Botryosphaeria panicle and shoot blight

Another hard-hitting disease of pistachio is Botryosphaeria (Bot) panicle and shoot blight which attacks and kills clusters in some California and Arizona orchards.

In the spring, blight infects young panicles and young shoots. This can cause springtime infection of the leaf petiole and defoliation. Fruit infection follows in the spring and summer, followed by cluster kill in the summer and fall months. 

Michailides warns, “This disease not only infects the current pistachio crop but can also kill buds for the next year’s crop.”

Growers and pest control advisers sometimes can mistakenly blame killed clusters on the citrus flat mite pest. With mite damage, the cluster dies but the cluster rachis (main cluster stem) remains green. A Bot infection turns the inside of the cluster rachis brown.

The pathogens which cause Bot disease include Botryosphaeria dothidea and seven more species in the same family (Botryosphaeriaceae).

Bot is spread by moving water in the orchard, insects (hemiptera), birds, pollen, and pruning equipment.

Bot eradication is difficult but not impossible, says Michailides. The disease can be managed with orchard pruning, cultural practices, and fungicides. The best spray time to spray is just prior to a rain event or 2-3 days later.

The UC IPM website suggests lowering sprinklers to keep irrigated water from reaching the tree canopy, or to shorten the duration of the irrigation from 48 hours down to 24 hours. Daytime irrigation for 12 hours over two consecutive days can significantly reduce disease outbreak.

Michailides says new classes of fungicides and pre-mix fungicides are very effective in managing this disease.