Fungicide Resistance in Alternaria alternata from Blueberry in California and Its Impact on Control of Alternaria Rot

Alternaria rot caused by Alternaria alternata is one of the major postharvest diseases affecting blueberries in California. The sensitivity profiles of A. alternata from blueberry field to quinone outside inhibitors (QoIs), boscalid, fluopyram, fludioxonil, cyprodinil and polyoxin D in California were examined in this study. EC50 values of 51 A. alternata isolates for boscalid varied greatly among the isolates, ranging from 0.265 to >100 µg/ml. EC50 values of 51 A. alternata isolates to fluopyram, fludioxonil, cyprodinil, and polyoxin D were 5.188 ± 7.118 µg/ml, 0.078 ± 0.021 µg/ml, 0.465 ± 0.302 µg/ml, and 6.238 ± 7.352 µg/ml, respectively. In total, 143 isolates were screened for resistance at 5 and 10 µg/ml for fludioxonil, cyprodinil, and fluopyram, 10 µg/ml for polyoxin D, and 10 and 50 µg/ml for boscalid. Based on the published discriminatory concentrations for phenotyping resistance, of the 143 isolates, all were considered resistant to boscalid; 32, 69 and 42 were sensitive, low resistant, and resistant to fluopyram, respectively; and all were sensitive to fludioxonil and cyprodinil. In a PCR-RFLP method for phenotyping, 60 out of the 143 isolates were classified as resistant to QoIs. Control tests on detached blueberry fruit inoculated with different Alternaria isolates showed that fludioxonil and cyprodinil significantly reduced disease incidence and severity; however, pyraclostrobin, boscalid, fluopyram and polyoxin D significantly reduced only disease severity. The obtained results will be helpful in making decisions on fungicide programs to control A. alternata isolates with resistance or reduced sensitivities to multiple fungicides.

Temporal and regional performance of dual and triple premixes for soybean rust management in Brazil: A meta-analysis update

Soybean rust in Brazil is currently controlled with several commercial premixes composed of demethylation-inhibitors (EPOXiconazole, CYPRoconazole, PROThioconazole, TEBUconazole), quinone-outside inhibitors (AZOXystrobin, TriFLoXystrobin, PYRAclostrobin, PICOxystrobin), and succinate demethylation inhibitors (BENZovindiflupyr, BIXaFen, FLUXapyroxad). We updated results on the performance of eight premixes evaluated in 177 cooperative trials conducted in 46 locations across 10 states during six crop seasons (2015 to 2020). All treatments were sprayed three times starting at R1 or R2. Percent control (C, %), from back-transforming meta-analytic estimates of the log of the ratio ranged from 56.2% (PICO + CYPR) to 76.8% (BIXF + TFLX + PROT). Estimates of mean yield difference (D, kg/ha) between fungicide-treated and untreated plots were greatest for BIXF + TFLX + PROT (1,080) followed by PICO + BENZ (1,010), PYRA + EPOX + FLUX (981.5), AZOX + BENZ (910), TFLX + PROT (891), PICO + TEBU (682), TFLX + CYPR (646) and PICO + CYPR (600). Significant declines in both C and D in a time period as short as four years were detected for AZOX + BENZ (35.3%; 550 kg/ha) and PICO + BENZ (15.5%; 359.8 kg/ha). Variance in D was partially reduced by the inclusion of baseline disease as covariate. In trials where baseline disease was ≥70%, yield was 250 kg/ha greater compared to low baseline disease. Disease control and yield response were generally better in the Southeast, where the frequency of profitable scenarios was 30% higher on average than in the Northwest. Results of this meta-analysis are critical to support decisions during planning fungicide programs.

Sensitivities of Cucurbit Powdery Mildew Fungus (Podosphaera xanthii) to Fungicides

This study was conducted to evaluate the sensitivity of cucurbit powdery mildew fungus (Podosphaera xanthii) to major fungicides used for managing this pathogen in the Midwestern United States. Fungicides azoxystrobin from the quinone outside inhibitors group, cyflufenamid from the phenylacetamide group, penthiopyrad from the succinate dehydrogenase inhibitors group, quinoxyfen from the quinolines group, and triflumizole from the demethylation inhibitors group were tested for their effectiveness for preventing infection of cucurbits by P. xanthii. In 2015 and 2016, 37 isolates of P. xanthii were evaluated for their sensitivity to azoxystrobin (Quadris 2.08SC), cyflufenamid (Torino 0.85SC), penthiopyrad (Fontelis 1.67SC), and triflumizole (Procure 480SC) on cucumber ‘Bush Crop’ cotyledon leaves. The number of isolates sensitive to tested concentrations of Quadris 2.08SC, Torino 0.85SC, Fontelis 1.67SC, and Procure 480SC was 8 (22%), 21 (57%), 20 (54%), and 23 (62%), respectively. During 2015 to 2018, Quadris 2.08SC, Torino 0.85SC, Fontelis 1.67SC, quinoxyfen (Quintec 250SC), and Procure 480SC were tested for their effectiveness for managing powdery mildew on pumpkin ‘Howden’ in the field. The results showed that powdery mildew was effectively managed in the plots treated with Procure 480SC and Quintec 250SC. However, management of the disease was less successful in the plots treated with Quadris 2.08SC, Torino 0.85SC, and Fontelis 1.67SC.

Widespread Resistance to QoI Fungicides of Colletotrichum acutatum from Strawberry Nurseries and Production Fields

Strawberry anthracnose, caused mainly by the Colletotrichum acutatum species complex, is a major disease in strawberry nurseries and production fields. The use of fungicides, such as the quinone outside inhibitors (QoIs), has been extensively deployed for the control of C. acutatum for the past 20 years. C. acutatum resistance to the QoIs was first reported in 2013 in Florida strawberry production fields. In 2015, anthracnose outbreaks were reported in strawberry nurseries and production fields across the United States. To elucidate the significance and geographical extension of C. acutatum resistance, fungicide use surveys were conducted, and isolates were collected in the affected areas. QoI-resistant isolates were collected from strawberry production fields and nurseries in six states in the United States. Fungicide use surveys indicated that, in some locations, the number of QoI applications exceeded the recommendation for this fungicide group. Thus, the current situation warrants immediate changes in anthracnose management strategies that integrate other chemical as well as nonchemical strategies to limit resistant population selection and prevent future anthracnose outbreaks.

Reduction in the in vitro sensitivity oF Drechslera tritici-repentis, isolated from wheat, to strobilurin and triazole fungicides

ABSTRACT Reports of failure in the chemical control of wheat yellow leaf spot led to determination of the sensitivity of Drechslera tritici-repentis (Dtr) to the fungicides quinone outside inhibitors (QoIs) and demethylation inhibitors (DMIs). The IC50 was obtained for strobilurins (azoxystrobin, kresoxim-methyl, picoxystrobin and pyraclostrobin) and for triazoles (cyproconazole, epoxiconazole, propiconazole, prothioconazole and tebuconazole), using five Dtr isolates. Seven concentrations of the fungicides were tested in the bioassay: 0.00; 0.01; 0.10; 1.00; 10:00 and 20.00 and 40.00 mg/L active ingredient (a.i.). Assays consisted of completely randomized design and four replicates. Each experiment was performed twice, using the average of the two tests for statistical analysis. The percentage inhibition data for conidial germination (QoIs) and for mycelial growth (DMIs) were subjected to logarithmic regression analysis, calculating the 50% inhibitory concentration (IC50) based on the generated equation. There was a reduction in the sensitivity of Dtr isolates to strobilurins. IC50 values ranged from 0.58 to > 40.00 mg/L. The lowest sensitivity of isolates was detected for azoxystrobin, kresoxim-methyl, picoxystrobin and trifloxystrobin. Pyraclostrobin was most efficient, showing IC50 between 0.58 and 1.03 mg/L. The IC50 ranged from 0.35 to 1.37 mg/L for epoxiconazole, from 0.49 to 1.28 mg/L for propiconazole and from 1.41 to 2.34 mg/L for tebuconazole. Prothioconazole was most potent, showing IC50 between 0.09 and 0.21 mg/L. The hypothesis that the control failure can be attributed to the reduced Dtr sensitivity to the fungicides QoIs and DMIs was confirmed.