we here define as low concentrations of virus in plant tissues, have been discovered in Thinopyrum spp. (formerly Agropyron). In this study we have characterized such resistance in [BC.sub.2] and [BC.sub.3] populations, derived by crossing cultivated wheat (Triticum aestivum L. emend. Thell.) with Th. ponticum (Podp.) Barkw. & D.R. Dewey Agropyron Elongatum [Syn (Host) P. Beauv.] and backcrossing to wheat. The objectives were to evaluate the levels of resistance genetically transferable to wheat lines using both enzyme-linked immunosorbent assay (ELISA) and cDNA dot-blot hybridization for the detection of low levels of virus in plant tissues. Uniformity in results using both techniques depends on procedural uniformity, but a high correlation (0.93) between ELISA values and densitometry scans was obtained. A range in resistance was observed among the BC[sub2] and BC[sub3] populations from highly resistant to moderately resistant to susceptible. Lines classified as resistant had densitometry absorbance values of 0.25 to 0.60 and ELISA values from 0.03 to 0.30. The low values .for both ELISA and dot-blot assays indicate that significant resistance can be maintained in more advanced generations. Such low virus levels also suggest that inhibition of virus replication or cell to cell movement may occur in resistant Thinopyrum spp.

Soybean [Glycine max (L.) Merr.] generally yields more when rotated with another crop rather than growth continuously. The specific reasons for this yield response are unknown. The objective of this study was to investigate the extent to which a soybean-corn (Zea mays L.) rotation limited the buildup of host-specific pathogens of soybean, and to evaluate whether plant diseases were involved in the rotation effect. Incidence and severity of seven diseases and seed yield were recorded at two locations in the field in 1987 and 1988 for soybean maintained in four cropping sequences ranging from annual alternation with corn to 1,2, or 5 yr of continuous cropping. Brown stem rot (BSR) caused by the fungus Phialophora gregata (Allington & D.W. Chamberlain) W. Gams was found to be the most prevalent disease during both years and at both locations. Rhizoctonia root rot (Rhizoctonia solani Kuhn) and stem canker (Diaporthe phaseolorum Cooke. & Ellis Sacc. var. Cavlivora Athow & Caldwell) were noted in 1987 at both locations among < 5% of all plants monitored. The BSR-prevalence ratings were relatively high (mean 60%) for Hodgson 78, a susceptible cultivar, and relatively low (mean 22%) for BSR101, a resistant cultivar. Seed yield data from the various cropping sequences for the two cultivars indicated no clear relationship between BSR severity and seed yield. In 1987, rotation with corn resulted in a yield benefit to both cultivars. In 1988, both yield and development of BSR were adversely affected by drought. A comparison of the seed yield of severely infected and uninfected plants within cropping sequence indicated that BSR had a minimal effect on soybean yield in 1988. Therefore, within the conditions of this study, the yield benefit to soybean from rotation with corn did not appear to be due to the reduced incidence of plant diseases.

The sorghum [Sorghum bicolor (L.) Moench] plant produces antimicrobial phytoalexins that are essential components of its defense mechanism against attempted infection by Colletotrichum graminicola (Ces.) G.W. Wils., the causal agent of anthracnose. Sorghum phytoaiexins are 3-deoxyanthocyanidin flavonoids shown to be luteolinidin, apigeninidin, and a caffeic acid ester of arabinosyl 5-0-apigeninidin. Crosses were made between resistant and susceptible sorghum cultivars to examine the genetic x isolate relationships of the phytoalexin response. Plants were grown in a growth chamber and in the greenhouse and inoculated with two virulent isolates and one avirulent isolate of C. graminicola 7, 21, 35, 49, and 63 d after emergence. Luteolnidin concentration was greater than that of the other phytoalexins in juvenile tissues; however, the ratios of the three compounds changed as the tissues aged and extractable amounts in tissues older than 21 d were lower than in juvenile tissues. All three phytoalexins accumulated to greater concentrations in resistant inbreds and resistant x susceptible [F.sub.1] progenies than in susceptible inbreds. These relationships were unaffected by the developmental stage of the host. Thus, analysis of phytoalexins in juvenile plants appears to be a potential tool for identification and incorporation of host resistance to C. graminicola in adult plants.

Sunnower rust, incited by Puccinia helianthi Schwein., is a major disease in sunflower (Helianthus annuus L.) growing areas of the world. Resistance to the four prevalent races is lacking in most of the commercial hybrids. Six accessions (PI 413023, PI 413037, PI 413048, PI 413118, PI 413171, and PI 413175) with a high frequency of rust-resistant plants were identiried. This research examined the allelic relationships among resistance genes present in these wild accessions and the inbred line HAR4. Plants in each Pi that were resistant to all four races were crossed, then backcrossed, to the susceptible inbred line RA89; BC,F, progenies resistant to all four races were identiried, and each progeny was heterozygous for the four dominant genes conferring resistance to the four respective races. The [F.sub.1]'s of HA89/HAR4 also had four independent dominant genes in heterozygous condition for resistance to the four races. The [F.sub.1]'s of HA89/RAR4 were then intercrossed vath the BC,F, rust-resistant plants derived from the six accessions. Resistant genotypes from the intercrosses were testcrossed with HA89. The resultant progenies indicated that the gene for resistance to each race of rust in flve of six wild accessions was different from those of HAR4, whereas genes for resistance in PI 413118 were the same as those in HAR4. The resistant [BC.sub.1F.sub.1's] of PI 413023, PI 413118, PI 413171, and PI 413175 were intercrossed. Testcross progeny evaluations revealed that each accession had rust-resistance genes unique to speciric races. The apparent genetic diversity of these rust-resistant accessions could be exploited to diversify the protection against the four prevailing North American sunflower rust races.

We evaluated the resistance of 631 plant introduction accessions (PIs) of alfalfa (Medicago sativa L.) from 50 countries, to two isolates (I-7 and I-8) of Peronospora trifoliorum de Bary, causal fungus of alfalfa downy mildew. The objectives were to investigate the association of alfalfa subspecies, basic germplasm source, fall dormancy, and regions of origin, with levels of resistance to P. trifoliorum. Differences between mean percentages of resistance to the fungus isolates were tested for statistical significance with t-prime tests, or LSD tests, adjusted in accordance with Bonferroni's inequalities. Five M. sativa subspecies were included: falcata, x varia, sativa, x hemicycla, and caerulea. The mean proportions of plants resistant to one isolate were not significantly different from the proportions resistance to the other isolate within any subspecies. Hence, genetic ancestry of the plants apparently was unrelated to isolate-specific resistance. Mean percentages of resistant plants ranged from 26.8 (subsp. falcata), to 6.3 (subsp. caerulea). Subspecies sativa accessions averaged 15.4% resistant plants. Significant differences between isolates within individual accessions were found in 160 accessions. The PIs with significantly more plants resistant to I-7 than to I-8 were from South and Central America, the Mediterranean area, and Australia. Plant introductions with significantly more plants resistant to isolate I-8 than to I-7 were from northern Europe and North America. This isolate-specific resistance did not appear to be associated with the geographic centers of origin of the basic types of alfalfa germplasms.

Isozymic variation in chickpea (Cicer arietinum L.) crosses involving ascochyta blight [incited by Ascochyta rabiei (Pass.) Lab.] resistant germplasm was studied with the objective of identifying linkages of allozyme variation with ascochyta resistance genes. Such linkages, if found, could allow indirect selection of genes for ascochyta resistance. Eight F, populations were scored for allozymes of four polymorphic isozyme loci. Five isozymes encoded by the loci Est-1, Est-2, Est-4, Est-5, Gal-1, and one previously identified isozyme locus, Pgd-c, were inherited codominantly. Linkage relationships among isozyme loci and genes for ascochyta resistance, growth habit (hg), and leaf type slv) were also investigated. No linkage was found among isozyme loci, or between isozyme loci and ascochyta resistance; however, a loose linkage was observed between the gene controlling growth habit (hg) and the gene encoding for the cytosolic form of 6-phosphogluconate dehydrogenase Pgd-c). Linkage of ascochyta resistance with the available isozymic variation was not found in this study. This was not unexpected, due to the limited number of loci segregating in the [F.sub.2] populations we used. The inheritance of several isozyme loci was determined, however, and a previous report of a gene duplication in the Est-4/Est-5 region was confirmed. Hybridization of cultivated chickpea with closely related C. reticulatum Lad., the presumed wild progenitor, might increase the number of polymorphisms that can be used to locate ascochyta resistance genes., The inheritance of isozyme variation in ascochyta blight-resistant chickpea lines was investigated to determine possible linkages between allozyme variation and ascochyta resistance. The results on four esterase, one beta galactosidase and one phosphogluconate dehydrogenase isozyme showed a codominant mode of inheritance. However, no genetic linkages were detected between ascochyta resistance and the isozyme loci. Similarly, no linkages were detected among the loci themselves. It is proposed that the hybridization of resistant chickpea with susceptible Cicer reticulatum Lad. will increase isozyme variation.

Because stalk lodging is an important problem in maize (Zea mays L.) production, improved methods of breeding for stalk lodging resistance are important. The objective of research reported in this paper was to determine the influence of six cycles of selection for rind puncture resistance, measured just prior to anthesis, on postanthesis rind puncture resistance, grain yield, grain moisture, and stalk rot reaction in two maize synthetics, RSL(MPEN) and RSSSC(MPEN), which represent two different heterotic groups. Response to selection was measured in the populations, per se, testcrosses of each cycle of each population to an appropriate inbred tester, and in crosses between cycles of the two synthetics. In all cases, selection significantly increased postanthesis rind puncture resistance. Stalk lodging was significantly reduced for all cases except RSL(MPEN) per se. Stalk rot incidence was significantly reduced in RSSSC(MPEN) per se. Few adverse effects were noted for other traits. Measuring rind puncture resistance just before pollination allowed effective improvement in stalk lodging resistance using a selection scheme which requires only one generation per cycle.

Biochemical markers linked to pest resistance genes would be useful to plant breeders. This study was conducted to determine if biochemical markers could be found closely linked to genes conferring leaf rust resistance in wheat (Triticum aestivum L.). Near-isogenic lines have been developed in 'Thatcher' for a number of genes, Lr1 to Lr31, conferring resistance to leaf rust, caused by Puccinia recondita Rob. ex Desm. Fifteen of these Thatcher near-isogenic lines were evaluated for the isozyme endopeptidase. Two of the near-isogenic lines (Lr1 and Lr19) had endopeptidase phenotypes that differed from the Thatcher backcross parent. The endopeptidase phenotype observed in the near-isogenic line possessing Lr19 was completely different from any other phenotype normally observed in wheat. The leaf rust resistant Thatcher near-isogenic line possessing Lr19 was crossed with the leaf rust susceptible cultivar 'GA 100' (PI538257). The 59 F(sub 2) progeny were grown in the greenhouse and evaluated for leaf rust resistance and endopeptidase phenotype. All F(sub 2) progeny exhibiting the Lr19 endopeptidase phenotype were resistant to leaf rust. Maximum likelihood estimation, using the F(sub 2) phenotypic data, indicated a very tight linkage with a recombination value of 0.01 + or - 0.009 map units for the Lr19 and Ep-D1d loci. In addition, no recombinant F(sub 2) genotypes were found among the 30 F(sub 2) plants for which F(sub 3) progeny were obtained. Thus, the gene Lr19, which confers resistance to leaf rust, and the gene Ep-D1d which encodes endopeptidase phenotype EP-D1d, are linked. This linkage will enable rapid detection of seedlings possessing Lr19 in wheat improvement programs.

Two parasitic weeds, Striga gesnerioides (Willd.) Vatke and Alectra vogelii Benth., cause heavy yield losses in cowpea (Vigna unguiculata (L.) Walp.) throughout the semiarid regions of sub-Saharan Africa. Several sources of resistance have been identified, including B 301, a landrace from Botswana that has shown complete resistance to both striga and alectra. Striga resistance in B 301 is conditioned by a single dominant gene. Our objective was to determine the inheritance of alectra resistance. B 301 was crossed to three susceptible cultivars (IT84S-2246-4, SUVITA-2, and IT82D-849) and F1, F2, and backcross seeds were obtained. The reaction of parental, F1, F2, and backcross plants to alectra infestation was evaluated. Test populations were planted in plastic pots of 13 cm diam. filled with a mixture of sand and soil and approximately equal to 800 alectra seeds. The roots of each plant were examined for alectra attachment 10 wk after planting, and the number of resistant and susceptible plants in each population were recorded. All plants of B 301 and backcrosses to B 301 were completely free from alectra. All plants of IT84S-2246-4, SUVITA-2, and IT82D-849 were susceptible. All F2 populations segregated 15 resistant : 1 susceptible, and backcrosses to the susceptible parents segregated 3 resistant : 1 susceptible indicating duplicate dominant gene inheritance. We propose the gene symbols Rav(sub 1) and Rav(sub 2) (resistance to Alectra vogelii) for this trait.

To understand better cotton (Gossypium hirsutum L.) plant compensation for early-season floral bud (square) loss due to insect damage, a field study was conducted in 1994 and 1995 at Marianna, AR. The control treatment was protected by insecticide applications, while tarnished plant bugs (Lygus lineolaris Palisot de Beauvois) and bollworms (Helicoverpa zea Boddie) were released in the plots of the other treatment three times before flowering. Square abscission at the first sympodial fruiting position was 5 and 33% for the control and infested plants, respectively, and yield was reduced 21% by insect infestation. Insect treatment resulted in 4% more light penetration through the canopy, which may have contributed to the 17% increase in photosynthesis of the eighth main-stem leaf from the terminal leaf as compared with the control plants. Canopy photosynthesis recorded 4 wk after the initiation of flowering was 21% higher in the infested plants. [CO.sub.2] labeling showed infestation also resulted in more 14C recovered in the terminal node (terminal leaf plus main stem above the terminal leaf) and less remaining in the branch at the same node as the source leaf, which corresponded to an increase in plant height, although node number was not affected. Since our insect-induced abscission treatments had similar effects as manual fruit removal treatments reported by others, future studies seem justified in using either approach. Early fruit loss in the U.S. Mid-south results in changes in carbon exchange and allocation, but poor late-season growing conditions often prevent yield compensation., Key insect pests of cotton have been shown to feed preferentially on flower buds (squares) and young bolls (Hearn and Fitt, 1992; Williams et al., 1987). Investigation of cotton plant [...]

Frogeye leaf spot (caused by Cercospora sojina Hara) is a common foliar disease of soybean (Glycine max L. Merr.) in the southeastern USA. Three major dominant genes (Rcs(sub 1), Rcs(sub 2), and Rcs(sub 3)) have been identified that confer resistance to frogeye leaf spot. Our objective was to determine the inheritance and possible allelism to Rcs(sub 3) of genes for resistance in 'Lee', 'Stonewall', and 'Ransom'. Lee, Stonewall, and Ransom were all intercrossed, crossed with 'Davis', which has Rcs(sub 3), and crossed with known susceptibles. F(sub 2) plants of crosses were inoculated with a spore suspension of race 5 of C. sojina, placed in a moist chamber for 2 to 3 d, and identified as susceptible or resistant based on leaf symptoms 14 d after inoculation. All F(sub 2) populations segregated 3 resistant: 1 susceptible or 15 resistant: 1 susceptible. This indicated different dominant genes for resistance to race 5 in Ransom, Stonewall, and Lee and that each was nonallelic to Rcs(sub 3) and to each other.

The greenbug (Schizaphis graminum (Rondani)) has been a major pest of sorghum (Sorghum bicolor (L.) Moench) since 1968, and several biotypes have evolved. Except for PI 266965 (S. halapense (L.) Pers., a tetraploid) and Cargill hybrid 607E, all previously reported sources of resistance to the widespread Biotype E are susceptible to Biotype I, a new biotype first reported in Kansas in 1990. Greenbug is a serious pest in Russia. To determine whether Russian sorghum contains resistance to Biotype I, 110 germplasm accessions obtained from Krasnodar Province in the fall of 1991 were assessed for their reaction to Biotype I greenbug. In two replicated growth chamber tests using individually caged plants, 10 accessions (3 grain sorghum and 7 sudangrass types) showed significantly higher (P = 0.05) levels of resistance than did the susceptible check when rated 13 to 14 d after infestation. The accession showing greatest resistance, PI 550610, a durra grain type originally from Syria, showed a level of resistance equal to that of the resistant hybrid check.

Only partial resistance is available for sheath blight, the second most important disease of rice (Oryza sativa L.) worldwide. This partial resistance has been considered to be polygenic. The partial resistance of the cultivars Jasmine 85 (J-85) and Teqing (TQNG) was studied by evaluating the resistance of [F.sub.1], [F.sub.2], [F.sub.3], [F.sub.4], and backcross [F.sub.1] generations of crosses between the resistant parents and the susceptible cultivars Maybelle (MBLE) and Cypress (CPRS). [F.sub.1] plants from the crosses were resistant. Segregating [F.sub.2] populations from both crosses showed ratios of 3:1 resistant/susceptible plants. Segregating backcross [F.sub.1] populations showed a ratio of 1:1 resistant/susceptible plants when the [F.sub.1] was crossed to a susceptible parent and no segregation occurred when the [F.sub.1] was crossed to a resistant parent. These results were consistent with the partial resistance from the two resistant cultivars being controlled by single dominant genes. When the resistant cultivars were crossed, the segregating [F.sub.2] population showed a 15:1 resistant/susceptible ratio. The results suggested that the two resistant parents each possessed a nonallelic dominant major resistance gene that segregated independently. Some of the [F.sub.4] lines from this cross appeared to have both resistance genes and a higher level of resistance than either resistant parent. This suggests that major genes conferring high levels of partial resistance to sheath blight may be incorporated together into lines to give near complete resistance. This makes the identification of major genes for partial resistance to sheath blight critically important to rice breeding programs, Sheath blight of rice, caused by Rhizoctonia solani Kuhn, is an economically important disease of rice, being second only to rice blast among the fungal diseases of rice worldwide (Marshall [...]

The most destructive fungal disease of soybean [Glycine max (L.) Merr.] is Phytophthora root rot. The most common method of protecting soybean from Phytophthora root rot utilizes specific resistance genes, but continuous selection pressure exerted on the pathogen frequently causes resistance genes in soybean to become ineffective. Infected plants which exhibit tolerance often yield nearly m well as uninfected resistant plants and prevent the buildup of races capable of overcoming resistance genes. Experiments were initiated to obtain information concerning the genetics of tolerance to Phytophthora root rot. The objectives were to: (i) compare parent-offspring regression, correlation, and realized heritability estimates of tolerance, (ii) compare heritability and phenotypic variation among single-, three-way, and double-cross populations, and (iii) correlate non-infected yields with tolerance and maturity. Ten [F.sub.3:4] populations were screened for tolerance in the [F.sub.3] and [F.sub.4] generations by the inoculum layer method. Heritability estimates were low, ranging from 0.17 to -0.10 when obtained through parent-offspring regression. These estimates were not separable from zero because of the magnitude of their standard errors. Parent-offspring correlation estimates were larger than parent-offspring regression estimates, and ranged from 0.43 to -0.19. Realized heritability was estimated by selecting the 20% highest and lowest tolerant lines within each population. These results were generally larger, ranging from 0.80 to -0.13, and suggested that relatively high tolerant lines may be identified through selection. The number of parents composing a population did not influence heritability or phenotypic variation within populations. Most correlations among yield, tolerance, and maturity were non-significant. Tolerance levels within populations should not vary with maturity. The inoculum layer method may not be sensitive enough to allow accurate screening of germplasm with low tolerance levels., Phytophthora sojae M.J. Kaufmann & J.W. Gerdemann (Syn. P. megasperma Drechs. f. sp. glycinea T. Kaun & D.C. Erwin) (Hansen and Maxwell, 1991), which causes Phytophthora root rot (PRR) of [...]