Bacterial Spot identified in almonds

A wet spring and many frosty nights had growers hopping nonstop during and after almond bloom and with it came a new almond disease.

According to Colusa County, California UCCE farm advisor John P. Edstrom, precipitation from March through May totaled 9.8 inches, 300 percent above normal in Arbuckle for each of the three months.

Growers were forced to repeatedly apply fungicides to protect the crop from most of the common fungal diseases, and they generally held off the fungal onslaught.

“However, our spray programs were not active against bacterial diseases and it ap¬pears that we have found a new one,” said Edstrom.

In many orchards, the farm advisor found unusual leaf symptoms developed early, resembling bacterial blast and shot hole. Lab results did confirm some bacterial blast (Pseudomonas syringae) but very little fungal shot hole.

“The leaf spotting seemed different than what is normal for blast or any other com¬mon almond disease,” he explained.

Additional lab work conducted on almond leaves by Dr. Themis Michailides at UC Davis isolated Bacterial Spot.

“We now suspect this organism caused many of the confusing leaf symptoms up and down the valley. However, further tests must be completed next spring before the identification of Bacterial Spot can be confirmed.

Bacterial spot, is a common problem in stone fruit and almonds throughout Europe, the Middle East, Australia and the Southeastern US. This disease can be very dam¬aging, with severity of infection depending on rainfall and dew formation. Fruit and foliage are both susceptible to infection in humid regions or areas with regular late spring and rains. In Australia, many growers have been forced to abandon the two most severely affected varieties, Fritz and Neplus due to extensive crop loss. Infected nuts develop corky lesions that ooze orange colored gum that either drop prematurely or remain on the trees after harvest as stick-tights. Circular or angular reddish-lesions develop on leaf blades. Leaf spots may be discrete or may coalesce along margins and result in a tat¬tered appearance. These symptoms are easily confused with shot hole, added Edstrom. Defoliation follows and per¬sists throughout the rainy period in Australia.

Mission and Monterey are also very susceptible while Nonpareil and Price intermediately susceptible. Intensive spray programs with copper and Mancozeb have not controlled Bacterial Spot "down under," In the South Eastern US, peach growers have applied Copper plus oxytetracycline as preventative fall sprays with some success. In all affected areas, selecting less susceptible varieties has been the best protection from bacterial spot.

The threat from Bacterial Spot to almonds in California is probably small unless we have a shift in spring rainfall patterns as we experienced this season.

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