Perhaps the fight against several diseases of pistachio is a modern day version of the legendary Shootout at the O.K. Corral.

In the scenario, University of California Plant Pathologist Themis Michailides places four diseases facing the western pistachio industry in his gun sight. He pulls the trigger and fires a round of solutions which can help pistachio growers reduce the amount of damaged or lost nuts - and income.

This analogy brings to the forefront four disease threats facing western pistachio growers - Alternaria late blight, Botryosphaeria panicle and shoot blight, Botrytis blossom and shoot blight, and Phoma blight - and scientific efforts to help growers effectively manage the diseases.

All four can damage or ruin pistachios and reduce yields. Management tools are available to help limit losses to disease.

Michailides discussed the diseases and solutions during the American Pistachio Growers Convention held in February in Coronado, Calif.

Alternaria late blight

Alternaria leaf blight is found in some California pistachio orchards, Michailides says. It occurs in high humidity situations in the orchard tied to micro sprinkler or flood irrigation, or orchards with low soil infiltration.

The first Alternaria symptom is small black spots found on the leaves and fruit in June. As the summer progresses, the spots enlarge into brown, necrotic lesions which develop a black powder; the spore source of the pathogen.

The spores spread to new leaves and secondary infection occurs. This cycle can be repeated 2-3 times per season, depending on the weather conditions in the late season (humid and warm).

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The disease leads to tattered leaves, defoliation, and lesions on the fruit. The end result can be shell and nut staining which renders a lower quality nut which in some cases may be unmarketable.

The disease pathogens include Alternaria alternata, A. tenuissima, A. arborescens, and Stemphyllium species.

Michailides says good management practices – cultural efforts and fungicide sprays - help control Alternaria.

“It’s best to use drip irrigation or micro sprinklers, not flood irrigation, to help reduce Alternaria,” Michailides said. “Hedging trees can help ventilate the orchard and release moisture into the air.”

He suggests that growers plant new orchards in the same direction as the wind flow – for example, north to south.

Be careful with fungicide use as the Alternaria fungi can easily develop resistance. A list of registered fungicides for pistachio is located on the website. 

Conduct sprays in May, June, and July. Bloom sprays and sprays in August provide minimal control. Michailides says the best time to spray fungicides is late June. July 4th is a good rule of thumb.

Michailides urged growers to, “Follow the fungicide label recommendations, do not use reduced rates, rotate fungicides of different classes, and aim for good coverage.”