Research activities working towards identifying causes of tree fruit decline
Griffiths, J., Ellouze, W. (2019) Research activities working towards identifying causes of tree fruit decline. Orchard Network Newsletter, December 2019. p. 6-7.
Fruit trees in Ontario and across the country are facing a serious threat which is killing many of them and research teams from Agriculture and Agri-Food Canada (AAFC) are working towards identifying the cause(s) of this devastating problem. New symptoms were first reported in 2014 in Pennsylvania apple orchards where the condition was named Rapid Apple Decline (RAD). Over the ensuing years RAD symptoms have been reported in the US and Canada. Certain growing regions have reported tree mortality of up to 40 per cent. Even more alarming was the appearance of similar symptoms in stone fruit orchards. This fruit tree condition has perplexed growers and scientists attempting to understand and identify the cause(s) and origin of this apple and stone fruit tree decline. Dramatic increases in tree mortality have continued in Ontario in the 2019 growing season. AAFC is funding a national research program to study RAD and identify the cause(s).
Apple decline has been reported across Canada in British Columbia, Ontario, Quebec, Nova Scotia, New Brunswick and Prince Edward Island. RAD, also known as Sudden Apple Decline (SAD), symptoms include loss of vigor, appearance of yellow-red leaves, rapid development of cankers extending upwards from the graft union, and sudden collapse of entire tree(s) within two weeks. Tree death was seemingly sporadic or random throughout the orchards. In the summer of 2018 and 2019, a total of 21 apple orchards where sampled in Canada; this included nine in British Columbia, seven in Ontario, two in Prince Edward Island and one in Nova Scotia. Tree samples were used to isolate potential fungal, bacterial and viral pathogens. Samples were taken from healthy and symptomatic trees. Fifteen fungal species were isolated belonging to Cylindrocarpon complex, Botryosphaearia, Dothidia, Diaporthe, Fusarium, Alternaria, and Trichoderma species. Molecular techniques were being used to confirm final species identification. Apple luteovirus 1, a newly discovered virus first reported in 2018 in the US, was detected in more than 50 per cent of the collected samples in Ontario. In the US, the virus was detected in SAD affected and healthy orchards. Thus, its association with the disease is unclear although it may be one of the contributing factors. Ethanol-baited traps for monitoring damaging Ambrosia beetles were installed last summer in apple orchards affected by SAD across Ontario. Species identification of trapped species of Ambrosia beetles has started. In spring 2020, the AAFC team in Vineland is planning to artificially expose apple trees, in a contained environment, to one or a combination of the collected pathogens to evaluate if the pathogens can cause SAD disease symptoms.
Stone Fruit Decline
The AAFC researchers in Vineland first became aware of a new and serious disease in apricots at the beginning of the summer of 2018. An apricot orchard in the Niagara region was brought to their attention that had up to half of newly planted trees dying. The AAFC team in Vineland began sampling from that site and quickly expanded to 13 different sites in the Niagara region. Since then, rapid and unexplained tree death has been recorded in apricots, peaches, plums, pears, and nectarines. The disease progression appears similar to SAD in that symptoms are primarily observed in young two- to eight-year-old trees with a rapid decline and sudden death within weeks. Frequently observed symptoms include individual branches dying off, increased suckering and rootstock overgrowth, increased gummosis and cankers on the tree, necrosis that begins at the graft union and progresses up the tree, and tree die back with a full load of mature fruit. This summer the team has intensively monitored and sampled from a plot of nectarines (Fantasia on Bailey rootstock) where at the end of the summer of 2018, there were 11 dead trees out of 170 total (7 per cent). By September 2019, they observed 42 dead trees (25 per cent), with another 118 showing symptoms of decline (69 per cent), and only 9 remaining healthy trees (5 per cent).
The AAFC Vineland team is continuing to process all tree fruit samples in an attempt to identify all potential pathogens present. In addition, abiotic factors such as herbicide/pesticide damage, drought or winter injury, and graft incompatibility are also being studied. The decline of tree fruits appears to be a complex and multi-factorial problem. Careful and thorough scientific investigations are being conducted at AAFC research centres in British Columbia, Ontario, Quebec and Nova Scotia; in a coordinated effort to identify the decline cause(s) and work towards solutions.