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 effect of the fungal pathogen, Colletotrichum coccodes, on velvetleaf intraand interspecific (with soybean) competition across a range of monoculture and 1:1 mixture densities. In pure stand, application of this velvetleaf foliar pathogen had little impact on seed yield of the weed. In these plots, velvetleaf intraspecific competition stimulated vertical growth and favored the rapid replacement of diseased leaf tissue that had prematurely senesced. In mixtures, however, C. coccodes inoculation differentially influenced the yield of both species. In two of three years, C. coccodes inoculation reduced velvetleaf seed yields by, an average, 60% compared with yields for control (uninoculated) plants. Velvetleaf suffered greater yield losses from soybean interspecific competition in the presence of C. coccodes, especially at the lower planting densities. The decline in velvetleaf yield was primarily attributed to the stunting effect of the pathogen, which allowed soybean plants to grow above the weed. Consequently, soybean yield losses within inoculated mixture plots were generally lower than for control plots, although significant increases (23%) in soybean yield were recorded 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 velvetleaf performance in pure stand, while having a significantly greater effect in a competitive environment with a soybean crop, has important ramifications as to the value and accuracy of initial efficacy testing that rates potential biocontrol agents based solely on their effect within pure stands of the target weed. Nomenclature: Colletotrichum coccodes (Wallr.) Hughes; velvetleaf, Abutilon theophrasti Medik. #3 ABUTH; soybeans, Glycine max (L.) Merr. 'Maple Arrow.' Additional index words. Biological weed control, disease, intraspecific competition, interspecific competition, mycoherbicide, weed/crop interference, ABUTH. 'Received for publication August 28, 1995, and in revised form March 22, 1996. 2Asst. Prof., Prof., and Asst. Prof., Dep. Plant Sci., Macdonald Campus of McGill Univ., Ste-Anne-de-Bellevue, Quebec H9X 3V9. Portion of Ph.D. thesis of senior author. 3Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds, Revised 1989. Available from WSSA, 1508 West University Ave., Champaign, IL 61821-3133. INTRODUCTION Velvetleaf is a major annual weed in soybean and corn (Zea mays L.) in the midwestern United States, southern Ontario and is increasing in Quebec (3). Within the last few years, velvetleaf has also been reported for the first time in several Canadian maritime provinces (9). Important crop yield losses caused by velvetleaf competition have been well documented (1, 2, 5, 10, 12, 13, 19, 20). The effect of soybean interference and velvetleaf intraspecific competition on growth and reproduction of this weed have not, however, received as much attention (1, 2, 10, 14, 24). Velvetleaf is difficult to control because of its rapid growth rate and prolific production of seed with extended dormancy. The high competitiveness of velvetleaf in soybean and corn is partly attributed to its capacity to establish a height differential with the crop, especially when both crop and weed emerge at the same time (1, 5, 27). Moreover, this robust weed is tolerant to many soybean and corn herbicides (12). Hence, weed control strategies in soybean and corn have been largely inadequate against velvetleaf (26). The prospect of controlling velvetleaf with the fungal pathogen Colletotrichum coccodes has been investigated within the last decade (15, 23, 32, 33, 34, 35). Typically, the velvetleaf isolate of C. coccodes causes gray-brown foliar lesions on infected velvetleaf. Initially, lesions appear as small flecks but later become large and necrotic (35). The areas surrounding lesions become desiccated, and diseased leaves are shed prematurely. In general, velvetleaf plants are killed only when inoculated at a relatively young age (i.e., cotyledon stage) (35). When C. coccodes is applied at later growth stages, the pathogen causes extensive necrotic lesions on inoculated leaves, but although infected plants are stunted, and development is delayed, velvetleaf recovers. A substantial part of the velvetleaf-C. coccodes biocontrol research has focused on laboratory and field experiments that have attempted to optimize inoculum production and application as well as to elucidate environmental conditions under which this pathogen provides better control (23,35). An integral component of the C. coccodes-velvetleaf biocontrol research also has been to determine the effect of this selective pathogen on velvetleafsoybean interspecific and velvetleaf intraspecific competitive interactions under laboratory and field conditions (11). This research is critical in light of the growing body of research demonstrating the mediating effect of host-specific disease on intraand interspecific competitive interactions (4, 7, 17, 21, 22). For example, Burdon et al.(7) were able to quantify the effect of the rust, Puccinia chondrillina Bubak & Syd., on competition between resistant and susceptible forms of skeleton weed (Chondrilla juncea L.) in Australia. In the absence of the rust, the two