“Careful monitoring of disease threats early on is the best way to deal with potential orchard health problems later in the season,” writes Gurreet Brar, University of California Cooperative Extension farm advisor for Fresno and Madera counties in the January/February issue of his newsletter, From the Shell.
One of the major threats to pistachios is Botryosphaeria panicle and shoot blight.
This fungal disease is extremely difficult to control, especially if allowed to increase over several years. In fact, infected wood can produce inoculum for as long as six years.
Symptoms appear in mid to late spring as black, 1- to 2-mm circular spots on shoots, rachises, and leaves. Shoots originating from contaminated or partially infected buds develop black lesions at the base.
March is a good time to evaluate buds or small twigs in your orchard to determine how much, if any, Botryosphaeria is present.
Bob Beede, UC Farm Advisor Emeritus, recommends walking through your orchard and collecting at least one hundred female flower buds in a paper or plastic bag.
“Take a stick with you and hit the persisting fruiting rachises on the tree to see if they knock off cleanly at their base, or if a stub is left,” he says. “If the latter, pull that fruit limb down, or cut if off, and cut into the wood around the base of the rachis to check for black streaks. Discovery of these symptoms would suggest that rachis was infected with Botryosphaeria this past season. Cutting the collected buds in half with a razor blade will help you determine how much of the disease is present in the orchard. Buds whose interior is black are often infected with Botryosphaeria. So, if you find several in your first sample, you might want to sample one or two more times to get a better average infection level.”
Brar also suggests considering dormant bud sampling (BUDMON). Based on the percentage of buds harboring spores during the dormant season, it can help assess the disease risk at harvest, especially if damage from Botryosphaeria was high the previous year.
To achieve the most effective control University of California IPM guidelines recommend fungicides, pruning, and irrigation management.
In sprinkler-irrigated orchards, lower the sprinkler angle 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 two 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 the 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 the disease the following year.
When the disease is severe, both pruning and fungicides are advised. Two to three applications of strobilurins (azoxystrobin, pyraclostrobin, or trifloxystrobin) during summer will control the disease. In orchards with high pressure from the disease, 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.
For help in selecting the right material and the proper timing of application for the most effective disease control, Brar recommends the Fungicide Efficacy Guide, published by the University of California. It’s available at http:// www.ipm.ucdavis.edu/PDF/PMG/fungicideefficacytiming.pdf.
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