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Abstract: Tan spot, caused by Pyrenophora tritici-repentis, is an important disease of wheat (Triticum aestivum L.) in the United States (U.S). Wheat growers sometimes rely on fungicide applications to manage foliar diseases. Pyraclostrobin is one of the recommended quinone outside inhibitor (QoI) fungicides and is commonly used to manage tan spot on wheat. Although P. tritici-repentis isolates has been reported as insensitive to QoI fungicides in Europe, little is known regarding the sensitivity of the P. tritici-repentis population to fungicides in the U.S. Twenty-eight isolates collected prior to 1997 and 136 isolates collected from pyraclostrobin treated fields in North Dakota were collected in 2007 and 2009 to determine if P. tritici-repentis isolates can use alternative respiration in the presence or absence of salicylhydroxamic acid (SHAM) in vitro and to determine the effective fungicide concentration that inhibited conidia germination by 50% (EC50). Six of 10 baseline isolates of P. tritici-repentis assayed had significantly higher (P ≤ 0.05) EC50 values when SHAM was not added to agar medium amended with pyraclostrobin, confirming that the fungus can utilize alternative respiration to overcome QoI toxicity in vitro. EC50 values of the baseline isolates ranged from 0.0012 to 0.0024 μg/ml (mean EC50 value of 0.0017 μg/ml and standard deviation is ±0.00039 μg/ml) while EC50 values of the 136 fungicide exposed isolates ranged from 0.0013 μg/ml to 0.0027 μg/ml (mean EC50 value of 0.0017 μg/ml and standard deviation is ±0.00030). These results indicate that the field exposed isolates have not shifted to reduced sensitivity to pyraclostrobin and that the baseline sensitivity values can be useful for a fungicide resistance monitoring program for P. tritici-repentis in the U.S. [Copyright &y& Elsevier]

Abstract: Ascochyta rabiei causes Ascochyta blight, a yield-limiting disease of chickpea (Cicer arietinum) world-wide. In 2007, fungal populations of A. rabiei resistant to the QoI group of fungicides were detected in the Northern Great Plains of the United States. Assays were conducted to determine fungal sensitivity for two alternative fungicidal modes of action. A total of 78 isolates of A. rabiei collected between 1983 and 2007 were screened to determine baseline sensitivity to the demethylation-inhibiting foliar fungicide, prothioconazole, and 100 isolates collected between 1987 and 2007 were screened for sensitivity to the methyl benzimidazole carbamate (MBC) fungicide, thiabendazole. Isolates were tested using an in vitro mycelial growth assay to determine the effective fungicide concentration at which 50% of fungal growth was inhibited (EC50) for each isolate–fungicide combination. Baseline EC50 values of prothioconazole ranged from 0.0526 to 0.2958μg/ml, with a mean of 0.1783μg/ml. Isolates of A. rabiei collected from 2007 to 2009 from North Dakota chickpea fields exposed to prothioconazole, were screened for prothioconazole sensitivity using the same assay. Mean EC50 values for these isolates were 0.3544μg/ml, 0.3746μg/ml, and 0.7820μg/ml, respectively. These values represent an approximate 2.0 (2007–2008) and 4.4-fold (2009) decrease in sensitivity from the baseline mean. EC50 values of thiabendazole ranged from 1.192 to 3.819μg/ml, with a mean of 2.459μg/ml. No significant decrease in fungicide sensitivity was observed for thiabendazole. To date, no loss of Ascochyta blight control has been observed with the use of either prothioconazole or thiabendazole. [Copyright &y& Elsevier]

Early blight, caused by Alternaria solani, and brown leaf spot, caused by a number of small-spored Alternaria spp. including Alternaria alternata sensu stricto , Alternaria arborescens, and Alternaria tenuissima, are observed annually in all midwestern potato production areas. However, Alternaria spp. have developed reduced sensitivity and/or resistance to many single-site mode of action fungicides such as quinone outside inhibitor (QoI; FRAC group 11), succinate dehydrogenase inhibitor (SDHI; FRAC group 7), and anilinopyrimidine (AP; FRAC group 9). Mean in vitro sensitivity EC 50 values (effective concentration where fungal growth is inhibited by 50%) of A. alternata (n = 16) , A. arborescens (n = 3) , A. tenuissima (n = 5) , and A. solani (n = 58) in response to the AP fungicides pyrimethanil and cyprodinil and the phenylpyrrole (PP) fungicide fludioxonil were determined via mycelial growth assays. Significant fungicide by isolate interactions were observed for all Alternaria spp. evaluated in vitro, indicating reduced-sensitivity of some isolates to individual fungicides. EC 50 values for three non-baseline A. solani isolates collected in 2010, 2011 and 2013 were within the baseline for all three fungicides. A significant correlation was observed between pyrimethanil and cyprodinil EC 50 values among A. alternata isolates, but no relationship was observed with the other fungicides or in A. solani. In greenhouse evaluations, a significant loss of disease control was observed for some non-baseline A. solani isolates, and this was more pronounced in the AP fungicides, pyrimethanil and cyprodinil. No significant correlation was observed between in vitro EC 50 value and area under the dose response curve based on greenhouse assays, likely due to the limited number of isolates evaluated. Further research is needed to determine if these reductions affect control of early blight and brown leaf spot in potato under field conditions. Results from this study indicate that fludioxonil and cyprodinil are potentially good additions into fungicide rotation programs or as co-pack chemistries for control of leaf spot diseases and fungicide resistance management. • AP and PP fungicides remain effective on potato early blight/brown spot pathogens. • Cross-sensitivity between pyrimethanil and cyprodinil in baseline A. alternata. • Significant loss of disease control seen for some non-baseline A. solani isolates. • Fludioxonil and cyprodinil are good candidates for addition to a potato IPM program. [ABSTRACT FROM AUTHOR]

Abstract: Reduced sensitivity to quinone outside inhibitor (QoI) fungicides conferred by the presence of the F129L mutation was identified in Alternaria solani isolates collected in Nebraska in 2000 and in isolates collected from the Midwestern states of Minnesota and North Dakota in 2001. Over 4200 isolates of A. solani collected over a 5-year period from 2002 to 2006 from 11 potato-producing states were evaluated for the presence of the F129L mutation utilizing azoxystrobin spore germination assays and real-time PCR. From this population, 96.5% of isolates were determined to have reduced sensitivity to QoI fungicides and/or to contain the F129L mutation. In addition to Midwestern states of Minnesota, Nebraska and North Dakota, A. solani isolates containing the F129L mutation were collected from Colorado, Michigan and Texas in 2003, Wisconsin in 2005, as well as from the Western United States including Idaho, Washington and Wyoming in 2005 and Oregon in 2006. The detection of these isolates in areas outside the Midwest suggests that the F129L mutation is stable and present under conditions that are less conducive for the pathogen and where it is under less selection pressure by QoI fungicides. Competitive fitness tests confirm that, while isolates of A. solani with the F129L mutation had a decrease in in vitro percentage germination compared with wild-type isolates, F129L mutant isolates produced higher disease severity in greenhouse trials than wild-type isolates. Field trials were performed in central Minnesota and central North Dakota in 2000 and 2001 when the A. solani population was dominated by wild-type isolates as well as in 2002, 2003, 2005 and 2006 when F129L mutant isolates dominated. These trials support in vitro and greenhouse results indicating that the F129L mutation has affected the field performance of all QoI fungicides. Field trial results suggest that these fungicides do not provide improved disease control over standard protectant fungicides such as chlorothalonil and mancozeb. [Copyright &y& Elsevier]

Pink rot is an important disease of potato with worldwide distribution. Severe yield and quality losses have been reported at harvest and in postharvest storage. Under conditions favoring disease development, pink rot severity can continue to increase from the field to storage and from storage to transit, causing further losses. Prediction of pink rot disease development in storage has great potential for growers to intervene at an earlier stage of disease development to minimize economic losses. Pink rot disease is estimated as percent rot confined on the interval (0 or 1, corresponding to 0% as no disease and 100% as maximum disease). In this study, beta regression is considered over the traditional ordinary least squares regression (linear regression) for fitting continuous response variables bounded on the unit interval (0,1). This method is considered a good alternative to data transformation and analysis by linear regression. The percentages of incidence of pink rot in tubers at harvest, yield, and days after harvest were used as study covariates to predict pink rot development from 32 to 78 days postharvest. Results demonstrate that the interaction between percentage of pink rot at harvest and yield is a significant predictor (P < 0.0001) of the beta regression model. A linear regression model was also designed to compare the results with the proposed beta regression model. Linear predictors observed in diagnostic plots with linear regression model was found to not be constant and an adjusted R2 (0.49) was obtained. The pseudo R2 (0.56) and constant variance for this study suggests that the beta regression function is adequate for predicting the development of pink rot during storage. The use of the beta prediction model could help growers decide whether to apply a fungicide to tubers going into storage or to market their crop before significant storage losses are incurred. [ABSTRACT FROM AUTHOR]

A 2-year field and laboratory experiment was initiated to study the competitive parasitic fitness of mefenoxam-resistant (50% effective concentration [EC50] > 100 μg ml-1) and mefenoxam-sensitive (EC50 = 0.07 μg ml-1) isolates of Phytophthora erythroseptica with equal aggressiveness. The competitive ability of the mefenoxam-resistant and -sensitive isolates was tested under no selection pressure (nonfungicide treated) as well as under the influence of mefenoxam and nonmefenoxam (phosphorous acid) fungicides. P. erythroseptica isolates were combined in four ratios of mefenoxam-resistant (R) to mefenoxam-susceptible (S) (0R:0S, 1R:1S, 3R:1S, and 1R:3S) and subsequently infested into the soil at the time of planting. In-furrow mefenoxam applications were applied to the soil immediately following infestation with P. erythroseptica. Phosphorous acid was applied at tuber initiation and 14 days after tuber initiation. Noninfested, non- fungicide-treated plots served as controls. P. erythroseptica isolates recovered from field-infected pink rot tubers at harvest and 3 to 4 weeks after harvest were tested for mefenoxam sensitivity in vitro. In vivo studies were performed by challenge inoculating a zoospore suspension in the four ratios described above onto potato tubers harvested from nontreated, phosphorous acid-treated, or mefenoxam-treated field plots. These field plots were not infested with P. erythroseptica at planting. Results from both field and in vivo studies demonstrate that mefenoxam-resistant isolates of P. erythroseptica are as fit as sensitive isolates in the absence of selection pressure or in the presence of a phosphorous acid fungicide treatment. Under mefenoxam selection pressure, mefenoxam-resistant P. erythroseptica isolates were more parasitically fit than -sensitive isolates. These studies suggest the lack of an apparent fitness penalty in mefenoxam-resistant P. erythroseptica populations under field conditions and that these isolates could be stable in most agroecological systems. Based on these results, mefenoxam-based fungicides are no longer recommended for the management of pink rot once mefenoxam-resistant P. erythroseptica populations are detected in a specific field. [ABSTRACT FROM AUTHOR]