How to Manage Pests
UC Pest Management Guidelines
Botrytis Blossom and Shoot Blight
Pathogen: Botrytis cinerea
(Reviewed 10/14, updated 10/14)
In this Guideline:
SYMPTOMS AND SIGNS
Botrytis blossom and shoot blight occurs in early spring and fruit blight later in spring. The first symptom to be observed is wilting of tender shoots; later leaves shrivel and dry. Young shoots die and the leaves remain attached, a symptom called flagging (shepherd's hook).
Botrytis blossom blight is more severe in male than female trees, especially in the 02-16 and 02-18 male selections. The fungus enters the flower and invades the wood where it causes cankers on current or two-year-old shoots. Cankers can coalesce and measure up to 10 inches (25 cm) long. When cool, wet weather prevails, diseased blossoms and basal portions of shoots are generally covered with buff-colored masses of spores. Large circular lesions can develop on blades of both female and male trees, and portions of the leaf blade (usually a V-shaped area near the terminal) mainly on male trees may also be infected and killed by the fungus. Late rains can result in infections of fruit clusters, killing parts or the entire cluster, which become beige in color.
COMMENTS ON THE DISEASE
Infections occur in spring on succulent current-season growth. Most Botrytis cankers occur at the base of shoots and most likely start from contaminated buds and bud scales. The fungus colonizes the bud scales and then grows and infects the developing tender shoot. Shoots wilt and form a shepherd's crook. Inflorescences, especially in male trees, are also attacked.
Blighted shoots provide inoculum during the current growing season and in the following spring. Under humid conditions, the fungus colonizes and sporulates on male flowers that are on the tree or already dropped to the ground. Other sources of inoculum include infected weeds, leaves, and immature fruit dropped to the ground, or other crops neighboring the pistachio orchard. The disease is problematic during cool, wet springs and causes damage by killing current season shoots and fruit, thus reducing fruiting wood for the following season and yields (fruit blight phase).
Orchard sanitation can help reduce the incidence of Botrytis blossom and shoot blight.
If spring weather is cool and wet during bloom, consider applying a fungicide.
UC IPM Pest Management Guidelines:
T. J. Michailides, Kearney Agricultural Center, Parlier