This is called the disease triangle. All three parts of the triangle must be present for the disease to occur. In order to control a disease, it is necessary to remove one of these three parts. Plant disease controls include crop resistance, cultural and physical methods, pesticides, and regulation. Therefore, one does not have to worry that the black spot on roses will next appear on the garden tomatoes.
Plant breeders strive to select varieties of host plants that have resistance to plant diseases.
Many hybrids of vegetable crops, ornamentals, etc. have been developed that have some level of crop resistance. Merely choosing and planting these hybrids will cut down disease problems. There are also varieties that show more tolerance for a disease. The plant may seem to have as many symptoms as the plant that is not tolerant, but somehow will still thrive and produce. There are cultural methods that can be used to help control disease.