EVALUATION OF FUNGICIDES APPLIED TO SOIL TO CONTROL NATURALLY-OCCURRING TAKE-ALL USING A BALANCED-INCOMPLETE-BLOCK DESIGN AND VERY SMALL PLOTS

Six sterol biosynthesis-inhibiting fungicides representing several combinations of properties were applied to soil to control naturally-occurring take-all (caused by Gaeumannomyces graminis var. tritici) in winter wheat in field experiments in two successive years. The average take-all severity category was never more than moderate in the different clay-loam and sandy loam sites used in each year. At each site in each year there were six treatments and an untreated control in an arrangement based on a balanced-incomplete-block design for six treatments in 10 blocks each with three treatments. Each block had three treated plots and a control plot and was paired with the complementary block of three treatments (plus control) to form a complete replicate, of which there were 30 per site. Take-all assessments in June or July showed that after incorporation into the seed bed (at 2 kg ha-1 and sometimes at 1 kg ha-1) in autumn. two non-volatile, strongly lipophilic compounds, nuarimol and triadimenol, with good intrinsic toxicity to the take-all fungus and slow rates of degradation, partially controlled take-all. However, another compound, flutriafol, with similar properties to nuarimol and triadimenol, controlled take-all less. Two slightly volatile, strongly lipophilic compounds, flusilazole and penconazole, with good intrinsic activity, were less effective (at 2 kg ha-1). A volatile, less lipophilic compound, PP 969, with less intrinsic activity, also partially controlled take-all, but only after application as a drench in the spring (2 kg ha-1). The most effective treatments were generally more effective the greater the level of disease (as indicated by assessments of disease in control plots), especially in spring assessments of disease. Although flutriafol did not perform as expected, it still seems reasonable to conclude that the requirements for a soil-applied fungicide to control take-all are likely to be: (i) good intrinsic fungitoxicity, (ii) some mobility in soil water (i.e. not strongly lipophilic), and (iii) season-long persistence.