EFFECT OF THE FUNGAL PATHOGEN, COLLETOTRICHUM-COCCODES, ON ABUTILON-THEOPHRASTI HEIGHT HIERARCHY DEVELOPMENT

1. The impact of a selective fungal pathogen, Colletotrichum coccodes, on aboveground biomass and development of height hierarchies in monoc ulture populations of Abutilon theophrasti (velvetleaf) was examined o ver an 11-week period, at each of three densities (125, 250 and 375 pl ants m(-2)), over three field seasons. 2. Fungal infection caused sign ificant 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 plot s did not differ significantly from biomass within uninoculated contro l 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 pe r unit area increased significantly with planting density 2 weeks afte r inoculation (WAI). In subsequent harvests (5 and 8 WAI), however, A. theophrasti unit area biomass was generally little affected by increa sing planting density regardless of inoculation treatment. 3. Height f requency distributions of control and inoculated A. theophrasti popula tions within each of the three planting densities were generally posit ively skewed (L-shaped) by the first harvest (2 WAI) and negatively sk ewed (J-shaped) 3 weeks later. A positive skewness indicates the prese nce of a large number of short A. theophrasti individuals and a relati vely few, taller plants within a population. The opposite trend is obs erved for a negatively skewed population structure. By the third harve st (8 WAI), however, height frequency distributions at the two lowest densities remained negatively skewed regardless of inoculation treatme nt, while at the highest density, frequency distributions typically re verted to being positively skewed. 4. By 5 and 8 WAI, height variabili ty, as measured by the Gini coefficient, increased significantly for m ost A. theophrasti populations subjected to the inoculation treatment. This trend was observed in all three years. In contrast, height varia bility between control and inoculated populations differed little 2 we eks after C. coccodes application. 5. In all three years, no consisten t trend was found between height variability and A. theophrasti monocu lture planting density. 6. This study demonstrates that the fungal pat hogen C. coccodes, which is being evaluated as a potential bioherbicid e against A. theophrasti, may play an important role in structuring he ight distributions of even-aged populations. Single C. coccodes applic ations were shown to accelerate height asymmetry within these monocult ure 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 applic ation, possibly in combination with plant growth regulators or herbici des, be employed.