Agriculture: Pistachio Pest Management Guidelines

Botrytis Blossom and Shoot Blight

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).

Management

Orchard sanitation can help reduce the incidence of Botrytis blossom and shoot blight.

  1. Prune blighted shoots, shoots with cankers, and dead or dying branches.
  2. Destroy or remove infected pruned-out wood and brush piles.
  3. Prune trees and clear the debris right after harvest and, if needed, during the dormant season.

If spring weather is cool and wet during bloom, consider applying a fungicide.

Common name Amount per acre REI‡ PHI‡
(Example trade name) (hours) (days)
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and timing, honey bees, and environmental impact. Always read the label of the product being used.
A. FLUOPYRAM + TEBUCONAZOLE
(Luna Experience) 6–8 fl oz 12 35
MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) and demethylation inhibitor (3)
B. FENHEXAMID
(Elevate 50WDG) 1–1.5 lb 12
MODE-OF-ACTION GROUP NAME (NUMBER1): Hydroxyanilide (17)
COMMENTS: Apply at 5 to 10% bloom and full bloom.
C. PYRACLOSTROBIN + BOSCALID
(Pristine) 10.5–14.5 oz 12 14
MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and carboxamide (7)
D. POLYOXIN-D ZINC SALT
(Ph-D) 6.2 oz 4 0
MODE-OF-ACTION GROUP NAME (NUMBER1): Polyoxins (19)
E. FLUOPYRAM + TRIFLOXYSTROBIN
(Luna Sensation) 5–7.6 oz 12 28
MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) and quinone outside inhibitor (11)
F. CYPRODINIL + FLUDIOXONIL
(Switch 62.5WG) 11–14 oz 12 7
MODE-OF-ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9) and phenylpyrrole (12)
COMMENTS: Make first application at early bloom and a second 14 days later.
G. PYRIMETHANIL
(Scala SC) 18 fl oz 12 30
MODE-OF-ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9)
H. THIOPHANATE-METHYL
(Topsin M WSB) 1.5–2 lb 72
MODE-OF-ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
COMMENTS: Apply at bloom. Restricted entry interval is 3 days.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1,4,9,11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.
Not applicable.
Text Updated: 10/14
Treatment Table Updated: 10/14