ECOLOGICAL IMPLICATIONS OF RECOGNITION EVENTS IN THE PRE-INFECTION STAGES OF ROOT PATHOGENS

Host-specificity and recognition by root-infecting pathogens are revie wed, with emphasis on fungi but drawing parallels with other root para sites, symbionts and rhizosphere micro-organisms. Two major levels of specificity are identified: family-specific parasitism, exemplified by many parasites that characteristically infect Gramineae, and the poss ibly recent segregation of pathogen species into crop-specific strains . In both of these cases there are examples where host-specificity is paralleled by host-recognition in the pre-infection stages, including host-specific triggering of propagule germination. The pre-infection s equence of zoosporic pathogens (Pythium, Phytophthora, Aphanomyces spp .) is reviewed. These fungi seem to have both general (low-affinity) a nd host-specific (high-affinity) receptor-based recognition systems fo r chemotaxis to root diffusates and for induction of encystment by hos t surface components. Zoospores orientate (dock) during encystment, an d the cysts germinate from a fixed point, whereas the non-motile spore s of at least some root parasites (e.g. Idriella bolleyi) can germinat e from any point, which is influenced by the host.The possession of bo th general and host-specific recognition systems by a parasite could e nable it to exploit rhizosphere niches or to invade compromised plants in the absence of preferred hosts. Evidence from graminicolous (famil y-specific) Pythium spp. suggests that host-specific recognition is qu antitative rather than absolute but, when compounded over successive c ycles of infection, is a powerful selective mechanism.