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First Report of Peronospora parasitica on Wild Rocket (Diplotaxis tenuifolia) in Italy

Several species of Diplotaxis (D. tenuifolia, D. erucoides, and D. muralis), known as wild rocket, are now widely cultivated in Italy. Wild rocket is used in Mediterranean cuisine as salad, a component of packaged salad products, and as a garnish for food. During the fall of 2002, a foliar disease of D. tenuifolia was observed in the field or greenhouse on several commercial farms in the Liguria Region of northern Italy. Symptoms appeared as small, irregular, dark brown-to-black speckling on the adaxial surfaces of leaves. The speckled areas sometimes expanded into larger spots. These symptoms were followed by leaf yellowing and the appearance of sporangiophores and sporangia on the lower and upper leaf surfaces. Sporangiophores were dichotomously branched with slender curved tips. Sporangia were ovoid, measuring 20 to 28 (average 22) μm long and 15 to 25 (average 19) μm wide. The causal agent of the disease was identified as Peronospora parasitica (3). Pathogenicity was established by inoculating 10 30-day-old plants of D. tenuifolia grown in pots in a peat/pumice/clay/composted bark mix (60:20:10:10), with a conidial suspension (10² conidia per ml). Ten noninoculated plants maintained under the same conditions served as the control. Plants were maintained in a glasshouse at air temperatures ranging between 10 and 26°C (average 16°C) and relative humidity at 85%. The pathogenicity test was done twice. Downy mildew symptoms developed within 12 days, and the same fungus was observed on inoculated plants. Noninoculated plants did not develop symptoms. To our knowledge, this is the first report of P. parasitica on D. tenuifolia in Italy. P. parasitca has been reported as the causal agent of downy mildew on D. muralis in England (1) and on cultivated rocket (Eruca sativa) in California (2).

References: (1) J. Fraymouth. Trans. Br. Mycol. Soc. 39:79, 1956. (2) S. T. Koike. Plant Dis. 82:1063, 1998. (3) D. M. Spencer. The Downy Mildews. Academic Press. New York, 1981.