Weeds as an inoculum source of Sclerotinia sclerotiorum.

Numerous weed species could be alternative hosts for diseases of cultivated plants, among which fungi play an important role (Anikster 1982, Jenkinson and Parry 1994, Roy et al., 1994, 1997). Velvetleaf (Abutilon theophrasti Medik.), ragweed (Ambrosia artemisiifolia L.) and rough cocklebur (Xanthium strumarium L.) are very aggressive weeds in arable crops such as soybean, sunflower, maize and sugar beet. Velvetleaf, ragweed and rough cocklebur plants infected with Sclerotinia sclerotiorum (Lib.) de Bary (Sclerotinia stem rot, white mold) were recorded on several locations in eastern Croatia during 2001 and 2002. Symptoms on velvetleaf plants occured on basal stem parts as well as on upper plant parts, fruits and seeds. Symptoms of white mold on ragweed and rough cocklebur plants occured only on stems. Isolates of S. sclerotiorum from diseased weed plants were used as inoculum sources for pathogenicity tests on soybean and sunflower. Pathogenicity tests were done in laboratory and field conditions. Inoculated plants were examined daily to record development of lesions, wilting and lodging. On sunflower plants first lesions in field and laboratory were recorded on the second day after inoculation with isolates from velvetleaf and ragweed and on the third day after inoculation with isolate from rough cocklebur. On soybean plants first symptoms in laboratory were recorded on the second day and in the field on the fourth day after inoculation with isolates from velvetleaf and ragweed. First lesions on soybean after field inoculation with isolate from rough cocklebur was recorded on the third day. Total number of lodging plants showed that all examined isolates were more pathogenic on sunflower than on soybean. Occurence of white mold in velvetleaf, ragweed and rough cocklebur can increase inoculum density of S. sclerotiorum in soil.