Competition between strains of Colletotrichum gloeosporioides f. sp. aeschynomene on northern jointvetch

Colletotrichum gloeosporioides f. sp. aeschynomene (Cga) causes anthracnose on northern jointvetch (NJV), Aeschynomene virginica, a weed common to rice and soybean fields. Previous research showed that a strain, 3-1-3, was not as competitive as strain Cla5a. The objective of this study was to determine if the relative proportion of the strains infecting NJV is altered by subsequent challenge inoculations. In the field and greenhouse, three lateral branches were inoculated with one or both strains, then challenged four days later by inoculation with one or both strains. Fitness components, such as lesion size and number, were measured and isolates were identified by RFLP and/or RAPD marker analysis to determine the proportion of each isolate in the population. Cla5a comprised 80% of the final population averaged across all treatments. Infection by Cla5a significantly reduced subsequent infection by 3-1-3. Only infection by 3-1-3 alone or wounding alone, followed by a secondary challenge of 3-1-3, showed an increased 3-1-3 population relative to the other treatments. These results indicate possible strain-strain and strain-host-strain mechanisms underlying changes in population structure.