Sudden apple decline in British Columbia: a potential link between fungal cankers, invasive Sesiidae moths, and abrupt hydraulic failure

Sudden apple decline (SAD) is a poorly understood disorder, resulting in rapid death of apple trees. We investigated the signs and symptoms, biotic and abiotic stressors, fruit quality impacts and internal tree hydraulics of afflicted trees. In 2018, orchard surveys were conducted in seven apple orchards in the Okanagan Valley reporting high tree mortality consistent with SAD. Of 350 trees observed, 28.4% were assessed as declining; necrotic stem lesions were observed on 87.5% of declining trees, and underdeveloped foliage was observed on 27.7% of the declining trees. 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. 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 water limiting bottleneck at the graft union, often associated with canker. 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 possible 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). We propose that impaired water transport across the graft union, due in part to the impacts from fungal canker and associated S. myopaeformis infestations, may be a contributing factor to hydraulic failure and the incidence of SAD in this region.