Plant Pathogens and the Theory of r- and K-Selection

r- and K-theory can be used to compare the life strategies of plant pathogens, at least during the parasitic phase. While, as a group, plant pathogens tend to be r-selected, individual species display certain tactics that may best be described by either r- or K-descriptors. Characteristics such as the ability to survive prolonged periods and the flexibility to time activities are more closely associated with "bet-hedging" than r- or K-selection. A conceptual model is proposed to show how pathogens can be positioned along an r-K continuum based on their relative allocation of resources to maintenance, growth, and reproduction. Superimposed on resource allocation is the nature of the parasite-host relationship, determined in part by whether the interaction is obligatory or facultative, whether nutrient is derived from living or dead cells of the host, and the extent of destruction of the host habitat. In general, obligately biotrophic parasites which exhibit narrow host ranges, specialization as biotypes or races, and which cause minimal tissue damage act to stress the host by causing physiological disruption such as altered photosynthesis, transpiration, or respiration. They are more K-selected than pathogens which may allocate host resources similarly, but which are facultatively parasitic, attack many hosts, and cause disturbance by actively destroying plant biomass.