IN YOUR ORCHARD
usually appears in late spring or early summer. The fungus attacks leaves but not fruit. On leaves, it produces small, bright yellow angular spots on the upper leaf surface, and reddish orange pustules on the lower leaf surface. The first fungicide application for rust control should take place as soon as disease symptoms are first observed in spring or early summer (e.g., if 1 out of 100 randomly collected leaves in a block or 1% of the leaves show symptoms, then an application is recommended). Treatments could be integrated with an Alternaria control program. In orchards with a history of rust, applications should be made before symptoms are observed. Maneb was a highly effective treatment (registration recently canceled) but is now replaced with mancozeb (Manzate). This fungicide and chlorothalonil (Bravo, Equus, Echo) can be applied until five weeks after petal fall and followed up again with another fungicide 10 weeks after petal fall. Usually, only one to two applications of FRAC 3 and 11 fungicides are necessary for a high level of disease control, but sulfur may need multiple applications due to its short residual activity. Alternaria leaf spot is becoming more problematic particularly in humid locations in the San Joaquin Valley; I have observed severe infections in humid pockets in Kern County and in the northern Sacramento Valley. Alternaria first appears in early summer on exposed leaves and is usually more severe on open-trained and spreading- canopy trees planted in an east/west direction. Alternaria can cause large lesions on leaves and can cause severe defoliation. If your orchard is in a problem area or monitoring indicates the presence of Alternaria, begin late-spring treatments about mid-April (five weeks after petal fall) followed by as many as three or more late-spring applications. Dr. Adaskaveg recommends using iprodione (Rovral, Nevado) in the southern almond growing areas where it warms up sooner than central and northern areas followed by other fungicides with differing modes of action. Dr. Adaskaveg has successfully used an Alternaria Disease Severity Model (DSV) to predict one to three infection periods from May to late June or early July. Because Alternaria resistance to QoI has been documented, do not use FRAC 11 fungicides in these orchards.
An integrated control program that also includes reducing orchard humidity is critical: this could include planting orchards north-south orientation to allow for prevailing wind movement through the orchard, orchard hedging to increase air movement through the canopy, adding gypsum to improve water penetration, and monitoring leaf stem water potentials to avoid over-irrigation. Partial deficit irrigation may be desirable in Alternaria plagued orchards.
An extremely damaging fungal disease, anthracnose ( Colletotrichum acutatum ) can be severe in warm, wet springs, with average daily temperatures above 63◦F. Usually, we don’t see anthracnose commonly in the San Joaquin Valley. On fruit, anthracnose can cause deep crater-like lesions; the affected area turns a rusty-reddish
Amber gum from hull spots
brown, older fruits often gum profusely, and the nut meat is usually destroyed. The fungus can also infect flowers during warm, wet rains and is reported to invade the wood and the branches upon which infected fruits reside weaken and die. In addition to destroying the crop, long-term damage and weakening of the tree may occur. Usually with warming temperatures in April and May, a good scab control program will usually control or reduce anthracnose. Orchards that have a history of anthracnose should be treated during bloom, starting even at pink bud (your brown rot spray to protect blossoms), to help reduce inoculum build-up. Ideal conditions for disease are warm rains and protecting trees before every rain is necessary for ideal control. In orchards that have a history of anthracnose, apply fungicide sprays every 10 to 14 days if rains persist after bloom. Late spring rains may necessitate additional applications into May. Alternate fungicides, as discussed, using FRAC numbers and rotating materials. Pruning out dead, infected wood reduces inoculum. If sprinkler irrigation is practiced, use low-angle nozzles to prevent the tree canopy from being wetted by sprinklers. Fortunately, we have new fungicides that were
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