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.
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.
Over the past week, I have received a number of phone calls pertaining to what economic threshold should be used for Threecornered Alfalfa Hoppers (TCAH) in soybean. Based on previous work conducted by Sparks and Newsom (1984), Sparks and Boethel (1987) and subsequent experiments conducted by the LSU AgCenter in the last five years, the LSU AgCenter recommended threshold is “starting at pod set, 3 nymphs per row foot or one adult per sweep”. This recommendation is based on published data and experiments conducted in Louisiana at LSU AgCenter Research Stations and Louisiana field locations. Independent of yield loss, TCAH can also cause an increase in the “green bean/stem syndrome” that often is present in soybean fields. Green bean syndrome’s exact cause is not known but we do know that stress plays a large part in the malady. Environmental stress compounded by insect or disease related stress is often the culprit. The picture below is from a TCAH experiment conducted at the Macon Ridge Research Station to investigate the effects of TCAH on dryland soybeans. The experiment was terminated at the onset of stink bugs and all non-target insects were controlled for the duration of the experiment. The green bean syndrome pictured below is directly related to TCAH feeding. The beans on the left were non-treated throughout the duration of the experiment, the beans on the right were keep free of hoppers. We also observed significant increases in green bean syndrome in both irrigated and dryland soybeans and observed significant differences in yield in dryland soybeans but not irrigated. The take home message is TCAH are economically important insects that have a documented, data backed economic threshold of 1 adult per sweep at pod set. If you have any questions or concerns feel free to contact your county agent or me for more information.
This edition covers early rice yields in southwest Louisiana, ratoon stubble management, sheath blight, AV-5055 headed rice bird repellent, LSU AgCenter seeks hybrid rice seed partner, and how to join the text message group. This edition and older editions can be found on the LSU AgCenter’s Rice website here: