If it rains a quarter inch or more during bloom, it may be necessary to apply a fungicide to control Botrytis and Botryosphaeria in pistachios. Disease potential will depend on past infection levels and repeated rain events.
Dr. Themis Michailides has determined that 0.2 inches of rain and temperatures at or above 55 degrees F constitutes a Botryosphaeria infection event.
Your inoculum pressure can still be assessed by examining the base of old cluster rachises for blackened tissue that extends into the one-year-old wood and looks like verticillium streaking from where the cluster attached to the branch.
Infected old clusters also tend not to break cleanly from the shoot. Instead, they leave a stub when one attempts to knock them off. Also, examine older wood for sunken areas, which, when cut into, also have blackened tissue running lengthwise in the limb.
These represent old infections, which can possess active pycnidia for up to six years. Botrytis attacks the male bloom more than the female, because the tufts of pollen are high in sugar and proteins, both good substrates for the growth of this fungus.
Male cultivars 02- 16 and 02-18 (old selections originally released with Peters to assist in its overlap of Kerman) are more susceptible than Peters due to their denser bloom character.
Kerman female trees show Botrytis infection in young, tender shoots. Diseased shoots wilt, and their tips curl like a shepherd’s hook. They turn dark, dull green, not black.
The base of the shoot also develops a cluster of buff-colored spores. Botryosphaeria shoot infections do not occur until later in the summer when it gets hot. See pages 37-40 of your BOT manual (most of you do not even remember the BOT manual) to contrast Botrytis to BOT. Several fungicides are now registered and effective against these diseases.
Consider the likelihood of treating for Alternaria later in the season, and save the fungicide most effective against Alternaria for use in June or July. You can compare fungicide efficacy at the UC website.
Management of Alternaria resistance has now become a major concern for PCA’s and growers.