Almond trees are dying in California. Researchers don’t fully know why, but they have recently turned up some important clues about the disease responsible for killing some trees and sharply cutting yields of others.

Researchers call the disease “almond union mild etch” because it leaves a mark where the almond-producing scion was grafted onto the rootstock. Grafting is a common technique used to obtain trees with superior yield and rooting ability.

While the condition may affect as few as 5 percent of California’s almond trees, it could have major economic implications. California produces 65 percent of the world’s almonds, worth $1 billion annually.

Symptoms of the disease include leaves that turn yellow in the summer and drop off early in the fall. Most of the affected trees may survive. But they are stunted and only produce two- thirds the nut crop of healthy trees. A plant pathologist with the Agricultural Research Service in Davis, Calif., began studying the problem in 1990, with initial funding from the state’s Almond Board.

The disease has appeared on six varieties of almonds, all grafted to Marianna 2624 rootstock. Alarmingly, this particular rootstock is noted for disease resistance, especially in soils were drainage is a problem. About 5 percent of the almond trees in California are grafted to Marianna 2624 rootstock.

Scientists with ARS and the University of California at Davis do not believe the disease is contagious. They have found no pathogenic virus, bacteria or phytoplasma organisms on the trees or root rot organisms in the soil.

Based on 4 years of field observation, the researchers believe the disease alone does not kill trees. They found the most severe tree losses where the disease was present in combination with other stress factors: soggy soil conditions, coupled with impaired grafting that restricts the flow of carbohydrates from leaves to roots. Weakened by stress, roots of affected trees become vulnerable to normally harmless organisms in the soil.

Scientific contact: Jerry K. Uyemoto, ARS Crops Pathology and Genetics Research Unit, Davis, Calif., phone (916) 752-0309, fax (916) 752-5674, JKUyemoto@ucdavis.edu.