Blast in Rice: Researchers Detect Resisitance to Fungicides

Linda Benedict  |  8/23/2007 12:51:12 AM

Donald Groth, Milton C. Rush and Don Lindberg

Pesticides are used in agriculture to control many different insects, weeds and pathogens that cannot be controlled by other practices, such as planting resistant cultivars, cultural management and biological control. Most pests have the ability to overcome pesticides by becoming resistant over time. This often leaves a gap in the farmer’s ability to control important pests. Monitoring programs are in place to detect these situations and allow time to develop new control practices.

Blast, often called rotten neck blast because the disease attacks the plant at the joint just below the seed head, is caused by the fungus Pyricularia grisea. This is one of the most significant rice diseases in Louisiana and the Mid-South. Some varieties offer resistance, but most do not. Control is enhanced by establishing and maintaining a flood as soon as possible, planting early to avoid late-season blast pressure, using recommended nitrogen fertilizer rates and not planting in sandy soils or in tree-lined fields. Losses due to blast are increasing because of current production practices that require draining fields for controlling insects, correcting herbicide damage or preventing straighthead disease. Farmers often have to depend on fungicides to protect their rice crop from severe blast damage, and the development of resistance to fungicides by the blast fungus poses a major risk.

Blast fungicide trials have been conducted at the LSU AgCenter’s Rice Research Station at Crowley since the 1970s. Small plots were usually 4 feet by 16 feet, consisting of seven strips with 7-inch row spacing. Seeding rates, fertility and pest control followed current recommended practices. Experiments were randomized with at least four replications. Varieties selected were susceptible to blast. The plots were fertilized with high nitrogen rates, planted late, drained at midtillering until the soil cracked and then reflooded and located where disease pressure was high to favor the incidence and severity of the disease.

Typically, fungicides were applied to the small plots using pressurized sprayers at the 2-inch boot stage and at 50 percent heading. Benlate (withdrawn from the market in 1997), Quadris and Gem were applied to plots, and an unsprayed control was included. Blast incidence was determined by counting the number of heads infected. Plots were combine-harvested and yields expressed in pounds per acre at 12 percent moisture. Milling samples were collected, and total and head rice percentages determined.

Historical data from 30 years of testing showed that from 1976 to 2001, control of blast by Benlate decreased from 60-70 percent to below 50 percent when light disease pressure was present and from 50-60 percent to 10-20 percent when heavy disease pressure was present (Figure 1). Decreases observed in Quadris and Gem over time indicated the blast fungus may be developing resistance to these fungicides (Figure 2).

At this time, fungicides that control blast appear to be performing well in commercial rice fields in Louisiana except for about three failures reported in the past four years. Before Benlate was removed from the market, blast control failures had been reported. The current practice of applying fungicide only once per year and not using a fungicide every year has limited pressure on the fungal pathogen population to develop resistance in commercial fields. If blast becomes more of a problem and rice needs multiple fungicide applications every year, fungicide resistance in the blast pathogen population could easily become a problem. Loss of currently registered fungicides to resistance would be a major blow to rice production. It is essential that testing of fungicide efficacy in Louisiana rice fields be continued and that management practices that reduce the speed of pesticide-resistance in major pathogens be studied.

Donald Groth, Professor, Rice Research Station, Crowley, La.; Milton C. Rush, Professor; and Don Lindberg, Retired Professor, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.

(This article was published in the summer 2007 issue of Louisiana Agriculture.)
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