SYMPTOMS AND SIGNS
Alternaria leaf spot appears as fairly large brown spots on leaves, about 0.5 to 0.75 inches (12-18 mm) in diameter. The spots turn black as the fungus produces spores. Leaf spot develops most rapidly in June and July, and trees can be almost completely defoliated by early summer when the disease is severe. The disease appears to be most severe where dews form, humidity is high, and air is stagnant.
COMMENTS ON THE DISEASE
Alternaria leaf spot can occur on almond trees grown anywhere in the Central Valley, but rarely is it severe enough in the northern San Joaquin Valley to require treatment. It has been most serious on trees in the southern San Joaquin Valley and in the northern Sacramento Valley.
MANAGEMENT
The disease occurs first and is most severe on exposed leaves. Trees trained to an open and spreading canopy usually have more severe Alternaria leaf spot. Trees planted with rows in an east-west direction also have more severe disease than do orchard with rows planted north-south. Varieties that are most susceptible include Carmel, Sonora, Monterey, Winters, and Butte. Monitor for signs of the disease in April through June. If monitoring indicates the presence of Alternaria, begin late- spring treatments about mid-April. In orchards with a history of the disease, treat in mid- to late April and 2 to 3 weeks later.
A Disease Severity Value or DSV model has been developed on tomato and modified for almond for forecasting Alternaria leaf spot. Index values are assigned for specific ranges of average temperatures during leaf wetness periods during a day. Apply fungicide if accumulated index values over a 7–day period reach a value of 10 or higher.
| Mean temperature during wetness | Leaf | wetness | duration | (hours) | |
|---|---|---|---|---|---|
| 15-17 | 0-6 | 7-15 | 16-20 | 21 | — |
| 17.1-20 | 0-3 | 4-8 | 9-15 | 16-22 | 23+ |
| 20.1-25 | 0-2 | 3-5 | 6-12 | 13-20 | 21+ |
| 25.1-29 | 0-3 | 4-8 | 9-15 | 16-20 | 23+ |
| DSV | 0 | 1 | 2 | 3 | 4 |
In some orchards, Alternaria sp. resistance to quinone outside inhibitor (QoI) fungicides (also known as strobilurins) and succinate dehydrogenase inhibitor (SDHI) fungicides have been documented; do not use FRAC mode of action Group number 7 or 11 fungicides in these orchards.
PUBLICATION
UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431
Diseases
J. E. Adaskaveg, Plant Pathology, UC Riverside
W. D. Gubler, Plant Pathology, UC Davis
Roger Duncan, UC Cooperative Extension, Stanislaus County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier B. A. Holtz, UC Cooperative Extension, San Joaquin County
Clipped from: http://ipm.ucanr.edu/PMG/r3101611.html