INFECTION BY THE FUNGAL PATHOGEN COLLETOTRICHUM-COCCODES AFFECTS VELVETLEAF (ABUTILON THEOPHRASTI)-SOYBEAN COMPETITION IN THE FIELD

Field research was conducted from 1990 through 1992 to evaluate the ef fect of the fungal pathogen, Colletotrichum coccodes, on velvetleaf in tra- and interspecific (with soybean) competition across a range of mo noculture and 1:1 mixture densities, In pure stand, application of thi s velvetleaf foliar pathogen had little impact on seed yield of the we ed, In these plots, velvetleaf intraspecific competition stimulated ve rtical growth and favored the rapid replacement of diseased leaf tissu e that had prematurely senesced, In mixtures, however, C. coccodes ino culation differentially influenced the yield of both species, In two o f three years, C. coccodes inoculation reduced velvetleaf seed yields by, an average, 60% compared with yields for control (uninoculated) pl ants, Velvetleaf suffered greater yield losses from soybean interspeci fic competition in the presence of C. coccodes, especially at the lowe r planting densities, The decline in velvetleaf yield was primarily at tributed to the stunting effect of the pathogen, which allowed soybean plants to grow above the weed, Consequently, soybean yield losses wit hin inoculated mixture plots were generally lower than for control plo ts, although significant increases (23%) in soybean yield were recorde d only in 1992, The inoculation treatment had relatively little impact on the number of seeds produced per fruit or seed unit weight in both species regardless of whether plants were grown in monocultures or in mixtures, The finding that C. coccodes has only a limited effect on v elvetleaf performance in pure stand, while having a significantly grea ter effect in a competitive environment with a soybean crop, has impor tant ramifications as to the value and accuracy of initial efficacy te sting that rates potential biocontrol agents based solely on their eff ect within pure stands of the target weed.