Botrytis blossom and shoot blight

Botrytis, caused by the fungus pathogen Botrytis cinerea, is found in some California pistachio orchards. The sporadic disease develops in cool and rainy situations at bloom in April and during May and June and infects tender clusters. Wilted shoots resemble a Shepherd’s crook.

Shoots die and the dead leaves remain attached (flagging). Late rains can lead to infected fruit clusters and the eventual partial death of clusters, UC Davis reports. Blossom blight is more severe in male trees than female trees.

“However as temperatures rise the infection of the clusters stops, leading to the partial killing of the clusters,” Michailides said.

Orchard sanitation, including blighted shoot pruning, can reduce the inoculum level in the orchard. Consider bloom fungicide sprays if the weather is cool and wet during the bloom period.

Phoma blight

Phoma blight is an anthracnose-like disease found in some Arizona pistachio orchards. The pathogen is the fungus Phoma fungicola. The disease moves very fast in the tree and kills clusters near harvest time.

Michailides has conducted several trials in the lab to determine fungicide efficacy against Phoma blight. He hopes to conduct disease-fungicides field trials this year in infected pistachio orchards with the cooperation of local Arizona pest control advisers.

For more information diseases in pistachios, contact Themis Michailides at tjmichailides@ucanr.edu.