BREEDING FOR DISEASE AND INSECT RESISTANCE

Fehr, Chapt. 21.
Refs. Hooker, A.L. 1983. Breeding to control pests. p 199-229 in Wood(ed)
Crop Breeding. ASA. CSSA.
Jenkins, J.N. 1981. Breeding for insect resistance. p 291-308 in Frey(ed)
Plant Breeding II.
Briggs and Knowles, Chapt. 27 and 28.

Breeding for disease or insect resistance differs from other characteristics -- dealing with changing the relationship with an evolving and variable pest or pathogen population. NOTE THE ENVIRONMENT IS UNPREDICTABLE, BUT THE PEST IS CHANGING -- THERE IS A DIFFERENCE BETWEEN UNPREDICTABILITY AND CHANGE.

Influence of the environment cannot be overemphasized. Genes for resistance cannot be identified unless the plant containing the gene(s) is interacting with the pathogen or insect pest in an environment in which susceptible plants normally would be diseased or injured. VERY IMPORTANT TO HAVE CHECK VARIETIES.

Easier to breed for resistance to some pests than others. e.g., viruses, rusts and mildews vs. root and storage tissue rots, aphids, greenbugs, plant hoppers vs. root chewing or grain storage insects.

RESISTANCE -- Resistant genotypes/cultivars/hybrids are inherently less damaged or infested by a pest than other genotypes/cultivars/hybrids under comparable environments in the field.

Resistance to establishment
e.g., hypersensitive reaction.
COMPONENTS OF DISEASE RESISTANCE
Colonization prevented
e.g., production of phytoalexins.

Stage resistance

1. Mature plant resistance e.g., rusts, mildews.
2. Young vegetative growth resistant & mature plant susceptible.

Field resistance . . . appearance of resistance in a population of plants in a field. Usually complex with a number of mechanisms operating. NOTE THIS IS DIFFICULT TO TEST IN A SMALL PLOT NURSERY DUE TO CLOSE PROXIMITY OF OTHER GENOTYPES.

Disease tolerance

Tolerant plant is able to endure disease attack without suffering a severe loss in yield. Resistant to products of the pathogen but not resistant to the pathogen itself, e.g., observed for rusts, viruses, bacteria. OFTEN CONFOUNDED WITH STAGE/ENVIRONMENT.

Disease escape

Plants/cultivars normally susceptible may not be inoculated. Consider how escapes change heritability.

Effect of the environment

Resistance may break down because of environmental conditions . . . temperature (air/soil), light intensity/duration, mineral deficiency.

Inoculation methods for disease

Natural infection . . . not the most reliable method -- disease intensity may fluctuate from year to year. Several locations an advantage (problem will be escapes and confounding with environment) -- sampling the range of biotypes and environments which the crop would normally be exposed to. For some diseases an effective artificial inoculation method may not have been developed.

• EXAMPLE IS MOBILE NURSERIES

• EVALUATION METHOD REQUIREMENTS (DISEASE/INSECT)

1. Repeatable.
   
   
2. Give gradations of reaction to pest -- allow selection.
   
   
3. Duplicate severity of disease/insect experienced under natural conditions.
   
   
4. Allow evaluation of a large number of plants.
   
   
5. Easy -- not biologically complicated nor utilize expensive equipment.
   
   
6. Adapted to the breeding and selection scheme for the crop.
   
   
7. Use seedling plants if feasible -- more plants can be tested, saving time and space.
   
   NEED TO DEVELOP A GENETIC UNDERSTANDING!

• CONSIDERATIONS IN BREEDING FOR INSECT RESISTANCE

Insect-plant interactions.

Insects can and do exercise choice.

• Choices influenced by environment, e.g., monoculture vs. multicropping.

host-crop
Insect Life cycle
alternate host-crop/weed

-- generation time & population dynamics. ex., Hessian fly in Georgia vs. Indiana.

• INSECT RESISTANCE MECHANISMS (Independent/in combination)

1. Antibiosis
   

   Certain characteristics of a resistant plant that cause adverse effects on the insects that feed on it (insects may die/lay fewer eggs/slower growth rates). e.g., DIMBOA present in leaves of ECB (European corn borer) resistant inbred lines of corn. This type of resistance places greatest selection pressure on the insect population for new biotypes.

2. Tolerance
   

   Tolerant plant is able to reproduce/repair insect injury so that yield is not significantly reduced despite supporting an insect population that would damage and reduce yields of a susceptible host under similar conditions. e.g., rootworm tolerance in corn . . . some genotypes are able to replace injured roots.

3. Antixenosis (non-preference)
   

   Plant is avoided because it is an undesirable host. Certain plant characteristics (e.g., surface texture or chemical constituent) cause differential oviposition and habitation by the insect on the host. This mode of resistance is very effective where the insect has choice. May not be a very effective mechanism in a monoculture situation. Need to recognize that this may be due to the previous crop in rotation.

• REQUIREMENTS FOR EVALUATION OF INSECT RESISTANCE

1. Good supply of eggs, larvae or adult insects. "Nurse crop principle" to enhance natural infestations. For artificial infestation insects reared in the lab should be similar to those in the wild (biotype, behavior etc.). Lab rearing often changes pest behavior.
   
   
2. Rapid, repeatable rating system related to the development of the insects and/or damage done by the insect.
   
   
3. Single plants vs. progeny-row evaluations depending upon the degree of damage, uniformity of infestation, inheritance of resistance.
   
   
4. Appropriate plant growth stage -- feeding site specificity.
   
   
5. Appropriate confinement of the insects.

IMPORTANT CONSIDERATIONS CONCERNING TYPE OF RESISTANCE
TO BREED FOR (DISEASE / INSECT):

RESISTANT CULTIVARS AS ONLY METHOD OF CONTROL:

• Must determine the degree of resistance that gives economically acceptable pest control -- will vary with crop e.g., vegetables vs. alfalfa.
  
  
• Decide whether realistic to place high selection pressure on the pest population by developing resistant cultivar, that may result in further biotype selection, or consider resistance that minimizes biotype selection e.g., tolerant or multiline cultivars.
  
  

RESISTANT CULTIVARS AS A COMPONENT IN AN INTEGRATED
PEST MANAGEMENT PROGRAM (IPM).

• If the resistant cultivar is to be the base component in an IPM, then may only need a low-level resistance.

PERMANENCE OF RESISTANCE CULTIVARS:

• Probability of biotype selection should be carefully considered when embarking on a resistance breeding program.
  
  Probability of biotype selection varies with . . .
  
  
  1. Pest species: e.g., Insect species that are parthenogenic, obligate feeders on a single host, have several generations per year, or are sedentary vs. migratory, are more likely evolve new biotypes.
     
     
  2. Mode of resistance: e.g., Antixenosis or tolerance put very little pressure on biotype development.
     
     
  3. Amount of land area grown to a single crop and relative proportion of cultivars bearing the same type of resistance.
  
  
  

Pathologist
Plant Breeder interaction is very important in tackling the task of pest control.
Entomologist

• RESISTANCE GENETICS:

Model 1: Need a virulence gene for each resistance gene to give susceptibility. If no resistance genes, then even an avirulent races will convey susceptibility.

Model 2: For susceptibility need both a virulence gene in the pathogen and a susceptibility gene in the host. An example is a disease with a toxin -- need the toxin produced (p₁) and a receptor (r1r1) in the host.