Influence of crop rotation on inocrlum density of Rhizoctonia solani and sheath blight incedence in rice.

Belmar, S. B. , Jones, R. K. , and Starr, J. L. 1987. Influence of crop rotation on inoculum density of Rhizoctonia solani and sheath blight incidence in rice. Phytopathology 77:1138-1143. A linear correlation (P = 0.01; r' = 0.85) was observed between the best described as aggregated in seven fields where individual soil core number of sclerotia of R. solani recovered from soil and sheath blight processing allowed for analysis of the population distribution. Inoculum incidence at the panicle initiation growth stage of rice. Cropping systems was spatially autocorrelated in these fields, whereas disease was not. In for 34 fields had a significant effect (P = 0.05) on the preplant inoculum matrix surveys conducted at two sites in 1984, inoculum density ranged density of R. solani and the percentage of sheath blight incidence. Mean from 0 to 44 sclerotia recovered per 440-cm 3 soil core, and disease incidence numbers of sclerotia recovered (per kilogram of soil) were 4.02, 1.43, and ranged from 0 to 26 diseased tillers per 50-tiller sample. Inoculum and 0.07 with an average disease incidence of 5.4, 2.7, and 0.4% for ricedisease were aggregated in the quadrat sampled sites at large (> 1 m) soybean-rice, soybean-soybean-rice, and pasture-pasture-rice cropping sample spacings and random at smaller sample spacings. A rapid and systems, respectively. Significantly higher (P = 0.05) inoculum densities reliable method of quantifying sclerotia of R. solani from field soil was and disease incidence were found in alternate year rotations out of rice than developed. in 2-yr rotations out of rice. Spatial patterns of inoculum and disease were Additional key words: aerial blight, cultural control, Glycine max, Oryza sativa, soybean, spatial pattern. Sheath blight of rice (Oryza sativa L.) and aerial blight of subsequent critical growth stage, it was necessary to devise a rapid soybean (Glycine max (L.) Merrill) are caused by Rhizoctonia and reliable method to quantitatively recover sclerotia from solani KUhn, teleomorph Thanatephorus cucumeris (Frank) Donk infested soil. Weinhold (37) described wet-sieving procedures to (syn. Pellicularia sasakii (Shirai) S. Ito, Corticium sasakii (Shirai) assay soils for R. solani. Elutriation procedures for quantitative Matsumoto). The pathogen belongs to anastomosis group 1 (2) assay of soils for viable propagules of R. solani have been described and causes an aerial blight of the foliar tissue of many by Clark et al (9) and further characterized by Martin et al (24). agronomically important crops (1,27,32,36) and weeds (12,34). Sclerotia of R. solani that cause sheath blight (sasakii forms) have Recent changes in rice culture, including the adoption of soybean been quantitatively recovered from soils with wet-sieving (20) and as a rotational crop, have enhanced the severity of sheath blight in hydrogen peroxide flotation methods (25). The sclerotia of this southern rice growing areas of the United States since the early fungus would appear well suited to extraction from soil by 1970s (17,21,22,28). elutriation because they are large (about 1 to 2 mm), discrete, and Sheath blight is initiated when buoyant sclerotia float in the buoyant. paddy and germinate, and hyphae infect tillers at the water line The objective of our study was to survey commercial rice fields (15,19,21,29). Sclerotia can survive and remain viable up to 21 mo and examine the relationship between preplant inoculum of R. in dry soil but survive less than 7 mo in saturated soils (30,31). solani and subsequent disease incidence at a critical growth stage. Basidiospores of T. cucumeris can also initiate infections in upland The influence of three popular cropping rotations was studied to rice (12,29) but are considered unimportant in the epidemiology of determine the effect of rotation length and rotational crop type on the disease on short-season cultivars grown under flooded culture the development of sheath blight in subsequent rice plantings. (21,27). The teleomorph has been described on sorghum (Sorghum Finally, the spatial patterns of inoculum and disease were vulgare (L.) Moench) (27) and on bermudagrass (Cynodon investigated as initial steps in developing an assay service to aid dactylon (L.) Pers. ) (10) but has not been characterized on rice in producers with management decisions. Preliminary reports have the United States. been published (3-5). Hashiba (14) identified temperature, relative humidity, and the quantity of sclerotia per unit of field area as systems components MATERIALS AND METHODS regulating the horizontal development and subsequent yield loss in rice caused by sheath blight. Temperature, humidity, cultivar Collection of soil samples from a field survey. Thirty-four rice susceptibility, and plant growth stage were found to influence fields, representing 728 ha, from eight counties in the Texas Upper vertical disease progress. An understanding of both horizontal and Gulf Coast were surveyed in 1984 (16 fields) and 1985 (18 fields). vertical components is necessary to estimate potential yield loss Sixteen fields had a cropping history of rice rotated with soybeans and evaluate control practices in commercial fields where in alternate years (R-S-R), 10 fields had cropping histories of 2 yr inoculum and disease are not evenly dispersed. Greatest loss in of soybeans followed by rice (S-S-R), and eight fields had native yield is evidenced when initial plant infection occurs during the late pasture for 2 yr followed by 1 yr of rice (P-P-R). Native pasture vegetative and early reproductive growth stages than when initial consists primarily of coastal bermudagrass (C. dactylon). All fields infection occurs at a later stage (6,14,16,21). were sampled 1 to 2 mo before planting rice during the sample year. To facilitate studies on the relationship between preplant Soil samples were collected during February and March each inoculum density and the incidence of rice sheath blight at a year with a modified sod planter (Young Industries, Mountain View, CA) that removed a 440-cm 3 soil core (7.6 X 7.6 X 7.6 cm). © 1987 The American Phytopathological Society Fields were sampled at 64-m intervals along a W-shaped sampling