As Botryosphaeria and Phomopsis canker and blight infections have spread throughout California’s walnut orchards over the past four years, University of California Cooperative Extension researchers have continued to learn more about what causes these diseases, how they spread and what steps growers can take to control these fungal threats.
UCC farm advisors Janine Hasey, Sutter/Yuba/Colusa Counties, and Rick Buchner, Tehama County, along with plant pathologist Themis Michailides, Kearney Research and Extension Center, Parlier, Calif., summarize these lessons in the Spring, 2014, issue of Sacramento Valley Walnut News.
Michailides’ research has been funded by the California Walnut Board.
Ten species in the Botryosphaeria (Bot) family and at least two species of Phomopsis have been associated with walnut disease. Six of the Bot species can infect spurs and shoots. However, all 10 species of Bot and the two Phomopsis species can infect the nut.
These fungi infect the nut, move into the peduncle (the stem of the nut) and then invade the spurs, killing next year’s buds. Cankers grow slowly in the winter when temperatures are low. But, at temperatures above 80 F, this process can occur within one week to 10 days.
Most symptoms are seen at harvest and later. Blighted spurs (twig blight) are common in fall, winter and early spring. During the season, brown blighted shoots aren’t seen under dry weather conditions, unless sprinkler water is hitting foliage or some other water source is spreading the disease.
Sources of fungal inoculum include hulls, peduncles, dead buds, dead spurs and cankers of walnut trees. Blackberry vines are one of many hosts of Botryosphaeria that can serve as inoculum sources.
Bot can infect the tree though leaf, bud and peduncle scars, broken branches, pruning wounds and blighted fruit. Scale insects, particularly Walnut Scale increase the potential for infection and development of cankers.
Due to the size of walnut trees and amount of fruitwood, both cultural and chemical controls are recommended to limit Botryosphaeria and Phomopsis damage. They include:
Preventing sprinkler irrigation from wetting the canopy.
Pruning dead branches back to healthy green wood and removing larger infected limbs.
Pruning mature trees after harvest when deadwood is easier to see, and before heavy rains can spread inoculum to coat tissues, pruning wounds, bud scales and other avenues of infection. Eliminating infected wood also reduces the disease threat.
Removing and burning prunings.
Shredding or chipping smaller wood remaining in the orchard ½-inch or smaller pieces. Bigger pieces of pistachio wood, for example, can produce viable spores for up to 1½ years.
Controlling scale insects.
Fungicides are effective only in preventing these two disease and only when sprayed on green susceptible tissues. In one study last year, some fungicide applications by growers in mid-May, mid-June and mid-July showed trends of reducing Bot infections.
Extension farm advisors are not making specific recommendations on fungicide use pending more research. However, potential fungicides registered in walnuts include
Pristine (replaced by Merivon), Luna Experience, Luna Sensation, Fontelis, Quilt Xcel, Abound, Quadris Top, Bumper, Quash, and Inspire Super. Copper and Manzate used for walnut blight will not control Botryosphaeria and Phomopsis fungi.
Look for more complete Extension recommendations for managing these diseases next year once current research is finished.