Sheath blight of rice

Phi ctonia spp. sheath blight of rice sharp eyespot of wheat... 

Polyoxins are used in the control of several pathogens, notably sheath blight of rice (Rhizoctonia solani). Polyoxin is specific to Alternaria, Botrytis, and powdery mildew. Differences in activity between members of the polyoxin group probably reflect dissimilar uptake characteristics, which are governed by specific peptides. Different peptides may limit polyoxin activity at the level of the uptake mechanism and may also determine their resistance traits. 

Validamycin A has been practically used to protect sheath blight of rice plant in the formulations of 3 % solution or 0.3 % dust. 

Polyoxins have a low toxicity to mammals. In oral doses of 15 g/kg, or by intravenous administration of 800 mg/kg, they are not hazardous to mice, nor are they toxic to fish. They are not phytotoxic—rice plants are unaffected even by treatment with 800 mg/1 of polyoxin. In Japan, polyoxin D is a commercial product for the control of sheath blight of rice. 

Rice bacterial blight (Xanthomonas oryzae Dowson) Sheath blight of rice (Pellicularia sasakii (Shirai) S. Ito)... 

Isol. from Micromonospora sp. Shows activity against sheath blight of rice. Less effective than Dapiramicin A. 

Nucleoside antibiotic. Isol. from Micromonospora sp. SF1917. Active against against sheath blight of rice less effective than Dapiramicin A. Needles + IH2O (MeOH aq.). Sol. AcOH, DMF fairly sol. MeOH, H2O poorly sol. Mc2CO, Et20. 

Iron toxicity is a syndrome of disorders associated with large concentrations of Fe + in the soil solution. It is only found in flooded soils. A wide range of concentrations produce the symptoms, from 1000 to only 10mgL in soils with poor nutrient status—especially of P or K—or with respiration inhibitors such as H2S. There are large differences in tolerance between rice varieties. The effects include internal damage of tissues due to excessive uptake of Fe + impaired nutrient uptake, especially of P, K, Ca and Mg and increased diseases associated with imbalanced nutrition, such as brown leaf spot (caused by Helminthospo-rium oryzae), sheath blight (caused by Rhizoctonia solani) and blast (caused by Pyricularia oryzae). 

Validamycin A is the only commercial fungicide active against this target (Figure 4.9). The compound is an aminoglycosidic pseudosaccharide and is a secondary metabolite produced in the fermentation of Streptomyces hygroscopicus var. limoneus. Validamycin A was introduced by Takeda Chemical Industries Ltd. for the control of Basidiomy-cetes and Corticiaceae, especially sheath blight, Rhizoctonia solani, in rice. 

Validamycin. Validamycin A (VM-A) is a new antifungal antibiotic recently developed in Japan for the control of rice sheath blight (50-52). It was isolated from the culture filtrate of Streptomyoes hygroseopicus var. timoneus, which also produced five... 

Several trehalase inhibitors have been isolated from natural sources these include deoxynojirimycin, validamycins, validoxylamines, salbastatin, calystigin B4, and trehazolin, the last one being the most potent. Trehazolin is used to control blight sheath of rice, caused by the plant pathogenic fungus Rhizoctonia solani, and it thus has potential fungicidal activity.12 15... 

Inhibitors of fungal glucan synthesis are of microbial origin, produced by Strep-tomyces hygroscopicus var. limoneus (HI) and of S. cacaoi var. asoensis (H2) (Table 12.13) [19]. Validamycin is a specific compound for the control of rice sheath blight, whereas polyoxins inhibit a target site that, potentially, is present in all As-comycetes and Basidiomycetes. Nevertheless, the market importance of polyoxins remains limited. 

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