Cercospora leaf spot of watercress

Cercospora armoraciae

Watercress (Nasturtium officinale R. Br.) is a semiaquatic plant in the Brassicaceae that is grown as a fresh-market, specialty leafy green, minor crop commodity. The 2014 watercress crop in Santa Clara Co. was 4.9 ha and the size of grower operations ranged from 0.04 to 1.5 ha. In July 2014, commercially grown greenhouse watercress plantings in this county exhibited symptoms of a foliar disease. Leaf spots were circular to oval or irregular in shape, whitish to tan, and observable on both adaxial and abaxial leaf surfaces. Sparse fungal sporulation was observed on both leaf surfaces. Spots ranged from 3 to 15 mm in diameter and were observed on both new and old foliage. Leaf spots sometimes coalesced and resulted in a blight-like symptom. In some greenhouses, the disease incidence reached 80%. Because diseased leaves are unmarketable and it was not feasible to sort healthy from diseased leaves, greenhouses with high disease incidence resulted in complete crop loss. For isolations, symptomatic leaves were soaked in 0.1% NaOCl for 1 min. Tissues from the leaf spot margins were placed onto malt extract agar (MEA) amended with tetracycline (0.1 g/liter). Isolations resulted in the recovery of one dominant type of fungus. 

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