Signs and Symptoms of sudden apple decline (SAD) in British Columbia: Impacts on tree physiology and the potential role of environmental stressors

Sudden apple decline (SAD) is a recent and little understood disorder, associated with wilted leaves and rapid death of apple trees. In 2018, orchard surveys were conducted in seven apple orchards in the Okanagan Valley reporting high tree mortality and potentially SAD. Of 350 trees observed, 28.4% were assessed as declining; of those, necrotic stem lesions were frequently observed (87.5%), underdeveloped foliage was observed less frequently (27.7%) and oozing wounds were rare (1.1%). A survey of a 1-10 year old apple germplasm orchard showed that the probability of trees exhibiting SAD increased with tree age, regardless of parentage. Parentage did not statistically significantly affect disease incidence. Across orchards, there appeared to be an association between infestation of apple clearwing moth (Synanthedon myopaeformis), the size of necrotic stem lesions, and incidence of SAD. Assessment of stem water transport showed a waterlimiting bottleneck at the graft union. The trees in decline also had lower midday stem water potential, lower photosynthetic rate, and lower fruit weight and dry matter. Grid (5-m) sampling of soils in four affected orchards showed a correlation between SAD-associated tree mortality and a given soil’s ability to retain water (e.g., soil depth, coarse fragment content, organic matter content). Recently, the Okanagan Valley has experienced an unprecedented drought (2017) and an increasing frequency in the number of days >35°C. We propose that heat and/or drought stress, compounded by impaired water transport across the graft union, may be a contributing factor to the incidence of SAD in this region.