Disease resistance is the ability of a plant to withstand attack from disease-causing organisms such as bacteria, fungi, or viruses. The extent of resistance can vary from being strongly resistant to infection to being only somewhat more tolerant of the disease than standard varieties. Resistance is not immunity. Improper culture of a resistant variety may negate that resistance.
Plant breeders have a tough job breeding disease resistance into crops because there are so many diseases and often several strains of a given disease. What is often done is to select the disease that causes the most problems and work on breeding resistance to that disease. Seed catalogs and packets indicate what, if any, disease resistance a variety has in descriptive text or with initials following the variety name. Disease resistance in tomatoes indicated by initials include: V - Verticillium wilt, F - Fusarium wilt (F1, race 1; F2, race 2), N - nematode, T - tobacco mosaic virus, A - Alternaria alternata (crown wilt disease), and L - Septoria leafspot.
