Downy Mildew of Arugula, Caused by Peronospora parasitica, in California

Over the past 5 years, commercial acreage of the leafy crucifer arugula (Eruca sativa) has increased greatly in the Salinas Valley (Monterey County), California, and the crop is an important component in packaged salad products. A foliar disease has affected this crop and at times reduced its quality to the extent that it could not be harvested. Symptoms consisted of small (1 to 4 mm in diameter), irregular, dark brown to black speckling on adaxial leaf surfaces. Speckling sometimes expanded into tan spots 3 to 8 mm in diameter. Corresponding abaxial surfaces usually supported white fungal growth typical of a downy mildew. Conidiophores had main trunks with dichotomous branches ending in slender curved tips. Conidia were ovoid in shape, measuring 22 to 25 μm long and 19 μm wide. The fungus was identified as Peronospora parasitica. Pathogenicity was established by gently pressing diseased leaves onto arugula test plants and incubating plants in a humidity chamber for 48 h. Downy mildew developed after 6 days and the same fungus was observed. Although this disease has occurred on this crop for several years, this is the first documentation of P. parasitica on arugula in California. A previously described foliar bacterial disease caused by Pseudomonas syringae (1) has similar symptoms, making field diagnoses difficult if fungal signs of the downy mildew are not present on the leaf. Reference: (1) S. T. Koike et al. Plant Dis. 80:464. 1996.