Pistachio growers might get some relief from Alternaria late blight pressure in their orchards, at least this year. That is, if drought conditions and limited available of water for irrigation continue. But, looking beyond that, the prospect is for even more pressure from this fungal disease in the future.

Alternaria, which can cause nut staining, early defoliation and, in some cases, moldy nuts, heads the list of disease concerns for California’s pistachio growers.

Growers whose orchards are in areas with a history of the disease or have areas of low, poorly draining soils are approaching the time when fungicide applications can help minimize the threat of Alternaria. The best time to attack this disease is from May through July, reports Themis Michailides, University of California plant pathologist at the Kearney Agricultural Research and Extension Center, Parlier, Calif.



Because the disease develops later in the season, bloom sprays are ineffective, he says. Treating orchards after symptoms appear does no good, either.

Continued dry weather this season would reduce development of two key factors that favor growth of Alternaria – high relative humidity levels and dew formation in orchards. Meanwhile, though, the increase in recent years in acreage of pistachios as well as almonds, another host crop, only adds to the challenge of managing this tough-to-control disease in the long term, Michailides notes.

More acres of these crops means more opportunities for the disease to develop and spread, he explains. And, in the case of pistachios, without extra equipment to harvest the added acreage, more nuts are likely to be harvested later when they are more vulnerable to Alternaria damage.

Although leaves can become infected by Alternaria earlier in the season, the critical time for development of the disease is from early August to mid-September when humid conditions prevail in the orchard, Michailides reports.  Infected leaves are characterized by dark brown spots or lesions, with blackened centers, which contain the disease-causing spores. Rubbing these lesions with a finger leaves a tell-tale black color on the finger.

That’s unlike lesions caused by Botryosphaeria dothidea. Because these fungi produce no spores on the surface of the lesions, rubbing them doesn’t blacken fingers.


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Late in the season both fungi can be present in the same lesion. In that case, microscopic analysis is needed to distinguish the two diseases. Michailides notes,

Wind disperses the Alternaria spores onto leaves, which cause secondary lesions, spreading the disease