Walking the rows of your almond orchards after harvest to check for signs of rust and shot hole can be time well spent; It can enable you to spot and head off any potential disease problem before it develops into a full-blown threat to your crop next year, says Gurreet Brar, University of California Cooperative Extension farm advisor for Fresno and Madera Counties.
As described in the UC IPM Online pest management guidelines for almonds (http://www.ipm.ucdavis.edu) rust occurs sporadically throughout almond-growing areas in California. The fungal disease causes leaves to fall prematurely and will weaken trees. In severe cases, the disease can defoliate trees quickly. Often, it’s seen in second- and third-leaf, non-bearing orchards where fungicides have not been applied. If not controlled, it reduces the following year's bloom.
Rust is more likely to become serious in orchards near rivers or streams or other locations where humidity is relatively high in spring and summer. Excessive levels of nitrogen can also increase the tree's susceptibility to the diseases
Rust fungus survives from one season to another through the infected leaves that keep hanging on the trees throughout the dormant period, Brar notes. These infected leaves can carry the inoculum to the spring and when spring rains provide humid conditions conducive for the fungal growth, the disease can spread quickly.
Want the latest agricultural news each day? Click here for the Western Farm Press Daily e-mail newsletter.
The rust spores are spread by air movement and infect other leaves to continue the disease cycle. Young twigs may be infected, but twig lesions are seldom seen on almond trees.
“In order to prevent the inoculum buildup and carry-over through the winter, we need to watch for any leaves that show rust lesions,” Brar says.
He advises checking the upper surface of the leaves for rust lesions. Look for small, yellow spots. On the lower side of the leaves, these spots are rusty red, due to eruption of the rust-colored spores through the leaf surface. Spread of rust is favored by humid conditions.
“If any leaves with the lesions are found, apply zinc sulfate, at the rate of 20 to 40 pounds per acre in early November,” Brar says. “This will hasten the leaf fall and, thus, prevent the inoculum from building up. Application of zinc sulfate also provides the trees with the necessary dose of zinc.”
Shot hole can cause reduce yield, cause defoliation and weaken trees. The fungus survives on infected twigs and as spores in healthy buds. Spores are moved by water to new sites. UC IPM Guidelines note that prolonged periods of wetness, either due to rain or sprinkler irrigation, are required for the disease to develop.
Shot hole lesions on the leaf begin as tiny reddish specks. They enlarge into bigger spots with tan centers and purplish margins, Brar notes.
“At this time of the year, look for the fruiting structures of this fungus,” he says. “They appear as a small dark speck in the center of the spots. These fruiting structures are typically formed after the fall rains begin. That’s when the temperature and humidity conditions usually favor the production of fungal spores.”
These fruiting structures can be easily seen with a hand lens, he adds. If you find shot hole lesions or fruiting structures, he advises treating the trees with zinc sulfate in late October to early November to hasten leaf fall and prevent a buildup of the shot hole inoculum, which can infect leaves the following spring. As with rust, the recommends applying zinc sulfate to treat for shot hole using the same application rate of 20 to 40 pounds per acre.