Fungal pathogens in pome fruit orchards and causal agents of postharvest decay

Abstract

Apples and pears (pome fruit) are important deciduous fruit species cultivated on a worldwide scale. Mild and humid climatic conditions favour fungal diseases on pome fruit. Pome fruit may remain for up to 12 months in storage, during which fruit rot diseases may develop. This thesis presents a number of new and emerging postharvest disease on apples and pears, and the important lenticel spot disease of pome fruit, caused by Fibulorhizoctonia psychrophila, in more detail. Typically, the causal pathogens of postharvest diseases infect fruits during the growing season and remain quiescent until disease symptoms occur after several months of storage. Epidemiological knowledge of these diseases is limited. This thesis describes Taqman PCR assays for quantification of N. alba, N. perennans, C. malorum and C. luteo-olivacea in environmental samples. Substrate colonization varied considerably between orchards. The temporal dynamics of these pathogens was followed in apple and pear orchards. Knowledge on population dynamics is essential for the development of preventative measures to reduce risks of fruit infections during the growing season. Fruit tree canker caused by Neonectria ditissima is a serious problem in apple production regions with climates with moderate temperatures and high rainfall throughout the year. A novel method for screening of apple and pear trees at the nursery stage for latent fruit tree canker infections caused by N. ditissima to be used prior to planting in orchards is described. As apple cultivars differ in their levels of susceptibility to N. ditissima, the appropriateness of two resistance parameters were examined. A new parameter, Lesion Growth Rate (LGR), appeared the best with respect to reproducibility and statistical significance. The presented methods can be used to develop strategies for the control of European fruit tree canker. Dead dormant flower buds of pear is a common phenomenon of economic importance in the major pear production areas of Europe. No indication was found that growth regulation can prevent the occurrence of dead flower buds, nor that P. syringae pv. syringae is the causal agent of dead flower buds disease in the Netherlands. We concluded in this thesis that dead flower buds of pear in the Netherlands should be regarded as a fungal disease caused by A. alternata SC and potentially also A. arborescens SC which may be controlled by specific fungicide applications. Finally, it is argued that postharvest diseases should be regarded as complex problems that require multiple interventions at different stages of the disease process in a systems intervention approach for their control. Such approach requires a deep understanding of the epidemiology of the causal agents in the orchard, fruit defense mechanisms, and the molecular biology of host-pathogen interactions in order to develop novel disease control methods.