Southern rootknot nematodes has been a challenge for Colquitt County cotton producers. According to Dr. Bob Kemerait, UGA Plant Pathologist, an estimated 60 to 70 percent of Georgia’s cotton fields are infested with at least one species of damaging nematodes. In a recent statewide survey of cotton fields (nearly 1800 samples were submitted by agents from randomly selected fields in 2002) approximately 69 percent of the fields were infested with root-knot nematodes, 2.8 percent with Columbia lance nematodes, 4.6 percent with reniform nematodes, and 0.6 percent with sting nematodes. During the 2020 year, root knot nematodes were noticed in a dryland cotton variety trial in Colquitt County. So we took this opportunity to learn more about this subject.
Nematode and cotton root samples were taken from this dryland variety trial after harvest. These samples were taken from plots containing 2 non resistant varieties (DPL 1646 and DPL 2038) and 3 nematode resistant cotton varieties (PHY 400, PHY 500, PHY 545). Each variety was replicated three times in this field. This project could not have been possible without Dr. Bob Kemerait, UGA Plant Pathologist. Below is an example of the rootgalling of a non resistant variety on the left and a resistant variety on the right.
Nematode samples were taken after harvest and the results are shown below. If you would like more information on sampling for nematodes and interpreting the results the UGA publication Guide for Interpreting Nematode Assay Results is an an excellent resource. According to the resource above, growers should consider treating for Southern rootknot nematode if those levels exceed 100 per 100 cc of soil at harvest.
Root samples were taken in each rep of all three cotton varieties and rated for galling. Below are the results from the samples.
What about yield? If you would like to look at the data from the variety trial at this location it can be seen below and its labelled as Colquitt DRY. If you would like to see other locations of the UGA On Farm Variety Trials please go to the link….
Below are some comments from Dr. Kemerait about the use of nematode resistant cotton varieties.
Question 1. If I have a nematode problem in my field, should I plant one of the nematode-resistant varieties?
Answer 1. As a grower you MUST consider this option (see further notes below). Before you make this decision, insure that the nematode problem in the field is caused by southern root-knot nematodes and not others, e.g. reniform, sting or Columbia lance.
Question 2. If I plant one of the root-knot nematode resistant varieties, do I still need to use a nematicide? Am I better off planting a “highest yielding variety” and treating with a nematicide?
Answer 2. The short answer is that these resistant varieties will certainly perform better than susceptible varieties in terms of decreased root-damage and reduced build-up of nematodes in the soil. This does not necessarily translate into increased yield. Recent data demonstrates that even the resistant varieties may benefit from use of a nematicide like Telone II when nematode populations are severe. However, growers who plant root-knot nematode resistant varieites are unlikely to see a benefit to treating the field with an additional nematicide (to include seed-treatment, Velum Total, AgLogic 15G or Telone II.)
Question 3. What is the value in planting a root-knot nematode resistant variety?
Answer 3. As compared to a susceptible variety, root-knot nematode resistant varieties will have less root galling and root damage and much lower populations of nematodes in the field at the end of the season. Planting a root-knot nematode resistant variety is almost like planting a non-host; nematode populations are greatly reduced for the coming season. Resistant varieties with two resistant genes (e.g., DP 1558 B2RF and PHY 487 WRF) have greater resistance to root-knot nematode varieties than do varieties with a single resistance gene (e.g., ST 4946 BRF).
Planting root-knot resistant varieties will a) decrease root damage which leads to better growth of the plants and b) reduced nematode populations which benefits the next time cotton is planted in the field. Root-knot nematode varieties will not always out-yield susceptible varieties; however resistant varieties are less likely to need the protection from nematicides.
Cotton varieties that have resistance to both southern root knot and renform nematodes are coming into the market place. We had the opportunity to evaluate PHY 443 W3FE in two on farm demonstrations last year. That info is posted below….
EXPO and Packer Park…
Hopefully this year more research will be conducted to evaluate the performance of these varieties.
If you have any more questions about managing root knot nematodes in cotton please contact your location county Extension agent.