Warm and wet weather as the Central Valley’s almond orchards burst into bloom makes widespread fungal diseases almost a sure bet.
“If growers get behind on their control and can’t get the fungicide sprays on, they might get hammered this year,” warns Dani Lightle, University of California Cooperative Extension (UCCE) farm advisor in the Northern California counties of Glenn, Butte, and Tehama.
Lightle says, “If pathogens get a foothold and it rains through bloom and after, there may be a lot of crop damage. You can’t catch up with these diseases.”
David Doll, UCCE farm advisor at Merced County, says fungicide applications are a preventative measure, not a control. Wet conditions during this year’s bloom created a perfect environment for fungal growth.
The pathogens are always present in an orchard, Doll explains, but they need a host and the right environmental conditions. Continued warm and wet conditions during bloom can open the doors for fungal infections.
“With no fungicide applications and current conditions, significant yield losses can be expected,” Doll said. “Depending on the variety, it could be 20-30 percent.”
On Feb. 1, the California Department of Pesticide Regulation approved the aerial application of fungicides in six North State counties due to the number of almond orchards inaccessible with ground spray rigs.
The exemption allows for fungicide applications in orchards with standing water. No pumping of water is allowed after the applications and the sprays must cease if a rain event is imminent.
The four main fungal diseases in almonds, which can do the most damage to the crop, are brown rot, anthracnose, shot hole, and jacket rot. All four are different and have different sensitivities to fungicides, according to Doll.
Anthracnose symptoms include blossom blight and fruit infections often with spur and limb dieback. Infected flowers appear similar to brown rot strikes. Infected nuts show round, orange-colored sunken lesions on the hull with symptoms appearing about three weeks after petal fall. Nuts can be infected later in the season if conditions are favorable.
Diseased nuts become mummified but remain attached to the spur. Shoots or branches with infected nuts often die. UC Integrated Pest Management (IPM) guidelines report that all cultivars are susceptible.
Management calls for fungicide treatments beginning at 5-10 percent bloom and repeated every 10-14 days if wet weather persists. Specific materials and application rates can be found on the IPM web site.
Almond blossoms are most susceptible to brown rot when fully open. Stigma, anthers, and petals are all susceptible to brown rot infection. Gum may secrete from the base of infected flowers.
This fungus survives on twig cankers and on remaining diseased flower parts and spurs. Spores are airborne or water splashed, and infections spread rapidly in wet weather with temperatures in the mid-70s.
Timing for control should be determined by the bloom of the most seriously affected cultivar. If infections were widespread the previous year, multiple fungicide applications may be necessary.
Symptoms of shot hole include spots on leaves, hulls, twigs, and flowers. Leaf lesions begin as tiny reddish specks. Spots on young leaves will fall out leaving a shot hole appearance. Older leaves retain the lesions.
Heavy infections can cause nutlets to drop, become distorted, or gum up. Infected trees will weaken, defoliate, and lose production.
There is a high risk of shot hole development in the spring if shot hole lesions with fruiting structures are found on leaves in the fall. Fruiting structures appear in the center of leaf lesions as small black spots, viewable with a hand lens.
Fungicide applications depend on weather conditions and the level of infection found in the fall.
Jacket rot, or green fruit rot, begins later in the bloom period when the fungus infects petals and anthers. The infection can spread to floral tubes or flower jackets causing them to wither and stick to developing nutlets. Entire nut clusters can rot if covered with the infected flower parts.
Jacket rot is not as prevalent as the three other fungal diseases and is more likely to appear in cooler weather conditions. Fungicide should be applied at full bloom to prevent jacket rot.
Lightle says targeting fungicide choices to the fungal disease of concern is vital. She encouraged use of the fungicide efficacy tables available at http://ipm.ucanr.edu/PMG/r3902111.html.