Search

The infection of the floral tissues of snap bean and other crops by Sclerotinia sclerotiorum, the causative agent of white mold, is by ascospores. Irrespective of the fungicide mode of action being evaluated, in vitro fungicide sensitivity tests are conducted almost exclusively using mycelial growth assays. This is likely because of difficulties and time involved in sclerotial conditioning required to produce apothecia and ascospores. The objective of this research was to compare estimates of fungicide sensitivity between mycelial growth and ascospore germination assays for S. sclerotiorum. Sensitivity assays were conducted using serial doses of three fungicides commonly used to control white mold: boscalid, fluazinam, and thiophanate-methyl. A total of 27 isolates were evaluated in replicated trials conducted for each fungicide and assay type. The effective concentration to reduce mycelial growth or ascospore germination by 50% (EC50) was estimated for each isolate, fungicide, and assay type. The median EC50 values obtained from ascospore germination assays were 52.7, 10.0, and 2.7 times higher than those estimated from the mycelial growth for boscalid, fluazinam, and thiophanate-methyl, respectively. No significant correlation was found between EC50 values estimated by the two methods. These findings highlight differences that may be important in evaluating the sensitivity of S. sclerotiorum given the fungicide mode of action and how they will be used in the field.

Rosellinia necatrix is the causal agent of white root rot (WRR), a fatal disease affecting many woody plants, including avocado (Persea americana). As with other root diseases, an integrated approach is required to control WRR. No fully effective control methods are available, and no chemical or biological agents against R. necatrix have been registered for use on avocado in South Africa. Fluazinam has shown promising results in the greenhouse and field in other countries, including Spain. The current study aimed to investigate the potential of a fumigant, chloropicrin, and biological control agents (B-Rus, Beta-Bak, Mity-Gro, and Trichoderma) against R. necatrix both in vitro and in vivo as compared with fluazinam. In a greenhouse trial, results showed that Trichoderma and B-Rus were as effective as fluazinam at inhibiting R. necatrix in vitro and suppressed WRR symptoms when applied before inoculation with R. necatrix. In contrast, Mity-Gro and Beta-Bak failed to inhibit the pathogen in vitro and in the greenhouse trial, despite application of the products to plants before R. necatrix infection. Fluazinam suppressed WRR symptoms in plants when applied at the early stages of infection, whereas chloropicrin rendered the pathogen nonviable when used as a preplant treatment. Plants treated with Trichoderma, B-Rus, and fluazinam sustained dry mass production and net CO2 assimilation by maintaining the green leaf tissues despite being infected with the pathogen. This study has important implications for the integrated management of WRR.

Garlic leaf blight caused by Stemphylium eturmiunum was first reported in Jiangsu Province in China. The dicarboximide fungicide (DCF) procymidone is reported to possess broad-spectrum action in inhibiting filamentous fungi and is widely used to control leaf disease of various plants. Of 41 Stemphylium eturmiunum isolates collected in this study from commercial garlic farms in Pizhou and Dafeng counties of Jiangsu Province, eight isolates were resistant to procymidone. The following three phenotypes were categorized according to in vitro responses to DCFs: sensitive, low resistance to iprodione and procymidone, and high resistance to all iprodione and procymidone. The fitness of all resistant isolates was decreased in accordance with data on mycelial growth, conidiation, and virulence. After treatment with 10 µg/ml of procymidone for 4 h, mycelial intracellular glycerol concentrations of resistant isolates were significantly lower than those of sensitive isolates. Positive cross-resistance was observed between dicarboximides and phenylpyrroles, but there was no cross-resistance between dicarboximides and fluazinam or difenoconazole in the two resistant phenotypes. Nucleotide sequence alignment of two-component histidine kinase genes from sensitive and resistant isolates indicated that amino acid mutations were located at the histidine kinase, adenylyl cyclase, methyl-accepting chemotaxis protein and at the phosphatase domain of the N-terminal region and the response regulator domain of the C-terminal region. To our knowledge, this is the first report of DCF resistance in Stemphylium eturmiunum, and these findings will help establish a rational strategy to manage DCF-resistant populations of Stemphylium eturmiunum in the field.

Gray mold, caused by the fungus Botrytis cinerea Pers ex Fr., is one of the most destructive spoilage diseases, severely affecting tomato production in Henan Province, China. Spraying fungicides from the flowering to the harvest stage is a necessary measure to reduce losses associated with B. cinerea infection. However, B. cinerea has developed resistance to fungicides in many countries. Boscalid is a succinate dehydrogenase inhibitor (SDHI) fungicide and was registered for the control of gray mold. In this study, a total of 269 B. cinerea isolates were collected from tomato in commercial greenhouses in different locations of Henan Province in 2014 and 2015. The sensitivity and resistance of B. cinerea field isolates were determined based on mycelial growth. The effective concentration 50 ranged from 0.11 to 15.92 µg/ml and 0.16 to 8.54 µg/ml, in 2014 and 2015, respectively. The frequency of low resistance to boscalid was 12.6 and 7.6%, and moderate resistance was 2.7 and 1.3% in 2014 and 2015, respectively. No highly resistant isolates were found in Henan Province, China. Mycelial growth, mycelial dry weight, spore production, and pathogenicity were not significantly different between resistant and sensitive phenotypes of the B. cinerea isolates. The results of cross-resistance testing showed no correlation between boscalid and carbendazim, procymidone, pyrimethanil, fluazinam, or fluopyram. In this study, the succinate dehydrogenase genes B (sdhB), C (sdhC), and D (sdhD) were analyzed and compared in sensitive and low-resistance and moderately resistant B. cinerea isolates to boscalid. Results showed that point mutations occurred simultaneously at sdhC amino acid positions 85 (G85A), 93 (I93V), 158 (M158V), and 168 (V168I) in 4 out of 10 sensitive isolates and 23 of 26 low-resistance and 5 of 5 moderately resistant B. cinerea isolates to boscalid. No point mutations were found in the sdhB and sdhD genes of all isolates. Furthermore, no point mutations were found in sdhB, sdhC, and sdhD genes in 3 of 26 low-resistance B. cinerea isolates to boscalid. Therefore, we speculate that the simultaneous point mutations in the sdhC gene may not be related to the resistance of B. cinerea to boscalid. These results suggested that there might be a substitution mechanism for the resistance of B. cinerea to the SDHI fungicide boscalid.

Spongospora subterranea is a soilborne plasmodiophorid that causes powdery scab in potato. It also transmits potato mop-top virus (PMTV), which causes necrotic arcs (spraing) in potato tubers. Three field experiments were conducted in naturally S. subterranea-infested soil to investigate the effects of two chemicals, Omega 500F (fluazinam) and FOLI-R-PLUS RIDEZ (biological extract), on powdery scab, PMTV, and changes in S. subterranea inoculum with six different potato cultivars. The efficacy of soil treatment with these two chemicals on tuber lesions, root galling, and pathogen population was also assessed in greenhouse trials. The chemical treatments did not reduce powdery scab, root gall formation, or S. subterranea inoculum in the field or greenhouse trials. Postharvest S. subterranea soil inoculum in fields varied across farms and among potato cultivars but the pathogen population consistently increased by the end of the growing season. The evaluated russet cultivars were more tolerant to powdery scab than the yellow- or red-skinned cultivars but all were susceptible to PMTV. In the field, powdery scab indices and soil inoculum changes were positively correlated, while postharvest S. subterranea inoculum was positively correlated with root galling in both greenhouse trials. Powdery scab and PMTV occurred in noninoculated potting mix, indicating that peat-based potting mix is a source for both pathogens. These results demonstrate that chemical management methods currently used by farmers are ineffective, that S. subterranea and PMTV in potting mix can cause severe epidemics in greenhouses, and that potato cultivar choices impact inoculum increases in soil.

Rhizoctonia solani is a widely distributed soilborne plant pathogen, and can cause significant economic losses to crop production. In chemical controls, SYP-14288 is highly effective against plant pathogens, including R. solani. To examine the sensitivity to SYP-14288, 112 R. solani isolates were collected from infected rice plants. An established baseline sensitivity showed that values of effective concentration for 50% growth inhibition (EC50) ranged from 0.0003 to 0.0138 μg/ml, with an average of 0.0055 ± 0.0030 μg/ml. The frequency distribution of the EC50 was unimodal and the range of variation factor (the ratio of maximal over minimal EC50) was 46.03, indicating that all wild-type strains were sensitive to SYP-14288. To examine the risk of fungicide resistance, 20 SYP-14288-resistant mutants were generated on agar plates amended with SYP-14288. Eighteen mutants remained resistant after 10 transfers, and their fitness was significantly different from the parental strain. All of the mutants grew more slowly but showed high virulence to rice plants, though lower than the parental strain. A cross-resistance assay demonstrated that there was a positive correlation between SYP-14288 and fungicides having or not having the same mode of action with SYP-14288, including fluazinam, fentin chloride, fludioxonil, difenoconazole, cyazofamid, chlorothalonil, and 2,4-dinitrophen. This result showed a multidrug resistance induced by SYP-14288, which could be a concern in increasing the spectrum of resistance in R. solani to commonly used fungicides.

Cucumber target spot, caused by Corynespora cassiicola, is a devastating fungal disease in greenhouses in China. Lack of resistant cultivars and unscientific use of fungicides aggravated the difficulty to manage this disease. In recent years, resistance of C. cassiicola to benzimidazoles, quinone outside inhibitors, and succinate dehydrogenase inhibitors has occurred in China. Here, we tested the fluazinam sensitivity distribution of 79 C. cassiicola isolates from different provinces in China based on mycelial growth inhibition. The EC50 values of fluazinam ranged from 0.1002 to 0.3129 µg/ml with a mean of 0.2136 ± 0.0495 µg/ml, and the sensitivity frequency was normally distributed (P = 0.2083, Shapiro–Wilk test). Meanwhile, the EC50 values for spore germination inhibition ranged from 0.0992 to 0.2278 µg/ml with a mean of 0.1499 ± 0.0504 µg/ml. This indicated that fluazinam exhibited an excellent in vitro fungicidal activity on both mycelial growth and spore germination. In addition, fluazinam also exhibited a good in planta control efficacy on detached cucumber leaves in the protective and curative assays. Moreover, the biological and physiological characteristics of C. cassiicola as affected by fluazinam were determined. Fluazinam not only significantly inhibited respiration and adenosine triphosphate production but also caused the increase of cell membrane permeability and the dysfunctions of cellular homeostasis. Interestingly, we found that fluazinam especially damaged vacuole structures, causing the redistribution of vacuole substances. Taken together, our findings provide not only essential references for resistance management of C. cassiicola but also interesting insights for further revealing the action mode of fluazinam against plant pathogens.

Asparagus stem blight caused by Phomopsis asparagi is a major hindrance to asparagus production worldwide. Currently, fungicides are used to manage the disease in commercial production, but resistance to common fungicides has emerged in the wild population. In the present study, 132 isolates of P. asparagi collected from different provinces in China were tested for sensitivities to pyraclostrobin, tebuconazole, and fluazinam. We also determined the efficacies of six fungicides against P. asparagi. The frequency distributions of EC50 values of the isolates tested were unimodal, but the curves for pyraclostrobin and tebuconazole had long right-hand tails. The mean EC50 values for pyraclostrobin, tebuconazole, and fluazinam were 0.0426 ± 0.0029, 0.6041 ± 0.0416, and 0.0314 ± 0.0013 μg/ml, respectively. In addition, the EC50 values for pyraclostrobin were very similar with or without salicylhydroxamic acid (SHAM), 20 μg/ml, indicating that SHAM is not needed to determine the sensitivity of P. asparagi to pyraclostrobin when using the mycelial growth inhibition assay. In greenhouse assays, Merivon (42.4% fluxapyroxad plus pyraclostrobin SC), Frown-cide (500 g/liter fluazinam SC), Cabrio (250 g/liter pyraclostrobin EC), and Nativo (75% trifloxystrobin plus tebuconazole WG) showed excellent preventive efficacy against P. asparagi. And these fungicides were more effective before inoculation than when they were applied after inoculation (P < 0.05). Therefore, these fungicides should be applied prior to infection to control stem blight. In field trials, Frown-cide, Merivon, Nativo, and Cabrio also performed good control effects, ranging from 75.2 to 86.0% in 2017 and 75.4 to 87.1% in 2018. We demonstrated that Frown-cide, Merivon, Nativo, and Cabrio had considerable potential to manage asparagus stem blight. In addition, rotations of these fungicides are essential for precluding or delaying the development of resistance and for controlling the disease.

The grapevine trunk diseases Eutypa and Botryosphaeria dieback threaten the sustainability of vineyards worldwide. This study aimed to develop practical and efficient wound protection strategies, which will lead to widespread adoption of preventative control for these diseases and increased longevity of vineyards. Five fungicides (tebuconazole, carbendazim, fluazinam, mancozeb, and flusilazole) were evaluated for efficacy against Eutypa lata and Neofusicoccum luteum infection on pruning wounds of ‘Sauvignon blanc’ in a New Zealand vineyard. All five fungicides controlled infection by both pathogens to varying degrees, with mean percent disease control of 70 to 90% for E. lata and 33 to 93% for N. luteum. This has led to the first known registration of a fungicide, fluazinam (Gem), for simultaneous control of both Eutypa and Botryosphaeria dieback. Furthermore, application of carbendazim with tractor-driven sprayers provided similar control of E. lata and N. luteum to that when applied by hand with a paintbrush. This constitutes the first report of Botryosphaeria dieback control with fungicides applied with tractor-driven sprayers.

Botryosphaeria dothidea, the causal agent of apple ring rot, is an important fungal plant pathogen that can cause serious reductions in crop yield, and fungicides still play a crucial role in management. In the present study, the sensitivity of B. dothidea to fludioxonil, fluazinam, and pyrisoxazole was assessed in 162 isolates. Moreover, the protective and curative activity of the three fungicides on detached apple fruit as well as the control efficacy in the field were determined. The results showed that the mean 50% effective concentration (EC50) values (± standard deviation) were 0.01 ± 0.008, 0.04 ± 0.03, and 0.02 ± 0.01 μg ml−1, with individual EC50 values of 0.002 to 0.05, 0.003 to 0.19, and 0.005 to 0.26 μg ml−1 for fludioxonil, fluazinam, and pyrisoxazole, respectively. In addition, the frequency distributions of EC50 values were both unimodal curves. However, significant correlations (P < 0.05) were found between fludioxonil and iprodione, between fluazinam and iprodione, as well as between pyrisoxazole and difenoconazole. In field trials conducted during 2016 and 2017, the control efficacy ranged from 75.91 to 87.41% when fludioxonil was applied at 100 to 150 mg active ingredient (a.i.) kg−1, 81.90 to 85.13% when fluazinam was applied at 400 mg a.i. kg−1, and 77.43 to 80.97% when pyrisoxazole was applied at 400 mg a.i. kg−1. The control efficacy of the fungicides in storage was higher than 60%, with the exception of fluazinam. These results demonstrated that fludioxonil, fluazinam, and pyrisoxazole have considerable potential to control apple ring rot.

White mold (Sclerotinia sclerotiorum) of leguminous crops in New York is generally managed with preventive applications of fungicides. However, no research has been conducted during the last decade to assess the sensitivity of the S. sclerotiorum population to fungicides or compare their performance under field conditions. The sensitivity of S. sclerotiorum to boscalid, fluazinam, and thiophanate-methyl was assessed in 151 isolates from 15 fields across New York using an agar dilution method with discriminatory concentrations. In addition, the effective concentration at which mycelial growth is reduced by 50% (EC50) was estimated for one representative isolate from each field. The efficacy of commercial formulations of each fungicide on white mold incidence in plants and pods was also tested in two field trials (2015 and 2016). The EC50 values ranged from 0.068 to 0.219, 0.001 to 0.002, and 1.23 to 2.15 µg/ml for boscalid, fluazinam, and thiophanate-methyl, respectively. Evidence of resistance was not found using the discriminatory concentration tests. The mycelial growth inhibition relative to the control ranged from 56 to 83%, 66 to 84%, and 53 to 83% at discriminatory concentrations of boscalid (5 µg a.i./ml), fluazinam (0.05 µg a.i./ml), and thiophanate-methyl (5 µg a.i./ml), respectively. Fourteen isolates with mycelial growth inhibition lower than 60% at 5 µg/ml of thiophanate-methyl, did not exhibit point mutations within a partial sequence of the β-tubulin gene. In the field trials, fungicides effectively reduced white mold incidence on plants by 75% (2015) and 93% (2016) and on pods by 81% (2015) and 87% (2016), both relative to the nontreated plots. However, fungicide applications led to significant increases in pod yield, relative to the nontreated plots, only in 2015 when the incidence of white mold on plants and pods were higher (85 and 49.2%) than in 2016 (31.3 and 10.3%). Although fungicide resistance was not detected, and thus control failures reported by New York snap bean growers may be due to other factors, further monitoring of sensitivity within the S. sclerotiorum population is encouraged as well as the use of rational systems to base their judicious and economic use.

Thiophanate-methyl (TM), fluazinam, and procymidone are fungicides extensively used for white mold control of common bean in Brazil. We assessed the sensitivity of Brazilian isolates of Sclerotinia sclerotiorum to these three fungicides using discriminatory doses and concentration that results in 50% mycelial growth inhibition (EC50) values. In total, 282 isolates from the most important production areas were screened and none was resistant to fluazinam or procymidone. The EC50 values varied from 0.003 to 0.007 and from 0.11 to 0.72 μg/ml for fluazinam and procymidone, respectively. One isolate was resistant to TM. The EC50 of the TM-resistant isolate was greater than 100 μg/ml, whereas the EC50 of the sensitive isolates varied from 0.38 to 2.23 μg/ml. The TM-resistant isolate had a L240F mutation in the β-tubulin gene. This is the first report of mutation at codon 240 causing resistance to a benzimidazole fungicide in S. sclerotiorum. The high-resolution melting analysis allowed the distinction of TM-sensitive and -resistant isolates by specific melting peaks and curves. The TM-resistant isolate had mycelial growth, sclerotia production, and aggressiveness comparable with that of the sensitive isolates, indicating that this genotype will likely compete well against sensitive isolates in the field. This study demonstrates that resistance to TM, fluazinam, and procymidone is nonexistent or rare. Resistance management practices should be implemented, however, to delay the spread of TM-resistant genotypes.

The effects of soil temperature and moisture, and nine pesticides commonly used in peanut production, on the mycoparasitic activity of Coniothyrium minitans on sclerotia of Sclerotinia minor were evaluated. In vitro mycelial growth and conidia germination of C. minitans were sensitive to azoxystrobin, chlorothalonil, fluazinam, pyraclostrobin, tebuconazole, and diclosulam. C. minitans survived and infected sclerotia of S. minor in the presence of azoxystrobin, chlorothalonil, diclosulam, fluazinam, flumioxazin, S-metolachlor, pendimethalin, pyraclostrobin, and tebuconazole. Mycoparasitic activity was reduced by all pesticides except S-metolachlor compared with the nontreated control. Optimum conditions for infection of sclerotia were temperatures from 14 to 22°C and soil moisture from -0.33 to -1 kPa × 102. Mycoparasitic activity of C. minitans remained high (98% sclerotia infected) at temperatures ranging from 14 to 22°C, but decreased at temperatures above 28°C. Viability of sclerotia was inversely related to the proportion infected by C. minitans (r = -0.9963, P = 0.001). Mycoparasitic activity also declined when soil moisture was greater than -1 kPa × 102 or less than -0.10 kPa × 102. These results indicate that C. minitans should not be applied when temperatures exceed 28°C, during extremes in soil moisture, or when there is a high risk of contact with pesticides before it becomes established in the soil.

Fungicides were applied to potato foliage at row closure (between rows) and at full bloom of primary inflorescences to control Sclerotinia stem rot during replicated trials in 2003, 2004, and 2005. Application at row closure followed labeled recommendations from manufacturers. Incidence of Sclerotinia stem rot did not vary significantly among fungicides when full labeled rates of thiophanate-methyl, fluazinam, and boscalid were applied at full bloom of primary inflorescences. Incidence of Sclerotinia stem rot was significantly less when fungicides thiophanatemethyl, fluazinam, or boscalid were applied to potato foliage at full bloom of primary inflorescences than at row closure or when fungicides were not applied in 2004 and 2005, and when thiophanate-methyl or fluazinam was applied to potato foliage at full bloom of primary inflorescences than at row closure or when fungicides were not applied in 2003. Mean percentage of control for the fungicides combined, relative to the nontreated control, was 43, 48, and 20% in 2003, 2004, and 2005, respectively, when application was made at row closure; whereas, it was 77, 83, and 80% in 2003, 2004, and 2005, respectively, when application was at full bloom of primary inflorescences. Mean disease incidences of infected stem were significantly less when fluazinam was applied at 100% bloom of primary inflorescences than at 20% drop of blossoms from primary inflorescences in 2004 and 2005. In summary, control of Sclerotinia stem rot was significantly better when fungicides were applied at full bloom of primary inflorescences than at row closure during all 3 years of the study.

Sclerotinia drop is a major disease of lettuce caused by two soilborne fungi, Sclerotinia minor and S. sclerotiorum. Fungicides such as dicloran (Botran), iprodione (Rovral), and vinclozolin (Ronilan) are currently available in the United States to manage this disease. Studies were conducted to investigate the relative effect of some new fungicides, including boscalid, fenhexamid, fluazinam, and fludioxonil, in comparison with vinclozolin, on growth of S. minor and S. sclerotiorum in agar plate tests as well as control of lettuce drop in the field. At a rate of 0.001 μg/ml, all tested compounds only suppressed mycelial growth of either pathogen from 0 to 20%. At 0.01 μg/ml, mycelial growth of S. minor was reduced 82 to 84% by fludioxonil and fluazinam and only 1 to 16% by boscalid, fenhexamid, and vinclozolin. At the same rate, mycelial growth of S. sclerotiorum was reduced 78% by fluazinam and from 0 to 12% by boscalid, fludioxonil, fenhexamid, and vinclozolin. At 0.1 μg/ml, all tested chemistries except vinclozolin inhibited mycelial growth of S. minor from 70 to 98%, whereas growth of S. sclerotiorum was suppressed 95 to 99% by fludioxonil and fluazinam, significantly less (40 to 47%) by boscalid and fenhexamid, and not at all by vinclozolin. At a rate of 1.0 μg/ml, all tested fungicides reduced mycelial growth of S. minor and S. sclerotiorum from 87 to 100% and 77 to 100%, respectively. Mycelial growth emerging from sclerotia of S. minor was reduced from 98 to 100% by all fungicides tested at a rate of 1.0 μg/ml, whereas growth from sclerotia of S. sclerotiorum was suppressed from 90 to 96% by fenhexamid, fludioxonil, fluazinam, and vinclozolin. In lettuce plots infested with S. minor, boscalid and fluazinam provided the highest level of disease control, significantly greater than that achieved with fenhexamid, fludioxonil, and vinclozolin. In the presence of S. sclerotiorum, the highest degree of disease suppression occurred with application of fluazinam, fludioxonil, and vinclozolin, whereas the least effective compound was fenhexamid. Boscalid and fluazinam were more effective against lettuce drop caused by S. minor than disease caused by S. sclerotiorum.

The activity of the registered fungicides fosetyl-Al and metalaxyl (subsequently replaced with mefenoxam by the manufacturer) was compared with other potentially useful compounds, azoxystrobin, dimethomorph, fluazinam, and zoxamide, for suppression of canker development on citrus bark after inoculation with Phytophthora citrophthora or P. nicotianae. The number of sweet orange trees on which cankers developed after inoculation with P. citrophthora and the average size of cankers when present were lower on plants treated with dimethomorph, fosetyl-Al, or metalaxyl compared with nontreated trees and those treated with azoxystrobin or fluazinam. When bark removed from treated trees was inoculated with P. citrophthora on the cambium surface at 5, 30, or 60 days after treatment (DAT), inhibition of lesion development on bark strips treated with dimethomorph, fosetyl-Al, or metalaxyl was significantly greater than that detected on bark treated with azoxystrobin, fluazinam, or zoxamide. When inoculated with P. nicotianae at 5 or 30 DAT, reduction of lesion size on bark strips treated with dimethomorph, fosetyl-Al, or metalaxyl was significantly greater than that detected on bark treated with azoxystrobin or fluazinam. Inhibition of lesion development by zoxamide was significantly less than that observed with metalaxyl at 5 DAT on bark inoculated with P. nicotianae; however, at 30, 60, and 90 DAT there was no significant difference in the performance of either fungicide. Reduction of lesion growth on the cambium surface compared with outer bark surface, when inoculated with P. citrophthora, did not differ significantly from 5 to 30 DAT for bark tissue treated with azoxystrobin, dimethomorph, fosetyl-Al, or metalaxyl. Among the nonregistered fungicides tested, dimethomorph provided the best level of Phytophthora gummosis control on citrus.

In vitro activity of azoxystrobin, dimethomorph, and fluazinam on growth, sporulation, and zoospore cyst germination of Phytophthora capsici, P. citrophthora, and P. parasitica was compared to that of fosetyl-Al and metalaxyl. The 50% effective concentration (EC50) values for)inhibition of mycelial growth of the three pathogens usually were lowest for dimethomorph and (metalaxyl, ranging from 90) levels for dimethomorph always were lower than the other four tested compounds, with values ranging from 0.32 to 1.6 μg/ml. Mycelial growth of P. capsici, P. citrophthora, and P. parasitica was least affected by azoxystrobin and fluazinam, with estimated (EC90) values >3,000 μg/ml. Reduction of sporangium formation by P. capsici, P. citrophthora, and P. parasitica in the presence of dimethomorph at 1 μg/ml was significantly greater than that recorded for the same concentration of azoxystrobin, fluazinam, and fosetyl-Al. For the three species of Phytophthora, zoospore motility was most sensitive to fluazinam (EC50 and EC90 values of 50 and EC90 values ranging from 299 to 334 and 518 to 680 μg/ml, respectively). Germination of encysted zoospores of P. capsici, P. citrophthora, and P. parasitica was most sensitive to dimethomorph (EC50 and EC90 values ranging from 3.3 to 7.2 and 5.6 to 21 μg/ml, respectively), intermediate in sensitivity to fluazinam (EC50 and EC90 from 18 to 108 and 67 to >1,000 μg/ml, respectively) and metalaxyl (EC50 and EC90 from 32 to 280 and 49 to 529 μg/ml, respectively), and lowest in sensitivity to azoxystrobin and fosetyl-Al (EC50 and EC90 from 256 to >1,000 μg/ml). The activity of azoxystrobin, dimethomorph, and fluazinam on one or more stages of the life cycle of P. capsici, P. citrophthora, and P. parasitica suggests that these compounds potentially could provide Phytophthora spp. disease control comparable to that of the established fungicides fosetyl-Al and metalaxyl.

Four hundred isolates of Sclerotinia minor were obtained from diseased peanut plants (Arachis hypogaea) in field plots untreated or treated three times with iprodione (1.12 kg/ha) in 1987. On glucose-yeast extract agar containing iprodione at 2 μg/ml, only 6% of the isolates grew. These isolates produced more mycelial growth than corresponding isolates not previously exposed to the fungicide in agar, indicating that insensitivity developed in vitro. In field microplots artificially infested with sclerotia of the insensitive isolates and planted to peanut for 7 yr, iprodione provided significant disease suppression even though insensitive isolates could still be recovered. In a separate 3-yr microplot study, the pathogenicity of an isolate (B-83-T2) of S. minor suspected of possessing field resistance to dicarboximide fungicides was compared to a sensitive isolate (S-2) after treatment of peanuts with dicarboximide fungicides (iprodione or vinclozolin), aromatic hydrocarbon fungicides (dicloran or pentachloronitrobenzene [PCNB]), and an experimental fungicide (fluazinam). Without fungicide treatment, disease incidence (stem lesions per microplot) at harvest averaged 19.9 in plots infested with isolate S-2 and 18.3 in plots infested with isolate B-83-T2 (no statistical difference), but yields were significantly lower (10%) in microplots infested with isolate S-2. When averaged across isolates, disease incidence was suppressed 96, 63, 42, 41, and 2O% by fluazinam, vinclozolin, iprodione, PCNB, and dicloran, respectively. Insensitivity in S. minor to the dicarboximide fungicides appears low but persistent and does not pose a threat to the continued use of dicarboximides or related fungicides in peanut production

The sensitivity of Sclerotinia minor to three dicarboximide fungicides (chlozolinate, iprodione, and vinclozolin), fluazinam (ASC-66825 and RH-3486), and MON-13108 was testedon fungicide-amended, glucose-yeast extract agar (GYEA). ED 50 values (dose for 50%inhibition of mycelial growth calculated from a dose-response line) for a field isolate (S-2) were 0.004, 0.004, 0.025, 0.08, 0.18, and 0.38 μg/ml for ASC-66825, RH-3486, MON-13108, vinclozolin, iprodione, and chlozolinate, respectively