THE EFFECT OF ENVIRONMENT ON PATHOGEN SURVIVAL
International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Andhra Pradesh 502 324, India
Survival is a key component in the life cycle of plant pathogens. The importance of weather in disease development has long been recognized. Environmental conditions influence not only the development of epidemics in a single growing season, but also survival of the pathogen until the next season. To retain its pathogenic status over a long time-frame, the pathogen must successfully withstand a myriad of biotic and abiotic stresses that it encounters during the formation of survival structures, their dormancy (if present), and finally rejuvenation. Specific environmental factors also influence the viability of infectious propagules during dispersal and after deposition on infection courts. Knowledge of pathogen survival is an important element in devising a sound disease forecasting system and effective deployment of disease management strategies in field and greenhouse crops.
Among the several weather variables the role of water and temperature has received considerable research attention, particularly for pathogenic fungi, and less so for radiation. Much of the literature deals with the survival of pathogens in the soil environment. Several fungi survive poorly at high soil water potential, providing an opportunity for disease control via flooding. Several physiological and microbiological processes are related to the pathogen's lack of ability to survive during flooding. The dynamics of changes in soil water potential also affect pathogen survival; the more rapid the change, the more detrimental it is for survival. This is also true for epiphytic bacterial plant pathogens. Much work has been done on the influence of water and temperature on dormancy and hatching of nematode eggs and anhydrobiosis.
Most pathogenic fungi survive poorly at extreme temperatures. Distribution of some pathogens are restricted because they cannot survive specific temperature conditions. For example, Texas root rot does not occur north of southern Oklahoma because Phymatotrichopsis omnivora is unable to survive freezing cold beyond a few days. Lethal and sublethal thermal points differ in different fungi. While lethal temperature kills the pathogen, sublethal temperature can reduce the inoculum potential of the surviving pathogen propagules and predispose them to antagonists. Soil water status and temperature often interact to influence pathogen survival. Soil solarization is an example of the use of high temperature and its interaction with moisture in inactivating pathogens and disease control. Survival of pathogen depends on the exposure of its propagules to the environmental factors. The pathogens survive better when associated with host tissue than in the free state. Temperature and water, through their effect on host tissues in which the propagules are embedded also influence pathogen survival.
Radiation effect on the viability of spores depends on several factors such as wavelength, light intensity, duration of exposure, spore wall thickness and pigmentation, moisture content and thermal death point of spores, and possibly other factors. This aspect has particular significance in long-range transport of spores and after their deposition on host surface. Radiation quality may also affect production of survival structures of a few genera of plant pathogens. The significance of radiation on pathogen survival under field conditions requires further research.
Environment also affects the survival of pathogen through indirect influence on microbes detrimental to the formation of survival propagules, such as in case of sorghum ergot . Pathogen survival may also be mediated by weather effects on host, particularly for obligate parasites such as in wheat rusts. Weather factors have been used to model winter and early spring survival of the wheat leaf rust fungus Puccinia recondita .