Sclerotinia basal stalk rot (BSR) of sunflower (Helianthus annuus L.) is a fungal disease of the roots that causes symptoms of wilt and a basal stem lesion. Evaluating root plate growth could improve our understanding of BSR. Separate studies were conducted to determine the effect of sunflower growth stage or genotype on root plate diameter in North American environments. Root plate diameter of cultivated hybrids at reproductive growth stages was 3 to 4 times larger than vegetative stages. Cultivated hybrids had larger root plate diameter than interspecific lines. These results have implications for artificial inoculation methods that evaluate genotypes for BSR resistance in the field or greenhouse. Disease escapes can occur if field-grown plants are inoculated too far from the root plate and/or too early at vegetative growth stages. Side-dressing mycelium-infested cereal grain closer (i.e., 10 cm) to plants at reproductive growth stages (i.e., R1 - R4) can increase disease pressure and reduce disease escapes. These guidelines for the field can be used to validate results from artificial inoculations in the greenhouse.
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