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Direct and Indirect Effects of Nitrogen Supply and Disease Source Structure on Rice Sheath Blight Spread. S. Savary, IRRI-ORSTOM Project on Characterization of Rice Pest Constraints, International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines, and ORSTOM, 911 avenue Agropolis, B.P. 5045, 34032 Montpellier cedex France; N. P. Castilla(2), F. A. Elazegui(3), C. G. McLaren(4), M. A. Ynalvez(5), and P. S. Teng(6). (2)(3)(6)Entomology and Plant Pathology Department; (4)(5)Biometrics Unit, International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines. Phytopathology 85:959-965. Accepted for publication 29 March 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-959.

Effects of nitrogen and type of disease source on focal expansion of rice sheath blight (caused by Rhizoctonia solani) were studied in two field experiments. Four disease source types, represented by inoculated individual hills at the center of experimental plots, were obtained by combining two inoculum doses with two heights of inoculation in the canopy. Compared with the low inoculum dose, there was a small but significant increase of disease severity on source hills with the higher inoculum dose, and no disease developed on noninoculated controls. Severity on the sheaths was higher on hills inoculated at the sheath level than on hills inoculated at the leaf level. Conversely, leaf severity was higher on hills inoculated at the leaf level than on hills inoculated at the sheath level. Increased nitrogen supply increased host plant tissue contacts (leaf-to-leaf and leaf-to-sheath), increased the capacity of the canopy to retain moisture, and increased the leaf nitrogen content in both experiments. The size of sheath blight foci was much larger in the rainy (3,000 to 30,000 cm2) than in the dry (250 to 1,100 cm2) season. In both experiments, foci expanded most rapidly in the high nitrogen supply (120 kg per ha) level compared with medium (80 kg per ha) and no nitrogen supply. Positioning of inoculum in the upper layer of the canopy (leaf level) on source hills resulted in faster spread of foci than positioning in the lower layer (sheath level). Multiple regression and path coefficient analyses suggested that nitrogen drives focal expansion in sheath blight essentially via indirect effects: increased tissue contacts in the canopy and higher leaf wetness.