INFECTION EFFICIENCY OF PHAEOISARIOPSIS-PERSONATA AND THE INFLUENCE OF DIFFERENT WETNESS PATTERNS ON GERM-TUBE GROWTH OF THE PATHOGEN

Controlled environment studies with Phaeoisariopsis personata, the cau sal agent of late leaf spot disease of groundnut (Avachis hypogaea), h ave shown that infection is enhanced if leaves are exposed to alternat e wet and dry periods (intermittent wetness) compared with continuous wetness. Detailed investigations to elucidate this phenomenon revealed more germ tubes per conidium and more branching of germ tubes with in termittent wetness than with continuous wetness. With intermittent wet ness there was clear evidence of tropic growth of germ tubes and branc hes towards stomata and subsequent penetration. With continuous wetnes s, germ tube growth did not appear to be directional and germ tubes co mmonly passed over the stomatal guard cells, therefore leading to rela tively few stomatal penetrations, For both wetness regimes, stomatal p enetrations continued to increase with increased leaf wetness for at l east 6 days after inoculation and there was a linear relationship betw een the number of stomatal penetrations and the number of resultant le sions. Infection efficiency was markedly increased when the spore load was reduced to 0.1 conidia per cm(2) (about one spore per leaflet).