Plant tolerance is the characteristic of a plant that allows a plant to avoid, tolerate or recover from attacks from insects, among other things, under conditions that would typically cause a greater amount of injury to other plants of the same species. These inheritable characteristics are what influence the ultimate degree of damage caused by a pest. Tolerance in terms of agricultural production means that despite stress from a pest or disease, the production levels will remain above the economic threshold.
Resistance means that a plant completely immunizes itself from a particular stress. This is typically a biotrophic pathogen infection. The host has a resistance gene which prevents the proliferation of the pathogen. The pathogen typically contains an avirulence gene which triggers plant immunity.
Resistance and tolerance are the best defense mechanisms of plants against pests.
There are two main types of resistance: ecological resistance/pseudo-resistance and genetic resistance. Ecological resistance is resistance related to favorable environmental conditions at a given location at a particular time. This type of resistance can be broken down into 3 forms:
- Host evasion – this phenomenon occurs when a host passes through the susceptible stages very quickly or during a period when pests are fewer. This type of resistance applies to an entire species population.
- Induced resistance – this type of resistance is the result of some type of changed condition for the plant, such as an increase in available nutrients or water
- Escape – this is more or less luck as there is an absence of infestation or injury to a host plant as a result of incomplete infestation.
Genetic resistance is resistance related to (you guessed it) plant genetics. There are several types of genetic resistance:
Resistance based on the number of genes
- Monogenic – controlled by a single gene which makes it easy to both incorporate and exclude in plant breeding programs
- Oligogenic – controlled by a few genes
- Polygenic – controlled by several genes
- Major Gene Resistance – controlled by one or a few major genes (vertical resistance)
- Minor Gene Resistance – controlled by many minor genes; the cumulative effect of minor genes is called adult/mature/field/horizontal resistance
Resistance based on biotype reaction
- Vertical Resistance – specific resistance against specific biotypes
- Horizontal Resistance – effective against all known biotypes; nonspecific resistance
Resistance based on miscellaneous factors
- Cross-Resistance – when resistance against a primary pest results in resistance to a secondary pest
- Multiple-Resistance – different environmental stresses (e.g. insects, diseases, nematodes, heat, drought, etc.) results in a new resistance
Resistance based on evolution
- Sympatric Resistance – resistance that is acquired through the coevolution of a plant and insect (which is why it is important to protect our native pollinators!); governed by major genes
- Allopatric Resistance – not governed by a co-evolution; governed by many genes
There are three mechanisms of resistance in plants.
Antixenosis (non-preference) resistance mechanisms are those by which host plant has characteristics that result in non-preference for insects in terms of shelter, oviposition, feeding, etc. There are morphological or chemical factors that influence insect behavior and results in the poor establishment of insects. Plant shape and color can also be an important influencing factor.
Antibiosis is the negative effect of a host plant on the biology of an insect. This can include decreased rates of survival, development and/or reproduction and is a result of biochemical and biophysical factors. Antibiosis may be a result of the presence of toxic substances, the absence of essential nutrients or a nutrient imbalance. Physical factors, such as thick cuticles, glandular hairs and silica deposits, also contribute to antibiosis.
Tolerance is a plant’s ability to grow and produce an acceptable yield despite a pest attack. Tolerance is typically attributed to plant vigor, regrowth of damaged tissue and a plant’s ability to produce additional stems/branches.
Both plant tolerance and plant resistance are extremely important to IPM efforts as it helps to reduce costs and amount of other control tactics that need to be implemented. This includes improving the efficacy of insecticides and promoting non-chemical benefits like shifts in predator-prey relationships and reduced pest populations.
The benefits of the use of resistant cultivars are numerous as this method has a high level of specificity, is eco-friendly, lower cost and adaptable to a given situation. The benefits are limited only by the amount of time required to develop new cultivars in breeding programs and by a lack of known resistance genes that can be used. To identify and capture relevant genes, crop wild relatives are often used.