How to Manage Pests
UC Pest Management Guidelines
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 Fusicoccum 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.
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). Preseason detection of the pathogen's incidence in buds (using a BUDMON test) helps predict the disease risk at harvest.
A test (BUDMON) is now available to help predict the risk of panicle and shoot blight at harvest. The test is done preseason (March) and detects the incidence of the pathogen in buds. A second text (ONFIT: overnight freezing incubation test) done with immature fruit in June/July is now available to help predict blighted fruit at harvest.
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 fungicides, pruning, and irrigation management. Lower the sprinklers so that water does not reach the tree canopy or shorten the duration of irrigation from 48 to 24 hours. Irrigating only during the daytime for 12 hours in 2 consecutive days reduces the disease significantly.
If your orchard has a history of panicle and shoot blight, plan to treat for this disease when panicles appear in spring. When disease incidence is low, pruning the blighted shoots and panicles during summer can help reduce or eliminate this disease for a few years. Pruning of infected parts 2 inches below the blighted margins in late summer and fall reduces disease the following year. When the disease is severe, both pruning and fungicide use are suggested. Two to three applications of QoI strobilurins (azoxystrobin, pyraclostrobin, or trifloxystrobin) during summer will control the disease, although in orchards with high pressure, sprays can start at bloom and continue during summer. Sprays with pyrimethanil (Scala) are only effective when the disease is low to moderate in severity.
UC IPM Pest Management Guidelines: Pistachio