The Issue 

Cornell Extension Specialists are being asked with increasing frequency to investigate both chronic decline and rapid collapse of apple trees in young, high-density plantings in all regions of New York State.  Symptoms of chronic decline can include poor growth, off color foliage, and a generally unthrifty appearance that worsens over several years.  The death of an (apparently) previously healthy tree over the course of just a few weeks has been termed Rapid Apple Decline (RAD).  While the symptoms of chronic decline are subtler in some cases, RAD is eye-catching.  Afflicted trees appear to "burn up" mid-summer after having set and sized a normal crop load.  RAD may be a subset of the chronic decline condition or a completely different problem; causation is unknown at this time.

In some of these cases, tree death, initially thought to be either chronic or RAD, upon further testing can be ascribed to Fire Blight (Erwinia amylovora) or Crown/Root Rot (Phytophthora sp.).  In other cases, the causation is much less clear.  The decline and demise of apple trees in orchards up to the 8th leaf is currently a hotly debated topic among producers, researchers, extension specialists, industry consultants and nursery businesses in several states across the country, as well as parts of Canada.  A long list of potential causes of decline has been compiled by fruit workers throughout the eastern U.S., including winter injury, herbicide injury, ambrosia beetles and dogwood borer.

From recent studies, Cornell virologist Dr. Marc Fuchs and a team of Cornell Cooperative Extension Regional Specialist have discovered a resurgence in the incidence of latent viruses in recently planted apple orchards across New York State.  A latent virus is defined as a virus that doesn't normally produce visible symptoms in an infected tree.  Studies have indicated the potential for reduced tree growth and yield in some instances.  As reported at the 2018 CCE-ENYCHP Fruit & Vegetable Conference in February, latent viruses has been identified in over 50% of tested trees surveyed across the state in 2016 and 2017, with 12% of samples found to contain two or more species.  Virus species identified include Apple Chlorotic Leaf Spot Virus (ACLSV), Apple Stem Pitting Virus (ASPV), and Apple Stem Grooving Virus (ASGV).  ASPV has been the most commonly found latent virus to date.  While not a latent virus because it produces visible symptoms in the graft union and is directly implicated in tree death, Tomato Ringspot Virus (ToRSV) has also been observed, but at a comparatively low incidence.

The high incidence of latent viruses in young New York State apple orchards is troubling.  The interaction of virus infected budwood with newer rootstocks which are relatively unproven in commercial plantings is a cause of concern and the subject of ongoing research efforts.  The consequences of two or more viruses infecting a tree in a commercial orchard are not well defined.   Many acres of high-density orchards are being top-worked from less profitable varieties over to more popular choices.  First identified in Japan (Yanase, H. et. al. 1975) Top-Working Disease is a condition where the grafted scion suffers from gradual decline associated with the presence of latent virus in the grafting material.  Virus testing bud sticks or the host tree is not commonly practiced in Eastern New York orchards prior to top-working.  Roughly speaking, considering the high incidence of latent virus found in orchards of 10th leaf and younger, both host trees and bud sticks that fall in this age range should be considered suspect and be tested.

We cannot discount the possibility that the observed declines are due to interactions between multiple stressors, or that we are facing the possibility of a previously unrecognized problem.  Apple Luteovirus 1 (ALV1) has been found in three of four tested ENY orchards to date, with additional orchard survey testing currently underway.  A group of virologists including Dr. Ruhui Li of the USDA, Dr. Kari Peter of Penn State, and Dr. Ekaterina Nikolaeva of the Penn Dept. of Ag. discovered and described the new Luteovirus species in late 2016 (Lui et al. 2018).  Some species in the Luteovirus family are known plant pathogens.  The pathogenicity status of ALV1 as of the writing of this article has not been determined, research on this topic is currently underway in both Pennsylvania and New York.

The Apple Orchard Decline Survey

After numerous orchard visits by specialists in different states, a working group of regional apple professionals coordinated by Dr. Kari Peter of Penn State University has developed a comprehensive list of symptoms to identify declining orchards. From this list, extension professionals in Pennsylvania and New York developed an online survey of apple tree decline to facilitate consistency in the collection of orchard data, and the identification of commonalities.  Thirty-seven orchard surveys were completed in 2017, with more in progress.  The NYSDAM Apple Research and Development Program granted an extension to Dr. Tess Grasswitz (CCE-Lake Ontario Fruit Program) and I to continue survey data collection through the summer of 2018.  To date, the results are inconclusive and do not point to any specific causes.  Chlorotic foliage in declining trees has been identified as a common symptom.  More surveys (i.e. data points) are needed to reliably identify causal factors.

If you have apple blocks showing decline symptoms, your comprehensive and accurate completion of this online survey will be an essential first step in helping us identify possible causes and to search for solutions to the problem.  Please include data for all your declining blocks: the more data points we have, the more we will learn.  Please be assured that your specific farm information will be kept strictly confidential.  This statewide apple decline survey project has been funded by NYS apple producers under the auspices of the NYSDAM New York Apple Research & Development Program.  To complete the survey, please click here.  The survey may take a few moments to load.  If you would prefer to complete the survey through an in-person interview during a farm visit, please contact Dan Donahue at djd13@cornell.edu and arrange an appointment before the end of August.

References

Yanase, H., Sawamura, K., Mink, G.I. and Yamaguchi, A. (1975). VIRUSES CAUSING APPLE TOPWORKING DISEASE (TAKA TSUGI BYO) IN JAPAN. Acta Hortic. 44, 221-230 DOI: 10.17660/ActaHortic.1975.44.35

Lui, Huawei, Wu, Liping, Nikolaeva, Ekaterina, Peter, Kari, Liu, Zongrang, Molov, Dimitre, Cao, Mengji, Li, Rui. 2018.  Virology Journal.  15:85

An example of chronic apple decline in a 7th leaf Zestar! orchard.

Photo Credit:  D.J. Donahue