ThePhytophthoradiseases of apple

SummaryThe most important Phytophthora pathogens of apple are P. cactorum and P. syringae, although other species may be locally significant. P. cactorum is widespread in the apple growing regions of the world and causes girdling bark rots on the scion (collar rot) and on the rootstock (crown rot), necrosis of the fine root system (root rot) and fruit rot. P. syringae is a significant pathogen of apple only in north-western Europe, where it causes collar rot but is most damaging as a cause of rot in stored fruit. Several other species of Phytophthora have been implicated in crown rot and root rot.Bark rots due to Phytophthora spp. have distinctive features, such as mottled green/ orange/brown coloration of necrotic tissue, but may be confused with other causes of bark death. Isolation of the pathogen is therefore essential for diagnosis, and is now made easier than formerly by the use of selective media. Phytophthora fruit rot is usually distinctive, and isolation from diseased tissue is readily achieved by conventional methods.P. cactorum and P. syringae are homothallic, soil-borne species. The principal peren- nating structure is the oospore. Zoospores are the main infective structures and are relatively short-lived. Mycelium and sporangia have intermediate survival characteristics. The longevity of ephemeral stages is favoured by moderate soil moisture and low temperatures. Growth and reproduction is associated with the colonization of living tissues. The two species have different temperature/growth characteristics. Oospore germination and sporangium production require soil moisture near to saturation, while zoospore release and motility depend on the presence of free water. The flexible growth response of these pathogens permits them to cope with fluctuating soil conditions.P. cactorum and P. syringae cause diseases of a range of other economic plants, woody and herbaceous. There is evidence for host adapted strains in P. cactorum.Phytophthora pathogens may be present in the soil when apples are planted or enter with planting material, in irrigation water, on animals or on implements. Once present, they may regenerate and produce durable increases in inoculum content of soil by colonizing live, abscissed apple organs such as leaves and fruit. Other plant species in orchards may also be important in inoculum regeneration.Infection of unthickened tissues is direct or via stomata or lenticels. Bark may be infected directly through wounds, growth cracks or lenticels, or indirectly via unthickened roots.Growth of the pathogens in host tissues is intercellular and intracellular. Susceptibility to bark rot is related to the effectiveness of phelloderm barriers in walling-off fungal growth. Attempts to relate the biochemical constituents of bark to resistance have been inconclusive.Disease incidence and development is affected by the availability of water above ground and in the soil, soil type, tree age, the height of the graft union, the interaction between scion and rootstock and soil management techniques. Seasonal influences on disease reflect climatic effects on pathogen activity and cyclic changes in tissue resistance. Bark diseases due to P. cactorum are associated with the growing season, while those due to P. syringae are associated with dormancy of the host.Host resistance and the manipulation of cultural conditions are the most effective means of disease control. The early selection for resistance in breeding material is complicated by the fact that susceptibility to collar rot develops only in mature trees, and the conditions required to induce crown rot experimentally in susceptible rootstocks are not fully understood. Other methods of control are the use of chemicals protectively or cura- tively and the use of Phytophthora-iree planting material. The possibilities of biological control are at an early stage of investigation.