Resistant varieties

Certain tobacco varieties resistant to nematodes are grown in some countries. Their degree of resistance to different species of Meloidogyne, and to some races in particular, is not identical.
 
Nicotiana tomentosa is a source of resistance to M. Incognita races 1 and 3. However, this resistance can be overcome in some cases, by races 2 and 4 of M. incognita and M. javanica and M. arenaria. Numerous American varieties possess this resistance. It has been observed that this resistance became ineffective in high temperatures (30-35 ° C). Resistant plants become highly necrotic in the presence of a particular strain of PVY.
 
Several other sources of resistance are reported in Nicotiana repanda, Nicotiana longiflora and local Zimbabwean cultivars. In some situations, this resistance contributes to control of M. javanica but not M. arenaria or race 3 of M. incognita (Zimbabwe). On the other hand, N. glauca is immune to M. javanica. The cross between N. repanda and N. tabacum does not seem easy to accomplish. Resistance genes present in N. repanda and N. longiflora were transferred after resolving some difficulties in breeding into the Virginia type, and more recently into Burley.
 
A third source of resistance to Meloidogyne is present in Nicotiana otophora acc. La Quinta. Race 4 of M. incognita can be controlled in certain situations. But no source of resistance against race 4 is known in N. tabacum. A program of interspecific crosses would be necessary.
 
Race 1 of M. incognita is certainly the most widely encountered race. In the U.S, there are also three other races 2, 3, and 4. Race 3 seems more common than race 1 in Zimbabwe, while in South Africa mainly races 2 and 4 are present. These few examples probably make one realise that the races of M. incognita are different depending on the country. In case one would like to use a resistant variety to control root-knot nematodes, it is very important to know what species and race of Meloidogyne one is dealing with.