Plant pathogens are divided into three groups

Biotrophs derive energy from living cells, they are found on or in living plants, can have very complex nutrient requirements and do not kill host plants rapidly;

Necrotrophs derive energy from killed cells; they invade and kill plant tissue rapidly and then live saprotrophically on the dead remains;

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R0<1, The population eventually declines to extinction 

R0=1, The population size remains constant

R0>1, The population increases

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λ represents the finite rate of population change over some time interval, often a year

λ = N1/N0

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Populations grow when R0 or λ>1, populations decline when R0 or λ<1, and are at equilibrium when R0 or λ=1

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The simplest method to measure population density is to visually count the number of organisms in a population in a given habitat.

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Quadrats - A square frame often, but not always, measuring 50 × 50 cm.