Phytophthora Root and Crown Rot

Pistachio is subject to root and crown rots and trunk and branch cankers. Trees show poor vigor; leaf yellowing, wilting, and scorching; and shoot and branch dieback. Infected roots become necrotic, with dark brown to black discoloration of the cortex and stele. Small roots eventually disintegrate. Cankers develop at the root crown and may extend above the soil line. Infected pistachio bark turns brown to black. Active cankers often ooze and release a light cream exudate in balls, strands, or puddles. The exudate is not diagnostic because other diseases and injuries to the trunk will result in sap release.

Occasionally, Phytophthora spp. cause perennial cankers on trunks and scaffolds. Such cankers stop at the union of the scion and the rootstock and often lead to tree death.

The pathogens survive in soil and can be carried in irrigation water obtained from surface sources. Prolonged periods of saturated soil are optimal for root infection. Disease development is enhanced in poorly drained soils, where orchards receive long durations of flood irrigation, or in trees in lower spots or along creeks and natural drainage creeks in the orchard. Several species of Phytophthora are known to attack pistachio roots and crowns.

Fungicides are not currently registered for control of Phytophthora root and crown rot in California pistachio. Management of this disease can best be achieved using strict planting practices and water management. Plant on raised berms in well-drained soil to allow for rapid water drainage following irrigation or rains.

Although the disease is not a major problem in California, growers have been losing trees to Phytophthora canker due to soil conditions. In heavy soils, a hard pan layer results in poor infiltration and lower spots in planting sites.

In general the Pistacia rootstocks are not considered very susceptible.