A Tale of Two Trees: Apple Decline Associated With Trunk Borers on Long Island

The Macoun apple, was named after Canadian fruit grower W.T. Macoun and developed in 1923 at the New York State Agricultural Experiment Station in Geneva by R. Wellington. Over the past 10 years we have seen this variety in decline throughout the Hudson Valley with high numbers of trees dying from what appears to be a variety of stress inducing conditions. There have been observations of ‘winter injury’ which include trunk cankers, trunk and scaffold scaling of the bark, dogwood borer in burr knot rooting initials, graft union necrosis from any number of unknown causes and Phytophthora rot, a genus of plant-damaging Oomycetes fungal pathogens. Dr. Rosenberger has suggested that in some cases, the use of Round-up (Glyphosate) may be at least in part, causing herbicide injury to trunks to induce stress. “Additional desiccation from herbicide exposure combined with normal water stress during hot dry periods may predispose the trunks to invasion by Botryosphaeria dothidea, a canker pathogen that is incapable killing the cambium in healthy functioning trees, but which becomes very pathogenic in drought-stressed trees”. As far as we can tell, the tree decline observed in Macoun, can be directly associated with some form of stress, arising from environmental (abiotic), insect or disease (biotic) or chemically induced factors.

During a recent tour of orchards in eastern and central Long Island, we found apple trees in full leaf, with developing fruit, that were turning yellow, drying out above and below the graft union and slowly dying. One site had experienced two hurricanes over the past three years, with standing salt water for 24 hours, and some level of what appeared to be winter injury. The yellowing tree on this first site was Honeycrisp on M-9 rootstock. It was dug and assessed with signs of cambian browning and dead wood below the bark layer. The M-9 rootstock, used widely in high density plantings, will produce many root initials or burr knots (seen in the image to the left), creating site very attractive to the complex of tree borers, especially the dogwood. This tree was one of hundreds of apples in this planting that had a significant number of live dogwood borers along the graft union, causing a constriction of water and nutrient movement to the tree canopy. As water requirements increased with demand for fruit production, the tree was unable to maintain needed supply of moisture to the canopy and began to yellow under stress.

The tree, a Macoun on M-26 rootstock on the second site appeared to be on well drained soil, planted on a slope with good air drainage. There was no sign of herbicide injury such as cankered, flaking or scaling bark or winter injury. Yet the tree was completely yellowed with browning fruit. The tree had completely shut down. The orchard had also experienced two hurricanes over the past three years, with standing water. Research in Ohio and North Carolina have shown that trees growing in saturated soils (water stress) tend to produce ethyl alcohol as the root zone goes anaerobic during wet periods. The ethyl alcohol acts as a host find mechanism to an insect complex known as Ambrosia Beetle. Trees undergoing this stress can become hosts for the black stem borer, Xylosandrus germanus, which was found infesting this lone Macoun tree in the middle of a block of what appeared to be healthy trees. The Long Island area is the site where Xylosandrus germanus is thought to have been first introduced into the US in 1932. (Gill et al. 1998). The black stem borer is a concern as it attacks apparently healthy plants that have undergone this form of ‘root zone anaerobic stress’. Entrance holes are round, approximately 1 mm in diameter, often having long toothpick-like strings of frass or compacted boring dust emerging from these holes.

Bucket trapping for newly emerged adults should begin at green tip using ethanol solution.

A recent article on Black Stem Borer can be found in Scaffolds, providing a western NY perspective on biology and management of this insect pest.