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Two isolates of Verticillium dahliae in vegetative compatibility group (VCG) 2B and one isolate in VCG 4A were tested for aggressiveness on peppermint, Scotch spearmint, and native spearmint alone and in combination with the lesion nematode Pratylenchus penetrans. The VCG 2B isolates were more aggressive than the VCG 4A isolate on all three mint species. Severity of wilt was greatest on peppermint and scotch spearmint with the VCG 2B isolates, and CFU of V. dahliae recovered from infected stems were greater in Scotch spearmint when inoculated with V. dahliae and P. penetrans than when inoculated with VCG 2B alone. Wilt severities and CFU in peppermint and Scotch spearmint were not increased when inoculated with VCG 4A and P. penetrans compared with VCG 4A alone. These data indicate that V. dahliae isolates from VCG 2B but not VCG 4A interact synergistically with P. penetrans on Verticillium isolate-susceptible mints. The resistance to Verticillium isolates in native spearmint was not affected by the presence of lesion nematodes.

The ability of several weed species to serve as hosts for tobacco rattle virus (TKV), the causal agent of corky ringspot disease of potato (CRS), and its nematode vector,Paratrichodorus allius, was investigated in greenhouse studies. ViruliferousP. allius multiplied on 24 out of 37 weed species tested, indicating they were suitable hosts of the vector. However, only 11 of these weeds were infected with TRV, as determined by ELISA. The nonhost status of a given weed species was not changed whether the viruliferous vector population originated from CRS problem fields in WA, OR, or ID. Several weeds served as hosts for the vector and virus including kochia, prickly lettuce, henbit, nightshade species (black, hairy, and cutleaf), common chickweed, and annual sowthistle. Virus-freeP. allius acquired TRV from the three nightshade species, volunteer potato grown from TRV-infected tubers, and prickly lettuce, and subsequently transmitted the virus to ‘Samsun NN’ tobacco indicator plants. Thus, some weeds may play a role in the epidemiology of CRS by perpetuating TRV and its vector in a problem field.

Tobacco rattle virus (TRV), the causal agent of corky ringspot disease of potato (CRS), was detected by reverse transcription-polymerase chain reaction (RT-PCR) in root tissues of field- and greenhouse-grown wheat (two) and corn (10) cultivars. TRV was also detected in some of the same samples by enzyme-linked immunosorbent assay (ELISA), but in a low frequency. Compared to ‘Samsun NN’ tobacco, wheat and corn appeared to be poorer hosts of TRV. However, these crops served as inoculum reservoirs of TRV. A nonviruliferous population of the nematodeParatrichodorus allius, the vector of TRV, acquired the virus from infected wheat and corn roots, and transmitted it to potato. ‘Russet Norkotah’ potatoes grown following TRV-infected corn and wheat, were blemished with arcs, concentric rings and diffuse browning, characteristic symptoms of CRS. The observations presented here suggest that wheat and corn as rotational crops will sustain TRV in a CRS problem field and the virus may cause severe damage on subsequent potato crops.

Soil samples from corky ringspot (CRS) problem fields of potato in the states of Washington and Oregon were collected and planted withNicotiana tabacum ‘Samsun NN’ tobacco to bait tobacco rattle virus (TRV) and to increaseParatrichodorus allius populations, the vector of TRV. Pathogenicity of three isolates of TRV was assessed on Russet Burbank and Russet Norkotah plants usingP. allius as the vector. The most severe CRS tuber symptoms were with TRV from Pasco, WA, followed by Umatilla, OR, and Mattawa, WA, indicating a distinct variation in virulence among virus isolates. The lowest number ofP. allius that transmitted TRV resulting in CRS symptoms on Russet Norkotah was three nematodes per 250 cm3 soil. When potato plants were exposed toP. allius at different times in their growth, tubers on older plants were more resistant than younger tubers to CRS. Severity of CRS tuber symptoms was correlated with age of potato plants in pot culture. Reproduction ofP. allius on potato roots did not influence the incidence and severity of tubers symptoms. Under similar conditions, Bintje, a variety known to be resistant to CRS, served as a host forP. allius, yet remained asymptomatic. The protocol employed in these studies is appropriate for testing potato germplasm for resistance to CRS.

Potato germplasm was assessed for resistance to corky ringspot disease (CRS) incited by transmission of tobacco rattle virus (TRV). TRV is transmitted by the stubby root nematode,Paratrichodorus allius, in the Pacific Northwest, and characterized by necrotic areas in the tuber. Four tests were conducted at two different problem fields in Umatilla, OR and Pasco, WA. The fields differed in the virulence of the virus isolates. Some clones and named varieties showed resistance only in the field with the relatively mild Umatilla isolate, while others remained symptomless at both sites. The host suitability of test potatoes to threeP. allius populations from the region was determined in greenhouse pot tests, and expressed as reproductive values [Rf = (final population of nematode at 55 days) / (initial population)]. TheR f values for the nematodes were not correlated with resistance ratings based on visual scoring of CRS symptoms in field grown potatoes. Thus, it appears that CRS resistance is based on reaction of potato genotypes to virus and not the vector. The availability of resistant clones inin vitro form is indicated.

Paratrichodorus allius, a vector of the tobacco rattle virus which causes the corky ringspot disease (CRS) in potato, was identified in soil samples from Oregon and Washington. Two populations from potato fields in Pasco, WA and Umatilla, OR reproduced successfully on alfalfa, wheat, and corn which are commonly rotated with potato, and most weeds associated with this crop in the Pacific Northwest. Greenhouse studies showed thatP. allius was sensitive to low soil moisture, and increased 10 and 100 fold on Samsun NN tobacco in soil above field capacity compared to 1/2 and 1/3 field capacity, respectively. Population dynamic studies for 1996 and 1997 on a potato-wheat cropping sequence showed thatP. allius declined after the winter wheat cover crop was disked in the spring of 1996, and remained low on potato at the 0–90 cm soil profile, with no evidence of downward migration. In 1997,P. allius declined early in the season on wheat, and remained low until June–July when it peaked before declining again. In soil columns,P. allius placed 30 cm below a confined tobacco root system was able to reach and transmit tobacco rattle virus. The nematodes below 30 cm transmitted the virus only when plant roots were allowed to grow to that depth.

Resistance toMeliodogyne chitwoodi races 1 (MC1) and 2 (MC2) andM. hapla (MH) derived fromSolanum bulbocastanum was introduced into the cultivated potato gene pool through somatic fusion. The initial F1 hybrids showed resistance to the three nematodes. Resistance to reproduction on roots by MC1 was accompanied by resistance to tuber damage in F1 clones. Tuber damage sometimes occurred, however, in hybrids of BC1 progeny resistant to reproduction on roots when MC2 and MH were the challenging nematodes. Resistance to reproduction was transferred into BC1 individuals, but a greater proportion of BC1 progeny was resistant to MC1 than to MC2 or MH. Resistance to MC1 appears to be dominant and discretely inherited. F1 and BC1 progeny were pollen sterile, but seed were produced from crosses using cultivated tetraploid pollen sources. Approximately 11 and 33 per cent of pollinations produced berries on F1 and BC1 pistillate parents, respectively. Seed yield increased fourfold overall in crosses with F1 compared to BC1 individuals.

Interspecific somatic hybrids were produced between tetraploid Solanum tuberosum and a nematode-resistant accession of the diploid species So/ahum bulbocastanum by protoplast fusion. Hybrid cells were selected using dual fluorescent labeling of protoplast preparations prior to fusion. Hybridity of regenerated plants was confirmed with a combination of morphological assessment, chromosome counting and isozyme analysis. Somatic hybrids had the same level of resistance to infection by race 1 of the nematode Meloidogyr~ chitwoodi as the S. bulbocastanum parent used in the fusion. Some of the somatic hybrids were fertile as females when crossed with tetraploid S. tuberosum breeding lines. Thus, these hybrids can be used in a potato improvement program to incorporate a valuable pest resistance.

Concord grape (Vitis labrusca) plants were inoculated with Macroposthonia xenoplax at levels of 100, 1,000, and 10,000 nematodes. After 4 months, plants inoculated with 10,000 M. xenoplax were stunted, and root systems were darker and had fewer feeder roots than those in other treatments. The lower nematode inoculation levels suppressed top growth but did not affect root growth. M. xenoplax reproduced well on Concord grapes.

Meloidogyne chitwoodi n. sp. is described and illustrated from potato (Solanum tuberosum) originally collected from Quincy, Washington, USA. This new species resembles M. hapla, but its perineal pattern is basically round to oval with distinctive and broken, curled, or twisted striae around and above the anal area. The vulva is in a sunken area devoid of striae. Vesicles or vesicle-like structures are present in the median bulb of females. The larva tail, being short and blunt with a hyaline tail terminal having little or no taper to its rounded terminus, is distinctively different from M. hapla. SEM observations revealed the nature of the perineal pattern and details of the head of larvae and males, and showed the spicules to have dentate tips ventrally. Hosts for M. chitwoodi n. sp. include potato, tomato, corn, and wheat but not strawberry, pepper, or peanut. The latter three crops are excellent hosts for M. hapla. The known distribntion of this new root-knot species presently involves certain areas of Idaho, Washington, and Oregon. The common name "Columbia root-knot nematode" is proposed for M. chitwoodi n. sp.

Meloidogyne chitwoodi and M. hapla were pathogenic to both roots and tubers of Russet Burbank potato. Both species affected root growth at 15, 20, and 25 C, but not 30 C. Meloidogyne chitwoodi reprotluced best at 15, 20, and 25 C and M. hapla at 25 and 30 C. Reproduction of M. chitwoodi was reduced at 30 C; reproduction of M. hapla was reduced at 15 C and less at 20 C. The reproductive potential of M. chitwoodi was higher than that of M. hapla at 15, 20, and 25 C. M. hapla reproduced better at 30 C than did M. chitwoodi. M. chitwoodi infected potato tubers in higher numbers than did M. hapla.

Mount St. Helens volcanic ash was incorporated into a loamy sand greenhouse soil mix to produce concentrations of 0, 0.5, 1.0, 2.0, 4.0, 8.0, 25, 50 and 100% ash. Chemical and physical properties of the various mixtures were determined. Three experiments were conducted in a greenhouse to determine if volcanic ash had any influence on root-knot nematode survival and infectivity. Tomato, Lycoperscion esculentum, seedlings cv. Columbia, susceptible to Meloidogyne hapla and M. chitwoodi were planted into pots of the soil-ash concentrations and infested with one of the two nematode species. Tomato seedlings were harvested 30, 50 and 60 days later and the roots examined for nematode infection and reproduction. Ash incorporation had no deliterious effect on root-knot nematodes in any of the experiments reported here. Nematode infection and reproduction on tomato were not affected at any ash concentration.

Metham sodium applied in October through center pivot irrigation systems was evaluated for control of Meloidogyne hapla at 374, 468, and 701 liters/ha and for control of M. chitwoodi at 468 liters/ha on potato. Metham sodium at the high rates effectively controlled M. hapla. No females were detected in the tubers at the high rates of nematicide application, whereas a mean of 19 and 69% of the tubers were infected at the low rate and in the nontreated controls, respectively. In the M. chitwoodi trial only 1.5% of the tubers in the treated plots were infected compared with 82% in the nontreated plots. Metham sodium effectively controlled M. chitwoodi to soil depths of 30, 61, and 91 cm.

Meloidogyne chitwoodi reduced the growth of winter wheat 'Nugaines' directly in relation to nematode density in the greenhouse, The relationship between top dry weight and initial nematode density suggests a tolerance limit of Nugaines wheat to M. chitwoodi of between 0.03 and 0.18 eggs/cm(3) of soil; the value for relative minimum plant top weight was 0.45 g and 0.75 g, respectively. Growth of wheat in field microplots containing four population densities (0.003, 0.05, 0.75 and 9 eggs/cm(3) soil) was not affected significantly at any inoculum level compared to controls during September to July, However, suppression of head weights of 'Fielder' spring wheat grown May-July occurred in microplots initially infested with 0.75 and 9 eggs/cm(3) soil. Reproduction (Pf/Pi) was poorer at these two inoculum levels as compared to the lower densities. In another greenhouse experiment, roots of wheat cultivars Fielder, 'Fieldwin,' 'Gaines,' 'Hyslop,' and Nugaines became infected by M. chitwoodi, but not by M. hapla. Reproduction of M. chitwoodi was less on Gaines and Nugaines than on Fielder, Fieldwin, or Hyslop.

Meloidogyne chitwoodi developed and reproduced more rapidly than M. hapla in potato roots at 15, 20, or 25 C when both species of nematodes were inoculated simultaneously at 250 or 1,000 juveniles of each. At 30 C significantly more M. hapla than M. chitwoodi females were found at the lower inoculum level after 41 days. More M. chitwoodi than M. hapla juveniles were extracted from soil at 15, 20, and 25 C, but only at the lower inoculum level at 30 C. Potato was considered a more suitable host for M. chitwoodi than M. hapla because of M. chitwoodi's greater reproduction at 15, 20, and 25 C. Corn and wheat cultivars tested supported M. chitwoodi reproduction at temperatures of 10, 15, 20, and 25 C, but fewest eggs were produced on these plants at 20 C. Temperatures of 10 to 25 C had little influence on the low reproduction of M. chitwoodi on four alfalfa cultivars. M. chitwoodi reproduced on the alfalfa entry Mn PL9HF.

Most of the 15 carrot cultivars tested were moderate to good hosts to Meloidogyne chitwoodi race 1, whereas all except Orlando Gold were nonhosts or poor hosts for M. chitwoodi race 2. All carrot cultivars were good hosts for M. hapla. The plant weights of the carrot cultivars Red Cored Chantenay and Orlando Gold infected with either race of M. chitwoodi were significantly less than uninoculated checks in pots. Under field microplot conditions, however, detrimental effects on quality were rarely observed. M. hapla was pathogenic to both cultivars in the greenhouse and the field. The tolerance level of Orlando Gold to M. hapla was lower than Red Cored Chantenay.

Second-stage juveniles (J2) of races 1 and 2 of Meloidogyne chiiwoodi and M. hapla readily penetrated roots of Thor alfalfa and Columbian tomato seedlings; however, few individuals of M. chitwoodi race 1 were able to establish feeding sites and mature on alfalfa. Histopathological studies indicate that J2 of race 1 either failed to initiate feeding sites or they caused cell enlargement without typical cell wall thickening. The protoplasm of these cells coagulated, and juveniles of race 1 did not develop beyond the swollen J2 stage. A few females of race 1 fed on small giant cells and deposited a few eggs at least 20 and 30 days later than M. chitwoodi race 2 and M. hapla, respectively. Failure of race 1 to establish feeding sites was related to egression of J2 from the roots. The M. chitwoodi race 1 J2 egression from alfalfa roots was higher than egression of race 2 and M. hapla. Egression of J2 of M. chitwoodi races 1 and 2 from tomato roots was similar and higher than that of M. hapla. Thus egression plays an important role in the host-parasite relationship of M. chitwoodi and alfalfa.

Migratory ability of second-stage juveniles (J2) of two Meloidogyne chitwoodi races and a M. hapla population were compared in soil-filled columns at 12, 18, and 24 C. J2 of all populations migrated farthest at 18 C and least at 12 C. Nematode survival was significantly reduced (P = 0.05) at 24 C.M. chitwoodi J2 migrated further and in greater numbers than M. hapla J2 at all temperatures. A comparison with and without a host plant demonstrated no preferential migration toward the plant. Water percolation through the migration columns stimulated upward migration.

Meloidogyne chitwoodi races 1 and 2 and M. hapla reproduced on 12 cultivars of Brassica napus and two cultivars of B. campestris. The mean reproductive factors (Rf), Rf = Pf at 55 days ÷ 5,000, for the three nematodes were 8.3, 2.2, and 14.3, respectively. All three nematodes reproduced more efficiently (P < 0.05) on B. campestris than on B. napus. Amending M. chitwoodi-infested soil in plastic bags with chopped shoots of Jupiter rapeseed reduced the nematode population more (P < 0.05) than amendment with wheat shoots. Incorporating Jupiter shoots to soil heavily infested with M. chitwoodi in microplots reduced the nematode population more (P < 0.05) than fallow or corn shoot treatments. The greatest reduction in nematode population density was attained by cropping rapeseed for 2 months and incorporating it into the soil as a green manure.

Seasonal vertical migration of Meloidogyne chitwoodi through soil and its impact on potato production in Washington and Oregon was studied. Nematode eggs and second-stage juveniles (J2) were placed at various depths (0-180 cm) in tubes filled with soil and buried vertically or in holes dug in potato fields. Tubes were removed at intervals over a 12-month period and soil was bioassayed on tomato roots. Upward migration began in the spring after water had percolated through the tubes. Nematodes were detected in the top 5 cm of tubes within 1-2 months of burial, depending on depth of placement. Potatoes were grown in field plots for 4 or 5 months before the tubers were evaluated for infection. One hundred eggs and J2 per gram soil placed at 60 and 90 cm caused significant tuber damage at the Washington and Oregon sites, respectively. At the Washington site, inoculum placed at 90, 120, and 150 cm caused potato root infection without serious impact on tuber quality, but inoculum diluted 2-8 times and placed at 90 cm did not cause root or tuber infection. Nematode migration was dependent on soil texture; 9 days after placement at the bottoms of tubes, J2 had moved up 55 cm in sandy loam soil (Oregon) but only 15 cm in silt loam (Washington). Thus, the importance of M. chitwoodi which occur deep in a soil profile may depend on soil texture, population density, and length of the growing season.

Responses of egg masses, free eggs, and second-stage juveniles (J2) ofMeloidogyne hapla and M. chitwoodi to ethoprop were evaluated. The results indicated that J2 were the most sensitive, followed by free eggs and egg masses. In general, M. chitwoodi was more susceptible to ethoprop than M. hapla. Ethoprop at 7.2 mug a.i./g soil protected tomato roots from upward migrating M. chitwoodi for 5 weeks. The zone of protection was extended to 10 and 20 cm below the root zone when 3.6 and 7.2 cm water were applied over 8 days. Ethoprop at 1.8, 3.6, and 7.2 mug a.i./g soil degraded faster and killed fewer M. chitwoodi J2 in potato field soil previously exposed to ethoprop than in unexposed soil or sterilized exposed soil. The enhanced biodegradation property of the exposed soil lasted 17 months after the last application of ethoprop. The limited downward movement of ethoprop in the soil, migration of M. chitwoodi J2 into the treated zone, presence of resistant life stage(s) at the time of application, and loss of efficacy due to enhanced biodegradation may have a significant effect on the performance of ethoprop.

An accession of Solanum hougasii, a wild tuber-bearing potato species native to Mexico, was found to be resistant to races 1 and 2 of Meloidogyne chitwoodi. A resistant selection was selfed and its progeny possessed the same combined resistance uniformly. A selected resistant seedling from the selfed progeny was crossed to cultivated tetraploid potato (S. tuberosum) to form an F₁ hybrid, and was backcrossed to cultivated tetraploid potato to form a BC₁ population in which resistance to the two races segregated. Progeny of the BC₁ were tested in inoculation experiments with four replicates for each progeny genotype for each race of nematode. Resistance was evaluated on the basis of extracted egg counts from the entire root system of pot-grown plants. Considering resistance to each race separately, for race 1, non-host (Rf ≤ 0.1) status was exhibited by approximately half of the BC₁. About one-third of the progeny showed non-host status to race 2. Egg production among progeny that showed non-host status for both races was higher with race 2 than with race 1. Analysis of co-segregation established that genetic control for the two races appears to be independently segregating. Although genes for resistance to race 1 derived from S. bulbocastanum and S. fendleri were previously described, this report is the first analysis showing independent genetic control in Solanum spp. for resistance to race 2 of M. chitwoodi only.

Several tuber-bearingSolanum species were screened for resistance to races 1 and 2 of Columbia root-knot nematode,Meloidogyne chitwoodi. Reproductive factor (Rf = Pf/Pi) was assessed and used to infer host resistance. Several clonal selections in one Plant Introduction accession ofSolanum hougasii showed high levels of resistance to both races. Crosses ofS. hougasii with tetraploid breeding clones yielded 14 vigorous F1 hybrids. Three of these interspecific hybrids expressed non-host (Rf 1.0) to race 2. Three hybrids were good hosts to both races, and one was a poor host to race 1 and non-host to race 2. The occurrence of resistance to both races in the F1 hybrids indicates success in the first step of introducing resistance to races 1 and 2 ofM. chitwoodi to the cultivated potato gene pool.

The mapping of resistance toMeloidogyne chitwoodi derived from Solarium bulbocastanum is reported. A population suitable for mapping was developed as follows. A somatic hybrid of nematode-resistant S. bulbocastanum and cultivated tetraploid potato was produced. This was backcrossed to tetraploid potato, and a single resistant BC1 was selected and backcrossed again to the same recurrent tetraploid parent. The mapping population consisted of 64 BC2 progeny scored for restriction fragment length polymorphic (RFLP) markers and 62 of these were evaluated for the reproductive efficiency of race 1 of M. chitwoodi. Forty-eight polymorphic RFLP markers, originally derived from tomato and mapped in diploid cultivated potato, were assigned to 12 chromosomes of S. bulbocastanum. Of the 62 progeny screened for nematode resistance, 18 were non-hosts and four were poor hosts. The rest were highly susceptible (good hosts). Analysis of the resistance (including non-hosts and poor hosts) as both a qualitative trait and as a meristic trait on which QTL analysis was applied supported the same genetic hypothesis. Genetic control was localized solely to factor(s) lying at one end of chromosome 11. The level of expression of resistance in the S. bulbocastanum parent and the resistant portion of the BC2 was essentially the same. This fact, together with the highly significant LOD scores for one end of the chromosome-11 marker array, supports a genetic model equivalent to monogenic dominant control.

Overwinter N leaching increases the potential for NO 3 contamination of ground water under irrigated desert soils. Our objectives were to (i) identify winter cover crops for recovering N and (ii) determine the N availability following green manure cover crops relative to N uptake by a succeeding potato (Solanum tuberosum L.) crop. A 2-yr field study was conducted on commercial fields on Quincy loamy sand (mixed mesic Xeric Torripsamments). Cover crops were seeded following sweet corn (Zea mays L.) as fall-incorporated sudangrass (Sorghum bicolor L., 'Sordan'); fall- and spring-incorporated white mustard (Brassica hirta Moench, 'Martegena'); and spring-incorporated wheat (Triticum aestivum L., 'Stevens'), rapeseed (B. napus L., 'Jupiter'), and rye (Secale cereale L.). Potato ('Russet Burbank') was planted 3 to 5 wk after spring incorporation of green manures. All cover crops except sudangrass accumulated 112 to 142 kg N ha -1 when planted in August at Plymouth, WA, but N uptake was decreased by >50% when planted in September at Quincy, WA. Overwintering cover crops lowered soil mineral N at 0 to 180 cm by 155 kg N ha -1 compared with bare fallow at Plymouth. Fall-incorporated mustard released greater inorganic N over the winter compared with spring incorporation. Soil NH 4 and NO 3 increased following overwintering cover crops by potato planting, providing timely increases in N availability to the potato crop. Winter cover crops can improve N cycling and reduce the amount of N below the root zone in potato-based rotations.

Leaves of the rapeseed (Brassica napus) cultivar Jupiter used as a soil amendment effectively reduced Meloidogyne chitwoodi population densities at the zone of incorporation. Soil in the zone of incorporation was protected from nematode colonization for 6 wk. Stem and root tissues of rapeseed were also effective when homogenized before being used as an amendment. Greenhouse-grown plants accumulated more glucosinolates and became more effective in suppressing M. chitwoodi with age (.)

Plant-parasitic nematodes have a significant economic impact on potato production in the temperate zones. The most damaging nematodes on potatoes are the potato cyst nematodes, Globodera pallida and G. rostochiensis. Of the non-cyst nematodes (NCN), the potato rot (Ditylenchus destructor), root-lesion (Pratylenahus penetrans), root-knot (Meloidogyne spp.) and stubby-root (Paratriehodorus and Trichodorus spp.) nematodes are most important. Literature on NCN problems in potato production, of which the occurrence or effect depends on crop rotation systems or cropping frequency, is limited. This is due to the wide host range of NCN, which makes crop rotation impractical. Thus, the most common means to control NCN on potatoes is by the use of nematicides. However, in the USSR crop rotation with adequate weed control, use of clean seed, and planting potatoes every 3 or 4 years had been reported to satisfactorily control D. destructor. In the northwestern United States, a three-year crop rotation along with nematicides is used to control M. chitwoodi and M. hapla on potatoes. Monoculture with potatoes has been shown to reduce the efficacy of soil fumigants in controlling M. chitwoodi. Crops commonly grown in rotation with potatoes in the northwestern United States are alfalfa and cereals. The cereals are used to suppress M. hapla populations but are good hosts for M. chitwoodi. Alfalfa is a good host for M. hapla and M. chitwoodi race 2 but unsuitable for race 1. Thus, a major part of the problems encountered with M. chitwoodi on potatoes is due to an inadequate crop rotation system.

Twenty clones showed nonhost reactions (R=