Association of the carrot pathogen alternaria dauci with new diseases, alternaria leaf speck, of lettuce and celery in California

Abstract

In 2014 an unidentified disease was found on lettuce and celery grown in the Salinas Valley, Monterey Co., CA. Lettuce leaf symptoms were round to oval spots, 2 to 4 mm in diameter, white to tan, with a brown border. Celery leaf spots were round to oval, 2 to 4 mm in diameter,white to tan, with a chlorotic border. Initially it was thought that the damage was caused by chemical drift; however, field personnel noted the association of the disease with nearby carrots infected with Alternaria leaf blight. Microscopic examination of lettuce and celery leaves showed obclavate, brown, multicelled conidia in the spot centers, and a fungus was consistently isolated from this tissue. Cultural, morphological, and molecular analyses confirmed the fungus to be Alternaria dauci. Isolates of A. dauci from lettuce, celery, and carrot were pathogenic on all three crops. Field surveys showed that disease severity was greatest on the side of the lettuce or celery field closest to carrots. This is the first documentation that the carrot pathogen A. dauci can cause a disease on lettuce and celery grown in the field. The disease is designated as Alternaria leaf speck. Alternaria dauci was consistently isolated from nearby spinach plants showing identical symptoms and signs. However, pathogenicity tests were inconsistent and additional studies are needed to document A. dauci on spinach.