Soybean Variety Response to Taproot Decline (TRD)

Paul Price, Purvis, Myra, Padgett, Guy B.  |  4/9/2019 6:59:21 PM

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.

Figure 1 Interveinal chlorosis and necrosis.jpg thumbnail

Figure 1. Interveinal chlorosis and necrosis.

Figure 2 Taproot decline of seedling.jpg thumbnail

Figure 2. Taproot decline of seedling.

Figure 3 Plant death caused by taproot decline.jpg thumbnail

Figure 3. Plant death caused by taproot decline.

Figure 4 Blackened root diagnostic of taproot decline adjacent to infested debris.jpg thumbnail

Figure 4. Blackened root diagnostic of taproot decline adjacent to infested debris.

Figure 5 Dead mans fingers produced by Xylariajpg

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.


Figure 5. “Dead man’s fingers” produced by Xylaria sp., causal agent of taproot decline.

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
Osage 8.391702
CZ 4818LL 18.879462
5N490R2 19.263012
S42RY77 20.944016
5N433R2 22.215409
5067 LL 22.559704
R07-6614RR 22.970824


Table 2. List of varieties moderately resistant to TRD as determined by inoculation and response.

Variety % Root Weight Reduction
Armor 55-R68 25.253945
RJS47016R 25.793535
CZ 5375RY 26.205598
HBKLL4953 27.339808
4880 RR 27.926596
P5752RY 28.094408
CZ 5225LL 28.605468
ARX4906 29.805397
Go Soy Ireane 30.762175
4995 RR 30.883269
AG 48X7 31.611326
P4788RY 32.46393
AG 46X6 34.502577
S47RY13 35.157094
5625 RR2 35.190462
S49XT07 35.483918


Table 3. List of varieties moderately susceptible to TRD as determined by inoculation and response.

Variety % Root Weight Reduction
P4814LLS 36.6288
CZ 4105LL 36.631044
GS48R216 37.120729
REV 57R21 37.152585
CZ 4222LL 37.789292
S49LL34 39.360691
P54T94R 39.928806
S12-2418 40.28502
S52RY77 40.607899
REV 51A56 40.734935
P41T33R 41.997581
S11-17025 43.578124
4967 LL
43.925284
S47-K5 43.984519
Armor 46-D08 44.015611
Armor 48-D24 44.107678
Go Soy 5115LL 44.470801
Armor 48-D80 45.47956
REV 56R63 45.566353
REV 49R94 45.659963
Rev 49L49 45.896947
S43RY95 46.122564
5N480R2 46.84488
5N406R2 47.288423
P4588RY 47.58291

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.

For more information on these topics or others, please contact your local extension agent, specialist, nearest research station, or visit www.lsuagcenter.com or www.louisianacrops.com.

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