Importance of Plant Spacing and Sclerotial Position to Development of Sclerotinia Wilt of Sunflower

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Abstract
Wilt of sunflower [Helminthus annuus] in Manitoba [Canada] was due to infection of underground tissue by S. sclerotiorum. The taproot-hypocotyl axis was the primary site of infection; lesion development on this tissue resulted in wilting of leaves and sudden death of the plant. The penetration site on the taproot lesion was within the zone of lateral roots. Studies in artificially infested field plots showed that sclerotia were the primary source of inoculum and that their spatial distribution affected the incidence of wilt. Wilt incidence was highest when sclerotia were buried next to seed and decreased with increasing distance between sclerotia and seed. The plant at the site of infestation developed wilt symptoms first and became a primary infection locus (PIL) from which the pathogen spread by root contact from plant to plant in sequential order. Plant spacing affected efficiency and time of spread to adjacent plants and the number of new infections developing from each PIL.