Among the almond diseases that can require monitoring and treating orchards in late spring and summer are Alternaria leaf spot and rust. Both are found throughout California’s almond-growing areas. Development of both diseases are favored by humid conditions in the orchard. And, in severe cases, both can defoliate trees quickly. For example, sometimes Alternaria can cause trees to lose almost all of their leaves by early summer.
Alternaria develops most rapidly in June and July, notes Gurreet Brar, University of California Cooperative Extension farm advisor for nut crops in Fresno and Madera Counties. Trees are particularly susceptible in conditions of dew, high humidity and stagnant air.
Already this season, Brar has seen cases of Alternaria in several orchards in his area. “The disease is becoming more prevalent as growers plant trees on closer spacings,” he says. “This, along with larger canopies, reduces ventilation within the orchard. Because of prevailing winds in the Central Valley, Alternaria tends to be more severe in trees planted on east-west rows that those planted in a north-south direction. Also, a history of Alternaria infection in an orchard increases the threat.”
Varieties most susceptible to the disease include Carmel, Sonora, Monterey, Winters, and Butte.
Brar advises growers to monitor orchards for signs of Alternaria from April through June.
Symptoms include fairly large brown spots on the leaves, about ½ to ¾ inch in diameter, which gradually turn black in the center as the fungus produces spores.
The shape of these spots may not necessarily be round. For example, in mid-May of this year, samples of irregularly-shaped brown lesions Brar collected in an almond orchard tested positive for Alternaria.
If you see signs of the disease, Brar recommends treating trees with a fungicide based on the disease severity value or DSV model. This approach uses specific ranges of average temperatures and number of hours when leaves are wet during the day to determine when to spray the trees. If the accumulated index values over a 7-day period totals 10 or higher, begin treatment, he says.
The DSV model and UC-Davis fungicide recommendations are available online at www.ipm.ucdavis.edu.
If monitoring in the spring indicates presence of Alternaria, these guidelines call for beginning treatment in mid-April. In orchards with a history of the disease, treat in mid- to late April and two to three weeks later. Otherwise, the most effective time to spray trees showing signs of Alternaria is in May and June, when leaf spot develops most rapidly, Brar notes.
Rust, which causes premature leaf fall and weakens trees, starts as small, yellow spots on the upper surface of leaves. On the underside of leaves, these spots turn rusty-red when the rust-colored spores produced in the lesions erupt through the surface.
As of early June, Brar had received no reports of possible rust symptoms in almond orchards in his area. “It can be most severe in orchards near streams or rivers and other areas of relatively high humidity or rain in late spring and summer,” he says. “Excessive nitrogen application rates also can increase a tree’s susceptibility.”
The rust fungus survives from one season to the next in infected leaves and possibly also in infected twigs. If not controlled, the disease will reduce bloom the following year, he notes.
Most bloom sprays will control the rust inoculum, Brar reports. However, without treatment then, symptoms may develop in late spring or early summer. In fact, rust often strikes second- and third-leaf non-bearing orchards where fungicides have not been applied.
More information on identifying and controlling rust is available on the UC IPM website.
“Fungicides can be very effective if you apply them before symptoms are visible in the summer,” Brar says. “If you have a history of rust in your orchards, we advise spraying five weeks after petal fall. Depending on severity of the disease, you may need to treat two or three more times beginning about four to five weeks later in late spring and summer.”
You can eliminate the need for fungicide sprays by applying 20 to 40 pounds per acre of zinc sulfate in late October to early November. This will hasten leaf fall, preventing rust inoculum from increasing.
“Rust inoculum can increase during fall rains and the fungus can spread to the twigs where it can survive the winter and become a problem the following spring,” he explains. “So, you want to get the leaves off the trees in late fall to prevent inoculum build-up.”