Alternaria Late Blight

Damage of pistachio from Alternaria late blight caused by three species of Alternaria has been severe in the last several years, particularly in orchards where fungicide resistant populations have developed.  Previously, fungicides in the class of strobilurins and also mixtures that contained strobilurins and a carboxamide such as boscalid  were very effective. Unfortunately, in some of the orchards where the disease has been severe, the efficacy of these fungicides lasted only a couple seasons. This is because the pathogens causing Alternaria late blight have the ability to mutate easily and develop resistance. Therefore, it is a continuous struggle to increase the efficacy period of newly registered fungicides and avoid the fate of previously effective fungicides in other classes.

The first symptoms of Alternaria late blight appear in late July/August when high humidity in pistachio orchards favors the formation of dew. The main damage to pistachios by the disease is the staining of the nut shell with occasional also mold on the kernels and the early defoliation of trees. Excessive defoliation upon shaking the trees at harvest may slow down the harvest and post-harvest operations. Some growers estimate a $1,000 loss per acre due to shell staining and lower nut quality resulted from Alternaria infections. Although the disease occurs annually in California, the severity varies from between years and across locations depending mainly on micro-environmental conditions. Furthermore, as orchards age and canopies expand to adjacent trees, this creates favorable conditions for the disease. Therefore, Alternaria late blight has been observed in more and more orchards causing epidemics.