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Mulches Used to Control Avocado, Citrus Root Rot

The Issue

Phytophthora root rot of avocado and citrus is a worldwide problem, causing the potential destruction of avocado orchards and reducing fruit quality and yield for citrus. It is estimated that some 60 percent of California avocado orchards are affected by the disease.

What Has ANR Done?

Professor of Plant Pathology John Menge of the University of California, Riverside began a series of field trials in 1999 to evaluate the effects of yardwaste mulches on citrus and avocado. Results indicated that avocado yields and growth increased by as much as 43 percent in Phytophthora-infested orchards when the mulches were applied. In addition, the mulches helped reduce the need for water, fertilizer, and weed-control agents. Some benefits were seen for citrus, but they were not as dramatic. Dr. Menge was able to determine that mulches are effective because they produce enzymes that decompose the cell walls of Phytophthora. Another major area of Dr. Menge's work involves developing disease-resistant rootstocks aided by genetic tools such as microsatellite analysis to determine the parentage of natural crosses. Some 50 new rootstocks have been developed with excellent root-rot resistance, and extensive field tests of these new varieties are ongoing. One new rootstock, the Merensky II, has been released to growers.

The Payoff

Avocado Growers Benefit from Control Strategies, Governments Aided in Waste Management

Virtually all Phytophthora-infected avocado orchards planted today use yardwaste mulches due to the research conducted by Dr. Menge. Avocado growers are finding that through a combination of control methods--including resistant rootstocks, mulches, and fungicides--they are able to replant in orchards where almost all the trees had died. Municipal and country governments also have benefited from Dr. Menge's research. Previously local officials were paying as much as $100 a ton to dispose of yardwaste. Now citrus and avocado growers purchase the yardwaste for their orchards, enabling cities and counties to earn a profit and to reduce the solid waste they deliver to landfills.


Supporting Unit: UCR Department of Plant Pathology

Professor John Menge
238B Fawcett Laboratory
UC Riverside
Riverside, CA 92521
(909) 787-4130 (Ph), (909) 787-4132(Fax)