Resistance of Botryosphaeria dothidea from pistachio to iprodione

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 mug/ml), whereas all the other isolates were sensitive (EC50 < 1 <mu>g/ml). B. dothidea isolates rea dily developed resistance to iprodione in vitro. Furthermore, these in vitr o-derived iprodione resistant (IR) isolates retained high virulence on pist achio. The EC50 values for these IR isolates were greater than 25 mug/ml, w hereas the EC50 values for the original wild-type counterparts were less th an 1 mug/ml. Iprodione resistance significantly declined when these IR isol ates were propagated on pistachio leaves in the absence of the fungicide. I R isolates also were resistant to vinclozolin, another dicarboximide fungic ide, but sensitive to tebuconazole and benomyl. IR isolates showed an incre ase in osmotic sensitivity on media amended with NaCl. Applications of ipro dione at 500 mug/ml, which is effective against naturally sensitive isolate s, failed to control disease caused by IR isolates in both the laboratory a nd greenhouse. The results indicate that, although naturally occurring IR i solates of B. dothidea may be rare in California pistachio orchards, the fu ngus readily develops resistance to iprodione in vitro and, more importantl y, retains high levels of virulence on pistachio.