Apples are one of the most common and beloved fruits. From their center of origin in Central Asia, apples have spread and are cultivated in temperate regions around the world. Besides providing a delicious and nutritious fruit, apple orchards also produce great economic benefits for the areas in which they are grown. Just for one example, the New York State apple industry is worth around $2 billion yearly and creates at least 12,000 jobs. However, growers are constantly battling against a diverse army of threats, including bad weather, insects, fungal and bacterial pathogens, and viruses.
A new major concern for growers in apple-producing areas in central, northeastern, and northwestern United States and in Ontario, Canada in recent years is the rapid decline of established apple trees. First symptoms are generally reduced growth and pale yellow to reddish leaves, followed by a chlorotic appearance of the full tree canopy. Since they have low productivity, symptomatic trees are most often removed; if kept in the orchard, the tree may eventually collapse entirely.
We took a comprehensive approach to identify the potential involvement of abiotic and biotic stresses, evaluating the physical and chemical properties of orchard soil and weather data over five years of the region. We sequenced the genomes of bacterial and fungal communities in the soil, rhizosphere, roots, and shoots and tested for the presence of viruses in scions and rootstocks of symptomatic and asymptomatic trees. Some pathogens can cause root rot and internal clotting of vascular tissues that lead to tree decline and, ultimately, death. For example, rootstock fire blight and root or crown rot in apples can kill trees by infecting the root system. However, we did not identify symptoms or pathogens of common soil-borne root diseases or apple rootstock blight in any of the declining trees.
Our results show that the occurrence of severe cold followed by drought, or either individually, might not directly cause rapid decline, but could have weakened the trees and led to the proliferation of insects and infection by opportunistic pathogens. Necrotic lesions and wood decay symptoms were primarily observed in the rootstock and not the scion of declining trees. In some rootstock and scion combinations, an inadequate graft union due to weak vascular connections, phloem degeneration, and vascular discontinuity can be impacted under extreme abiotic stresses, leading to a slow collapse of the tree. Additionally, the tight space available to roots in high-density plantings creates intense competition for nutrients and water, especially for inadequate root-systems, which are thus unable to upkeep heavy crop, foliage, and biomass under extreme weather. These negative effects can be aggravated in soils with poor water holding capacity.
There are indications for several causes of the rapid decline of established trees; our results point towards extreme weather events in this situation, but cannot clearly rule out the involvement of other factors. Since apple trees are a perennial crop, the loss of a single tree represents a significant investment over its long life and a great deal of time, about 4-5 years, and money are needed to replace the loss. If a threat like rapid apple decline is not sufficiently understood, there is nothing that can be done to address it. There is a need for broader and more in-depth studies, incorporating both more sites and more factors to more closely define the syndrome. This will, in turn, allow for more resilient orchards through breeding resistant rootstocks and scions as well as improved management practices.
These findings are described in the article entitled Potential role of weather, soil and plant microbial communities in rapid decline of apple trees, recently published in the journal PLOS One.
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