Environment: abiotic and biotic factors that influence growth, development and health of an organism. Components of the environment interact with the target organism, but interactions occur among components of the environment.

Organism (Genotype) x Environment = Interaction phenotype

The concept of genotype x environment interaction can modified to characterize plant interactions with causes of health problems. Disease is a likely outcome when a host plant encounters a causal agent and together interact in a specific, but dynamic environment. Agrios - pages 4-5

DISEASE TRIANGLE

(CHEMICAL, PHYSICAL, BIOLOGICAL) (INFECTIOUS & NON INFECTIOUS AGENTS)

Greater plant health can be achieved by understanding principles, concepts and technical aspects of the Disease Triangle.

Non-infectious Causal agents of disease: abiotic factors (usually extremes or imbalances)

Temperature

Water

Light

Toxic chemicals

Mineral deficiencies

Soil pH

Environmental factors and the effect on disease cycle - climatic conditions, soil factors, soil type, cultural practices, and biotic factors.

The effect of environment on the host, the pathogen and the interaction of host and pathogen need to be distinguished.

1) Abiotic factors may cause direct damage to host.

2) Environmental factors can predispose the plant making it more susceptible to a pathogen.

3) All pathogens, like all living organisms have optimal requirements of water, temperature, nutrients and other factors needed for growth and reproduction.

The following environmental factors can influence the activities of the pathogen or host.

A. Location factors

1. Soil texture and structure

2. Temperature and moisture

3. Plant nutrition, fertility and soil pH

4. Soil-applied agricultural chemicals

5. Field location; topography

6. Regional climate

B. Climate factors

1. Temperature

2. Moisture--relative humidity, rain, dew

3. Wind

4. Solar radiation

C. Cultural practices

1. Planting site

2. Time of planting

3. Tillage systems

4. Sequence of plant species or varieties

5. Plant spacing

6. Pesticides/fertilizers applied

Host reaction to pathogen and environment - is naturally inherent and may change with stage of crop growth. The relationship between disease severity and stage of growth will greatly influence the amount of economic loss due to disease.

Host factors associated with disease development:

1. Host genotype -

· Host or nonhost situation

· Host - degree of resistance or susceptibility.

2. Form of resistance expressed by a plant variety.

3. Stage of plant development when inoculation occurs.

4. Nutritional status of host.

5. Stress due to abiotic or biotic agents.

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