Resistant varieties are an important cultural disease management practice to include in a disease management program. Varieties have been bred with resistance to many important diseases caused by different types of pathogens (fungi, oomycetes, bacteria, viruses, nematodes), but unfortunately not all. Conventionally breeding a variety with resistance necessitates having a source of resistance which is often a wild plant relative. There are important facts to know about resistant varieties to ensure obtaining the degree of control of the targeted disease needed to obtain desired yield and fruit quality. It is important to recognize that resistant varieties should not be relied upon as the sole management practice for any disease. For more information about resistant varieties, look for university evaluations and ask the seed distributor.
Important facts about resistant varieties
Resistance is rarely complete (immunity).
Typically some symptoms develop, especially when conditions are highly favorable for disease development.
Resistant varieties vary in their ability to suppress disease development.
There is variation among varieties bred to be resistant to different diseases, variation among different crop types bred to be resistant to the same disease, and also variation among varieties bred to be resistant to the same disease. Two terms used in seed catalogues to describe some varieties are high resistance (HR), which is used for varieties that highly restrict pathogen infection and development, and intermediate resistance (IR). There will be greater need to include fungicides in a disease management program with resistant varieties that have IR than HR.
The cucurbit crop group provides examples of these variations. Resistance to powdery mildew varies among the cucurbit crop types. It is so good in resistant cucumber varieties that symptoms typically do not develop and crop susceptibility to this disease may be forgotten until an old variety lacking this resistance is grown. Squash and pumpkin varieties marketed as powdery mildew resistant are usually described in seed catalogues as having intermediate resistance and they have exhibited limited suppression in variety evaluations. Varieties with resistance from both parents (homozygous resistance) typically exhibit better suppression than those with resistance from one parent (heterozygous), which are more common. Resistance to powdery mildew in cantaloupe is highly effective but race specific, which means the resistance genes bred into cantaloupe are for specific races of the powdery mildew pathogen. If a race is present that the variety does not have resistance for, it may become as severely affected as a variety with no resistance. Ability to suppress downy mildew varies greatly among cucumber varieties.
Grow resistant varieties separate from susceptible varieties.
This is especially important with varieties that have intermediate resistance because the resistance can be overwhelmed by the amount of inoculum from adjacent varieties lacking resistance when the disease is not well managed on them. An example is Septoria leaf spot in tomatoes. Very few varieties are currently available with resistance to this common disease in the northeastern U.S. where many growers who direct market often grow several tomato varieties with diversity of fruit type to satisfy their customers.
Pathogens can evolve to overcome plant resistance.
This is mostly a concern with race-specific resistance. A new race is designated each time the pathogen evolves such that it can infect a resistant variety. With race-specific resistance, the plant was bred to have the ability to recognize the pathogen, typically a protein (called an effector) produced by the pathogen to enable it to infect a susceptible variety without being detected. A simple genetic change in this protein may be sufficient to enable the pathogen to infect a resistant variety. Examples of this type of resistance are bacterial spot of pepper and downy mildew of spinach. Many races of both pathogens have evolved. Races are designated with a number. Race 0 refers to the pathogen that can only infect susceptible varieties.
Sometimes changes in the pathogen affecting resistance occur less frequently and they can be associated with new pathogen genotypes (strains) being brought into the U.S. Beginning in 2004, downy mildew developed, often becoming quite severe, on cucumber varieties that had been providing very good suppression of this disease for many years due to a new pathogen genotype.
Resistant varieties are best used as a component of a management program.
This is because of the previous facts. The management program for all diseases should include other appropriate cultural practices, such as rotation, as well as fungicides. Also, plants should be examined for symptoms weekly to determine need for fungicides and to assess the overall efficacy of the program.
Additional information about resistant varieties
- Almost all resistant varieties that are commercially available today were developed through conventionally breeding. There are a few summer and zucchini squash varieties genetically engineered to resist viruses. One method used to develop these resistant varieties is to engineer the plant to produce the protein that encapsulates the virus particle, which the virus removes when it enters a host plant to replicate. Virus protein produced by the resistant plant re-encapsulates the virus particle, thus it cannot replicate. Scientists have developed resistant varieties by using genetic engineering techniques to move resistance genes between plants that cannot be conventionally crossed, for example, this was done to develop a tomato with bacterial spot resistance from pepper. Scientists are now using new knowledge about the genes involved in the plant-pathogen interaction during infection and new technology, in particular CRISPR-Cas9 gene editing, to develop plants with resistance. One approach is to turn off (silence, knock out) the Susceptiblity gene that some plants have been found to possess.
- Fungicides that have as a mode of action inducing resistance in treated plants may have greatest activity with a variety that has intermediate resistance. These include Actigard and several organic biopesticides: LifeGard and many others with a Bacillus species as the active ingredient. This is based on limited research results to date and the idea that a plant with some genetic resistance may be more responsive to this induction.
- Exposing a resistant variety to the target pathogen does not ‘strengthen’ its resistance (a grower question).
More information/prepared by:
Margaret Tuttle McGrath
Associate Professor
Long Island Horticultural Research and Extension Center (LIHREC)
Plant Pathology and Plant-Microbe Biology Section
School of Integrative Plant Science
College of Agriculture and Life Sciences
Cornell University
mtm3@cornell.edu
March 2022