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California Pistachios: Don’t Confuse Mites With Diseases At Harvest

Owen Taylor
By Robert Beede, U.C. Farm Advisor, Emeritus, Kings County, California September 3, 2017

California Pistachios: Don’t Confuse Mites With Diseases At Harvest

There may be some Botryosphaeria in the south valley this year, considering the rain we had during the spring. Also, some orchards had quite a bit of stinkbug activity right around the first of July, which can cause isolated BOT strikes in the fruit clusters from their feeding.

Be on the lookout for it during harvest, so that a strike-cutting program can be initiated in the fall, if necessary. Alternaria has not shown up yet (as of August 23) in orchards with two sprays, so the new materials appear very effective.

As leaf tissue ages, its susceptibility to Alternaria infection increases due to decreasing sugar content. Warm temperatures and high humidity increase the Alternaria potential.

Look for yellowing leaves, which have black necrotic lesions and spores in the center. This differs from the common yellowing that occurs on fruiting spurs, which is the result of nutrient extraction by the developing kernels.

Also examine the leaf stem (petiole) and main vein. Rub the area with your fingers to see if some of the black comes off. If so, this is Alternaria. Assess how much exists in the canopy and look for small black lesions on the hull tissue.

Remember that Alternaria DOES NOT kill nut clusters and shoots. Botryosphaeria does that. BOT also does not rub off on your fingers when you handle the infected tissue. Leaves uniformly brown low in the canopy can be easily mistaken as Alternaria infections when in fact they are simply dying from lack of light or water stress.

How do you tell? Look for the black spores that rub off on your fingers. If there are no spores and the leaves are UNIFORMLY brown rather than having angular sections of brown with black spores in the center, they are shoots that have simply shaded out.

They could also be infested with Pacific mite, which is rare in pistachio, but I have seen lots more of it this year.

Alternaria can cause economic damage from defoliation and nut staining. Following harvest, you should determine if pruning, irrigation and soil management practices might be modified to reduce the problem.

Poor pruning and slow water infiltration are common causes. Consider applying gypsum in June rather than in the winter to improve the surface soil structure.

My desire to minimize Alternaria infection through good cultural practices is based on the limited materials we presently have registered for this disease and the rapid resistance that develops from their frequent use.

Mites And Diseases – Which Is Which

Do not confuse citrus flat mite, Pacific mite, or rain damage for Alternaria. Several calls typically occur at harvest concerning dried clusters on the tree, which cannot be removed by the shaker.

Citrus flat mite causes patches of chocolate brown discoloration on the hull and rachis tissue rather than the distinct, round lesions about 1 mm in diameter associated with Alternaria.

Flat mite discoloration is also only on the surface, so scratch the tissue to see if it is green underneath. Citrus flat mite also does not attack leaf tissue and cause black necrotic margins. This tiny, orange colored mite can turn entire clusters brown and render them unharvestable.

Citrus flat mite damage can be confused with BOT, BUT flat mite does NOT cause gumming or blackening of the cluster like BOT. Wettable sulfur in June or July is the cure for flat mite. Also, if you are near a dairy, do not confuse fly speck for flat mite.

Pacific mite defoliates trees in patches of the orchard. Browning begins inside the tree canopy, and works it way out, causing leaf loss with mite levels as low as one or two per leaflet. Pacific mite and Gills mealybug problems are on the rise with multiple pyrethroid sprays against NOW.

Reports from the field indicate Gills mealybug has really spread this year. Super clean nuts are coming with added cost. Softer NOW management programs are the key to future cost control.

Do not confuse leaf scorch common on the male “Peters” variety for Alternaria or Botryosphaeria. Male scorch is thought to be caused by heat and it may predispose the males to Alternaria, but this disease did not cause the initial leaf browning.

Owen Taylor
By Robert Beede, U.C. Farm Advisor, Emeritus, Kings County, California September 3, 2017