Effect of the fungal pathogen, Colletotrichum coccodes, on Abutilon theophrasti height hierarchy development.
The impact of a selective fungal pathogen, Colletotrichum coccodes, on above-ground biomass and development of height hierarchies in monoculture populations of Abutilon theophrasti (velvetleaf) was examined over an 11-week period, at each of three densities (125, 250 and 375 plants m-2) for 3 years (1990-92) at Ste. Anne-de-Bellevue. Fungal infection caused significant reductions (30-44%) in unit area above-ground biomass within 5 weeks of inoculation in 2 of the 3 years. However, 8 weeks following C. coccodes inoculation, A. theophrasti biomass within inoculated plots did not differ significantly from biomass within uninoculated control plots in any of the three years. As expected, mean biomass per plant generally declined with increasing monoculture density regardless of inoculation treatment and harvest date. In all three years, biomass per unit area increased significantly with planting density 2 weeks after inoculation (WAI). In subsequent harvests (5 and 8 WAI), A. theophrasti unit area biomass was generally little affected by increasing planting density regardless of inoculation treatment. Height frequency distributions of control and inoculated A. theophrasti populations within each of the three planting densities were generally positively skewed (L-shaped) by the first harvest (2 WAI) and negatively skewed (J-shaped) 3 weeks later. A positive skewness indicated the presence of a large number of short A. theophrasti individuals and of relatively few, taller plants within a population. The opposite trend was observed for a negatively skewed population structure. By the third harvest (8 WAI), height frequency distributions at the two lowest densities remained negatively skewed regardless of inoculation treatment, while at the highest density, frequency distributions typically reverted to being positively skewed. By 5 and 8 WAI, height variability, as measured by the Gini coefficient, increased significantly for most A. theophrasti populations subjected to the inoculation treatment. This trend was observed in all three years. In contrast, height variability between control and inoculated populations differed little 2 weeks after C. coccodes application. In all three years, no consistent trend was found between height variability and A. theophrasti monoculture planting density. This study demonstrated that the fungal pathogen C. coccodes, which is being evaluated as a potential bioherbicide against A. theophrasti, may play an important role in structuring height distributions of even-aged populations. Single C. coccodes applications were shown to accelerate height asymmetry within these monoculture populations, thus resulting in a relatively few, tall individuals becoming dominant. Given the importance of light competition for this vigorous agricultural weed, effective control of these dominant plants is critical. Thus, it is recommended that a second C. coccodes application, possibly in combination with plant growth regulators or herbicides, is given.