Dept. Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1 G 2W1

Background and objectives
Taraxacum officinale Weber is an important perennial weed worldwide. This weed has been controlled with chemical herbicides, e.g. 2,4-D, but concerns regarding pesticide use have stimulated the exploration of alternative weed control methods [1]. Plant pathogens have been considered for the control of weeds for more than a century. However, environmental limitations such as requirements for prolonged leaf wetness duration and specific temperature regimes often influence the efficacy of plant pathogens [2]. The indigenous fungal pathogen Phoma herbarum Westend has shown promise for efficacy on T. officinale under controlled environmental conditions, but failed to provide satisfactory control under field conditions in previous field experiments. Therefore, the objectives of the present study were to evaluate adjuvants for enhancement of disease severity by P. herbarum.

Materials and methods
Based on previous studies, the following treatments and adjuvants were selected for evaluation: 5% guar gum, 5% pectin, 5% NatipideŽ (Rhone-Poulenc Rorer, Nattermann Phospholipid GMBH, Cologne), 5% durum semolina, 5% gluten flour (all adjuvants were prepared with 20% mycelium of 5-day-old liquid cultures of P. herbarum in potato dextrose broth, PDB), 20, 50 and 80% mycelium in PDB, and adjuvant control treatments without the fungus. Two trials, one in fall 1996, and one in spring 1997 were conducted as randomized complete block designs with five replicate plots per treatment, and ten sub-sampled plants per plot. Two plant ages were tested: seedlings were 4 weeks old, and transplants were 6 weeks old at the time of treatment. Plants were sprayed with manually operated pump sprayers from approximately 20 cm until run-off. Disease severity (DSR) was evaluated on individual T. officinale plants using the Horsfall-Barratt rating system 1, 2, and 3 weeks post-inoculation (WPI). Multiple regression analysis was used to compare disease severity for each treatment overtime using the Statistical Analysis System (SAS Institute, Cary, NC).

Results and conclusions
In the fall field trial, multiple regression and graphical analyses of the DSR versus time revealed that initial increases in disease severity were significantly higher for gluten flour and guar gum than for the standard treatment (20% mycelium v/v in PDB). The increase in DSR from the first to the second WPI, and from the second to the third WPI, was significantly (P=0.05) higher for 80% mycelium in PDB, gluten flour, durum semolina, pectin and guar gum, than for the standard treatment. The overall increase in DSR from the second to the third WPI was less than from the first to the second WPI. In the spring field trial, disease severity was generally lower than in the fall field trial and mean DSR ranged from 0 to 6.9. Durum semolina, gluten flour and guar gum resulted in significantly greater DSRs than the standard treatment on seedlings and transplants. Disease in spring 1997 was less severe, the initial disease increase for 80% mycelium, durum semolina, pectin and guar gum was significantly larger than for the standard treatment. However, no other comparisons of intercepts and linear and quadratic slopes were significantly different to the standard treatment. No phytotoxicity was observed in any adjuvant control treatments. Rainfall, relative humidity, temperature and incoming solar radiation for the duration of both field trials were summarized and indicated disease conducive conditions. In conclusion, formulations can greatly enhance disease severity by P. herbarum under field conditions and increase its potential as a biological weed control agent.

1.Riddle GE, Burpee, LL, and Boland GJ, 1991. Weed Sci. 39, 109-118.
2.Charudattan R, 1991. Microbial Control of Weeds.