Detection of isolates of Alternaria alternata with multiple-resistance to fludioxonil, cyprodinil, boscalid and pyraclostrobin in California pistachio orchards

• The majority of Alternaria alternata isolates were resistant to boscalid (86.3%) and pyraclostrobin (92.9%).
• Most were still sensitive to fludioxonil (77.8%) and cyprodinil (89.7%).
• Isolates with single or multiple fungicide resistance were found.
• Efficacy of Switch could become affected in the long term.
• Resistance must be monitored and managed continuously in at risk orchards.

In this study, 229 isolates of Alternaria alternata collected from commercial pistachio orchards with (n = 198) and without (n = 31) a prior history of Switch (cyprodinil + fludioxonil) applications were examined for their sensitivity to fludioxonil and cyprodinil using in vitro mycelial growth assays. The 50% effective dose (EC50) values for the baseline isolates for cyprodinil and fludioxonil ranged from 0.001 to 1.184 μg/mL (average = 0.214) and from 0.010 to 4.875 μg/mL (average = 0.770), respectively. Out of 72 isolates assayed from the exposed population, 93% had EC50 values for fludioxonil ranging from 0.001 to 5.370 μg/mL (average = 0.472), but five isolates were highly resistant to this fungicide (EC50 > 100 μg/mL). About 98.55% of these isolates were sensitive to cyprodinil (0009 < EC50 < 0.415 μg/mL; average = 0.098). Only one isolate was resistant to cyprodinil (EC50 > 10 μg/mL). No cross-resistance relationship between cyprodinil and fludioxonil was observed (P = 0.2925, r = −0.1074). To determine the occurrence of multiple resistances, the radial growths of 126 isolates, collected from orchards with a history of Switch, Pristine, and strobilurin exposures, were measured on agar media amended with a single discriminatory dose of 10 μg fungicide/mL. High proportions of these isolates were resistant to boscalid (86.3%) and pyraclostrobin (92.9%). Most isolates were still sensitive to fludioxonil (77.8%) and cyprodinil (89.7%), but some were found to be independently or simultaneously resistant to all fungicides tested. Under conditions of excessive exposure to Switch resistant phenotypes may develop in A. alternata populations and ultimately affect fungicide performance. Therefore, monitoring studies must be continued and strict adherence to anti-resistance strategies followed and implemented in at risk orchards.