Does Preharvest Stress of Plants Affect Postharvest Decay

In discussion related to the planning of the present workshop on 'disease resistance in fruit', the question arose as to whether fruit harvested from stressed plants have increased, decreased, or unchanged susceptibility to pathogens whose disease symptoms are expressed after harvest. For example, does preharvest water deficit stress affect the incidence and severity of stem-end rot (endophytic infection) or anthracnose (latent infection) on harvested ripening mango fruit? A literature survey failed to yield any research papers relating preharvest plant stress to the host-pathogen biology of the harvested fruit. Thus, it was decided to offer a largely speculative account of how preharvest stress might affect postharvest decay. The practical intent of this presentation is to stimulate infonned debate and directed investigation of the proposition that 'preharvest stress may alter the ability of host fruit tissue to suppress the development of quiescent infections'. For instance, in terms of constitutive and induced defence mechanisms, might fruit from plants subjected to abiotic stress have enhanced levels of constitutive defence compounds (e.g. phenolics) but reduced capacity to synthesise new defence compounds (e.g. proteins related to pathogenesis)? ALL fresh horticultural products succumb to pathogens at some time after harvest. This increase in disease susceptibility occurs in association with a shift in the balance of physicochemical processes of the plant tissue away from those favouring the host and towards those favouring the pathogen. That is, host metabolism shifts from a net anabolic to a net catabolic state in response to 'harvest stress'. Harvest stress associated with detachment involves pennanen! disruption of source/sink relationships characterised by water, nutrient (mineral, carbohydrate), and growth regulator Duxes. Harvested host tissue becomes progressively less able to mount defence reactions against pathogens and other stresses and, in fact, encourages decay in association with * Department of Plant Production, The University of Queensland, Gatton College, Queensland 4345, Australia. ACIAR, GPO Box 1571, Canberra, ACT 260 L Australia. Cooperative Research Centre for Tropical Plant Pathology, The University of Queensland, St Lucia, Queensland 4072, Australia. 39 physiological deterioration (Sommer 1989). Defence reactions against pathogens of plant tissue include the maintenance of constitutive antifungal chemicals (e.g. phcnolics), the synthesis of inducible antifungals (e.g. pathogenesis related proteins), and the laying down of physical barriers to pathogen invasion (e.g. callose) (Issac 1992). From an evolutionary perspective, physiological deterioration manifest during fruit ripening and plant organ senescence is functionally important as it favours seed dissemination in the first instance and nutrient recycling in the second.