Cool weather tips
Cool weather during the early season affects your weed management program. The cooler the temperature, the slower weeds take up and metabolize herbicides, which results in slower weed death and frequently less mortality. Thus, careful scouting (yes, here I go again) is an essential ingredient to successful weed control.
Seedling rice pests like aphids and thrips seem to prefer cool conditions, so be on the lookout for these critters. As I suggested in last month’s article, get a hand-lens, which will more easily allow you to see these small insects. If you want a hand-lens, please contact me at firstname.lastname@example.org.
Why is water management important to retaining our current pest management tools and gaining new ones? The simple reason is that we grow rice in an aquatic environment, and the water exiting our fields can have direct and indirect impacts on creatures inhabiting or utilizing native and man-made waterways, including bayous, rivers, wetlands, lakes, estuaries and oceans.
We are much better at managing our water than we used to be, but we need to be constantly aware of the environmental and legal repercussions of releasing water from rice fields treated with pesticides. All rice pesticide labels have a required period of time between application of the pesticide and release of water from the field.
If an impending rain is imminent, make sure you have “freeboard” on your water boxes/gates to contain the water from the rainfall event. All it takes is a single spill-over of water from a treated field to cause harm to our entire industry. When this happens, we jeopardize all of our hard-earned pest management tools. I know of at least two important rice insect pest management tools we have lost due to possible improper use, and both of these unfortunate incidences were related to water issues.
As far as disease management, practically any stress, biotic or abiotic, can negatively impact disease severity. Maintaining a healthy rice plant with proper nutrient and cultural inputs helps alleviate disease problems. Dr. Don Groth, LSU AgCenter rice pathologist, says most rice diseases are dependent on moisture in the canopy, so cultural practices that increase canopy thickness encourage disease.
We now have effective fungicides for many of our rice diseases. These tools are expensive and should only be applied when needed, which means you must scout your fields for disease presence and severity. Why apply a fungicide that’s not economically justified? Needless applications of any pesticide can lead to development of resistance in the target organism. Proper timing of fungicides is crucial to successful disease control. Dr. Groth says salvage sprays after significant damage occurs usually do not produce yield increases because tissue is already killed and yield potential reduced.
On a cooperative note, Don and I just completed a multi-year study looking at the interaction between rice water weevil and sheath blight. Basically, we found that rice water weevil injury does not predispose rice to sheath blight damage.
As I mentioned in a previous article, Texas AgriLife hired an Extension Pathologist – Dr. Young-Ki Jo – with rice responsibilities. Dr. Jo has been on board for about a year and is evaluating novel fungicides for sheath blight and other rice disease control. Again, Don is helping Young-Ki with these experiments. We appreciate all the good help we receive from LSU AgCenter and the Rice Research Station at Crowley.
Finally, the Beaumont Center is in the process of hiring a research rice pathologist who will work cooperatively with Dr. Jo and other rice scientists and clientele.
Treat for bakanae
Rice planted just prior to very cool conditions may be subject to environmental factors that are unfavorable for rice growth and development and possibly predispose plants to disease early in the season. 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.
Be vigilant for signs of seedling disease by inspecting poorly germinating seeds or slow-growing seedlings by pinching the seed to determine if it has 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 helpful in reducing disease incidence to manageable levels and is now considered a standard practice. Planting within 12 to 24 hours of draining is essential as fungal inoculum may increase on seed being held in trailers starting about 24 hours after draining.
Treated seed may suffer from increased bakanae incidence if drained seed is held for prolonged periods.
Rice blast incidence is still mostly concentrated in areas where it has occurred annually in the northwest Sacramento Valley. I recommend not growing M-104 or M-205 in areas that are traditionally affected by rice blast as they are much more susceptible to leaf blast than varieties such as M-202 and the resistant variety M-208.
There is no substitute for field scouting when deciding if a fungicide application for diseases, such as aggregate sheath spot and rice blast, is needed. Inspect your crop weekly for disease symptoms, even if you think it is too early. As rice approaches the boot stage, increase scouting intensity for rice blast and aggregate sheath spot diseases to determine your risk based upon disease incidence, disease severity, weather forecasts and consultation with your pest control adviser to determine your need for a fungicide application.
Cultivars & disease
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 rainfall 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. CL 131 and CL 151 are both very susceptible to sheath blight. From mid-season on, these cultivars need to be monitored closely for sheath blight. On these two cultivars, research has shown that you get a 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, then make another application (either Quilt or Stratego) at late boot.
Cocodrie and Sabine are rated susceptible to sheath blight. These cultivars will need a fungicide application for sheath blight control, but typically only one application. 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.
A lot of the later-planted rice last year had false smut. False smut is always an issue on rice that is planted at the end of May or the first of June. The way to prevent this disease from being a problem is planting early. Propiconizole fungicides (Tilt, Quilt or Stratego) only provide suppression. Therefore, if you have late-planted rice and false smut is an issue, don’t say the fungicide did not work.
The role of water
In the last issue of Rice Farming, I made the statement that Ricestar HT herbicide should not be used on medium grain rice varieties. Without going into detail on the reason for that statement, I have since learned that I was wrong. Ricestar HT can be used on medium grain varieties. I apologize for any confusion that this may have caused anyone.
Before the introduction of rice fungicides, rice disease management was restricted to selecting a resistant variety (if one existed) and using cultural practices that minimized the effect of disease. After 1980, when fungicides became more common, the use of cultural practices to reduce disease began to fade away. Last year, we witnessed a demonstration of how incorrect water management can lead to serious disease problems.
CL 151 was planted on a little over 300 acres in Louisiana last year and was generally regarded as an excellent variety. However, in one field, serious rice blast disease developed. When scientists from the Rice Research Station visited the field, they found ideal conditions for disease development. The farmer had inadvertently mimicked what Dr. Don Groth does in his blast nursery to encourage the development of disease so experimental lines can be evaluated. Because it was a Clearfield variety, the farmer delayed establishing a permanent flood until nearly mid-season.
Blast is one disease that we know is discouraged by maintaining a deep flood. Keeping the field dry that long was ideal for blast development.
Water management for sheath blight is not clear cut. Prior to fungicide introduction, about all that could be done was to select a variety tolerant or resistant to it, avoid over fertilization with nitrogen and hope for a dry year. The sheath blight fungus infects rice when parts of the fungus floating on the water’s surface come into contact with a rice stem.
These fungal parts are called sclerotia. They germinate, and the fungus penetrates the leaf sheath or stem. Varying water levels helps to reduce to some degree infection because it prevents the sclerotia from being in the same spot long enough to attach itself to the plant. If we could raise and lower the water level often enough and fast enough, it might help, but it is simply not practical.
Several of the older varieties escaped some sheath blight injury because they were so tall. The fungus would attack at the water line, but there was often almost three feet of plant between the water surface and the panicle. By the time the fungus had grown up, the plant grain fill was nearly complete and injury was less. Of course, almost no one wants to go back to those very tall, lodging-susceptible, lower- yielding varieties.
There have been a number of articles devoted to using less water or using other forms of irrigation rather than flooding a field as a means of conserving water. I like water in rice fields. In my experience in 14 years of monitoring verification fields on about 4,000 acres and in visiting farmer fields for longer than that, rice grows better in water. It only makes sense to me since most of the world’s rice grown for the past 3,000 years was lowland or paddy rice. Because rice has evolved under these growing conditions, today water plays a greater role than just weed control.
I understand the interest in reducing water use in rice production. The primary purpose of flooding rice has been weed control. The introduction of Clearfield rice technology has made it possible to control weeds without the benefit of a flooded field. The combination of these two developments has stirred the waters again regarding growing rice without a flood.
This is not a new concept. There are some areas of the world where, for various reasons, rice is grown without a flood. This is called upland rice. The varieties used for upland rice production are different from lowland rice and usually have lower yield potential.
Until our rice breeders come to me with a variety that can be grown as an upland variety, I will continue to discourage any form of rice production in Louisiana except paddy rice. In our verification fields, we try to establish permanent flood as early as possible and maintain it until we reach physiological maturity. One reason for this philosophy is that I really do not like to fix things that are not broken.