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

Abstract

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.

Thirst or Malnutrition: The Impacts of Invasive Insect Agrilus mali on the Physiological Status of Wild Apple Trees

“…Such correlation between δ 13 C and precipitation has also been demonstrated in other studies [33][34][35], suggesting higher stomatal conductance in moister environments. Similar confluent and sequential stresses were also reported as potential causes for the decline of Atlas cedar [36] and cultivated apple trees showing symptoms of "rapid apple decline" in North America [37]. Our study also implies that pest infestation aggravated the decline in WUE, especially in drought conditions.…”

A bushel of viruses: Identification of seventeen novel putative viruses by RNA-seq in six apple trees

“…The cause of apple decline has remained elusive in both the eastern United States and in Washington state. A study by Singh et al [4] ruled out the involvement of ASGV, ASPV, and ACLSV, while in this study we have found several more known viruses that require investigation. But what else?…”

“…At this point there is no common thread distinguishing declining trees from healthy trees. A recent examination of a variety of factors, including weather, soil chemistry, viral infection, and microbial communities was conducted and no obvious difference was observed between symptomatic and asymptomatic trees [4]. This suggests that some other, as yet unidentified factor, is ultimately responsible for decline.…”

“…These viruses were also observed in asymptomatic trees, making it unlikely that they are the sole cause of this disease [2]. A recent study of declining trees in the eastern United States examined weather conditions, soil conditions, common viruses, and the microbial rhizosphere population [4]. No differences were observed between symptomatic and asymptomatic plants, excepting a decrease in one bacterial population in the rhizosphere of declining trees that may have been indicative of water stress [4].…”

“…A recent study of declining trees in the eastern United States examined weather conditions, soil conditions, common viruses, and the microbial rhizosphere population [4]. No differences were observed between symptomatic and asymptomatic plants, excepting a decrease in one bacterial population in the rhizosphere of declining trees that may have been indicative of water stress [4]. Next generation sequencing (NGS) has enabled researchers in the last few years to assess the virome of a plant [5].…”