Bacterial spot

Bacterial spot, caused by a bacterium (Xanthomonas arboricola pv. pruni), is another spring disease showing up with a high incidence in many California Almond orchards. Last year, the disease created a critical situation in some orchards in Colusa, Merced, Stanislaus and San Joaquin Counties, leading to significant fruit loss.

The Fritz variety of almonds is very susceptible, but isolations have been made on a number of other varieties, which are not as severely affected. A high degree of wetness from rainfall or sprinkler irrigation is favorable for infections. In-season symptoms include numerous fruit lesions that develop amber gumming and result in excessive fruit drop. Leaf spots and defoliation are also symptoms of the disease.

February is a good time to observe if the disease was present in orchards last year. Look for infections on blossoms. Unusual lesions on blossoms can be attributed to several things, but if brown rot sprays have been made and there are still unusual lesions, contact your local farm advisor.

Symptoms of the disease from the previous season can also be found as raised, circular bumps on fruit mummies still attached to the tree. These lesions have high levels of the pathogenic bacterium; in fact, mummy sanitation is a key component of control. Twig lesions and contaminated buds are also overwintering sites.

UC plant pathologists Jim Adaskaveg and Themis Michailides and UC farm advisors Brent Holtz (San Joaquin), David Doll (Merced) and Roger Duncan (Stanislaus) have done much to characterize the extent and nature of this disease in California Almonds; however, management strategies used on other crops in some areas of the U.S., as well as on almonds in Australia, need to be investigated and adapted for California.


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UC Riverside plant pathologist Adaskaveg is leading Almond Board–funded research investigating dormant and springtime applications with bactericides. This work should soon offer California-specific recommendations, but for now, UC personnel are basing their advice on experiences in Australia, where bacterial spot was first confirmed in 1994–95, and on information from stone fruit crops in the southeastern United States, where the disease has been endemic for many years.

As this is a relatively new, emerging disease, questions should be directed to UC farm advisors.

Brown rot

With the broad efficacy and reachback of today’s fungicides, growers can hold off on brown rot bloom sprays until full bloom in drier weather, and get by with a single — or perhaps no application — depending on weather conditions and location.

Delaying fungicide treatments to when 40% to 80% of the flowers are open under drier conditions can save on fungicide applications and also help reduce exposure to foraging bees during pollination, while still getting excellent brown rot blossom blight control and coverage for other spring diseases, says Adaskaveg.

Growers can minimize exposure of bees and pollen to sprays by avoiding applications when pollen is available and bees are foraging. This normally is best accomplished by spraying after mid-afternoon and at night.

“We have reviewed fungicide efficacy data, and with today’s efficacy in both pre- and post-infection activity, we can be more discriminatory about our applications,” Adaskaveg said. “If rain is not in the forecast, you can hold off on your sprays until 40% to 80% bloom in the northern part of the state, and get by with a single spray, and in the south, get by with perhaps no spray at all.”

Adaskaveg said that given the excellent efficacy of today’s fungicides, recommendations are moving away from an early pink-bud application at 5% bloom toward a delayed single application under less favorable conditions for disease.


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