Effect of Rice Blast and Sheath Blight on Physical Properties of Selected Rice Cultivars

Cereal Chem. 77(5):535–540 Observations in 1997 indicated a significant reduction in kernel bulk density and head rice yield of rice cultivar LaGrue due to blast (Pyricularia grisea). A more detailed study on rice cultivar M202 in 1998 confirmed such observations but it also showed negative effects of blast on other physical properties of rice. Rough rice from blast-infected panicles was drier by 7–10 percentage points and 10% thinner than rough rice from blast-free panicles. Blast also caused incidences of chalky, unfilled, and fissured kernels that were 21, 30, and 7 percentage points higher, respectively. The effects of sheath blight (Rhizoctonia solani) on kernel thickness and moisture content of rice cultivars Cocodrie, Cypress, Drew, and LaGrue were similar to the effect of blast on M202. Sheath blight generally reduced kernel bulk density but did not significantly affect head rice yield of the cultivars in 1997 and 1998 (except in one sample of Drew). There was a general trend toward higher incidences of unfilled, chalky, and fissured kernels in sheath-blight-infected samples. The data indicated that blast could be a significant preharvest factor in causing high variability in physical properties as well as in reducing the milling quality of rice. Sheath blight is also a potentially significant preharvest factor in affecting these properties in situations where sheath blight pressure is high. Rice blast and sheath blight are considered to be the most economically significant fungal rice diseases in the world (Groth et al 1988, Cu et al 1996). Rice blast is caused by Pyricularia grisea (Cooke) Sacc., while sheath blight is caused by a soilborne fungus, Rhizoctonia solani Kuhn. Blast is destructive under favorable conditions in both tropical and temperate areas (Ou 1985). Reliable estimates and exact figures of grain yield losses caused by blast are very few (Ou 1985, Hwang et al 1987). Of the two major forms of blast, panicle blast has a stronger negative effect on panicle biomass than leaf blast (Torres and Teng 1993). Heavy infections of the panicles are often detrimental to rice yields (Ou 1985) because panicle blast constricts the main node of the panicle to reduce translocation (Torres and Teng 1993). Leaf blast has an indirect effect on panicle biomass by reducing the photosynthetic activity in infected rice plants due to a reduction in the green leaf area (Burrel and Rees 1974, Padhi et al 1978). There is little information on the effect of blast on the physical properties of rough rice. It has been reported that for every 10% of neck blast incidence, there was ≈6% yield reduction and 5% increase in chalky kernels which lowered the rice quality by one or two classes (Katsube and Koshimizu 1970). Sheath blight is considered a major constraint to rice production where rice is grown under intense production systems in both temperate and tropical production areas (Cu et al 1996). Sheath blight is a serious disease of rice in the southern United States, especially in fields of long-grain rice (Marchetti 1983, Marchetti and Bollich 1991). In 1988, annual damage due to sheath blight was estimated by the U.S. Rice Foundation to be $67 million (Groth et al 1988). Yield loss can occur at any stage but is higher when infection occurs at panicle initiation, booting, and flowering (Sharma et al 1990, Cu et al 1996). Sheath blight infection from panicle initiation to flowering resulted in yield loss by reducing the mean grain weight and the number of filled grains (Cu et al 1996). Sheath blight also interferes with grain filling (Marchetti 1983) and can reduce rough rice yield by 39%, but that loss can increase to 50% in terms of kg/ha of milled whole grain rice because grains can be weakened and subsequently break during milling. A possible 46% yield loss in milled rice has been estimated if sheath blight lesions reach 90% of the plant height (Ahn and Mew 1986). Like blast, the effect of sheath blight on the physical properties of rice is not well documented. Damage due to blast and sheath blight could be more extensive than what is already known. Previous research data have already indicated the effect of these diseases on photosynthesis, translocation of photosynthates, grain filling, and respiration (Roy 1982, Marchetti 1983, Baastians 1991). The objective of this study was to determine the effect of these two diseases on 1) rough rice kernel uniformity in terms of kernel physical properties such as individual kernel moisture content and individual kernel thickness distributions; and 2) kernel bulk density, head rice yield, and incidences of unfilled, chalky, and fissured kernels. Information from this study could be used to explain whether part of the variability in processing qualities of rice that are often experienced by end-use processors even within a cultivar, harvest lot, and location could be due to blast or sheath blight. MATERIALS AND METHODS Blast Initial studies to determine the effect of panicle blast on the physical properties of rice were performed during the 1997 cropping season. Rice samples from naturally infected panicles, as well as samples from blast-free panicles of rice cultivar LaGrue were collected by hand from three producers’ fields in Lodge Corner (two sites) and Ulm, AR. The samples were analyzed for bulk density (mass per unit volume) and head rice yield. Environmental conditions during the 1998 cropping season were not favorable for blast development in the state of Arkansas. Hence, only one cultivar and one location were available for sampling. Rough rice samples from blast-free and naturally infected panicles of rice cultivar M202 were collected from varietal performance trial plots in Pocahontas, AR, on September 12, 1998. The design structure for this varietal performance trial was a randomized complete block design with four replicates. Blast-free and blast-infected panicles of M202 were collected by hand and hand-threshed individually. Four samples (or replicates) of rough rice each weighing ≈300 g from blast-free and blast-infected panicles were collected. Additionally, 20 blast-free and 20 blast-infected panicles per replicate were collected. From these panicles, the number of unfilled kernels was counted to determine the percentage of unfilled kernels per panicle. Data for bulk density and head rice yield of LaGrue were analyzed as a location × disease level factorial experiment by using the PROC GLM procedure of SAS (version 6.12, SAS Institute, Cary, NC). 1 Published with the approval of the director of the Arkansas Agricultural Experiment Station, Fayetteville, AR. Mention of a commercial name does not imply endorsement by the University of Arkansas. 2 Former research associate and professor, respectively, Food Science Department, University of Arkansas, Fayetteville, AR. 3 Corresponding author. E-mail: [email protected] Phone: 501/575-2841. Fax: 501/575-6936. 4 Professor, Rice Research and Extension Ctr, University of Arkansas, Stuttgart, AR. 5 Extension plant pathologist, Cooperative Extension Service, USDA, University of Arkansas, Little Rock, AR. Publication no. C-2000-0808-03R. © 2000 American Association of Cereal Chemists, Inc.