No abstract

In laboratory tests, Rhizoctmia solani grew best on agar at 30 "C and pH 5-5. Mycelial growth was strongly inhibited by benomyl, chloroneb and quintozene, less so by thiophanate-methyl. The optimum temperature for inhibition was 25 "C for quintozene; the other three fungicides gave greatest inhibition at the lowest temperature tested (20 "C). Benomyl and thiophanate-methyl were most inhibitory at pH 7-8, chloroneb and quintozene at pH 5-6.In pot trials using mung bean, long melon, egg-plant, common pea and sugar beet, R . solani caused maximum disease at 20 "C and in wet and alkaline soils. As seed treatments, benomyl and thiophanate-methyl gave optimum control at 20 "C and pH 7-6; chloroneb and quintozene were most effective at 30 "C and pH 5-4. All four fungicides gave maximum control on plants growing in wet sandy loam. Comparisons of host effects showed that, on all the four hosts tested, thiophanate-methyl wettable powder at 0.25 % (0.175 % a.i.) gave 90 % control, chloroneb w.p. at 0.3 yo (0.195 yo a.i.) gave 80 yo control on mung bean and sugar beet, benomyl w.p. at 0-3 % (0-15 % a.i.) was satisfactory on mung bean, egg-plant and sugar beet but not on long melon, quintozene w.p. at 0.3 % (0.225 % a.i.) gave effective control on sugar beet only. I N T R O D U C T I O NRhixoctonia solani Kuhn, a soil-borne plant pathogen of world-wide distribution, causing root rot and damping-off of a variety of plants, can be controlled by fungicides applied as seed treatments or soil drenches. There is some evidence that control by the commonly recommended fungicides is affected by host plant, seasons and locations. R. K. Grover & B. L. Chopra (unpublished) found that the control of R. solani rot of Phaseolus mungo by seed treatment with carboxin varied at different times of the year. Grover (1973) indicated the effect of environmental soil factors in determining the toxicity of five systemic fungicides to R. solmi Studies were therefore initiated to investigate the reasons for discrepancies in control of R . solani on different hosts and under different environmental conditions, using three systemic fungicides (benomyl, chloroneb and thiophanate-methyl) and one non-systemic (quintozene).

Seven 6‐alkyl‐3‐n‐butyl‐7‐hydroxy‐4‐methylcoumarins and three other related substituted coumarins were synthesised, characterised and tested for their toxicity towards mycelial growth of seven phytopathogenic fungi in culture. Rhizoctonia solani, Alternaria alternate, and Drechslera oryzae exhibited maximum sensitivity to these compounds whereas Pythium aphanidermatum, Colletotrichum falcatum, Fusarium solani and Macro‐phomina phaseolina were less sensitive. The 6‐n‐butyl and 6‐n‐pentyl derivatives possessed strongest toxicity towards all fungi except C. falcatum but the corresponding 6‐n‐propyl derivative was inhibitory to only D. oryzae and A. alternata.

SUMMARYOf 41 fungicides tested in the laboratory, copper carbonate, copper sulphate, mercuric chloride, Agrosan GN, quintozene, kasugamycin, carboxin, pyracar‐bolid, carbendazim, chloroneb, benomyl, Ohric, RH 893 (2‐n‐octyl‐4‐isothiazole‐3‐one) and Terrazole were most inhibitory to the mycelial growth of Rhizoctonia solani on Czapek's agar plates and had EC50 values of less than 1 μg a.i./ml, while copper oxychloride, Udonkor, zineb, ziram, F 319 (3‐hydroxy‐5‐methyl isoxazole) and anilazine were much less toxic, ziram being least inhibitory with an EC50 of 214 μg a.i./ml.Of 17 fungicides tested in the greenhouse as seed treatments, thiabendazole, carbendazim, benomyl, thiophanate‐methyl, dichlozoline and Ohric gave 80–90% control of damping‐off of mung bean seedlings. A single soil drench with thiophanate‐methyl and two drenches with benomyl gave about 90% disease control, More seedlings with R. solani infection survived when thiophanate‐methyl was used as a post‐inoculation soil drench than when benomyl or chloroneb were used.

Rh&octonia solani caused maximum mortality of mung bean seedlings at 20~ and the disease incidence decreased with increase of temperature; 30 ~ was optimum for mycelial growth of the fungus in vitro. The fungus grew best in nutrient broth of pH 5.5 but infected mung bean and pea seedlings more severely in neutral and alkaline river sand than in the sand adjusted to acidic reaction. The disease incidence was higher in adequately moist sandy loam and less in soil under moisture stress. Incidence of cowpea seedling rot was higher in heavy-textured loam and silt loam soils than in light-textured sandy-and loamy sand.