Postpollination Stigmatic Constriction, the Basis of Ergot Resistance in Selected Lines of Pearl Millet

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Abstract
Five ergot-resistant pearl millet lines, developed at the International Crops Research Institue for the Semi-Arid Tropics (ICRISAT) and showing varying degrees of protogyny, and two highly susceptible hybrids were tested for ergot [Claviceps fusiformis] resistance. Where protogyny of florets of individual inflorescences lasted for 48 hr or less, the lines were resistant to ergot infection. Disease escape was mediated through the development of a localized stigmatic constriction that occurred 6 hr after self-pollination. In contrast, stigmas of highly susceptible hybrids remained receptive for up to 6 days, when an aging constriction occurred, located similarly at the pollination-induced abscission site. Development of an aging constriction in stigmas of susceptible lines, prior self-anthesis, also resulted in self-incompatibility. Extension of the period between stigma emergence and anthesis allows establishment of the pathogen within the unfertilized ovary. Where protogyny lasted for more than 48 hr, resistance could be conferred only by cross-pollination prior to gynoecial aging. It appears that ergot resistance is based on a pollen escape phenomenon linked to normal events occurring during pollination. Breeding at ICRISAT for ergot resistance under high inoculum pressure has resulted in selection of individuals in which stigmas emerge only a few hours before self-pollen is shed. Consequently, stigmatic constriction is rapidly self-induced.