In autumn 1998, mature ragweed (Ambrosia artemisiifolia) plants, 1-1.5 m tall were collected from the borders of 4 sunflower fields in which Sclerotinia rot of sunflower, caused by S. sclerotiorum, was frequently observed during the season, and also from 6 other roadside sites in Hungary. Ragweed plants exhibiting symptoms characteristic of Sclerotinia rot i.e. wilting foliage and light brown, dry lesions on the stems, were found only near 2 sunflower fields. Black, round to irregular or oblong sclerotia were also observed on the infected ragweed plants both externally on the stem lesions and internally, in the pith cavity. Sclerotia measured up to 5 mm in diameter and were 5 to 14 mm long. After isolation on potato dextrose agar, the pathogen produced abundant aerial mycelium and large sclerotia characteristic of S. sclerotiorum. Pathogenicity was confirmed by inoculation of potted seedlings and mature plants of ragweed with wheat grains colonized with mycelia of S. sclerotiorum. Seedlings were killed in 2-3 days while mature plants wilted after 5 to 6 days. Nonsclerotial mutants of the fungus are being produced to be tested as potential biocontrol agents of common ragweed, which has become not only the most widespread, but also the most important allergenic plant species in Hungary since the early 1990s.
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