Winter-damage on apple trees

South Tirol is an important region for apple production in Europe. The mild climate allows a high productivity but during the last decades, winter damage on apple trees was observed in 3 to 4 year intervals at numerous sites in South Tirol. This winter damage in apple orchards is economically relevant as e.g. in season 2004/2005 costs of more than 6 Mio Euros were caused. There are several indications that the observed dieback of crown parts or even trees was related to frost drought. The situation is comparable to that of trees at the timberline, where winter damage was analysed in previous projects. Based on our experience with trees growing at the alpine timberline, we hypothesize that winter damage in South Tirol apple orchards is strongly influenced by the duration of water uptake blockages, the extent of transpirational water losses, the trees` water storage capacity as well as the climatic conditions in autumn. We expect damage in living tissues as well as xylem embolism to cause prolonged drought stress in spring. In the proposed project, these aspects will be analysed and avoidance strategies will be developed. In field measurements at five apple orchards in South Tirol, climatic conditions and effects on tree water relations (water potential, hydraulic conductivity, water storage, transpiration) as well as winter injury will be quantified, and in experimental approaches important parameters will be analysed in detail. These data will be compared with hydraulic characteristics (vulnerability to embolism, drought resistance of living tissues, water storage capacity) of studied varieties. In consequence, numerous varieties will be screened for resistance to frost drought, whereby hydraulic as well as related anatomical parameters will be analysed to develop a valuable screening protocol for variety selection. Cultivation techniques to avoid winter damage will also be tested in this project. The proposed project will be done in close cooperation with the "Südtiroler Beratungsring für Obst- und Weinbau". It will enable an analysis of relevant environmental factors and physiological mechanisms causing winter damage in apple orchards and the development of avoidance strategies in breeding and cultivation. Beside this economically relevant aspect, it will be an important contribution to our studies on winter water relations of plants.

South Tyrol is one of the most important apple producing regions in Europe. The mild climate enables high productivity and an excellent quality of fruits. However, at irregular intervals winter damage with delayed leaf-flushing, dieback of crown parts or even of whole trees occur. These damages can entail high economic losses. For instance, in winter 2004/2005 more than 200.000 trees had to be replaced, causing costs of over six million Euro. Based on several observations, it was hypothesised that this phenomenon may be caused by drought stress in winter and spring. To test this hypothesis, five experimental sites were set up along the Vinschgau valley and various analyses were carried out on six apple varieties. From spring 2009 to spring 2014, annual courses of native embolism and water as well as starch content of wood, bark and buds were measured. These data were then correlated with microclimatic (e.g. air- and soil temperature, humidity, wind speed, radiation) and phenological (e.g. cambium activity, bud phenology) parameters. Furthermore, we assessed the vulnerability of branch xylem and of leaves to drought-induced embolism and analysed the drought tolerance of different tissues. In a field experiment, the influence of prolonged soil frost on tree hydraulics and phenology was investigated. Results show that the water transport system of apple trees is severely impaired in winter and full recovery only occurs in late spring when already high water losses due to transpiration occur. This is typical for many angiosperm trees. As long as sufficient water supply from the soil is ensured this does not affect plant survival and growth. However, the particular climatic conditions in the Vinschgau valley frequently lead to an early bud break and thus to high transpirational water losses while water supply from the frozen or cold soil is still reduced. Results of the field experiment demonstrated that prolonged soil frost at contemporaneously high air temperatures has strong effects on tree hydraulics and phenology. A possible damage of plants though strongly depends on the apple variety and tree age. Vulnerability analyses revealed that drought tolerance varies considerably between varieties and younger trees are more vulnerable than older ones. Obtained results strongly indicate that winter damage on South Tyrolean apple trees is related to drought stress. However, for severe winter damage with fatal dieback of trees to occur several adverse factors have to coincide. The project enabled new and essential insights into tree hydraulics and is of relevance for basic and applied research. Based on this project, a catalogue of strategies to reduce the risk for severe winter damage on fruit trees was developed.