Epidemiological Assessments and Postharvest Disease Incidence

Abstract

Postharvest plant disease can be measured by “incidence,” by recording the presence or absence of symptoms, and “severity,” the degree to which the symptoms are expressed. Weather and other environmental conditions play a significant role by causing stress in plants and lowering natural defenses, and by creating conditions suitable for pathogens to infect the plants. Specifically for postharvest diseases of fruit, infections can start as early as fruit set and continue until harvest. Although weather conditions influence the epidemiology of a disease in the field, the incidence of postharvest disease depends on the incidence of latent infections that initiate in the field during the season, contamination with fungal propagules during harvest, the effectiveness of postharvest treatments, and storage and marketing conditions. True latent infection, defined as a parasitic relationship that eventually induces macroscopic symptoms (Verhoeff, K., Annu. Rev. Phytopathol. 12:99-107, 1974) plays a major role in both the incidence and severity of postharvest disease. If conditions are favorable, incidence and severity of latent infections will be higher and the risk for postharvest disease development will increase and vice versa. For example, in California kiwifruit there is a positive relationship between the incidence of latent infection of sepals or stem ends, and the incidence of gray mold of fruit in cold storage. We visualize kiwifruit and other kinds of fruit as recording devices that copy the environmental conditions as latent infections. And in some cases, quantification of these latent infections can predict postharvest disease (i.e. BOTMON (Botrytis monitoring in kiwifruit sepals and/or fruit stems and in stems of grape berries) and ONFIT (overnight freezing incubation technique in stone fruit, other fleshy fruit, and in nut crops)). The source of inoculum that can drive an epidemic of a disease in the field can also affect the ­incidence of postharvest decay, by affecting the incidence of latent infection. Reducing the source of inoculum (sanitation) can reduce the incidence of latent infection of fruit, with the ultimate result in reducing postharvest disease (i.e., stone and pome fruit). Environmental conditions during bloom in various crops can have detrimental effects on the incidence of postharvest disease (i.e., grapes, pomegranates, prunes, etc.) by affecting the levels of latent infection or affecting the plant host directly. Altering cultural practices that may affect environmental conditions in the field or the physiology and histology of fruit can also affect the incidence of fruit diseases both in the field and postharvest. The development of efficient, accurate, and rapid molecular techniques (including real-time PCR assays) can facilitate the detection and quantification of disease inoculum and latent infection of fruit (i.e., stone fruit) and help predict incidence of postharvest disease. In addition, development of allele-specific RT-PCR methods for rapidly detecting fungicide resistant fungal pathogens will help growers to manage fungicide resistance and make correct decisions to reduce postharvest disease. The goal of our laboratory is to develop less expensive molecular techniques that determine latent infections and assess populations of fungi resistant to fungicides and enable us to process large numbers of samples at our laboratory and to provide the protocols to private laboratories.