Nematodes in Cotton

Root-Knot Nematode

Pathogen and Range

Three root-knot nematode (RKN) species damage cotton, the southern root-knot nematode (Meloidogyne incognita), Javanese root-knot nematode (M. javanica), and guava root-knot nematode (M. enterolobii). Root-knot nematodes infect a wide range of plant hosts, including many potential crop rotation options for cotton growers. Soybean, corn, tobacco, sorghum, sweet potato, several vegetable crops, and multiple weed species can be potential hosts for RKN, complicating management options.

Root-knot nematodes live in the soil and feed on young roots. Females live inside the roots and produce egg masses that range from 500 to 3,000 eggs. The eggs hatch under moist conditions, and then juveniles spread through soil water and enter root tips. A single root tip can contain as many as 20 juveniles. Damage to the roots results from chemical changes caused by RKN feeding that can enlarge root cells up to ten times their normal size. These large cells prevent the root from developing and taking up the water and nutrients the growing plant needs. The damaged and enlarged roots form galls that resemble knots. During the growing season, the RKN lifecycle completes every four to six weeks. Root-knot nematode development is greatly slowed or stopped when soil temperatures are below 50 ˚F or above 100 ˚F.^1,2

Root-knot nematode (RKN) infested areas of a field are not uniformly distributed, and cotton plants growing in these areas may be chlorotic and stunted (Figure 1). Infected plants can wilt by midday due to the damaged roots’ inability to take up water, and damage to the roots can also lead to secondary infection by bacteria or fungi. Root-knot nematodes produce visible galls on the roots of cotton plants. Symptomatic plants can be carefully dug up and the soil gently removed to reveal galls on infected roots (Figure 2).^1,2

Diagnostic Notes

Root-knot nematode populations are often lowest at planting and highest at cotton plant maturity. Survivability can be very low during the winter, with up to 99% of RKN individuals unable to survive freezing conditions. However, their ability to reproduce during warm conditions can increase the RKN population more than 100 times between planting and harvest.²

Root-knot nematode populations are more commonly found in coarse-textured, sandy soils.¹ RKN may be found in the same fields as Columbia lance nematode, but it is usually not found with reniform nematodes unless corn is used in the crop rotation sequence.³

Fields with RKN can increase the incidence and severity of Fusarium wilt, if the disease is present. In general, fields without RKN require 100 times more inoculum of the Fusarium pathogen to cause the same damage as fields with RKN. The Fusarium wilt/root-knot disease complex often leads to large areas of dead plants, which normally would not occur with either Fusarium wilt or RKN alone. The severity of some root rots may also be higher in the presence of RKN.²

Potential Yield Loss and Control

Cotton plants can tolerate a small amount of RKN damage without negatively impacting yield. However, this tolerance is related to the availability of nutrients and water, as well as exposure to insects and diseases.

Soil sampling for nematodes should be completed around harvest, when nematode populations are typically at their highest. The following article can provide more information on the procedure for sampling for nematodes in cotton, Cotton Nematode Predictive Sampling and Prevention.

Nematodes are present in the soil of infested fields and can be disseminated to non-infested fields by soil on equipment, trucks, or shoes. Sanitation is important to prevent the spread of nematodes to other fields. If possible, work in fields known to have nematodes last.¹

Root-knot nematodes can be managed by planting a RKN-resistant cotton variety, through crop rotation, and with the application of nematicides. Root-knot nematode-resistant varieties provide protection throughout the season. Fields can also be rotated to a non-host crop like peanut or a soybean product that is resistant to RKN. Nematicides are additionally available and should be applied pre-plant or at planting.