Disease and insect pests
Every season, the situation for disease and insects in rice is entirely different. Each is influenced greatly by environmental conditions, but they’re always there looking to take a bite out of our bottom line.
The 2022 season and its rising input costs scream this is the time to follow integrated pest management practices. This means we need to use scouting and thresholds to determine whether it’s economically in our best interests to act.
Let’s start with sheath blight, arguably our No. 1 disease in rice. Other diseases can cause greater direct losses, but we deal with sheath blight on a more regular basis.
Finding sheath blight in a field does not mean we should treat for it. Use a threshold to determine when to treat based on cultivar susceptibility, frequency of distribution and height in the plant canopy. For moderately susceptible cultivars, when greater than 50% of field stops are positive for sheath blight, or for susceptible cultivars when greater than 35% of field stops are positive, treatment may be warranted.
That covers the cultivar susceptibility and the frequency of distribution. The final condition is height in the canopy. Even if we meet the previous two criteria to consider treating, we need to evaluate how high in the canopy sheath blight has progressed. Our goal is to reach 50% heading with the upper three leaves in the canopy clean to outrun yield loss.
If sheath blight is present at above-threshold levels, we need to keep a very close eye on it. If it is staying low in the canopy, we can delay treatment and hopefully outrun it, which will save us the fungicide application.
For blast and the smuts (kernel smut and false smut), they occur in a manner that prevents us from using thresholds. We’re forced to treat for these preventatively. In fields with a history of these diseases, avoid susceptible cultivars and excess nitrogen applications. For blast, maintaining an adequate flood depth is critical. Susceptible cultivars grown in fields with a history of these diseases may be candidates for preventative treatment.
Rice stink bugs are certainly an annual issue, but we never know what we’re going to get. The most important consideration for this year will likely be the point at which we can safely stop treating for stink bugs. When 60% of the kernels on panicles have reached the hard dough stage (straw-colored kernels), and rice stink bugs are below treatment thresholds, we can stop treating for the pest. As a reminder, the threshold is 5 stink bugs per 10 sweeps the first two weeks of heading and 10 stink bugs per 10 sweeps the second two weeks of heading.
Hopefully, we deal with a below average pest management season in 2022. Contact your county Extension agent or a specialist if we can help.
Sheath blight issues in furrow-irrigated rice
The time we’ve all been waiting for is finally here. I opened with that line last month, and it appears I was wrong. Writing this at the end of April, we MIGHT be 1% planted in Missouri and that’s being hopeful.
A few dry days to close out the month have been much needed, and some of the lighter ground is finally planted. Hopefully, by the time you’re reading this, we have some real rice acreage in the ground. If not, we’ll be looking at even more of a reduction in rice acreage for 2022.
Caused by the fungus Rhizoctonia solani, sheath blight has been an issue in furrow-irrigated rice. The mindset of most experts four or five years ago was that it would be much less of a problem in furrow-irrigated rice. It’s safe to say we got that one wrong.
While there is not a flood to float inoculum into contact with the sheaths, the disease inoculum is still present in the field. Reports from farmers and consultants indicate that sheath blight is a much more common problem in furrow-irrigated rice than we would have thought. In fact, the largest issue is at the top of the field.
Now let’s dive into that issue. While the sclerotia float and infect the plant at the water line in flooded rice, it can start at the soil line where a flood is not held. Also, keep in mind the furrows do hold water for a significant period of time.
Once infection occurs in furrow-irrigated rice, it may be more likely to spread due to slightly higher nitrogen rates and increased seeding rate. Also, the lack of a flood may keep the temperature higher under the canopy while the humidity remains very high due to muddy soil, creating an environment for sheath blight to thrive.
A strobilurin fungicide is still the best method of suppression for sheath blight. Avoiding a dense stand and excessive N rates are good cultural practices but may be needed to optimize furrow-irrigated rice production. Growing a taller cultivar may also help, but that brings up another point — the rice is not growing as tall at the upper end of a row rice field, making it more susceptible to sheath blight damage vertical progression.
The take-home message remains the same in furrow-irrigated rice — the upper two or three leaves must be threatened to cause significant yield reduction. Treatment threshold is 35% positive stops for a susceptible or very susceptible cultivar, and 50% positive stops for a moderately susceptible cultivar.
Ultimately, we would never recommend an automatic fungicide application in rice. However, it is imperative to scout even our furrow-irrigated rice for sheath blight infection and be ready to pull the trigger if needed. If you have any questions, please feel free to contact me, and as always, eat Missouri rice!
Disease management in California
Thanks to the dry weather we enjoy in the Central Valley of California during summer, we do not have to deal with a large number of rice diseases. There are four main diseases that can create problems: stem rot, aggregate sheath spot, kernel smut and blast. Stem rot and aggregate sheath spot are fairly common, but in many fields do not reach high enough levels to cause concern. Kernel smut is usually found at low levels, but in 2018, we saw a large epidemic in the northern part of the valley. Blast can be a severe problem when we experience warm nights, high humidity and long periods of dew in the morning.
Variety can have an effect on disease severity. An experiment conducted last year showed that longer season varieties, like M-211 and M-209, are more tolerant of stem rot than shorter season varieties. This was not the case for aggregate sheath spot, which was not affected by variety. Long grains are more susceptible to kernel smut than medium grains, and among the medium grains, M-209 is the most susceptible. M-210 is the only blast-resistant variety currently available. This variety is basically M-206 but with a gene for blast resistance. M-209 is probably the most blast-susceptible variety.
Good fertility levels will help to reduce disease severity. Excess nitrogen makes plants more susceptible to stem rot, blast and kernel smut. Potassium deficiency can result in higher levels of stem rot and aggregate sheath spot. Good straw decomposition can reduce the stem rot and aggregate sheath spot inoculum levels in the soil, resulting in less disease during the season.
Fungicides can help reduce the levels of disease and avoid yield losses. Trials I have conducted during several years have shown that application of azoxystrobin — the active ingredient in Quadris — between mid-boot and early heading, can reduce the incidence and severity of stem rot, aggregate sheath spot and blast. The trials showed that applying the fungicide at propanil time did not significantly reduce disease levels. If kernel smut is a concern, the fungicide propiconazole — the active ingredient in Tilt — can be tank mixed with azoxystrobin at the mid boot stage.
Kernel smut symptoms appear just before maturity. A black mass of smut spores replaces all or some of the endosperm of the seed. Often the spores ooze out of the grain, leaving a black mass along the seam of the hulls.
The fungus, Tilletia barclayana, overwinters as spores in soil of affected fields and in seed. The disease is easily observed in the morning when dew is absorbed by the smut spores. The spore mass expands and pushes out of the hull, where it is visible as a black mass. When this spore mass dries, it is powdery and comes off easily on your fingers.
Rain washes the black spores over adjacent parts of the panicle. Affected grains are a lighter, slightly grayish color compared with normal grain. Usually, only a few florets may be affected in a panicle, but fields have been observed in Louisiana with 20% to 40% of the florets affected on 10% or more of the panicles in a field.
Smutted grains produce kernels with black streaks or dark areas. Milled rice has a dull or grayish appearance when smutted grains are present in the sample. Because fewer kernels break when parboiled rice is milled, kernel smut can be a severe problem in processed rice.
This disease is usually minor in Louisiana, but it can become epidemic in local areas. Spores of the fungus are carried on affected seeds and overwinter in the soil of affected fields. The pathogen attacks immature, developing grain and is more severe when rains are frequent during flowering. Fungicide applications at early booting can be affective for controlling this disease. Significant quality and yield reductions are possible.
Disease development is favored by high nitrogen rates. Research results indicate the 2- to 4-inch panicle in the boot applications of demethylation inhibitors (propiconazole and difenoconazole) reduce damage significantly. Applications after boot split have little, if any, activity.
Remember, growth stages can advance quickly, so it is important to scout for the rice growth stage at the same time as you scout for disease. Also, you will need to allow time to obtain a fungicide, schedule the application and allow for good weather conditions to apply the fungicide at the correct time. The use of foliar fungicides is justified in many cases. The history of the field is a major factor to consider in making this decision.