Department of Plant Pathology, University of California, Davis, and Kearney Agricultural Center, Parlier, CA 93648
Out of 79 field-collected isolates of Botryosphaeria dothidea, causal agent of panicle and shoot blight of California pistachio, 1 had low resistance to the dicarboximide fungicide iprodione (effective fungicide concentration to inhibit 50% of mycelial growth [EC50] = 2.726 μg/ml), whereas all the other isolates were sensitive (EC50 < 1 μg/ml). B. dothidea isolates readily developed resistance to iprodione in vitro. Furthermore, these in vitro-derived iprodione-resistant (IR) isolates retained high virulence on pistachio. The EC50 values for these IR isolates were greater than 25 μg/ml, whereas the EC50 values for the original wild-type counterparts were less than 1 μg/ml. Iprodione resistance significantly declined when these IR isolates were propagated on pistachio leaves in the absence of the fungicide. IR isolates also were resistant to vinclozolin, another dicarboximide fungicide, but sensitive to tebuconazole and benomyl. IR isolates showed an increase in osmotic sensitivity on media amended with NaCl. Applications of iprodione at 500 μg/ml, which is effective against naturally sensitive isolates, failed to control disease caused by IR isolates in both the laboratory and greenhouse. The results indicate that, although naturally occurring IR isolates of B. dothidea may be rare in California pistachio orchards, the fungus readily develops resistance to iprodione in vitro and, more importantly, retains high levels of virulence on pistachio.