Potential role of weather, soil and plant microbial communities in rapid decline of apple trees

An unusual decline and collapse of young established trees known as “rapid apple decline” (RAD) has become a major concern for apple growers, particularly in the northeastern United States. This decline is characterized by stunted growth, pale yellow to reddish leaves, and tree collapse within weeks after onset of symptoms. We studied declining apple trees to identify potential involvement of abiotic and biotic stresses. We used 16S and ITS to profile bacterial and fungal communities in the soil, rhizosphere, roots, and shoots and tested for the presence of six viruses in scions and rootstocks of symptomatic and asymptomatic trees. The viruses detected were not associated with RAD symptoms. Bacterial and fungal populations were highly variable in plant tissue, soil and rhizosphere samples, with bacteroidetes, firmicutes, proteobacteria, acidobacteria, and actinobacteria the predominant bacterial classes in various samples. ‘Alphaproteobacteria-rickettsiales’, a bacterial class usually reduced in water-limiting soils, had significantly low abundance in root samples of symptomatic trees. Basidiomycota and Ascomycota fungal classes were the most common fungal classes observed, but neither showed differential enrichment between symptomatic and asymptomatic trees. Analyzing weather data showed an extremely cold winter followed by drought in 2015–2016, which likely weakened the trees to make them more susceptible to varied stresses. In addition, similar physical and nutritional soil composition from symptomatic and asymptomatic trees rules out the role of nutritional stress in RAD. Necrotic lesions and wood decay symptoms dispersing from bark or vascular cambium towards the heartwood were observed primarily below the graft union of declining apple trees, suggesting that the rootstock is the originating point of RAD. We speculate that differences in abiotic factors such as moisture levels in declining roots in combination with extreme weather profiles might cause RAD but cannot clearly rule out the involvement of other factors.

一种被称为"苹果速衰症（rapid apple decline, RAD）"的异常衰退与倒伏现象，已成为苹果种植者面临的主要难题，在美国东北部产区尤为突出。该衰退病症的典型特征为植株生长受阻、叶片呈淡黄色至红褐色，且在症状出现数周内即可发生倒伏。为探究非生物胁迫与生物胁迫在该病症中的潜在作用，本研究针对表现衰退症状的苹果树展开了相关分析。本研究通过16S与ITS测序技术，分别对土壤、根际、根系及枝条中的细菌与真菌群落组成进行表征分析，并对感病与健康植株的接穗与砧木中的6种病毒进行检测。检测结果显示，所检出的病毒与RAD症状并无关联。植物组织、土壤及根际样本中的细菌与真菌群落组成均存在显著差异，各类样本中优势细菌类群依次为拟杆菌门（Bacteroidetes）、厚壁菌门（Firmicutes）、变形菌门（Proteobacteria）、酸杆菌门（Acidobacteria）与放线菌门（Actinobacteria）。通常在水分受限的土壤中丰度降低的α-变形菌门-立克次体目（Alphaproteobacteria-Rickettsiales），在感病植株的根系样本中丰度显著降低。担子菌门（Basidiomycota）与子囊菌门（Ascomycota）为检测到的主要真菌类群，但二者在感病与健康植株间均未表现出丰度差异。气象数据分析显示，2015至2016年间经历了极寒冬季与后续干旱，该气候条件可能削弱了植株抗性，使其更易受到各类胁迫的影响。此外，感病与健康植株的土壤物理性质及营养组成并无显著差异，这排除了营养胁迫在RAD发生过程中的作用。研究人员在衰退苹果树的嫁接口下方区域，主要观察到了从树皮或维管形成层向心材蔓延的坏死病斑与木材腐朽症状，这表明砧木可能是RAD的发病源头。本研究推测，根系水分状况等非生物因子的异常结合极端气候条件，可能是RAD的致病诱因，但目前尚无法完全排除其他因素的参与。