SPORULATION BY RACE-0, RACE-2, AND RACE-3 OF COCHLIOBOLUS-CARBONUM ON SYNTHETIC MEDIUM AND STERILIZED CORN LEAVES

Isolates of races 0, 2, and 3 of Cochliobolus carbonum collected from two locations in North Carolina were grown in vitro at 20, 24, or 28 C on potato-lactose agar (PLA) or on water agar with either autoclaved green corn leaves or senesced corn leaves. Races, temperatures, and media all significantly affected sporulation. Temperature X race and substrate X race interactions were not statistically significant. Therefore, it seems unlikely that differences in sporulation by races 2 and 3 on corn are influenced differentially by temperature or simple nutritional factors. Averaged over all factor levels, race 2 sporulated best, followed by race 3 and then race 0. The races ranked in the same order when compared for parasitic fitness in an earlier survey of race frequency changes over time in the two fields where these isolates were collected. In another experiment with-318 C carbonum isolates from these two fields plus 14 from a field in Tennessee, race 2 mycelium grew significantly faster on PLA than mycelium of race 3 or race 0; any difference between race 3 and race 0 was not significant. Thus, mycelial growth rate may be a less reliable indicator of fitness than sporulation in vitro.