Field trials were conducted over two growing seasons in South Dakota to determine the effect of artificial inoculation of hybrid sunflower (Helianthus annuus) with Phoma macdonaldii on seed yield and other agronomic characters. Treatments were a factorial arrangement of inocula (sterile water or 2 x 10(5) conidia per milliliter of P. macdonaldii) injected into the lower stem and time of inoculation (2 wk before anthesis, anthesis, and 2 wk after anthesis), as well as an unioculated check. Inoculation with P. macdonaldii consistently caused greater internal decay, external symptoms, and premature ripening than did injection with sterile water. Seed yields were not significantly reduced by inoculation with P. macdonaldii, but 100-seed weights were significantly reduced when inoculation occurred at anthesis or 2 wk postanthesis. Percent oil content of seeds was slightly increased by inoculation with P. macdonaldii at anthesis. Regression analysis indicated that yield was reduced 0.5% and 100-seed weight reduced 0.3% for every 1% increase in premature ripening, and 100-seed weight was reduced 3.7% for each unit increase in severity of external symptoms (0-4 scale). P. macdonaldii is capable of causing causing extensive internal stalk decay and premature ripening of sunflower, but seed yield losses are slight. The small losses in seed weight indicate that premature ripening occurred late in the seed-filling stage.
