Patrick D. Colyer and Philip R.Vernon

Fusarium wilt and the root-knot nematode are both serious diseases of cotton that cause substantial losses across the Cotton Belt. Both pathogens are common in most cotton-producing areas and often inhabit the same fields. These two pathogens often infect cotton simultaneously, forming a complex that increases the incidence and severity of Fusarium wilt. The Fusarium wilt/root-knot nematode complex is one of the most widely recognized and economically important disease complexes in the world.

The symptoms caused by the complex are the same as those produced by the pathogens individually. Although cotton seedlings infected by the Fusarium wilt pathogen may be killed, most symptoms appear near mid-season. The symptoms of Fusarium wilt on older plants include wilting and chlorosis (yellowing) followed by necrosis (brown, dead tissue) of the foliage (Figure 1) and overall stunting of the plant. The vascular system of infected plants is discolored and readily visible when the stem is cut (Figure 2). Often, infected plants mature earlier and have fewer bolls and reduced seedcotton yield. Severe infection can kill plants. Research has demonstrated that plants may be infected with Fusarium wilt, but the only symptom observed is vascular discoloration.

Above-ground symptoms of the root-knot nematode are not as obvious, but include stunting and yellowing or reddening of the foliage. Infected plants appear to be suffering from nutritional deficiency. The most distinctive symptom of the root-knot nematode infection is the formation of galls on the roots (Figure 3).

Difficult to Manage

Management of both diseases is difficult. Crop rotation, host resistance and the application of nematicides are considered the best approaches to managing these diseases individually or together. Crop rotation is often recommended to reduce the incidence of Fusarium wilt, but the ability of the fungus to survive in the soil for long periods in the absence of cotton limits the effectiveness of rotations. Furthermore, long rotations are not economically feasible for most cotton growers. Because of the ability of the nematode to increase the incidence of wilt, rotations designed to reduce nematode populations may be successful in reducing the incidence of wilt.

Research has demonstrated that root-knot nematode-resistant soybeans, grain sorghum and peanuts are not hosts for the root-knot nematode and are good rotation crops for managing soil populations of the root-knot nematode. Most grasses and legumes used as winter cover are also susceptible to root knot nematode; however, since they are grown during periods of low soil temperatures, they are not conducive to nematode growth and infection. Because many weeds are hosts, fallowing is not effective unless weeds are controlled.

Tillage Impact Unclear

The impact of tillage on the disease complex is not clear. Tillage is thought to have little effect on the wilt pathogen because of its ability to survive in the soil for extended periods. Tillage may disturb the root-knot nematode and expose it to mortality, but it may also spread the nematode inoculum. A study was conducted on a field with a history of the disease complex in Bradley, Ark., to compare reduced tillage with conventional tillage following a winter fallow or the winter cover crops, hairy vetch and common vetch, on the incidence and severity of the complex. Incidence of wilt was not affected by tillage or winter cover. The severity of root-knot nematode was not affected by winter cover but was higher with reduced tillage. Although wilt was not increased in this study, the increase in the severity of root galling by the nematode associated with reduced tillage could lead to an increase in Fusarium wilt.

Nematicides

Soil fumigation, which may affect both fungal and nematode survival in the soil depending on the fumigant used, has successfully reduced the incidence of wilt. Fumigation, however, is expensive and may not be economical for most cotton production areas. The ability of nonfumigant nematicides, like aldicarb, that are applied in the furrow at planting to reduce nematode populations and root galling, has been demonstrated widely.

In a study at the Red River Research Station, the effect of aldicarb (Temik 15G) on severity of the disease complex in eight cotton cultivars with different levels of resistance to the disease complex was tested in 1994 and 1995. Disease severity for the root-knot nematode was determined by rating galling on the roots and for Fusarium wilt by rating stem discoloration. Across all cultivars, the application of aldicarb reduced root galling and stem discoloration. These differences resulted in increased seed cotton yield and lint percentage.

Host Resistance

The use of host resistance to manage the complex has been moderately successful. Although there are no wilt-immune cultivars, several commercial cultivars have moderate to high levels of wilt resistance. They should be planted in fields with a history of Fusarium wilt. Because the root-knot nematode increases the incidence of wilt, and infection by the nematode can increase the susceptibility of cultivars that are normally resistant, planting cotton cultivars with resistance to the nematode will help reduce the incidence of wilt. Regardless, wilt-resistant cultivars have a lower incidence of wilt than susceptible cultivars in the presence of the nematode. Unfortunately, only moderate resistance to the root-knot nematode is available in commercial cultivars.

Cultivars are evaluated for resistance to the disease complex annually at the Red River Research Station in a field plot with uniformly high levels of the root-knot nematode and the wilt pathogen. The results demonstrate the low level of root-knot nematode resistance available in cotton cultivars. Only three cultivars with acceptable levels of resistance to the complex have been identified: Stoneville LA 887, Paymaster 1560 and Acala Nemx. Stoneville LA 887 and Paymaster 1560 were developed in Louisiana and are well adapted to our growing conditions, but Acala Nemx was developed in California and is not adapted to the Mid-South. Based on results of these annual evaluations, it has also been determined that the transgenic relatives of Stoneville LA 887 and Paymaster 1560 cultivars do not react like their nontransgenic parents and are more susceptible to the disease complex. Apparently, the resistance to the complex was reduced during the development of these transgenic lines. In summary, management of the Fusarium wilt/root-knot nematode complex remains difficult. Resistance would be the simplest management strategy, but, until cultivars with higher levels of resistance are available, tillage, crop rotation and the application of nematicides are alternatives. These methods are more effective at reducing the infection by the root-knot nematode than controlling the wilt pathogen. An integrated approach that includes all or most of these management options is the best strategy.

This article was published in the spring 2003 issue of Louisiana Agriculture.