Define plant resistance and describe different types of plant resistance.
Genetically inherited qualities that result in a plant of one cultivar (genotype) being less damaged than a susceptible plant lacking these qualities. Resistant genotypes are less damaged than susceptible genotypes subjected to the same herbivore pressure.
Identify the traits plants possess that contribute to plant resistance
- Plant biochemicals traits (dhurrin inhibits flea beetle and larvae feeding).
- Plant morphological or structural traits (trichomes on soybeans interfere with feeding behavior of potato leafhopper).
- Plant traits that affect natural enemies (attractants)
- Plant tolerance traits (lower yield loss)
Describe the effects of plant resistance on insect herbivores. In what ways do resistance-related traits affect insect pests or plant-insect interactions?
Antibiosis: comprises those plant traits that negatively affect the fitness (growth, development, fecundity) of pest insects
Antixenosis: comprises those plant traits that negatively affect the behavior of pest insects.
Tolerance: comprises those plant traits that allow a plant to recover from/compensate for injury
Consider how resistant plant varieties can be used in IPM.
Screening: 1. Evaluation of germplasm. 2. Identification of donors.
Breeding: Cooperation w/ breeder.
Implementation: How fit into IPM?
Inducible x Constitutive resistant traits
Direct vs Indirect effect
Inducible Resistance/tolerance-related plant traits are only expressed by the plant after pest attack.
Constitutive resistance/tolerance is expressed regardless of whether the plant has been attacked by herbivores.
Direct resistance-related traits affect pests directly (fitness), while others affect pests indirectly by facilitating the activities of natural enemies.
Goal of screening
To characterize genotypic variation in plant resistance to an arthropod pest in order to:
1) identify resistant genotypes that can be used immediately in an IPM program
OR
2) identify resistant genotypes that can be used as donors in a breeding program
In Painter’s approach, screening is often coupled with studies to determine the type of resisance (antibiosis, antixenosis, tolerance). Painter’s approach does not emphasize the mechanisms of resistance (what traits responsible for resistance).
Goal of breeding
[This step unnecessary if resistance is found in an agronomically acceptable variety]
Goal: to introgress = incorporate genes responsible for resistance into agronomically acceptable background
Cooperation with breeders usually necessary – development of new varieties requires considerable infrastructure, intellectual and other
Polygenic basis of resistance.
If two genotypes differ substantially in their resistance to an herbivore, more than one gene is likely to be responsible for this resistance.
Goal of implementation
HPR superficially simple to implement: the tactic is in the seed!
HPR can affect pest populations in different ways, depending on the pest and resistant cultivar:
- reduce initial colonization
- reduce tissue removed
- slow populations growth
- increase pest mortality
- reduce yield loss per unit injury
In practice, integrating plant resistance with other tactics may be difficult.
Implementation and integration with other tactics
- Resistance is rarely strong enough to stand alone
- Resistance usually must be integrated with other tactics
- Must understand effects on pest populations to effectively integrate
- HPR generally compatible with other tactics
- However, there may be conflicts, esp. with biocontrol
Not enough information on integration
When is plant resistance likely to be most useful?
- When effects on pest are strong
- When the pest is a “resident’ in the crop (plant resistance has cumulative effects)
- When plant resistance is compatible with other tactics
Advantages and Disadvantages of HPR
Advantages:
- Low cost to growers
- Simple to use if primary strategy
- Integratable in theory with other tactics
- Benefits accrue over space and time (area-wide), sometimes sustainable
Disadvantages:
- Long and difficult process to develop; requires considerable infrastructure
- Difficult to use low levels of resistance optimally
- Some incompatibilities
- Some forms unstable, not sustainable
Case study: Hessian Fly
- Resistance controlled by single gene in insect and corresponding gene in plant: gen for gene
- Insect gene probably control salival protein, plant gene a protein that recognizes the protein
- Resistance in plant gives complete resistance, easy to phenotype, easy to breed
- Within a few years of release of R variety, populations with resistance to the resistance (“biotypes” can develop)
- Over 20 R genes have been identified, and at least 16 fly biotypes have developed. This has necessitated breeding of new varieties with new R genes
- Can be used as primary management tactic
- Can be combined with cultural practices, insecticides
