The genotype-specific cross-resistance relationships between the SDHIs boscalid and penthiopyrad and the lack of cross-resistance between these fungicides and fluopyram should be taken into account when selecting SDHIs for gummy stem blight management.

These findings suggest the development of resistance to azole antifungals in A. fumigatus may be present where agricultural azoles are used in the USA.

Field experiments were conducted in 2000 and 2001 on Georgia Green, Florida MDR-98, and C-99R peanut (Arachis hypogaea) cultivars in Tifton, GA, to determine the effects of tillage practices on early leaf spot (Cercospora arachidicola) epidemics under standard fungicide regimes and fungicide regimes with fewer applications. Leaf spot epidemics were suppressed in reduced tillage (strip-till) plots compared with conventional tillage plots and were suppressed in MDR-98 and C-99R cultivars compared with the standard runner-type cultivar, Georgia Green. Within tillage and cultivar combinations, leaf spot intensity typically was lower in plots treated with fungicides at standard intervals (seven total applications) than in those treated at extended intervals (four total applications). However, in most cases, leaf spot control in extended interval treatments in the strip-till system was comparable to that in the standard interval treatments in conventional tillage. Based on these results, the number of fungicide applications could be reduced without compromising control of leaf spot when reduced tillage is used, especially if combined with moderately resistant cultivars. Suppression of leaf spot epidemics in the strip-till plots did not coincide with higher yields in either year. In 2001, yields were lower in strip-till plots than in conventional tillage plots. Yields were typically higher in the cultivar C-99R than in Georgia Green, regardless of the tillage treatment.

Field experiments were carried out to evaluate the effects of integrated management of early leaf spot, caused by Cercospora arachidicola, and spotted wilt, caused by Tomato spotted wilt virus (TSWV), on peanut (Arachis hypogaea) using host resistance, two tillage systems, and varying fungicide programs. Effects on pod yield and economic return were assessed. Genotypes C-11-2-39 and Tifrunner demonstrated the best field resistance to TSWV, whereas cvs. DP-1 and GA-01R and line C-28-305 were among the genotypes with the best leaf spot resistance. Epidemics of both diseases were comparable or suppressed in strip-tilled plots compared with conventionally tilled plots. Leaf spot intensity decreased with increased fungicide applications, but to a lesser degree with use of resistance and strip tillage. Yields and net returns were similar between tillage treatments in 2002 and lower in strip tillage in 2003. Genotypes with the greatest yields and returns were C-11-2-39, C-99R, and GA-01R. Returns were comparable among the four-, five-, and seven-spray programs in both years, despite differences in yield. The standard production system, Georgia Green in conventional tillage with seven sprays, resulted in lower returns than half the integrated systems tested in 2002, but had comparable or higher returns than nearly all systems in 2003. When significant, yields and returns were correlated with spotted wilt intensity to a greater degree than leaf spot intensity.

Thomas, A., Langston, D. B., Jr., and Stevenson, K. L. 2012. Baseline sensitivity and cross-resistance to succinate-dehydrogenase-inhibiting and demethylation-inhibiting fungicides in Didymella bryoniae. Plant Dis. 96:979-984.Didymella bryoniae, which causes gummy stem blight (GSB) of watermelon, has a history of developing resistance to fungicides, most recently the succinate-dehydrogenase-inhibiting (SDHI) fungicide boscalid. To facilitate fungicide resistance monitoring, baseline sensitivity distributions were established for demethylation-inhibiting (DMI) fungicides tebuconazole and difenoconazole and the SDHI fungicide penthiopyrad, and reestablished for the SDHI fungicide boscalid. In all, 71 isolates with no known prior exposure to SDHIs or DMIs were used to determine the effective concentration at which mycelial growth was inhibited by 50% (EC50). EC50 values for boscalid, penthiopyrad, tebuconazole, and difenoconazole were 0.018 to 0.064, 0.015 to 0.057, 0.062 to 0.385, and 0.018 to 0.048 ng/ml, with median values of 0.032, 0.026, 0.118, and 0.031 fig/ml, respectively. Significant positive correlations between the sensitivity to penthiopyrad and boscalid {P < 0.0001, r = 0.75) and between tebuconazole and difenoconazole {P < 0.0001, r = 0.59) indicate a potential for cross-resistance between chemically related fungicides. In 2009, 103 isolates from fungicidetreated watermelon fields were tested for sensitivity to boscalid and penthiopyrad using a discriminatory concentration of 3.0 ng/ml. Of the isolates tested, 82 were insensitive and 14 were sensitive to both fungicides. Because of the significant potential for cross-resistance between closely related fungicides, growers will be advised not to use both SDHIs or both DMIs successively in the same fungicide spray program.