Disease-resistant and tolerant varieties are the cheapest, easiest, and most efficient way to reduce disease losses.
Varieties should be selected that possess resistance or tolerance to one or more disease organisms. For some diseases, such as the soilborne vascular wilts and the viruses, the use of resistant varieties is the only means of ensuring control.
Certified marijuana seed of resistant varieties is available and sold commercially. The use of varieties of plants resistant to particular diseases has proved to be very effective, i.e., stem rust of wheat, rust of dry bean, and Rhizoctonia root rot of sugar beet. Most plant breeding is done for the development of varieties that produce greater yields of better quality.
When such varieties become available, they are then tested for resistance against some of the most important pathogens present in the area where the variety is developed and where it is expected to be cultivated. If the variety is resistant to these pathogens for that area, it may be released to the growers for immediate production.
If, however, it is susceptible to one or more of these pathogens, the variety is usually discarded, or sometimes it is released for production if the pathogen can be controlled by other means, e.g., chemical, but more often it is subjected to further breeding in an attempt to incorporate into the variety genes that would make it resistant to pathogens without changing any of its desirable characteristics.
There are degrees of resistance to certain diseases, some varieties being completely immune, others partially susceptible. Resistant varieties may become susceptible to new races of a pathogen, i.e., dry bean varieties Beryl and Olathe were resistant to rust races present at the time of their release, but are now susceptible to new rust races.