SPRING TREATMENTS FOR SCAB

SPORES OF THE FUNGAL PATHOGEN that causes the disease >almond scab begin to form in overwintering twig lesions in late March to mid-April and are likely to be spread by wind and mid- to late-spring rains, particularly in the northern San Joaquin and Sacramento valleys.

The most effective timing for fungicide treatments for Cladosporium carpophilum, the fungus that causes scab, is in the spring, two to five weeks after petal fall, according to plant pathologist Jim Adaskaveg, UC-Riverside. In a presentation at the annual Almond Industry Conference last December, Adaskaveg noted there is widespread resistance of populations of the pathogen to several classes of fungicide, particularly the strobilurins (or QoI fungicides).

"This resistance emphasizes the importance of having several fungicide classes available that are active against a specific pathogen, and the need for growers to use resistance-management strategies whenever several alternatives to manage a disease are available," he said.

These strategies include a rotation program with fungicides in different classes of chemistry, and avoiding the use of single-site mode-of-action fungicides once the disease is developing. Another resistance-management option is applying a delayed-dormant treatment with liquid lime sulfur or copper-oil, which will be reviewed in more detail in an upcoming article in California Almonds this fall.

Promising new in-season treatments for scab include Inspire, Quash, Elite, and Indar (pending their late-season registration). The new single-site, mode-of-action fungicides should be in rotation with petal fall applications of multi-site materials like Bravo, Ziram, Captan, or Maneb.