PP128 Brown Spot in Florida Rice1 Lawrence E. Datnoff and Richard S. Lentini2 1. This document is PP128, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date May 1994. Reviewed April 2003. Visit the EDIS Web Site at http://edis.ifas.ufl.edu. 2. Lawrence E. Datnoff, Associate Professor, Plant Pathology; Richard S. Lentini, Sr. Biological Scientist; Everglades Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Belle Glade, FL 33430. The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition. The term "plates," where used in this document, refers to color photographs that can be displayed on screen from CD-ROM. These photographs are not included in the printed document. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean Brown spot is one of the most important rice diseases in Florida. The disease can adversely affect the yield and milling quality of the grain. Under environmental conditions conducive to disease, yield loss estimates range from 16 to 40 percent in Florida. SYMPTOMS Brown spot symptoms initially appear as small circular to oval spots on the first seedling leaves. Leaf spots are observed throughout the growing season and may vary in size, shape and color depending on the environmental conditions, age of the spots, and the degree of susceptibility of the rice variety. Small spots are dark brown to reddish brown while large spots have a light, reddish-brown or gray center surrounded by a dark to reddish-brown margin (Plate 1). Older spots may have a bright yellow halo surrounding the lesion. Spots on the leaf sheath and hulls are similar to those on the leaves. Plate 1. Early brown spot lesions are difficult to distinguish from blast disease lesions, but mature blast lesions are usually spindle or diamond shaped (Figure 1). Severely infected leaves may die before maturity and these plants will produce lightweight or chalky kernels. Infection occurring directly on the kernels will significantly reduce grain yield and quality. Infected glumes and panicle branches have a black discoloration. Figure 2.
Brown Spot in Florida Rice 2 CAUSAL AGENT Brown spot is caused by the fungus Bipolaris oryzae (formerly known as Helminthosporium oryzae). It can infect all plant parts. The morphology and life cycle of the fungus is complex and beyond the scope of this paper. Plant pathologists, however, can usually identify the causal agent by looking at the conidia (spores) microscopically. Briefly, conidia of Bipolaris oryzae are very small, club shaped to cylindrical, generally curved, light brown to golden brown, with 6 to 14 transverse cell walls (Plate 2). Plate 2. SPREAD OF THE DISEASE Bipolaris oryzae is seedborne. It can also survive on infected rice straw and stubble. It spreads from plant to plant in the field by airborne spores. Species of some 23 grass genera have been reported to be susceptible to infection by Bipolaris oryzae. It is not known how many of these species may act as alternative hosts in the spread of the disease to rice. Disease development is favored by high relative humidities (86-100%) and temperatures between 68 to 78 F. Leaves must be continuously wet for 8 to 24 hours for infection to occur. PREVENTION AND CONTROL Severe leaf spotting is often associated with weak plants growing under stressful conditions such as dense plant stands, water stress, inadequate fertilization or herbicide toxicity. Plants that grow in soils with nutritional deficiencies or in soils where nutrient uptake is hindered are more susceptible to infection. The severity of brown spot can be intense on Histosol(organic)-grown rice in Florida. Although there are varietal differences in susceptibility to brown spot, the best management strategy is balanced nutrition. Soils known to be low in plant-available silicon should be amended with calcium silicate slag before planting. Improved silicon nutrition not only enhances plant resistance to brown spot, but will increase rice yields. The usual slag application rate is two tons per acre, although higher rates have been shown to be even more effective in reducing brown spot incidence and increasing yields. Small plot experiments conducted by the University of Florida have shown that using calcium silicate slag and fungicides may limit yield reductions in infected fields. In one recent experiment, nontreated plots exhibited yield reductions of 16%, 39.3% and 43.8% when compared to plots treated with a fungicide (Tilt) alone, slag alone, or a combination of fungicide and slag, respectively. Note that although fungicide treatment alone exhibited an increased yield over non-treated plots, the effects due to slag treatment alone were much greater, and the addition of fungicide with the slag treatment did not improve yields significantly. Therefore, slag treatment alone may be the best method for managing this disease on Florida's Histosols. Fungicidal seed treatment, however, has proven very effective in reducing seedling brown spot disease.