How to Manage Pests
UC Pest Management Guidelines
Alternaria Late Blight
Pathogens: Alternaria alternata, A. tenuissima, A. arborescens, and Stemphylium spp.
(Reviewed 10/14, updated 10/14)
In this Guideline:
SYMPTOMS AND SIGNS
Alternaria late blight occurs as black angular or circular lesions on leaves of both male and female trees. When the pistachio fruit are maturing, black lesions with a red halo appear on hulls. Black spores develop in the center of the leaf lesions when humid conditions prevail in orchards. Rubbing the leaf lesions with a finger will blacken the finger. In contrast, rubbing a finger on a lesion caused by Botryosphaeria dothidea does not blacken fingers because there are no spores produced on the surface of these lesions. However, late in the season both fungi can be present in the same lesion and microscopic identification will be needed. Leaf infections can cause severe premature defoliation and create problems during shaking the trees at harvest.
Black lesions are also present on petioles and main veins of leaf blades. On immature fruit the symptoms appear as small black lesions about 1/25 inch (1 mm) in diameter. On mature fruit, both small 1/25 to 1/13 inch (1–2 mm) and large 1/5 inch (5 mm) black lesions are present on the epicarp, usually surrounded by a reddish purple margin. Multiple lesions on leaves and fruit cause leaf blight and deterioration of hulls, respectively. Deterioration of hulls results in shell staining. Excessive growth of the fungus can invade also the kernel, resulting in kernel decay.
COMMENTS ON THE DISEASE
Alternaria is a problem in orchards irrigated by sprinklers or flooding. It can also be a problem in orchards with low soil infiltration and those irrigated with microsprinklers, particularly in lower areas in these orchards where relative humidity can be high and dew formation frequent during late August and September. The pathogen causes latent infections on leaves and fruit. The disease is more severe on leaves from fruit-bearing shoots than those without fruit.
Losses occur mainly because of fruit staining, kernel decay, and from early defoliation, which can be severe enough to cause difficulties during harvest. In addition, the fungus can colonize the inner surface of the shell and endocarp, causing moldy nuts. Leaf and fruit lesions are common on both Kerman and Red Aleppo cultivars, as well as on the leaves of Peters and on leaves of 02-16 and 02-18 male selections. The newly released pistachio cultivars, Golden Hills, Lost Hills, and Randy seem to be as susceptible as Kerman and Peters cultivars.
Late July to early August is the time when one can see the first symptoms of Alternaria late blight, depending on the orchard's humidity conditions. By mid-September the disease can develop to epidemic levels. Orchards with cover crops have more Alternaria blight than orchards with mechanical or chemical mowing. Sunburned fruit become more susceptible to Alternaria blight than nonsunburned fruit.
Starting in mid-July through harvest monitor foliage for Alternaria late blight symptoms. If only one fungicide application is done, the best timing is late June to early July (see TREATMENT TIMING FOR KEY DISEASES).
Resistance of Alternaria spp. to Group 11 and 7 fungicides has been detected in some pistachio orchards. Take steps to avoid the development of fungicide resistance. Rotate among the fungicide classes and avoid repeated use of one class of fungicides. Other practices that often lead to resistance are using reduced concentrations and poor coverage.
Alternaria blight is difficult to control and requires a combination of management approaches. If water management is used, weigh disease severity against the impact of deficit irrigation on shell splitting.
UC IPM Pest Management Guidelines: Pistachio
T. J. Michailides, Kearney Agricultural Center, Parlier