Agronomic Services — News ReleaseFOR IMMEDIATE RELEASE
FRIDAY, JULY 5, 2002
Contact: Jack Imbriani
NCDA&CS helps farmers manage cotton’s
RALEIGH — Does your cotton look stunted, discolored or wilted? Are there bare spots in the field and lots of weeds? Such poor growth could be due to any number of problems, but one of the more likely causes is nematodes.
hidden enemies — nematodes
Since cotton has made a comeback in North Carolina, it is common to find fields that have been in cotton production for as many as six to eight consecutive years. Growers don’t have to worry as much about boll weevils any more because of the extensive trapping done statewide. However, many farmers seem to have forgetton that nematodes can cause significant damage as well.
“About 20 percent of the samples we receive from cotton fields contain hazardous levels of nematodes,” said Jack Imbriani of the N.C. Department of Agriculture and Consumer Services’ Nematode Assay Section. “Nematodes are especially a problem on sandy soils. I would advise all cotton producers in the eastern part of the state to take samples annually and remember to rotate crops because many nematodes can be kept at manageable levels by a good rotation.”
Nematodes are sometimes called “hidden enemies” of cotton production. By damaging roots, they interfere with nutrient and water uptake. Damage is usually not obvious right away.
As nematode damage progresses, plants may exhibit nutrient and moisture deficiencies. A nematode-damaged root system will not take up water or nutrients from the soil even when sufficient amounts are present. However, when plants wilt or become discolored, it is not automatic to think of nematodes as a possible cause.
Identifying nematode problems in cotton is difficult because there is no single, easily recognizable symptom. General problems like wilting, discoloration, stunting, patchy growth or weed prevalence can all indicate a nematode infestation but none of these symptoms are definitive. Although root-knot nematodes produce galls on roots, the galls may be relatively small and few in number so they are easily overlooked.
In most cases, soil and root samples must be taken and tests conducted to prove that nematodes are the source of the problem. Proactive growers collect samples for nematode assay prior to planting so they can make management decisions based on results. If nematode problems arise after the crop is planted, it will be too late to correct them.
For this reason, some growers use nematicides every year without knowing for sure if populations in their fields are hazardous. This practice is both financially and environmentally unsound. Routine application is expensive and may promote build-up of nematode resistance.
For $2 per sample, the NCDA&CS Agronomic Division can test for the presence of nematodes. The lab generates a report that lists the species of plant-parasitic nematodes present, their approximate populations, the degree of hazard they pose to the specified crop, and best management practices, if necessary. “This is the basic information needed to manage nematode populations adequately,” Imbriani said.
Cotton is damaged by at least four different kinds of nematodes. Root-knot, sting, Columbia lance and reniform nematodes are the best-known threats to cotton in North Carolina. However, data from NCDA&CS Agronomic Division indicates that two other kinds of nematodes—lesion and stubby root—may also be causing significant damage.
Lesion and stubby root nematodes are present in about 60 percent of the state’s cotton fields and reach potentially damaging population levels in about 8 percent of those. Research is needed to study the significance of these nematodes in cotton production and the population levels required for damage.
Root-knot nematode is not as prevalent in cotton fields as stubby root and lesion are, but it is more likely to cause damage. The NCDA&CS nematode assay lab finds root knot in 55 percent of all samples taken from cotton fields. Of the fields infested, more than a third have populations high enough to cause damage.
In North Carolina, the number of cotton fields with moderate to high root-knot population levels has increased from 11 to 17 percent since 1991. Monoculture and lack of resistant cotton varieties are probably reasons for this increase. Root-knot occurs throughout the state.
Columbia lance, sting and reniform nematodes are less common than root knot but tend to cause severe problems wherever they do occur. These nematodes are primarily problems in very sandy soils, but the reniform nematode can damage cotton on heavier soils as well. At this time, Columbia lance and reniform nematodes occur only in the southeastern quarter of the state.
Persons interested in collecting and submitting samples for nematode assay can find instructions at the Web site: www.ncagr.com/agronomi/pdffiles/samnemas.pdf. During the growing season, you can collect and submit a sample any time you suspect a problem. If you don’t see a problem, it is still a good idea to take a routine sample immediately after crop harvest in the fall—when nematode populations are likely to be at their highest. For further information, call the NCDA&CS Agronomic Division at (919) 733-2655 or contact your county Cooperative Extension office.