How to Manage Pests
UC Pest Management Guidelines
Botryosphaeria Panicle and Shoot Blight
Pathogens: Botryosphaeria dothidea; Neofusicoccum mediteraneum, other Botryosphaeriaceae fungi; conidial stage: Neofusicoccum spp.
(Reviewed 10/14, updated 10/14)
In this Guideline:
SYMPTOMS AND SIGNS
Vegetative and flowering buds that were killed during the previous fall or winter do not emerge in spring. In mid-spring (end of May to June) buds that were partially infected the previous season produce fruit clusters and shoots that develop blight from the fungus in buds. The rachises of these blighted clusters turn black as do the shoots. When temperatures increase in May through July, the fungus moves into shoots of the previous year, causing blighting of fully developed clusters. These blighted shoots, leaves, and clusters turn brown.
Secondary infections of clusters originate where the rachises branch; they start as small black lesions that later coalesce and cause fruit blight. Secondary infections of fruit start as round, black, pin-sized lesions, some of which will expand and decay the hulls. In late August through September, infected fruit are covered with pycnidia (black flasklike structures containing the fungus spores of the Neofusicoccum sp.) and obtain a silvery color, in contrast to the noninvaded blighted fruit, which are brown.
Infections on leaves also start as small black lesions that later coalesce and cause leaf blight. From August through October, large necrotic lesions with pycnidia in the center develop on leaves of male and female trees. Infection of petioles start as longitudinal black areas and cause blight of the entire leaf or of individual leaflets and defoliation. Scars of abscised buds or leaves can also be infected, resulting in sunken cankers above and below the scars. Infected rachises usually hang on the tree for 3 to 4 years, providing inoculum for the following growing season(s). On branches, lenticels can also be infected, but the infections remain small and do not invade the shoot.
To distinguish from Alternaria spp., rub leaf lesions with a finger
However, late in the season both fungi can be present in the same lesion and microscopic identification will be needed.
COMMENTS ON THE DISEASE
Sources of inoculum for this disease are rachises, shoots, and petioles killed during the previous growing season that remain on the trees. Cankers can also provide inoculum for as long as 6 years. Spores from these sources cause primary infections on the vegetative and flowering buds. Secondary infections subsequently occur on shoots, rachises, fruit, and leaves from spores produced in the primary infected plant parts. The pathogen can cause latent infections on buds, leaves, and fruit. Symptoms of the disease are triggered to develop by temperatures over 86°F (30°C).
Spores are spread in water from spring and summer rains, via water from sprinkler irrigation, or other means (birds, hemipteran insects, etc.). The optimum temperature range for disease development is 80° to 86°F, and the disease can become very severe during late spring to summer when temperatures and relative humidity in pistachio orchards are high.
Botryosphaeria panicle and shoot blight is extremely difficult to control, especially if allowed to increase over several years. The best approach employs monitoring, pruning, fungicides, and irrigation management.
To reduce disease incidence, lower sprinklers to prevent water from reaching the tree canopy or shorten the duration of irrigation from 48 to 24 hours. You may irrigate only during the daytime for 12 hours in 2 consecutive days to significantly reduce disease.
When disease incidence is low, pruning blighted shoots and panicles, shoots with cankers, dead and dying wood, and removing infected wood from the orchard can help reduce or eliminate inoculum for a few years. During late summer, after harvest, and during dormancy prune out infected areas 2 inches past blighted margins.
After harvest remove and destroy unharvested nuts and mummies to reduce sources of inoculum. At minimum shake trees and remove mummies as soon as possible before rain. Mummy removal will also reduce navel orangeworm.
Organically Acceptable Methods
Organic growers need to emphasize careful pruning to remove as much infected plant tissue as possible. Applying the fungicide Regalia is an organically acceptable method.
Monitoring and Treatment Decisions
During dormant and delayed dormant pruning activities, look for Botryosphaeria cankers noting trees of concern. Continue to look for shoot strikes two to three times during the growing season.
Two tests are available that predict disease risk at harvest. A preseason bud-monitoring test (BUDMON) detects bud infection incidence and predicts risk for panicle and shoot blight at harvest. A growing-season test (ONFIT = overnight freezing-incubation technique) of immature fruit provides treatment thresholds. For information on diagnostic labs that perform these tests ask your local farm advisor or pest control adviser.
If your orchard has a severe history of Botryosphaeria panicle and shoot blight or you find cankers, plan to apply a fungicide when panicles appear in spring. When the disease is severe, both pruning and fungicide treatment are suggested.
Apply two to three applications of QoI strobilurins (azoxystrobin, pyraclostrobin, or trifloxystrobin) starting at bloom. Pyrimethanil (Scala) is only effective when the disease is low to moderate in severity.
Time your fungicide applications before a rain event (if missed, an application can be applied 2 to 3 days after rain occurs); it has been shown that applications before or after rain reduce the number of calendar sprays.
Also consider the results of BUDMON or ONFIT tests to decide whether additional sprays are necessary.
UC IPM Pest Management Guidelines: Pistachio
T. J. Michailides, Kearney Agricultural Center, Parlier