In general the sandier a soil is the more damage nematodes will cause. This is due to the moisture stress already present in the soil. Nematodes prune the root system making it difficult for the plant to take up the moisture which is already in short supply. 

Results from a test by Scott Monfort, when he worked in Arkansas, give a good example of this principle. 

In his test a cotton crop grown in a soil with 40 percent sand could sustain a two bale cotton crop even when 2,000 Southern root-knot nematodes per 100 ml. of soil were present. In the area of the field with 60 percent sand just 200 nematodes per 100 ml. of soil would significantly reduce yield. 

Monfort is currently Extension peanut specialist at Clemson University and works with Mueller at the Edisto Agricultural Research and Extension Center in Blackville, S.C.

Irrigation is another factor that affects nematode damage thresholds. The combination of scenarios is almost endless when you add irrigation into the mix, Mueller says.

Variables aside, the general rule of thumb is to classify nematode populations in a field as either above or below the damage threshold levels.  

“If they are above the damage threshold we would estimate at least a 10 percent yield loss,” Mueller says 

“There is a second threshold above which we would expect a yield loss of 20 percent or more. In fields which exceed the low threshold we have recommended low rates of Temik 15G in the past. 

“With a shortage of Temik 15G we now recommend the seed treatments. For populations above the high threshold we have recommended combinations of seed treatments plus Temik 15G or using Telone II. 

“In some cases where nematode populations are too high to control or yield potentials for a field are low we strongly recommend to growers they rotate to a non-host crop to get the nematode situation under control, Mueller says.  

For Southern root-knot nematodes it usually takes at least 100 nematodes per 100 milliliters of soil to cause a yield loss. 

At populations above 250 per 100 milliliters of soil yield losses could exceed 20 percent. 

For Columbia lance nematodes, the low threshold is 80 and the high threshold is 175 per 100 milliliters of soil.  

For reniform nematodes, the low threshold is 250 and the high 625 or more.

For sting nematodes 1 per sample is the standard threshold.

“A deep, dark secret of nematology is that these numbers aren’t plus or minus five percent — maybe 20 percent or more depending upon when the sample was taken and what levels of stress are present in a field,” Mueller says.

“To be more accurate, individual growers should use their knowledge of individual fields to adjust thresholds,” he adds.

“When I see a high threshold, I think the grower is looking at 25 percent or more yield loss. In these fields we can justify the $40-50 per acre for Telone II that is needed to make a profitable crop,” he says.

Nematodes are an easy-to-miss killer of crop yields and the damage they cause is even easier to dismiss and underestimate.

The key to manage these tiny yield killers is knowledge: Know what specie is in your field and sample to determine how many, then make an informed decision as to how to most profitably manage the problem.

(The University of Georgia's Bob Kemerait has a plan for reducing nematode damage in cotton in 2012. To hear what he has to say, visit http://southeastfarmpress.com/cotton/farm-bob-kemerait-talks-nematode-control-cotton-2012).

 

rroberson@farmpress.com