In the past two weeks, instances of true armyworms (TAW) in wheat, corn and pastures have increased across the state. TAW are similar in appearance and size to fall armyworm (FAW). TAW possess a mottled brown head capsule (Figure 1) while FAW have an inverted “Y” on their head capsule. TAW develop into six instars, with larval development taking roughly 20 days and generational turnover occurring in 30 days. This insect is not well adapted to hot temperatures, and survival decreases significantly when air temperature is above 86 degrees F. TAW prefer grass hosts but will feed on broadleaves. TAW primarily feed at night, making observation during the day difficult. Larva consume 80% of the total foliage required for development in the last three to five days as larva. Larva congregate at the base of plants and on the soil surface to avoid midday temperatures. There are several natural enemies of TAW in Louisiana field crops. Predacious insects, parasitoids and pathogens occasionally will control TAW populations before a foliar overspray is required
TAW infesting Bt corn rarely causes economic injury, and Bt proteins available in field corn work very well controlling TAW. Non-Bt corn can experience significant injury from TAW, and fields should be scouted regularly to avoid defoliation. TAW can graze non-Bt corn to the ground; however, if the growing point is still beneath the soil (up to roughly V5), corn seedlings will recover quickly.
TAW can significantly injure wheat if worms are allowed to defoliate the flag leaf before soft dough or clip wheat heads at any stage. The LSU AgCenter threshold for TAW in wheat is when five worms per square foot are found and foliage loss is occurring.
In hayfields and pastures, TAW can cause significant injury to grass crops if left uncontrolled. TAW injury is identical to FAW, and routine scouting in the spring is recommended. The LSU AgCenter threshold is one worm per sweep.
Pyrethroid insecticides control TAW very well in corn, wheat and pastures. As a general rule, large worms are harder to control than small worms.
If you have any questions or concerns, please contact your local AgCenter agent for more information.
Many producers are having to replant corn due to poor stands. There are three main ways to remove a failed corn stand.
Use tillage equipment to physically remove the existing corn.
Apply 0.0469 lb clethodim/acre and wait six days before planting the second corn crop. That equals 6 ounces of a 1 lb/gal clethodim, 3 ounces of a 2 lb/gal clethodim or 2 ounces of a 3 lb/gal clethodim. Waiting six days before planting is critical to prevent injury.
Apply 0.625 lb paraquat/acre plus atrazine at 1 pint/A or diuron at 1 pint/A or metribuzin at 3 oz/A. Good coverage is essential. Even then, don’t expect outstanding control with this choice.
The first edition of Louisiana Rice Notes is now available. This edition covers planting progress, effects of warmer than average March weather on current crop, young rice farmers helping out, and new AgCenter rice publications which are available online.
A new Louisiana Rice Notes newsletter is now available. This edition covers the recent heavy rainfall and pending storms, nitrogen fertilizer questions, conventional rice following Provisia, Louisiana variety and hybrid trends over the last 18 years.
Trey Price, Associate Professor, & Myra Purvis, Research
Associate, Agronomic Crop Pathology, Macon Ridge Research Station
Boyd Padgett, Professor, Agronomic Crop Pathology, Dean
Lee Research Station
Taproot decline (TRD) of soybean, caused by Xylaria sp., usually is not noticed until pod fill when interveinal chlorosis and necrosis (Figure 1) become evident from the turn row. However, the disease may cause seed rot, seedling disease (Figure 2), and plant death (Figure 3) at any point the growing season. Infected seedlings and vegetative stage plants usually go unnoticed because they are quickly covered by rapidly growing neighboring plants. Infected plants will break at the soil line when pulled. Roots will appear black when excavated (Figure 4), and are usually in contact with blackened debris from the previous season. Reproductive structures of the pathogen known as “dead man’s fingers” may appear at the base of affected plants or on other debris during periods of high humidity producing spores that resemble powdered sugar (Figure 5). Disease distribution within the row usually will have a focal point of dead plants, surrounded by those with foliar symptoms, and neighboring healthy plants. These areas may overlap creating a clustered and streaky distribution within a given field. Fields in soybean for two years or more are at risk to taproot decline, and yield losses can be significant. For more information concerning taproot decline, please read the first report at the following link: https://doi.org/10.1094/PHP-01-17-0004-RS.
Many requests for a list of
susceptible/resistant varieties have been received prompting the release of
preliminary data. During the past two
off-seasons in the greenhouse, we have challenged varieties from the 2016
Official Variety Trials against the pathogen, Xylaria sp. The process is briefly described
hereafter. We used sterilized millet
infested with the pathogen to infest growing medium. Inoculum was standardized using inoculum
concentration experiments (data not shown).
A total of 145 varieties were screened.
During each “run”, 4 replications of 40 varieties (4 seed/4” pot,
planted in a linear furrow) were either inoculated at planting or left
non-inoculated then removed to flood-irrigated greenhouse tables for three
weeks. Plant roots were harvested, dried
to final moisture, and weighed. The
experiment was repeated once, and paired t-tests (α=0.05) were used to compare inoculated (n=8) vs.
non-inoculated (n=8) root weights for each variety. For simplicity, we present the results here
as the percentage of root weight reduction.
Paired t-tests indicated that
significant root weight reduction occurred at 48% and higher. Based on percent root weight reduction,
varieties were divided into four categories: susceptible (>48%), moderately
susceptible (36-48%), tolerant (24-36%), and resistant (<24%). Out of 145, 97 varieties were deemed
susceptible with percent root weight reduction ranging from 48 to 85%. There were 25 moderately susceptible, 16 moderately
resistant, and 7 resistant varieties.
For brevity, we will not present the susceptible varieties in this
report. A list of all varieties included
in the screening can be found here. Resistant, tolerant, and moderately
susceptible varieties with corresponding percent root weight reduction are in
Tables 1, 2, & 3, respectively. Field
confirmation of these results is ongoing.
Preliminary data from inoculated field trials indicates that varieties
deemed resistant in the greenhouse show no significant response. Varieties deemed susceptible in the
greenhouse show significant responses to inoculum in the field.
Table 1. List
of TRD-resistant varieties as determined by inoculation and response.
% Root Weight Reduction
Table 2. List
of varieties moderately resistant to TRD as determined by inoculation and
% Root Weight Reduction
Go Soy IREANE
Table 3. List
of varieties moderately susceptible to TRD as determined by inoculation and
% Root Weight Reduction
Go Soy 5115LL
In addition to variety selection, data from research trials, numerous observations, and other anecdotal accounts indicate that tillage and/or rotation will reduce TRD incidence and mortality. To date, there are no recommended seed treatments for taproot decline. Ongoing research indicates that a few fungicides applied in-furrow at planting may be effective on the pathogen. Taproot decline is soil/debris borne; therefore, avoiding spread via equipment is recommended. More research is needed to develop and further refine management strategies for taproot decline.
Louisiana has received a renewal of the 24(c) special local needs label that increases the seasonal use rate of Acephate from 1.5 lbs Ai/Acre to 2.0 lbs Ai/Acre in soybeans. Information pertaining to use rates, application restrictions and timings can be found on the specific cropping label. The label is available in the link below. The label must be in possession of the user at the time of application. If you have any questions please contact your county agent or extension specialist for more information.