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 www.ipm.ucdavis.edu 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 4^th 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.”

'Bot' disease threat 

Botryosphaeria panicle and shoot blight

Another hard-hitting disease of pistachio is Botryosphaeria (Bot) panicle and shoot blight which attacks and kills clusters in some California and Arizona orchards.

In the spring, blight infects young panicles and young shoots. This can cause springtime infection of the leaf petiole and defoliation. Fruit infection follows in the spring and summer, followed by cluster kill in the summer and fall months. 

Michailides warns, “This disease not only infects the current pistachio crop but can also kill buds for the next year’s crop.”

Growers and pest control advisers sometimes can mistakenly blame killed clusters on the citrus flat mite pest. With mite damage, the cluster dies but the cluster rachis (main cluster stem) remains green. A Bot infection turns the inside of the cluster rachis brown.

The pathogens which cause Bot disease include Botryosphaeria dothidea and seven more species in the same family (Botryosphaeriaceae).

Bot is spread by moving water in the orchard, insects (hemiptera), birds, pollen, and pruning equipment.

Bot eradication is difficult but not impossible, says Michailides. The disease can be managed with orchard pruning, cultural practices, and fungicides. The best spray time to spray is just prior to a rain event or 2-3 days later.

The UC IPM website suggests lowering sprinklers to keep irrigated water from reaching the tree canopy, or to shorten the duration of the irrigation from 48 hours down to 24 hours. Daytime irrigation for 12 hours over two consecutive days can significantly reduce disease outbreak.

Michailides says new classes of fungicides and pre-mix fungicides are very effective in managing this disease.

Botrytis, Phoma blight

Botrytis blossom and shoot blight

Botrytis, caused by the fungus pathogen Botrytis cinerea, is found in some California pistachio orchards. The sporadic disease develops in cool and rainy situations at bloom in April and during May and June and infects tender clusters. Wilted shoots resemble a Shepherd’s crook.

Shoots die and the dead leaves remain attached (flagging). Late rains can lead to infected fruit clusters and the eventual partial death of clusters, UC Davis reports. Blossom blight is more severe in male trees than female trees.

“However as temperatures rise the infection of the clusters stops, leading to the partial killing of the clusters,” Michailides said.

Orchard sanitation, including blighted shoot pruning, can reduce the inoculum level in the orchard. Consider bloom fungicide sprays if the weather is cool and wet during the bloom period.

Phoma blight

Phoma blight is an anthracnose-like disease found in some Arizona pistachio orchards. The pathogen is the fungus Phoma fungicola. The disease moves very fast in the tree and kills clusters near harvest time.

Michailides has conducted several trials in the lab to determine fungicide efficacy against Phoma blight. He hopes to conduct disease-fungicides field trials this year in infected pistachio orchards with the cooperation of local Arizona pest control advisers.

For more information diseases in pistachios, contact Themis Michailides at [email protected].