Written by Brent Holtz, UCCE San Joaquin County Symptoms of perennial canker of almond. Photo by F. Niederholzer. Perennial Phytophthora Canker has killed many almond trees. Two plant pathogenic fungi, Phytophthora cactorum and Phytophthora citricola are primarily responsible. This disease is lethal, perennial, and is not associated with pruning wounds which differentiates it from “Pruning Wound Canker.” Perennial Phytophthora Canker kills almond trees by girdling the scion and is usually initiated in a conducive environment either below the soil surface or on the tree trunk where the trunk and branches join. Phytophthora cactorum is usually associated with infections initiated near the soil surface, while Phytophthora citricola typically causes aboveground infections initiated near tree branch crotch pockets. Dr. Greg Browne, USDA-ARS, has found that the source of inoculum for above ground infections is likely debris from the orchard floor containing fungal spores that are blown onto trees during harvest. This debris and spores are most likely washed off the tree during rains, with some accumulating in main-branch crotch pockets or depressions where main branch scaffolds join the tree trunk. These scaffold pockets containing soil, water, and fungal spores most likely offer a conducive environment for Phytophthora infections to take place. Dr. Browne’s research has provided almond growers with some control measures that can be taken to prevent Perennial Phytophthora Canker. He has shown that phosphonates (e.g. inorganic and organic salts of phosphonic acid) have provided systemic activity against a number of diseases caused by many of the Phytophthora species of plant parasitic fungi. The phosphonic acid derived from phosphonates probably disrupts Phytophthora growth and appears to intensify almond tree defenses against infection. Fortunately, the phosphonate can be translocated both upwards and downwards in the tree due to its mobility in both xylem (water going up) and phloem (sugars going down) tissues.