Damage caused by nematodes are difficult to estimate because damage to roots may not be apparent in above ground symptoms, yet significant reductions in yields can occur with moderate levels of nematodes. Nematodes may increase the incidence of other diseases such as black shank, bacterial wilt and Fusarium wilt. The reduced use of fumigants during wet springs always results in dramatic increases in nematode damage and demonstrates the importance of soil fumigation!
The most important nematodes on tobacco are the root-knot nematodes. The most prevalent is the southern root-knot nematode, Meloidogyne incognita. However, another species (M. arenaria) also infests some fields. Meloidogyne arenaria (sometimes called peanut root-knot) is important because it is very damaging to tobacco and there is presently no resistance to this pest. Varieties that are resistant to the southern root-knot (M. incognita) are not resistant to M. arenaria. However, rotation is effective for both root-knot species and again should provide the basis for management of nematodes. If you notice gall development on root-knot resistant varieties, you should have the nematode identified. Your Extension agent can assist you with the details for this determination. Surveys indicate nearly 1/3 of sampled tobacco fields contain populations of root-knot nematodes (such as the peanut root-knot nematode) that will produce galls on resistant cultivars.
Nematicides may also be effective in reducing nematode numbers in soil. It is best to base the control strategy on rotation, with use of resistant varieties when appropriate and nematicide treatments to supplement the rotation strategy. If rotation cannot be practiced, or only short rotations (1 year) are utilized, the use of nematicides and resistance becomes essential. Combining rotation, resistant varieties, and nematicides or fumigants is the best control practice. The following table illustrates the effect of rotation on root-knot nematodes. The test was conducted to demonstrate the effect of rotation on relative populations of M. incognita and the more damaging M. arenaria nematodes in a field initially infested with 50% M. incognita and 50% M. arenaria. Note that cotton and corn favor shifts to the less virulent M. incognita, which can be managed with resistance and chemicals.
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