Friday, September 24, 2021

Stay ahead of weeds, and don’t let them go to seed

Whitney Brim-Deforest
DR. WHITNEY BRIM-DEFOREST CALIFORNIA
Extension Agronomist
University of California, Davis
wbrimdeforest@ucanr.edu

This season started fast and a bit earlier than normal. Temperatures have also been relatively warm, meaning that rice planted early in the season was able to get well established, likely with vigorous early growth. Weeds will also respond to the warm temperatures, germinating earlier and growing more rapidly due to the faster accumulation of growing degree days.

For watergrass control, we are having increasing issues every year due to the spread of multiple herbicide-resistant late watergrass and barnyardgrass, as well as a possible new biotype that appears to be resistant or tolerant to most of our grass herbicides. Yield decreases from grasses are higher than any other weed of rice, so it is important to get them under control early.

The longer they are allowed to compete with the rice, the greater the yield impact. A preemergent herbicide or the use of a stale seedbed in heavily infested fields will ensure that the rice can better outcompete the grasses early in the season.

For good control, the most effective herbicide plan involves multiple early season applications with a mix of modes of action. A late-season foliar application (cleanup application) may also be necessary.

We now know that sprangletop can emerge in anaerobic (flooded) conditions, even when fields are continuously flooded. Sprangletop was previously thought to be able to emerge only when a field had been drained.

However, due to the slower emergence under flooded conditions compared to flushed conditions, early granular herbicide applications may not fully control sprangletop. (The herbicide will have dissipated before the weeds have emerged).

Therefore, a later granular application applied around the two- to three-leaf stage of rice or a foliar application applied after tillering is necessary for good control. Remember that it is ineffective to apply an herbicide once the weeds have headed (gone to seed), as the seeds may already be viable and will not be killed by the herbicide.

Preventing weeds from going to seed is important for long-term weed management, as the seeds are deposited in the soil seedbank and may be viable for many years.

Sedge control depends on the species. For smallflower umbrella sedge, herbicide resistance is widespread. Therefore, it may be necessary to make multiple applications, one early in the season and a follow-up later in the season with a different mode of action.

For ricefield bulrush, resistance is not yet widespread, and one herbicide application is usually enough to control it.
Broadleaf control is important, but thankfully, herbicide resistance is only found in redstem (redberry) and some arrowhead, and it does not appear to be widespread nor is it multiple-herbicide resistant.

Ducksalad and monochoria are still susceptible to labeled herbicides. The most difficult part of controlling broadleaf weeds is they are not uniform across the rice-growing region. It is therefore important to know the species in each field, as many herbicides are not broad-spectrum on broadleafs.

Don’t let late-season issues get in your pocket

Jarrod Hardke, University of Arkansas
DR. JARROD HARDKE
ARKANSAS
Asst. Professor/Rice Extension
Agronomist
University of Arkansas Cooperative
Extension Service
jhardke@uada.edu

Disease and insects in rice always seem to try to get in our pocket late in the season. A few general guidelines can help us best manage these problems while minimizing their impact on our profitability.

Sheath blight and the smuts (kernel and false) are the most common disease issues each season. With sheath blight, we have the ability to scout and spray based on cultivar susceptibility and disease progress. Remember that after midseason when you have greater than 50% positive stops on moderately susceptible cultivars or greater than 35% on susceptible or very susceptible cultivars, fungicide treatment may be warranted.

However, just because sheath blight can be found doesn’t mean it’s at treatment level. It also needs to be threatening the three uppermost leaves in the canopy. If we can make it to 50% heading (panicle beginning to emerge from 50% of main tillers) with the upper three leaves clean, then we have outrun sheath blight.

So the threshold is based on cultivar susceptibility, percent positive stops for sheath blight in the field and progress of the disease up the plant, which is dependent on favorable conditions. If we fail to meet these three requirements, then a fungicide application is generally not economically warranted.

For the smuts, unfortunately, fungicide applications must be made preventatively before we know if we have the disease for certain. Fields with a history of either kernel or false smut, planted to susceptible cultivars and that received excess nitrogen fertilization are candidates for a fungicide application to prevent disease development.

The optimum application timing is mid-boot as flag leaves are coming out or just out. If you delay to split boot, then a fungicide application is unlikely to be effective.

Rice stink bug is always a moving target in terms of pressure each year. We recommend scouting beginning when 75% of plants are heading. The first two weeks after heading, treat when five or more rice stink bugs per 10 sweeps are found. The second two weeks after heading, treat when 10 or more stink bugs per 10 sweeps are found.

An additional point to make on rice stink bugs is when to stop spraying for them. Recent Arkansas research has shown that when 60% of panicles are hard dough (straw-colored kernels) or beyond, rice stink bug can no longer cause economical damage.

When we say, “hard dough, let it go,” we’re referring to the majority of the panicle being in hard dough.

Good luck managing these late-season issues in 2021 — hopefully conditions will be in the crop’s favor and not the pests.

Lessons learned for soybean rust preparation

young-ki jo
DR. YOUNG-KI JO
TEXAS
Professor & Extension Specialist
Department of Plant Pathology & Microbiology
Texas A&M University
ykjo@tamu.edu

First, I would like to take a moment to mention the retirement of my mentor, collaborator and colleague, Dr. M.O. Way. The void created in his absence can be felt in many places; already I miss his monthly articles in Rice Farming magazine describing current issues and providing insightful advice for Texas rice and row crops. His contributions and care to the Texas rice industry for the past 38 years will be remembered for a long time to come.

Dr. Way’s retirement has led me to reflect on our last collaborative soybean field trial in the Texas A&M AgriLife Research Center at Beaumont. Our original research plan for 2020 was disrupted by pest and disease outbreaks that seriously damaged our test crops. Our soybeans were planted in mid-May and we originally planned to test fungicide effectiveness for common foliar diseases.

But later that summer, green cloverworm (Hypena scabra) caterpillars devoured most of the leaves, leaving only leaf skeletons on the field plots that were not protected by insecticides (Figure 1). This blow was worsened in October when the field spared from the insects was hit hard by soybean rust (Phakopsora pachyrhizi) (Figure 2). Later, the soybean plots that were not protected by fungicides were highly infected with rust and mostly defoliated, leading to 25%-30% yield loss.

Green cloverworms are common in Texas and relatively easy to control with insecticide applications. The major challenge is to detect the presence of caterpillars early since young instars of pale green larvae can easily go unseen. Implementation of timely insecticide use before they cause economic damage is the key for managing foliage-feeding caterpillars.

cloverworm feeding
Figure 1. Feeding damage on soybean foliage by green cloverworm in September 2020 — photos by Young-Ki Jo

In contrast to the common cloverworms, an outbreak of soybean rust in the summer had never been observed in Jefferson County. Reports have been found of this occurring at low frequency in the late season but in southern Texas.

After surprising weather conditions such as high wind, above-average precipitation and cool temperatures in the late summer and early fall, the yellow leaves and numerous rust-colored speckle symptoms of the disease were clearly seen. When infected leaves were touched, visible floating dusts — the rust spores — drifted off.

Eventually the infected leaves were dropped and only bare stalks remained. Such foliar damage directly will result in decreased yield losses with the magnitude of damage increasing as the earlier infection occurs.

There are three major tips to avoid the worst-case scenario in which high infection during the early plant growth stages leads to significant yield losses.

First, use rust-resistant soybean plants; scientists are currently working to develop stronger varieties and make them commercially available to U.S. farmers. Second, plant your crop early in the season using early maturing cultivars; mature plants are less vulnerable to the harmful effects of the disease.

soybean rust
Figure 2. Soybean rust outbreak in October 2020. Yellow leaves caused by the disease are apparent in the right row (untreated control) compared with the left row which was protected by fungicides.

Third, the most practical and effective means of managing soybean rust is the preventative application of fungicide on R2-4 growth stages. Chlorothalonil, strobilurin or triazole fungicides have shown to be effective but early application remains crucial.

This severe incident of soybean rust was seen for the first time in eastern Texas, meaning we may have another disease to take into account when planning soybeans this year. The conducive environmental conditions for rust are cool weather with high moisture often brought by hurricanes in the summer.

Hopefully, this coming Atlantic hurricane season will be different from the record-breaking one in 2020. The good side of the winter storms hitting Texas in February is that the soybean rust will not survive in freezing temperatures, giving us a head start in managing rust this year. But be ready to take more preventative approaches learned from the last year’s events.

Early scouting for foliage-feeding caterpillars and soybean rust cannot be overemphasized. If you find many small green caterpillars or early symptoms of rust in the lower plant canopy at early growth stages, consult with county agents and experts and react in a timely fashion.

Scout, stay on top of rice diseases

ronnie levy
DR. RONNIE LEVY
LOUISIANA
Extension Rice Specialist
RLevy@agcenter.lsu.edu

Rice disease development varies due to the variety or hybrid you have chosen to plant, the presence of the disease pathogen and the current environment for that pathogen to develop. The most common diseases in rice that we can use a fungicide to manage include blast, sheath blight, Cercospora (narrow brown leaf spot) and smut (kernel smut and false smut).

Scouting is an important in-season tool used to determine the presence and severity of rice diseases, particularly for sheath blight and blast. And it should start early in the season. When scouting a field, make sure your disease evaluation is based on the whole field, not just a localized area.

There is no easy way to do this. You must cover the entire field, making as many stops as necessary to check for present diseases.

At each stop, you will want to evaluate if the disease is present on approximately a 3-foot section by opening the canopy. If you find disease, you may want to subjectively rate the severity of the disease from 1 to 10. A 1 rating would indicate that only one or two lesions were present, with a 10 rating being a complete infestation.

For sheath blight, cultivars that range from very susceptible to susceptible will experience an economic loss if 5% to 10% of the tillers are infected during vegetative stages. For moderately susceptible cultivars, the level is 15%. At these levels, consider using a fungicide. For blast control, apply a foliar fungicide at early heading (50% to 70% heads emerging) when leaf blast symptoms are present.

sheath blight
For sheath blight (pictured), cultivars that range from very susceptible to susceptible will experience an economic loss if 5% to 10% of the tillers are infected during vegetative stages. For moderately susceptible cultivars, the level is 15% — photo courtesy LSU AgCenter

Leaf blast does not always precede rotten-neck blast, and preventive applications of a fungicide may be warranted if a blast-susceptible variety is grown. The best timing against Cercospora is between panicle differentiation and boot growth stages.

The later the rice is planted, the earlier the fungicide must be applied for Cercospora. Boot applications of the proper fungicides can reduce kernel smut or false smut. Fungicide applications for smuts after boot split have little if any activity.

Fungicide timing is critical for disease control. Growth stages change quickly, so it is important to scout for the rice growth stage while scouting for disease.

Rice disease control using a single fungicide application is becoming more difficult because of fungal resistance to fungicides, multiple diseases requiring different timings for effective control and higher multiple applications being warranted.

Rice producers are encouraged to use full label rates, rotate modes of actions and use multiple fungicide applications when justified to effectively — and economically — manage rice diseases.

Additional information on rice disease control can be found at www.lsuagcenter.com/ricediseases.

Related Articles

E-News Sign Up

Connect With Rice Farming

Quick Links

Archives