First report of leaf blight of cilantro, Coriandrum sativum, caused by Alternaria poonensis in the United States.

Abstract

In 2014, a leaf blight of cilantro was observed in two commercial fields in Ventura Co., CA. Symptoms included water-soaked lesions 5 to 10 mm dia that became necrotic and coalesced with age to consume entire leaves. The following year, six nearby fields were affected. Damage in these fields was estimated at 15 to 50% incidence making symptomatic cilantro unmarketable. In 2017, the disease was observed in 2 fields in Monterey Co., CA. Concurrent reports of Alternaria dauci on cilantro in Florida with similar symptoms (Poudel & Zhang 2018) suggested a similar etiology. However, continued occurrence from 2017 to 2024 revealed characteristics distinct from A. dauci, so further studies were conducted. Fungi were recovered from original diseased cilantro on potato dextrose agar (PDA) using standard mycological techniques. Ten single-spored isolates were incubated on potato carrot agar (PCA) for production of spores used for morphometrics and for pathogenicity testing. Also cultured were the representative strain of A. poonensis, EGS 47-138 (Simmons 2007), first reported as causal agent of cilantro leaf blight in India (Raghunath 1963), and 5 strains of A. dauci isolated from carrot in Monterey Co. On PDA, all cilantro isolates produced light gray colonies and a diffusible maroon pigment in the medium. All A. dauci isolates produced light gray colonies and a diffusible violet pigment. On PCA, spore dimensions were obtained from 20 mature conidia per isolate. Mean spore dimensions (length X width) for the CA cilantro isolates, A. poonensis, and A. dauci isolates were 57.5 X 20.3, 56.0 X 19.6, and 88.4 X 21.8, um, respectively. Mean beak lengths were 158, 152, and 170 um, respectively. To assess pathogenicity, isolates were tested on cilantro (cv Slow Bolt) and carrot (cv Short N Sweet). Seeds were sown in potting soil in a greenhouse and thinned to 5 plants per 10 cm pot. Spore solutions were prepared by flooding PCA plates with sterile H2O, then adjusting the spore solution to 4 x 104 conidia/ml. At 5 to 6 leaf stage, plants were sprayed with solutions until runoff, enclosed in plastic bags for 24 hr, then misted intermittently for 10 d. After 10 d, the foliar area diseased (FAD) on cilantro ranged from 30 to 70% for cilantro isolates and 0 to 3% for A. dauci isolates. In contrast, the FAD on carrot ranged from 0 to 3% for cilantro isolates and 60 to 80% for A. dauci isolates. Control plants sprayed with water remained healthy. The fungus was re-isolated from inoculated cilantro to fulfill Koch´s postulates. Disease symptoms, culture characteristics, and pathogenicity were similar to A. poonensis (Raghunath 1963, 1969). DNA was extracted from each isolate and the allergen Alt a1 and calmodulin genes were sequenced using established procedures (Lawrence et al. 2013). Representative sequences were deposited in GenBank. For Alt a1, cilantro isolates had 100% identity (435/435 bp) and differed by 1 bp from A. dauci isolates, which had 100% identity (accessions OR763779 and OR763781, respectively). For calmodulin, cilantro isolates had 100% identity (593/593 bp) and differed by 3 bp from A. dauci isolates, which had 100% sequence identity (accessions OR763786 and OR763788, respectively). This is the first report of A. poonensis on cilantro in the United States. Because of high quality standards, this disease limits production. Alternaria leaf spot also creates diagnostic challenges because symptoms are similar to common bacterial leaf spot caused by Pseudomonas syringae (Cooksey et al. 1991).