Effects of Propiconazole on Morphology, Development, and Epidemiology of Rhizoctonia Solani, Causal Agent of Rice Sheath Blight.

Abstract

Propiconazole was inhibitory to mycelial growth of Rhizoctonia solani in vitro. Sclerotial weight per colony was also reduced by propiconazole treatment but, in relation to total amount of mycelium, sclerotia formed earlier in 1 to 10 $\mu$g/ml plates than in control plates. Colony diameters from germinating sclerotia obtained from cultures growing at 0.5 to 10 $\mu$g/ml, were significantly greater than those of control colonies. Prolonged inhibitor activity resulted in increased branching, swollen or strongly tapered hyphal tips, formation of 'beaded' hyphae, and rupturing of hyphal walls when intrahyphal hyphae emerged. Abnormal fluorescent patches, representative of abnormal cell wall deposition, were present in propiconazole-treated hyphae. At the ultrastructural level, abnormal wall inclusions were observed after exposure to 1 $\mu$g/ml. Infection cushions formed on glass coverslips from hyphae growing on propiconazole-amended medium at.25 to 5 $\mu$g/ml, were visibly more abundant than those formed from control hyphae. Although external morphology of infection structures was the same for all treatments, the cytoplasm showed signs of degeneration at 1 and 5 $\mu$g/ml. Often, penetration pegs underneath infection cushions on the inner surface of propiconazole-treated rich sheaths, did not penetrate the inner epidermis but developed into regular hyphae that continued to grow on the sheath surface. Collapsed hyphae occurred near deposits of propiconazole residue on the inner epidermis and, occasionally, directional growth away from a deposit was observed. At lower propiconazole concentrations, mycelial growth and infection cushion formation on the inner sheath surface were significantly greater than on control sheaths. Single applications of propiconazole at the green ring stage of rice and sequential applications at the green ring and boot stage effectively reduced rice sheath blight incidence over time. Severity of sheath blight after green ring applications was significantly greater than in the nonsprayed control, whereas boot applications of propiconazole followed by benomyl, iprodione, or pencycuron at heading significantly reduced disease severity. Consistent positive yield responses compared with the control resulted from all boot and heading applications, whereas green ring applications generally resulted in a negative yield response.