Root Rots Result from Poor Water Management

Trees infected with this soilborne pathogen lose the ability to take up water and nutrients. Trees girdled by an infection can decline rapidly and die. (photos courtesy F. Trouillas.)

Water management is the basis for controlling Phytophthora root and crown rot in pistachio trees.

In his presentation for the UC Advances in Pistachio Production Short Course, plant pathologist Florent Trouillas said calls from growers about Phytophthora infections on pistachio trees have increased in recent years. A 2015-17 survey found three species of Phytophthora infecting pistachio orchards in the San Joaquin Valley.

Trouillas, who conducts research at the UC Kearney Agriculture Research and Extension Center, said improper irrigation for the orchard soils, including over irrigation in the spring and planting trees in marginal soils, are two of the reasons for the increase in infections from this soilborne disease.

Phytophthora root and crown rot can affect any age pistachio tree. Symptoms include gumming at the base of the tree (Photo 1). Trees infected with this soilborne pathogen lose the ability to take up water and nutrients. Trees girdled by an infection can decline rapidly and die (Photo 2). Infections occur in wet conditions. Alternating cycle of wet and dry can exacerbate Phytopthora root rots, Trouillas said.

Puddling around crowns of trees, saturated soils and poor drainage in orchards favor the occurrence of Phytophthora crown and root rots.

Planting trees on berms, placement of drip or micro sprinklers away from tree trunks one or two years after planting and improving orchard infiltration rates can help reduce infections. Prevention measures include foliar or drip applications of phosphites in the spring when trees are fully leafed out and in the fall prior to leaf drop to control Phytophthora infections. There may be MRL (maximum residue level) issues with some products. Mefenoxam (Ridomil Gold) is not currently registered for pistachio in California.

Macrophomina charcoal rot is another soilborne disease affecting pistachio trees. Instances of this disease were found in Kern County and western Fresno County orchards planted in heavy soils and on UCB1 rootstock.

Trouillas said that there is little information about Macrophomina phaseolina affecting perennial woody crops. The pathogen has been isolated also in declining table grapes and cherry rootstocks.

A third soilborne disease, Fusarium, can be a secondary pathogen. Trouillas said that plant stress, including old Phytophthora infections, might allow Fusarium species to become virulent in pistachio.

A pistachio rootstock susceptibility study tested PG1, UCB1 and Platinum for susceptibility to soil pathogens. They were inoculated with Phytopthora species, Fusarium and Macrophomina phaseolina and incubated for ten months. Platinum rootstock showed the least lesion length growth with all pathogens combined. PG1 rootstocks fared slightly worse and UCB1 was found to be most susceptible.