Diseases are serious constraints to rice yield and quality. Estimates of yield losses to diseases in the Texas Rice Belt are about 12 percent. The increasing trend of planting at lower seeding rates and earlier to reduce planting costs and improve the likelihood of producing a ratoon crop places more importance on protecting seed from diseases, insects and other biotic stand reducers. This is why seed treatments have gained more significance in recent years.
Bacterial panicle blight is seed-borne. However, fungicidal seed treatments do not control this disease since the causes are bacteria – not fungi. But, planting early can help manage this disease. Cultivars vary in susceptibility to bacterial panicle blight. For instance, Bengal, CL131, CL151, CL161 and Cocodrie are susceptible while CLXL729, CLXL730, Jupiter and Neptune are moderately resistant.
Our most serious disease of rice in Texas is sheath blight, which is caused by a fungus. The introduction of semidwarf varieties and the general increase in nitrogen fertilization have elevated this disease to its current damaging levels. This situation illustrates the dynamic nature of pest management in rice. Changes in agronomic practices relative to cultivar, fertility, water management, planting methods, harvesting etc. can have a profound influence on pests.
Secondary pests can assume added significance or exotic pests can be introduced. This is why research and Extension activities are crucial to the sustainability of the US rice industry. Jefferson, Saber, Cheniere, many medium grain varieties and hybrid varieties are less susceptible to sheath blight than many other cultivars. Dr. Don Groth, LSU AgCenter rice pathologist, and I conducted cooperative research that showed rice water weevil damage does not predispose rice to sheath blight damage, which is good news.
Speaking of secondary pests, narrow brown leaf spot is receiving more attention in the Texas Rice Belt. This disease attacks both the main and ratoon crops. Fungicides containing propiconazole should be applied at the mid- to late-boot stage for control.
Recently, a new fungicide received a rice label. I know rice pathologists throughout the South helped obtain this new tool for our rice farmers. This new tool is Quilt Xcel, which contains 0.56 lb/gal more azoxystrobin than Quilt. This means you don’t have to spike with additional Quadris or Tilt to obtain the recommended amount of 10-12 fl oz/A Quadris + 6 fl oz/A Tilt. Dr. Groth has data to show 21 fl oz/A of Quilt Xcel provides long-lasting control of foliar diseases.
The 2010 Texas Rice Production Guidelines should be available in late May. It also will be online at http://beaumont.tamu.edu.
The Texas AgriLife Research and Extension Center at Beaumont and the David R. Wintermann Rice Research Station at Eagle Lake will have their annual field days this year on July 8 and June 29, respectively. Come visit with our faculty/staff, including our new scientists, Drs. Fugen Dou, Young-ki Jo and Shane Zhou!
I thank Dr. Don Groth for assistance in preparing this article.
A question of timing
Since Cercospora, in the form of net blotch, hit us without warning, fungicides have been applied as insurance because by the time the symptoms of net blotch show up it is too late. According to Dr. Groth, there is about a 30-day period from infection to appearance of symptoms. In areas of the state where false smut and kernel smut are problems, fungicide applications had become routine several years earlier. However, this does not mean fields no longer need to be scouted for disease. We spend a large amount of time between internode elongation (green ring) and heading looking for disease and deciding on fungicide usage. The biggest difference between the methodology of several years ago and now is not a question of whether a fungicide will be used, but what fungicide, how much and when. We still need to scout to make this decision.
Another change in our scouting is that once we discover disease in a particular area of the field, we designate it as a “hot spot” and monitor the progression of the disease there rather than continuing to walk the entire field. In some years and some fields, we may not discover that “hot spot” until fairly late in the season.
In general, if we are not concerned about the smuts or Cercospora and have not detected either blast or sheath blight by 50 percent heading, we will not use fungicides. If we do not pick up blast or sheath blight and are still concerned about the smuts or Cercospora, we will recommend a fungicide containing propiconazole only. So far, this has only happened in fields planted to hybrids.
The strobilurin-containing fungicides have little effect on control of the smuts or Cercospora, but propiconazole-containing fungicides do. Conversely, propiconazole-based fungicides are not very effective on sheath blight or blast. The result has been the combination of these, both as commercial packages and by mixing the two products. Because we are still economically in a single application market for the most part, it is often difficult to decide when to apply a fungicide. In an effort to try to clear up some of the timing questions, Drs. Groth and Hollier put together a chart indicating rice plant growth stage, disease and fungicide timing. This information is in the following table. For example: to time fungicide for greatest efficacy in a single application to control Cercospora, the best window is between boot and heading. The most difficult fungicide application to time is one to control blast, and the most flexible fungicide application is one to control sheath blight.
One part of the problem has been the determination of “heading.” If any part of a panicle is exserted from the boot, then that stalk is considered headed. When it looks like 50 percent heading from the pickup truck window, it is way past that stage. In our experience in the verification program, the easiest way to detect or demonstrate 50 percent heading is to cut a handful of plants then sort them according to whether or not any part of the panicle is showing or not.
Another factor is the rapid growth stage changes occurring from boot to heading. If growing conditions are good, you have to check fields at the very minimum of once a week to anticipate when it will be time to apply a fungicide.
Don’t forget to scout
Scouting is the most important tool in making the right fungicide decision. With rice diseases, every year is different and every cultivar is different. As a result, there is no one size fits all disease control program. Having a routine scouting program will help you make the right decision on when to apply a fungicide.
Diseases like sheath blight will always be a problem because the aquatic environment is perfect for diseases. However, frequent rain showers, heavy dews and mild temperatures can significantly increase the severity of sheath blight and blast. Having feet in the field will help you know the presence and severity of these key diseases.
Cultivars all differ in their ability to tolerate diseases. CL111, CL 131 and CL151 are very susceptible to sheath blight. From mid-season on, these cultivars need to be monitored closely for symptoms of sheath blight.
On these two cultivars, research has shown that you get better return on your investment with making two fungicide applications over making one single application. If sheath blight is a problem shortly after mid-season, I generally recommend applying six to nine fl oz/A of Quadris, and make another application (either Quilt or Stratego) at the late-boot timing.
Cocodrie and Sabine are rated susceptible to sheath blight. These cultivars need a fungicide application for sheath blight control, but typically only one application. Cheniere, Wells, XL 723 and XL 729 are cultivars that are only moderately susceptible to sheath blight.
Therefore, a fungicide application will be on a case-by-case basis.
Fungicide rates for sheath blight control will depend on how long you need to protect the crop. If you are applying a fungicide in the preboot timing, 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.
Varieties are key
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 and kernel smut.
Variety selection is first and foremost important for managing all diseases. 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 most important for diseases, such as rice blast and bacterial panicle blight, that have little or no response to fungicides. 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 CL 151) can be grown very successfully with careful grower management.
A favorable environment is necessary for diseases to develop to any extent. However, 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. Most diseases including blast, kernel smut, false smut and panicle blight tend to be worse in late-planted rice. As the planting window draws to a close, give consideration to the increased disease risk later in the season.
Fungicides have proven to be effective against some of the diseases, and even then to varying degrees. The strobulin fungicides (Quadris, Gem) are very effective against sheath blight while the propaconazoles (Tilt, Bumper, others) are effective against kernel smut. The propaconazoles 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 Xcel) for a one-stop shop. The strobulins are effective against rice blast although they result in much better control when optimum water management is used. In each instance, growers should carefully scout their fields to avoid surprises.
Just a note about furrow-irrigated rice and sprinkler-irrigated rice. I know that there is some interest in these systems and ,for some situations, may have some utility. However, from a disease standpoint, it is extremely risky. Greater pressure is placed on the existing blast resistance genes in either the hybrids or in varieties such as Templeton. While these are the varieties I would choose for these systems, the blast fungus has demonstrated an incredibly strong ability to “adapt and overcome.” If these practices increase and more acreage is placed in these types of systems, we will likely see the resistance break down. I’m just saying, don’t be surprised when it happens.
Good luck on your 2010 rice crop. For more information, contact your local county Extension office or don’t hesitate to contact me.
Signs of disease
We still have time to get rice planted, but there is going to be a sense of urgency over the next few weeks. UC Cooperative Extension recommends a preferred seeding date of April 20 to May 25 and an optimum seeding date of May 1 to May 10 for most California rice varieties.
Early planted rice often struggles through cool temperatures that slow germination and seedling emergence. Under these conditions, plants are predisposed to seedling diseases, which may be caused by several organisms and produce symptoms of seedling rot or seedling blight under water-seeded conditions. The later-planted fields may avoid these issues, as they experience warmer temperatures that favor rapid seed germination and stand establishment.
Watch for signs of seedling disease by inspecting poor germinating seeds or slow-growing seedlings by pinching the seed to determine if it has been rotted and looking for fungal growth on the seed or seedling. Seed that are infected shortly after seeding often don’t germinate because the endosperm or embryo is rapidly destroyed. Growth of seedlings may be greatly impeded when seeds are infected following germination. Symptoms of seedling disease may include stunting, yellowing or rotting of the seedlings.
Bakanae is primarily a seedborne disease, and it is recommended that all water-seeded rice in California be treated with a sodium hypochlorite soak solution using Ultra Clorox at the recommended rate during the normal seed-soaking procedure. This type of treatment, while not 100 percent effective, has been instrumental in reducing disease incidence to manageable levels and is now considered a standard practice. All California rice seed should be treated for bakanae disease unless there are extenuating circumstances such as drill seeding or organic production.
Rice blast disease is mostly concentrated in Colusa and Glenn Counties. I recommend not growing M-104 or M-205 in areas that are traditionally affected by rice blast as these varieties are much more susceptible to leaf blast than others such as M-202. The rice blast-resistant variety M-208 should be grown in fields where rice blast is an annual occurrence.
There is no substitute for scouting when deciding if action needs to be taken such as field drainage for seedling diseases or a fungicide application for diseases such as aggregate sheath spot and rice blast. Inspect your crop weekly throughout the season for disease symptoms.