Alternaria Found within Merced County

Relatively rare in the northern San Joaquin Valley, I was in an orchard today with Alternaria Leaf Spot. Caused by the pathogen Alternaria alternata, this disease usually appears by mid summer and causes defoliation. Significant problems with Alternaria have been observed in the southern San Joaquin Valley and northenr Sacramento Valley. In some of these orchards, resistance to strobilurins fungicides has occurred, which indicates the need to properly rotate fungicide chemistries when spraying to control this disease. Please see this UC Almond IPM page for more information.

Here are some pictures to help you identify Alternaria leaf spot:

Leaf symptoms of Alternaria on Carmel
Another shot of Alternaria Leaf Spot on Carmel

Leaf Lesion of Alternaria found on an almond leaf. Note the zonate
appearance of the lesion and black specks – these are the identifying symptoms of this disease.
Another photo showing the zonate appearance of the lesion.

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