BREEDING OBJECTIVES DISEASE AND PEST RESISTANCE

BREEDING OBJECTIVES DISEASE AND PEST RESISTANCE 2010

Reduced use of Agricultural Chemicals Must develop biological resistance to pests & diseases

BREEDING OBJECTIVES DISEASE AND PEST RESISTANCE Which pests and diseases affect crops. Effect of plant pests and diseases. Types of plant resistance. Mechanism for pest and disease resistance. Testing for pest and disease resistance.

TYPES OF PEST AND DISEASE Air borne fungi Soil borne fungi Bacteria Viruses Eelworms Insects Other, Incl Mammals

Biology Epidemics Spread Damage Examples Notes AIR-BORNE FUNGI Specialized; Complex life cycles; Usually pathotype differentiated. Recurrent; Seasonal, depending on host and weather. Wide and rapid; new pathotypes rapidly disseminated. Destroying photosynthetic apparatus, some quality (fruit blemishes) Potato blight, wheat, barley & maize rusts, powdery mildew (many crops), cereal smuts, rice blast Most conspicuous and damaging of plant diseases. Claimed most plant breeder attention.

Barley Powdery Mildew (Erysiphe graminis)

Potato Early Blight (Alternaria solani)

Biology Epidemics Spread Damage Examples Notes SOIL-BORNE FUNGI Obligate & specific; Long persistence in soil; complex life cycle (wart) or simple (verticillium) Chronic rather than epidemic. Slow since soil-borne; Usually man-assisted, new races generally restricted by region. Stunting, or killing plants, poor growth, initiation of secondary storage diseases. Potato wart, Brassica club root, Many Vertillilium and Fusarium wilts, cereal take-all. Can be devastating (i.e. Panama disease in banana) but usually of lesser importance to air-borne fungi.

Potato Scab (Streptomyces spp.)

Biology Epidemics Spread Damage Examples Notes BACTERIA Usually species specific; Semi-obligate strains, with little pathotype differentiation. Chronic rather than epidemic after initial infection. Slow; Usually man-assisted. Various; Stunting or killing plants (soil-borne vascular wilts); Kill foliage (cotton blackarm) Potato ring-rot; vascular wilts of many crops; ratoon in sugar cane; cotton blackarm Rather few bacterial species involved; little breeding effort; Screening of difficult.

Potato ring-rot (Corynebacterium sepedonicum)

Biology Epidemics Spread Damage Examples Notes INSECT Many with wide host range; some specific (wheat Hessian fly, several aphids) Characteristically season, can be chronic in tropical perennials Seasonal, can be limited to a few miles; Rapid for many winged insects. By defoliation (many examples); by stunting (aphids), destruction of fruit or grain. Colorado potato beetle, wheat Hessian fly, many aphids, cabbage seedpod weevil. Principal damage by many insects is later infection by other diseases. Virus vectors.

Aphids (Homoptera: Aphididae)

Biology Epidemics Spread Damage Examples Notes VIRUSES Always obligate, with levels of species-specificity; pathotype differentiation in many crops Chronic in perennials, can be epidemic in annual crops if transmission permits. Slow in planting material; rapid depending on insect vector or mechanical transmission. Usually by stunting, rarely lethal. Potato virus X, Y; tobacco mosaic, barley yellow dwarf, many clonal fruit viruses. Next in importance to air-borne fungi; Good deal of breeding efforts with moderate success.

Pea Early Browning Virus

Biology Epidemics Spread Damage Examples Notes EELWORM Specific, obligate pathogens; some pathotype differentiation. Soil persistent and chronic when infected. Slow, since soil-borne; usually man-assisted. Stunting growth through root damage, sometimes associated with gall-formation. Potato cyst nematode, cereal stem & root eelworms, sugar beet nematode, wire worms. Some nematodes can vector viruses; Causes damage that allows infection from other diseases.

Potato Cyst Nematode (Globodera pallida)

EFFECT OF PESTS AND DISEASES Destruction of leaf tissue and reducing photosynthesis rate or efficiency. Air-borne fungi (i.e. blight, mildew, rusts)

EFFECT OF PESTS AND DISEASES Reducing plant stands by killing whole plants and leaving gaps in the crop. Vascular wilts, soil-borne fungi, boring insects

EFFECT OF PESTS AND DISEASES Stunting plants by metabolic disturbance, nutrient drain or root damage. Many viruses, insects (aphids), eelworm

EFFECT OF PESTS AND DISEASES Killing branches or plant parts. Boring or feeding insects.

WHICH PESTS AND DISEASES ARE MOST IMPORTANT? Examine historical data from past researchers. Seek help from growers and other industry representatives. Conduct suitable research.

t/ha Effect of insect infestation on seed yield 2000 1800 1600 1400 1200 1000 800 600 400 200 0 B. napus B. rapa B. juncea S. alba

TYPES OF PLANT RESISTANCE Specific or Vertical resistance Controlled by a single gene. Results in distinct resistance classes. Resistance is usually absolute (yes or no).

TYPES OF PLANT RESISTANCE Non-specific, Field, General, or Horizontal resistance Controlled by multiple genes. Results in continuously variable levels of resistance. Usually resistance is not absolute.

VERTICAL RESISTANCE For each resistance gene in the host there is a gene in the pathogen that determines whether the pathogen is: Avirulent Unable to overcome the resistance and hence unable to infect or injure the host plant. Virulent Able to overcome the resistance and hence able to infect or injure the host plant.

Relationship between resistance genes and virulent genes is called Locks & Keys Locks (dominant resistance genes) can only be opened with the right keys (recessive virulent genes).

SINGLE GENE RESISTANCE (LOCK & KEY) Plant Genotype Pest genotype aabb Any virulent gene A_bb No virulent genes A_bbcc a a B B C c A_B_cc a a B B c c A_B_C_dd A a b b c c d d A_B_ccD_ a a b b C C d d Plant response

SINGLE GENE RESISTANCE (LOCK & KEY) Plant Genotype Pest genotype Plant response aabb Any virulent gene Susceptible A_bb No virulent genes A_bbcc a a B B C c A_B_cc a a B B c c A_B_C_dd A a b b c c d d A_B_ccD_ a a b b C C d d

SINGLE GENE RESISTANCE (LOCK & KEY) Plant Genotype Pest genotype Plant response aabb Any virulent gene Susceptible A_bb No virulent genes Resistant A_bbcc a a B B C c A_B_cc a a B B c c A_B_C_dd A a b b c c d d A_B_ccD_ a a b b C C d d

SINGLE GENE RESISTANCE (LOCK & KEY) Plant Genotype Pest genotype Plant response aabb Any virulent gene Susceptible A_bb No virulent genes Resistant A_bbcc a a B B C c Susceptible A_B_cc a a B B c c A_B_C_dd A a b b c c d d A_B_ccD_ a a b b C C d d

Gene Pyramid-ing AA BB AAbb x aabb = AaBb Self 15:1 or 16:0

Gene Pyramid-ing AAbb x aabb > Homozygosity AABB:aaBB:AAbb:aabb Self : find all Resistant

Potato Late Blight Find resistance gene (R 0 ) (Solanum demissum) Identify virulent pathotype (r 0 r 0 ) Find resistance gene (R 1 ) Identify virulent pathotype (r 1 r 1 ) Combine both genes (R 0 R 1 ) Identify virulent pathotype (r 1 r 1 r 2 r 2 )

MARKER ASSISTED SELECTION Easy, quick and inexpensive to score. Neutral in terms of phenotype. No deleterious effects on fitness. High levels of polymorphism. Stable expression over environments. Assessable early in plant development. Non-destructive assessment. Codominant in expression.

P 1 Probe P 2 P 1 P 2 F 2 R R R R R

HORIZONTAL RESISTANCE Horizontal resistance more durable than vertical resistance. Ability to control a wide spectrum of races. New pathotypes have difficulty overcoming all resistance loci. Probability of combining all (or many) resistance alleles into a single genotype are low.