In general a two to three year crop rotation schedule away from a crop and its relatives, will significantly reduce inoculum levels. Some of the bacterial diseases may only require a one year rotation, while some of the more persistant fungal diseases may require rotations of five years or more. Nor will crop rotation control pathogens such as Fusarium oxysporum that can survive in the soil for long periods in the absence of a susceptible host. Such diseases often can be controlled by using only disease-free or treated seeds or transplants, in combination with crop rotation. Resistance does not necessarily mean immunity to a disease.
Effectiveness can range from reduced rates of disease development and spread to complete immunity.
So growers should not be surprised if some disease develops on what is labled a resistant variety. Chances are that it has less disease on it than a susceptible variety. Growers should also be aware that for some pathogens several races exist, and the variety may be resistant to some of the races, but susceptible to others. In some cases the effectiveness of the resistance may depend on other factors such as the amount of inoculum present or the favorableness of the environment for disease development. Unfortunately, resistance is not available for all the major diseases on all vegetable crops, and a variety that is resistant to one disease may be susceptible to another.