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Disease Note

An Epidemic of Downy Mildew (Peronospora destructor) of Onion in Texas. M. E. Miller, Texas Agricultural Experiment Station, Weslaco 78596, and J. M. Amador, Texas Agricultural Experiment Service, Weslaco 78596. Plant Disease 69:268, 1985. Accepted for publication 8 November 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-268d.

In February 1983, an epidemic of downy mildew caused by Peronospora destructor (Berk.) Casp. was observed on onions (Allium cepa L.) in the lower Rio Grande valley, Texas, for the first time in over 30 yr.  Fungal sporangia were apparently transported to the area by a storm system that produced 10–13 cm of rain on 25 February.  Weather conditions the following week were ideal for sporulation and infection, with temperatures ranging from 9–14 to 16–24 C.  Downy mildew was widespread in the area on all onion cultivars.  By mid-March, temperatures were unsuitable for sporulation (ranging from 12–19 to 22–36 C) and no new lesions were found.  The fungus apparently did not oversummer, since no infection sites were found during the 1984 onion-growing season.

References: Hildegrand, P. D., and Sutton, J. C. Phytopathology 72:219, 1982.