Phytophthora Root and Crown Rot

Symptoms associated with Phytophthora root and crown rot in pistachio include chlorotic foliage (leaf yellowing) and defoliation. Infected trees also show a general poor vigor. Basal crown rot or cankers develop below the soil line and may extend up into the rootstock portion of the trunk, which is visible when the outer bark is removed with a hatchet.

Trees affected with crown rot usually die rapidly, and death typically occurs during the hot summer months. Trunks exude profuse light beige to light cream-colored gumming in strands. Gum balls that turn black may develop, especially when infection has reached the scion. Phytophthora root and crown rot is common in third- and fourth-leaf orchards that receive excessive amounts of irrigation water, but it can affect and kill trees of all ages. Infected roots become necrotic, with dark brown to black discoloration of the cortex and stele. Small roots eventually disintegrate. Infected pistachio bark and cambium turn brown to black.

Phytophthora root and crown rot is commonly associated with prolonged periods of high soil moisture in poorly drained soils, including soils with a hardpan, heavy soils, and saline and saline-sodic soils. Phytophthora species survive in the soil during dry periods as oospores or chlamydospores (thick-walled resting spores). Many Phytophthora species are present in canal and river water, and the use of infected surface water sources can introduce Phytophthora species in orchards. To date, Phytophthora species have not been found in well water, and surface sources of irrigation water, including rivers and canals, are major sources of Phytophthora inoculum in pistachio orchards.

Phytophthora inoculum is present in most soils, however, and any practices that result in saturated soil conditions, such as long durations of irrigation or overirrigation, can lead to disease occurrence in orchards. Several species of Phytophthora are known to attack pistachio roots and crowns. The most common species found in California pistachio orchards are Phytophthora niederhauserii, P. mediterranea, and P. crassamura. The most affected rootstocks are clones and seedlings of UCB-1.

Proper water management, particularly avoiding overirrigation, is the basis for managing Phytophthora root and crown rot. Correct sites with poor drainage. Use cultural practices that avoid prolonged saturation of the soil, such as planting on berms, shortening irrigation time, and improving water penetration, to reduce the incidence of Phytophthora diseases. Apply irrigation to meet trees’ water demand and avoid saturating the soil for periods longer than 24 hours. When roots have grown outside the root ball and active tree growth has begun, move drip emitters away from the trunk to avoid excessively wetting the crown.

Platinum is the most tolerant rootstock to Phytophthora root and crown rot, when compared to PG1 and clonal UCB-1 rootstocks, and should be the preferred rootstock when establishing orchards in situations at risk for Phytophthora root and crown rot. Nevertheless, the susceptibility of Platinum (a single descent of a PG2 seedling) to Verticillium wilt is unknown.

Several fungicides are effective against Phytophthora root and crown rot in pistachio.