Written by Brent Holtz, UCCE San Joaquin County
|Symptoms of perennial canker
of almond. Photo by
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.
In trials performed by Dr. Browne, a preventative foliar spray with phosphonate in the fall or spring of roughly 2 pounds per acre active ingredient (2.2 kg ai/ha) suppressed the development of Phytophthora cankers after wound inoculations of the fungus were made 15 days to 5 months after spraying. The Phytophthora cankers on trees sprayed with phosphonate before inoculation were 22 to 98 % smaller than those on trees that received no phosphonate. Dormant foliar applications of phosphonate were ineffective, probably because the dormant trees lacked the expanded l eaves necessary to absorb phosphonate. Dr. Browne also found that preventative chemi gation of phosphonate into irrigation water in the spring or summer also inhibited Phytophthora canker expansion. Chemigation was only effective during periods of high crop evapotranspiration and not during periods of low water use.
The sources of phosphonate used in Dr.Browne’s experiments included Phostrol (Nufarm Americas, Inc.) at 0.41 gallons/acre (3.8 liters/ha) and NutriPhite P foliar (4-30-8) applied at 0.46 gallons/acre (4.3 liters/ha). NutriPhite P Soil (0-60-0) was applied via chemigation at 0.46 gallons/acre ( 3.9 pounds phosphonic acid/acre).
In orchards where Phytophthora has been a perennial problem, I strongly suggest that you apply preventative phosphonate sprays in either the fall or spring.