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Etiology and Epidemiology of Stemphylium Leaf Spot and Purple Spot of Asparagus in California. Peter G. Falloon, Plant breeder, Crop Research Division, D.S.I.R., Private Bag, Christchurch, New Zealand; L. M. Falloon(2), and R. G. Grogan(3). (2)Research assistant, Crop Research Division, D.S.I.R., Private Bag, Christchurch, New Zealand; (3)Professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 77:407-413. Accepted for publication 10 March 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-407.

Stemphylium vesicarium was isolated from all asparagus production areas throughout California except the Imperial and Coachella valleys. Most California isolates from spears, green fern, or debris from the previous summerís growth as well as three isolates of S. vesicarium from Switzerland and two isolates resembling the conidial state of S. majusculum from asparagus in England were pathogenic on young asparagus seedlings. Isolates of Alternaria spp. from asparagus in California and England and one isolate of S. vesicarium from onions did not cause disease on asparagus seedlings. Disease ratings of spears harvested from the field during spring of 1983 and 1984 indicated that disease was more severe following periods of wet weather or sprinkler irrigation. Symptoms on spears were reproduced without prior wounding of spears, and disease was more severe when high humidity conditions were extended from 24 to 48 or 72 hr after inoculation. This is contrary to earlier reports suggesting that sand-blasting is requisite for disease development. All penetrations of tissue from germinated spores of Stemphylium and Pleospora were exclusively through stomata. Stomata started to close within 5 min after harvest of spears and all were closed within 60 min. Thus, reproduction of spear symptoms failed unless water stress was relieved or prevented before inoculation. Pseudothecia of Pleospora on fern debris were shown to be an important source of inoculum for purple spot of asparagus spears.