A new Louisiana Rice Notes newsletter is now available. This edition covers current conditions, prevented planting, and fungicide availability for 2019.
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.
|Variety||% Root Weight Reduction|
Table 2. List of varieties moderately resistant to TRD as determined by inoculation and response.
|Variety||% Root Weight Reduction|
|Go Soy IREANE||30.762175|
Table 3. List of varieties moderately susceptible to TRD as determined by inoculation and response.
|Variety||% Root Weight Reduction|
|Go Soy 5115LL||44.470801|
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.
Harvest data for our soybean core block variety demonstrations have begun to come in from across the state. The data for these 10 trials is now available on the LSU AgCenter website at the link listed below. I will continue to update the website with incoming data from the remaining trials.
Preliminary OVT data is also beginning to come in and is currently being analyzed before being made available to stakeholders. This information will also be available on the LSU AgCenter website when ready.
Check Soils for Compaction Layers
Josh Copes, Dennis Burns, R.L. Frazier, and Dan Fromme
Over the past couple of growing season, soil compaction has been a hindrance in many fields across Louisiana. Soil compaction was evident by observing reduced crop growth and development in fields and confirmed by inserting a penetrometer into the soil. Soil compaction is the compression of soil particles that reduces pore space thus creating a dense layer of soil that can impede plant root growth. Soil compaction can be caused by heavy machinery traffic and horizontal tillage operations when the soil is too saturated. There have been instances where a deep vertical till implement was used to alleviate a soil compaction layer only to create a new one, less than four inches deep in the row middle, when the rows were rebedded. This was probably a result of rebedding when the soil was too wet. Soil compaction reduces crop rooting ability, restrict water infiltration rate, reduces the volume of soil that plant root will be able to mine essential nutrients, and ultimately can reduce yield.
Machinery size is steadily increasing and will only lead to more frequent soil compaction issues. Silt loam soils are typically prone to compaction. There is perhaps a misconception that shrink and swell type clay soils are not prone to compaction layers due to being “deep broke” as they crack open during periods of drought. Regardless soil compaction layers have been observed in cracking clay soils. Fields where soil compaction could be an issue can be identified by visual observation where a reduction in crop growth rate is evident, early season nutrient deficiency symptoms occur, wilting of crops in certain areas of the field and not in others. Compactions areas can especially be identified during periods of cool weather early in the growing season where the crop develops at a reduced rate compared with the rest of the field with a similar soil type.
You can test for compaction layers by simply probing the soil (tops of beds/rows) in several areas within a field using a soil penetrometer. In order to mark the depth of the compaction zone, push the penetrometer down to the compacted zone and place a finger where the probe meets the soil surface. As a guideline, use the penetrometer when there is sufficient soil moisture for planting. Also, make sure that deeper soil compaction layers are not present. To avoid soil compaction limit field operations when soils are too wet. This can be difficult in Louisiana but creating hardpans will reduce yield. Deep vertical tillage is the fastest method to alleviate soil compaction layers. Deep or tap rooted winter cover crops can also help loosen a compacted soil over time and may help prevent a compaction layer from occurring by increasing soil organic matter and maintaining soil structure.
Below are some photos taken this year in fields with compaction layers. Fields with soil compaction layers should be identified and deep broke this fall when soil moisture conditions are favorable to lift the soil so the hardpan can be disrupted. If you have any questions or concerns please give us a call.
Photo 1. J-Rooted Cotton due to Soil Compaction Layer
Winter Cover Crops: Planning for Cover Crop Success
Josh Copes, James Hendrix, Lisa Fultz, Syam Dodla, and Naveen Adusumilli
Crop harvest is in full swing across most of Louisiana. As we move into October, now is the time to begin planning your winter cover crop management strategy. Cover crops are used for several purposes including: protecting soil from erosion, improving soil structure, scavenging and cycling of soil nutrients, increasing organic matter, helping to alleviate hardpans, etc. Cover crop selection will depend on the goals a producer would like to accomplish by planting a winter cover crop. Having a clear objective for planting a cover crop, will also aid in cover crop management. For example, if minimizing soil erosion is the main objective, selecting a cereal cover crop, would be a good choice. The fibrous root system of cereals will help prevent top-soil from leaving the field. Cereal winter covers are good nutrient scavengers as well. In contrast, a tap-rooted cover crop like forage/tillage radish is better suited for deep nutrient scavenging and potentially aids in loosening a soil compaction layer or preventing one. Mixes of cereal and legume covers can reduce early season N fixation issues in corn. Preliminary data collected by AgCenter scientists has shown that in soybean, legume cover crops can supply N for early growth needs until nodules develop. Other important considerations when selecting a winter cover crop includes: cash crop to be grown following cover crop termination, and winter cover crop termination. Be sure to plant only quality seed, this will help eliminate weed seed contamination issues. Seeding rates should be adjusted for germination percentage or pure live seed per pound. When planting legumes, make sure the rhizobium inoculant strain is correct for the legume that is to be planted and always inoculate. If planting pre-inoculated legume seed be sure to get pure live seed per pound and adjust seeding rates accordingly; some pre-inoculated seed are larger and therefore have less pure live seed per pound.
Cover crops should be planted as soon as possible following main crop harvest. When planted earlier in the fall, growth/biomass production will be maximized prior to cold weather which will slow growth and development of the cover crop. Planting your cover crop soon after harvest, is especially important if corn will be planted. Early cover crop termination, when planting corn, combined with late planting of a cover crop (November) will reduce overall biomass production, therefore minimizing the benefits of the cover crop. Legumes are generally slow growing if planted too late (November), and biomass production will be minimal prior to the onset of cold weather. If fields are enrolled in a NRCS conservation program, that requires cover crops, be sure to follow the NRCS’s cover crop guidelines. Below is a link that contains NRCS seeding rates and planting dates for common cover crops grown in Louisiana. The planting window for most winter cover crops will be October 1 to mid-November. Ranges for average first frost dates for Monroe, Shreveport, Alexandria, and Baton Rouge are November 15, 18, 19, and 29th, respectively (https://www.farmersalmanac.com/average-frost-dates). Posted below, hyper link 2 and 3 are some useful tools may aid in further refinement of accomplishing the intended goals for your farm.
- NRCS planting dates and seeding rates for common cover crops grown in Louisiana:
- Cover Crop and Tillage Scenarios (Potential Scenarios and their implications on incentives payments.):
- Q & A of Conservation Policy and Crop Insurance Surrounding Cover Crops:
- Cover Crop Economics Decision Tool:
Don’t Neglect Fall Weed Management
Josh Copes, Daniel Stephenson, Donnie Miller, and Lauren Lazaro
Trends in earlier crop harvest has resulted in adequate time for weeds to set seed between harvest and a killing frost. This time period can range from one to four months. The average first frost date in North and Central Louisiana is November 15 and 19, respectively. Since a lot of money and effort is spent in controlling weeds during the growing season to negate yield loss, timely weed control practices following harvest is important. The objective of post-harvest weed management is to reduce viable seed return to the soil seedbank, thus ensuring fewer weeds to fight in future cropping seasons. Post-harvest weed control is especially important in fields containing herbicide resistant weeds. A good example to illustrate the importance of post-harvest weed management is the ability of glyphosate-resistant Palmer amaranth to produce mature seed in as little as 30 days after emergence during late summer and early fall. Many other grass and broadleaf weeds are capable of setting viable seed in a similar time frame. Some common weeds infesting fields after harvest include barnyardgrass, morningglory species, prickly sida/teaweed, browntop millet, Palmer amaranth, and waterhemp. Special attention should be made to ditch banks and other non-cropland areas infested with Palmer amaranth and/or waterhemp, since their seed is easily spread in water.
For weeds present in the field at harvest time, mowing and/or tillage should be conducted as soon as possible upon harvest to ensure viable seed set is reduced. Very little time will be required for these weeds to set a substantial amount of seed. Rainfall will influence subsequent germination of weed seed and therefore the need for additional weed control. Furthermore, rainfall following cultivation could increase weed seed germination, however, if the weeds are controlled, the soil seedbank would be reduced. Producers in no-till systems will have to rely on mowing and herbicides to prevent weed seed production.
In a stale-seedbed production system, herbicide applications should be targeted from late-September through October when the time period from application to first killing frost is shortened. In minimum tillage systems, or where weeds emerge after field prep operations, herbicides should be applied before or shortly after flowering. This implies that weeds will be large and more difficult to control, and therefore water volume should be maximized to ensure good weed coverage, as this is critical for good weed control. Multiple post-harvest herbicide applications for control of summer annual weeds should be avoided, so as to minimize herbicide selection pressure that can lead to herbicide-resistance. Utilizing multiple effective modes of action will help minimize selection pressure, e.g. 2,4-D plus glyphosate or glufosinate plus 2,4-D etc. Herbicide choice should depend on weed species present in the field. Some soil residual herbicides can be applied in the fall following harvest. However, rotation interval restrictions must be followed and length of residual control will be influenced by soil temperature and saturation. Do not expect winter long weed control from soil residual herbicides applied from August to early October. Likewise, the lack of rainfall to properly activate residual herbicides can negatively impact treatment effectiveness.
Fall herbicide applications can be effective for control of perennial weed species such as johnsongrass, bermudagrass, alligatorweed, and redvine. Johnsongrass escapes are becoming more apparent across the state. Studies conducted by LSU AgCenter weed scientists have determined that fall applications should be made from September 15 to October 15 when environmental conditions favor weed growth (http://www.lsuagcenter.com/portals/communications/publications/agmag/archive/2006/summer/longterm-management-of-perennial-weeds-starts-in-the-fall). For johnsongrass, bermudagrass and alligatorweed control, 1.0 lb ai/acre of glyphosate should be applied. Two lb ai/acre of glyphosate or dicamba are effective control options for redvine. Glyphosate (2.0 lb ai/acre) plus dicamba (1.0 lb ai/acre) can also be an effective control option. Fields should be scouted the fall following herbicide application to determine whether an additional application is needed. Do not mow or till fields for several weeks following herbicide application.
Some weeds are capable of setting viable seed within 30 days after emergence during late summer and early fall. Post-harvest weed control is especially important when combatting glyphosate-resistant weeds such as Palmer amaranth, waterhemp, or johnsongrass. Problem fields should be identified and receive top priority for preventing seed return to the soil seedbank. Once harvested these problem fields should be mowed or tilled shortly after harvest to prevent and/or reduce seed set. Fields should then be regularly scouted for emerging weeds and additional control tactics applied prior to seed set. This will require close inspection of weed species to determine when they are flowering. Once a weed species is observed flowering a weed control operation should be implemented. Depending on weather conditions following harvest, weed control tactics may need to be implemented approximately every 3 to 4 weeks until a killing frost has occurred. If glyphosate-resistant Palmer amaranth or waterhemp is an issue, a management tactic (i.e. mowing, tillage, herbicide application) should be employed every 3 to 4 weeks. Budgets are typically tight in the fall and spending additional money on weed control when no crops are in the field is difficult, but by identifying fields in need of post-harvest weed management and by implementing field prep in a timely, well-spaced manner can go a long way in reducing future weed numbers in your fields. Below are a list of herbicides labeled for use following main crop harvest and for non-cropland use (ditch banks etc.). Always read and follow label guidelines and restrictions.
If you have any questions please contact us or your local county agent.
Herbicides Labeled for Post-harvest Weed Control
Glufosinate – Liberty 280 SL
Glyphosate – Roundup PowerMax
Gramoxone 2 SL
2,4-D LV4 and 2,4-D Amine
Dual II Magnum – Italian Ryegrass (Sept.1 to Dec. 1)
Herbicides Labeled for Non-Cropland Areas/Farmstead Use
Click on the link below to see this week’s featured nutrient, nickel. This nutrient profile is a part of a weekly series dedicated to the function of the 16 essential nutrients in soybean. After excluding carbon, hydrogen, and oxygen, we are left with a thirteen part series in which we will explore how nutrients are used throughout the plant as well as how to identify deficiency symptoms and develop nutrient management decisions.
Explore LouisianaCrops.com to find the all 14 soybean nutrient profiles. Contact your local extension agent with any questions you may have.