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Long-term work yields avocados resistant to disease

The Issue

The most devastating disease of avocado in California and the world is Phytophthora cinnamomi, or avocado root rot. In unprotected orchards, this disease can destroy virtually every tree within a few years, and put at risk the $400 million dollar California avocado industry. Fungicides have been used extensively to try to control the disease, but the ultimate control is a rootstock that is resistant to the fungus that causes the disease.

What Has ANR Done?

Beginning in the 1950s, George Zentmyer of UC Riverside explored Latin America for avocado trees that showed resistance to avocado root rot. Since Mexico and Central America are the origins of avocado, it was thought uncultivated trees there might have some tolerance to the fungal pathogen.

At the same time, Zentmyer was on the look out in California for “escape” trees, trees in orchards that survived while surrounding trees succumbed. These resistant trees could also be used in the breeding program. UCCE farm advisors were instrumental in identifying "escape" trees. Field trials that "challenged" the new rootstocks were conducted with farm advisors throughout the avocado growing area.

Out of this breeding and selection process came the first rootstock with significant avocado root rot resistance – “Duke 7.” The Duke 7 rootstock has been propagated extensively as a clonal rootstock by avocado nursery operators in Southern California and has proven to be a moderately vigorous rootstock, producing uniform trees. Many hundreds of thousands of trees on Duke 7 clonal rootstock have been planted in Southern California. Since then, nearly every year, rootstocks with greater and greater resistance have been identified. These rootstocks are now being identified as to their resistance to other, lesser diseases and to the high salinity waters often found in Southern California.

The Payoff

University sustains the California avocado industry

Avocado root rot had spread to a lesser or greater degree to nearly every orchard in California. The early fungicides that were used to slow the spread were ineffective. The only way the industry could continue to thrive was to find rootstocks that were resistant to the disease and to other production problems. UCCE helped develop and introduce these new rootstocks to growers and demonstrated how they needed to be managed. Virtually all new avocado plantings in the last 10 years have been with rootstocks developed by UC scientists. They have helped create a sustainable avocado industry that exports throughout the country.


Supporting Unit:

Santa Barbara/Ventura Counties UCCE
Ben Faber
669 County Square Dr., Suite 100
Ventura, CA 93003