Sheath blight study justifies scouting.
My first exposure to disease management in rice was in the summer of 1980 when I worked for DuPont as they introduced Benlate fungicide for use on rice. That May, there was a major rainfall event, producing from eight to 20 inches of rain overnight, then virtually no rain was received until the end of July. Fungicides were the last thing on farmers’ minds then, and discussion of rice disease was centered on sheath blight and blast.
Other fungicides came on the market in short order. Some of those I remember are Moncot, Rovral, Tilt and Top Cop. The introduction of the strobilarins (Quadris and Gem) opened a whole new world in disease control because they were so much more effective. When the net blotch phase of Narrow Brown Leaf Spot (Cercospora) showed up a few years ago, it revealed a weakness in the new chemistry and brought propiconazole (the active ingredient in Tilt) back. By then, the patent had expired on Tilt, resulting in many generic versions on the market today.
In the early days of disease management, we stressed scouting and using fungicides only when necessary. It was usually difficult to get growers to use the fungicides available at the time because the yield response was often difficult to determine; fungicides were expensive; and their application came at a time of year when funds were dwindling.
The newer fungicides produced dramatic results, so much so that many farmers now apply fungicides on a routine or “automatic” basis. In our Rice Research Verification Program, it has become more difficult for us to persuade growers not to apply fungicides than to apply them.
Dr. Don Groth, plant pathologist at the LSU AgCenter’s Rice Research Station in Crowley, has conducted an interesting study in the past few years. In addition to evaluating efficacy of the fungicides, he analyzed the data for economic response under certain conditions. When all plots were sprayed regardless of varietal response to disease and without scouting, there was an average of about 600 pounds per acre yield response. If only the susceptible varieties were sprayed automatically, the yield response was a little over 700 pounds per acre. If scouting was used and all varieties were sprayed, then the response was a little less than 800 pounds per acre. When scouting was combined with spraying only the susceptible varieties, the response jumped to about 1,000 pounds per acre. In this case, the primary disease was sheath blight; however, the study justifies the need to scout and make a decision on an as needed basis.
Remember that bacterial panicle blight is not controlled by fungicides. Fungicides control fungi, not bacteria. Unfortunately, we have nothing to recommend to control this disease other than to select resistant varieties and plant early to avoid heat at pollination.
Timing is key
Fungicide timing is a very critical component in disease control and management. Soon after mid-season, a weekly scouting program needs to be implemented to address the main rice diseases. Scouting is the key to proper fungicide application timing. The most common diseases that we face in Mississippi are sheath blight and kernel smut.
Sheath blight can easily be found by lesions at the water line after rice begins joint movement. Presence and severity of this disease can increase if there is a prior history of sheath blight, lack of crop rotation and lack of prior fungicide use. Also, frequent rainfall showers, heavy dews and mild temperatures can significantly increase the severity of sheath blight.
Knowing the presence and severity of sheath blight is important in keeping yield losses to a minimum. If sheath blight begins aggressively moving up the plant, a fungicide application will be necessary to prevent it from consuming the entire rice plant.
Kernel smut is another important disease in Mississippi. This is a soil-borne disease that infects florets and causes black rice kernels. Yield losses can be seen with kernel smut; however, it mainly reduces the quality of rice. Kernel smut is a disease that produces no signs or symptoms before the kernel becomes inundated with black spores. Therefore, a preventative application of a propiconazole (Tilt) containing fungicide is necessary for the control of this disease.
There is approximately a 28- to 35-day period from mid-season until heading. If sheath blight becomes present and severe within two weeks after mid-season, it will be best to go ahead and apply a strobularin fungicide (Quadris or Gem) by itself and then follow up with an application of a premix of a strobularin and propiconazole (Stratego or Quilt) prior to heading. We have seen a yield benefit from split applications when sheath blight is severe. This split application will prevent kernel smut as well.
If you are within a couple weeks of heading, the sheath blight begins to progress, and a kernel smut prevention application needs to be made. A single application of a premix of a strobularin and propiconazole (Stratego or Quilt) can be made prior to heading. Fungicide rates will depend on how long you need to protect the crop.
If you are applying a fungicide soon after mid-season, a higher fungicide rate will be needed to protect the crop through heading. As you get closer to heading, a lower rate may be used since the length of residual control needed will be less.
Before discussing disease management issues, I will give a brief update on the Texas rice crop. Basically, April was extremely dry, windy, overcast and cool. These conditions have resulted in slower- than-normal rice growth and longer-than-normal persistence of symptoms of herbicide injury in our rice crop. Lower temperatures mean slower metabolic breakdown of herbicides. Also, windy conditions and lack of adequate soil moisture can stress rice, which can exacerbate herbicide injury. Aerial applicators also are battling the wind. Eventually, normal climatic conditions will return, and rice will respond positively. So, don’t panic; rice is very resilient and can withstand a lot of stress and bounce back quickly.
Disease management is crucial to both main and ratoon crop rice production. Last year, Dr. Young-Ki Jo, Texas AgriLife Extension Service rice pathologist, applied various fungicides at different rates to sheath blight-inoculated main crop plots. He compared yields among the treatments in the main and ratoon crops. In some treatments, yield increases of more than 1,000 lb/A compared to the untreated were produced in the ratoon crop. Dr. Jo attributes this increase to production of more viable tillers in the ratoon crop. So, controlling diseases in your main crop can have great benefits for your ratoon crop.
Dr. Jo also is working on a novel method (cold plasma technology) of controlling seed-borne pathogens, like the bacteria that cause bacterial panicle blight. To quote Dr. Jo, “This technology sounds like something out of a science fiction novel.” According to Dr. Jo, cold plasma is essentially an excited gas, which has anti-microbial properties. He and Dr. David Staak, mechanical engineer at Texas A&M, are developing methods to expose rice seed to this cold plasma. So far, results are encouraging, but much more research needs to be done.
Dr. X. G. (Shane) Zhou, Texas AgriLife Research rice pathologist, also is researching bacterial panicle blight, which was particularly bad in Texas last year. In 2010, Dr. Zhou evaluated more than 60 rice varieties and elite lines at Beaumont and Eagle Lake by spraying bacterial (Burkholderia glumae) suspensions on plots at or near flowering. The majority of the varieties and lines, including CL111, CL142-AR, CL181-AR, CL261, Cocodrie,
Jazzman and Templeton, were susceptible or highly susceptible. Jupiter, Neptune, Presidio, Rondo, and the hybrids XL723, CLEARFIELD XL729 and CLEARFIELD XL745 were among the varieties showing partial resistance.
In addition, Dr. Zhou is investigating beneficial bacterial strains that are antagonistic against bacteria causing bacterial panicle blight. Some of these strains sprayed on flowering rice reduced the severity of bacterial panicle blight by as much as 50 percent with an associated increase in yield of as much as 17 percent. Dr. Zhou is continuing to expand this type of research in 2011 to develop an integrated pest management (IPM) program for bacterial panicle blight.
I am very excited about the work of these young rice scientists. I know other young (and not so young!) rice scientists in all rice-producing states are dedicated to solving the problems our rice farmers encounter in their fields. Research and development are absolutely essential to this mission.
Editor’s note: Dr. Mo Way, Dr. Young-Ki Jo and Dr. X.G. (Shane) Zhou co-authored this article.
Are we having a blast or what?
Rice blast was particularly problematic in California in 2010. The geographic distribution, incidence and severity of rice blast were much greater than in recent years. There are several factors that may have contributed to this situation. For any plant disease, you must have a susceptible host, favorable environment and the presence of the pathogen for disease to occur. These factors all seem to have conspired together in 2010. But there are several other factors that influence the susceptibility or tolerance of rice plants to rice blast disease and these should be kept in mind for the most effective integrated blast management program.
Currently, M-208 is the only commercial variety available in California with a specific resistance gene to the IG-1 race of the pathogen first identified in 1996. No other rice varieties grown in California have specific resistance to rice blast. However, these other varieties do differ in their tolerance to infection by the pathogen.
M-104 and M-205 appear to be the least tolerant of the most widely grown commercial varieties, while M-202 and M-206 are somewhat more tolerant.
Creating aerobic conditions in the soil predisposes rice plants to rice blast disease. Drill seeding and draining for stand establishment or herbicide applications in water-seeded systems increase the risk of infection and susceptibility to rice blast. Additionally, rice plants grown in deeper water exhibit increased resistance to the disease over those grown in shallower water depths. From an irrigation standpoint, maintaining a deep, continuous flood is one of the best management options for minimizing the risk associated with rice blast disease.
Stress may also predispose rice plants to blast by significantly altering the plant’s ability to tolerate infection by the pathogen. Nutrient deficiencies such as potassium and silica have been shown to significantly increase rice blast disease incidence and severity. Other common stresses that may impact disease tolerance in California may include salinity, extreme temperatures and herbicide injury. Managing the rice crop to avoid plant stress is a significant and often overlooked tool for minimizing risks associated with rice blast disease.
Rice blast is very complicated and has the ability to increase in incidence and severity very rapidly under the right conditions. Growers should manage for this disease in a holistic approach rather than just relying on fungicide applications for effective management. Little changes here and there in cultural practices can add significantly to the management of this disease.
Consider all options
In spite of the advances in rice technology, diseases continue to rob rice farmers of yield and quality, while increasing the cost of production. Use of high-yielding varieties with reduced disease resistance in combination with high nitrogen fertilizer rates have increased yields, but have substantially increased risk. With favorable weather for disease development, farmers have had to deal with increased incidence of rice blast, sheath blight, false smut, kernel smut, and most recently, bacterial panicle blight.
If you can select varieties or hybrids that are resistant to the major diseases, this can save money later that would normally be used for fungicides. However, some minor diseases may be more effectively managed with other cultural practices. Variety selection is important for diseases that have little or no response to fungicides, such as rice blast and bacterial panicle blight.
Resistant varieties (such as Templeton or the hybrids) should be planted in fields with limited irrigation water, history of rice blast, late-seeded rice, extensive tree lines and other factors that encourage disease development. In fields where water is plentiful and the flood can be maintained, susceptible varieties (such as Francis and CL151) can be grown very successfully with careful grower management. Varieties resistant or partially resistant to bacterial panicle blight include RiceTec hybrids, Jupiter and Taggart.
A favorable environment is necessary for diseases to develop to any extent. For example, the rainy, overcast conditions we have experienced during the past two growing seasons have been very favorable for disease development. Hot, dry conditions tend to reduce incidence of rice blast but often increase incidence of bacterial panicle blight. Management practices also tend to impact diseases. A deep flood is critical for minimizing blast in susceptible varieties, such as Francis, Wells and CL151. Excessive nitrogen fertilizer tends to aggravate most diseases. Blast, kernel smut, false smut and bacterial panicle blight tend to be worse in late-planted rice.
Fungicides have proven to be effective against some of the diseases, and, even then, to varying degrees. The strobularin fungicides (Quadris, Gem) are very effective against sheath blight while the propiconazoles (Tilt, Bumper, others) are effective against kernel smut. The propiconazoles also have activity against false smut when applied at higher rates, although perhaps not as good as kernel smut control. The pre-mix products have almost become the standard (Stratego, Quilt XL) for a one-stop shop. The strobularins are effective against rice blast although they result in much better control with optimum water management.
The best ways to avoid bacterial panicle blight include: 1) plant a resistant variety, 2) plant early [prior to April 20], 3) avoid very susceptible varieties (CL151, CL131, Wells) in continuous rice rotations, 4) completely destroy previous rice stubble if continuous rice is necessary and 5) adequately control stink bugs.