December 5, 2022
Diseases and nematodes will cause headaches for row crop growers in 2023. That is not speculation. That is fact, as in the words of Justin Wilson, “I gar-own-tee!”
Effective tactics to minimize losses to diseases and nematodes, both in terms of yield and expense, require planning ahead of planting. My greatest challenge in getting growers to adopt these tactics has less to do with “efficacy” and more to do with instilling faith that “more” isn’t always “better”.
“What is it?” “When should I spray?” “What should I spray?” These are the questions growers funnel through county agents to me. While the urgency is obvious, “best” strategies for management begin long before the first symptoms are observed or a fungicide program is initiated.
Nematodes cause significant losses each year in many crops. Tactics to reduce damage from nematodes include rotation to non-host crops, planting nematode-resistant varieties, and use appropriate nematicides at effective rates. To make the BEST management decisions requires that a grower knows at least two things.
First, you need to know the types of nematodes found in your field; second you need to know the size of the nematode population. Cotton can be severely affected by southern root-knot, sting, and reniform nematodes. However the host range varies between these nematodes as do the economic thresholds.
Corn is a host for the southern root-knot nematode but is not a host for reniform nematodes. The economic threshold for reniform nematodes is 250 juveniles per 100 cc of soil, while for southern root-knot and sting nematodes, the thresholds are 100 and 1, respectively. Pulling soil samples for nematode analysis soon after harvest helps a grower better determine management strategies for 2023.
Nematodes are often unevenly distributed in a field. Root-knot and sting nematodes are more likely to be found in sandier areas while reniform nematodes may occur where soils have sand and silt. Nematicides, e.g. Telone II, AgLogic, Counter 20G, Velum, and Vydate-CLV, are applied uniformly across a field. Use of nematicides will increase profitability, but will also increase production costs. When applying Telone II, some growers use yield maps from previous seasons to decide placement of the fumigant. Growers can optimize the use of nematicides by creating zone maps that include some assessment of soil texture. A Veris soil sampling rig measures soil conductivity which can be correlated with soil particle size. Maps created in pat on soil conductivity allow growers focus their efforts in sampling for nematodes. Specifically applying nematicide like Telone II to higher risk zones and not to lower zones reduces production costs for the grower.
Tomato spotted wilt was a significant problem for peanut growers in 2022. Use of Peanut Rx will not eliminate spotted wilt, but it will help growers to reduce the risk to this disease. Amending production practices according to Peanut Rx allows grower to reduce risk to leaf spot and white mold as well. Reducing risk to white mold and leaf spot from “high” to “low” safely allows a reduction in the number of fungicides applied during the season by extending the interval between applications. Several companies, to include Syngenta, Bayer Crop Science, CORTEVA, Valent, FMC, BASF, and NICHINO, offer “prescription programs” where the recommended number of applications varies based upon risk levels as determined using Peanut Rx. For more information see www.peanutrx.org.
Sentinel plots and maps
Southern corn rust and Asian soybean rust can cause serious yield loss; well-timed fungicide applications protect yield. Because rust diseases typically do not overwinter in the Southeast and must be reintroduced, the need for fungicides and the timing of such applications varies from year to year. Monitoring for these diseases occurs annually in Mississippi and in Alabama, and in Georgia, where sentinel plots are funded by the Georgia Commodity Commissions for Soybeans and for Corn. Growers are encouraged to follow reports of these diseases to make best decisions for timing of fungicide applications. Reports of southern corn rust and soybean rust can be found at https://corn.ipmpipe.org/southerncornrust/ and https://soybean.ipmpipe.org/soybeanrust/.
Winter climate has significant impact on the risk for damage from nematodes and diseases in the following season. A colder and wetter winter is beneficial for growers. Freezing temperatures, the earlier the better, kill cotton roots that can sustain reproduction of plant parasitic nematodes. Activity of plant parasitic nematodes slows and eventually stops at soil temperatures plummet.
Freezing temperatures can also kill “volunteers” bridging diseases from one year into the next. Wetter weather hastens rot of the crop debris that harbors pathogens. Unfortunately, the winter of 2022-223 is predicted to be the third consecutive “La Niña”; La Niña winters in the southeastern United States are typically warmer and drier than normal. “Warmer and drier” will increase the threat of disease and nematode problems next year.
The most effective disease and nematode management programs require planning that begins before seed is planted and before fungicides and nematicides are applied. Making informed decisions, especially with use of risk maps and prescription programs, can be daunting for a grower. It is at times like this that working with consultants can be very beneficial. To echo Justin Wilson, “I gar-own-tee!”
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