The symptoms generally appear in summer and are characterized by the presence of angular or circular spots of about 3 to 7 mm in diameter on leaflets. In some severe cases or in particularly susceptible varieties, the spots become blotches of up to 3 cm in diameter. Black lesions are also present on petioles and main veins of leaf blades. Severe infestation can cause leaves to wither and fall. On immature fruits, minute brown or black spots appear. On mature fruits, the spots are somewhat larger (1 to 5 mm in diameter) and are surrounded by a reddish halo. Mold starts to develop on fruits, leading to their decay. In severe cases, this significantly reduces the quality of the product.
The symptoms are caused by a group of three fungi of the genus Alternaria, among others Alternaria alternata. They hibernate in the soil or on plant debris. When conditions are favorable, they produce spores that are transported by wind, rain onto susceptible trees. High temperatures associated with high humidity, including dew formation, alternating wet and sunny periods, and nutrient deficiency are major factors in the development of the disease on pistachio. The symptoms can be confused with those of panicle and shoot blight caused by Botryosphaeria dothidea. To tell them apart, rub the affected leaves when humid conditions prevail: if it blackens the fingers, then it is a late blight caused by Alternaria alternata.
Spray Bordeaux mixture on the trees and cuttings after pruning affected parts. Alternatively, apply a concoction of garlic or horsetail on the leaves and fruits. You can use solutions based on Bacillus subtilis, which is a predator of the fungus.
Always consider an integrated pest management with preventative measures and biological treatments if available. Treatments against this disease should begin in the early summer, before the ripening of fruits. Products based on the active ingredients thiophanate-methyl, maneb, copper products work well. At least two treatments are needed to reduce infestations. The effectiveness of treatments depends on the timing of application, the adjustment to the age of the tree, and the application of the recommended dose.