Southeast cotton producers are reminded that harvest is the ideal time for nematode sampling in their fields. Recent surveys have shown the increasing severity of nematode problems in some areas of the region.
In a University of Georgia random cotton nematode survey conducted this past year, 779 of 976 samples submitted, or nearly 80 percent, contained parasitic nematodes. More importantly, parasitic nematodes were above threshold values in 34 percent of the samples.
Populations of parasitic nematodes on cotton will be at their largest numbers at harvest, according to Bob Kemerait, University of Georgia Extension plant pathologist and Cliff Brewer of the University of Georgia Nematology Lab.
“Growers who wish to identify fields where nematodes are adversely affecting their crop should pull nematode samples prior to mid-December and certainly before the first freeze of the season,” say the specialists.
Identifying elevated levels of nematodes now will allow growers to make management decisions for 2005, they add. Based on the types of nematodes recovered and the population size, growers will have information that can be used for crop rotation decisions and for the use of nematicides. The following are a few reminders on how to get the best results from a nematode sample:
1) Insure that soil samples are obtained in the root zone of the cotton plants, not in the row middles. The largest number of nematodes will be associated with the root system.
2) Pull multiple soil samples and, once finished with the collection, mix the multiple cores together to insure a representative sample (only 1 pint needs to be shipped for each combined sample, so make sure mixing is thorough).
3) In a field with obvious trouble spots, it is recommended to submit separate samples from the “good” areas and the “bad” areas in order to get a better understanding of the population fluctuations in the field.
4) Consider including root specimens in the sample. This can be very important, especially if soil samples come back with low populations of parasitic nematodes, though symptoms in the field are evident. There are times when fewer nematodes are recovered from a soil sample than the county agent, grower or consultant expects. In such situations, it can be very beneficial to also have a sample of the infected roots. These root samples can be assessed in the diagnostic lab, much like soil samples. Obtaining good soil samples has an element of “hit or miss” — recovery from symptomatic roots can be much easier.
5) Remember that nematodes are living organisms. Think of your soil sample as you would a container of red wiggler fishing worms. If they get too hot in the sun while locked in your truck, they will perish, and the sample will be of little benefit.
6) Ship the samples as quickly as possible to the nematode assay lab of your choice. If your sample will be delayed in shipment, it would be wise to store it in the refrigerator, or at least in a very cool place, as this will improve the survival of the nematodes.
Occasionally, soil samples are sent from a field where parasitic nematodes are very likely causing damage, but the results do not indicate a problem, say the nematode specialists.
“First and foremost, it must be remembered that assaying soil samples for nematodes is part science, part art and part luck,” they say.
Assuming that the samples are collected and handled properly, there is always the possibility that two samples collected from the same area of the field will give different nematode counts due to a variation in the populations of the nematodes.
“Also, in some instances, galling may be observed on the root system. However, few nematodes have migrated from the roots into the surrounding soil. And, of course, there is always the chance that a mistake is made somewhere in the process of sampling, shipping, storing, extracting, assaying and recording results which results in a sample with few recorded nematodes.”
To minimize the risk of under-reporting the populations of parasitic nematodes, it's recommended that growers consider the following:
1) Follow the recommendations for collecting nematode samples.
2) Include root samples with the soil samples. You don't need entire root samples, as portions of symptomatic roots in the soil work fine.
3) If for any reason you doubt the results from a sample you have submitted, re-sample and send the soil to the lab again. The goal of the University of Georgia's Nematology Lab is to provide an excellent service and accurate results. The lab will confirm results from production fields.