Who Let the Dogs Out. Dogwood Borer Management on M9 Rootstock.

From the Hudson to the Champlain Valley, its hard to find a single orchard of slender spindle high density apple on M-9 rootstock free of dogwood borer (DWB), Synanthedon scitula. It’s rare to find Dogwood borer on healthy bark or pruning wounds in apple trees, however, the adult will lay eggs on root initials or burrknot on certain rootstock varieties, giving rise to DWB adult egg laying and larval feeding within this susceptible area of the tree. The M-9 rootstock is quite susceptible to burrknots, very sensitive to fireblight and its shallow root system drought sensitive. Its near relative, Bud-9 has much less burrknot initials, having greater resistance to fireblight and Phytopthera & quite cold hardy. The detailed publication of the NC140 rootstock trial provides descriptions of rootstock and interstems selection strengths and weaknesses.

Given the ‘slender’ size of the rootstock, its likely that most DWB infestations will reduce vigor and yield, increase stress and potentially increase disease from DWB in young plantings on M-9 if left unmanaged. Telltale signs of infestation will be found within and along the edge of burrknots growing on the exposed portion of clonal rootstocks. Pupa cases protrude from the rootstock well after the adults have emerged and reddish frass from larvae feeding often accompanied by ozze within ‘wet’ areas of the burrknot will often be inhabited by larva in May with the onset of pupation beginning in June.

My concern this season is not just the presence of DWB in rootstock but what appears to be sites of disease, likely established last season where DWB are actively present. Phytophthora are fungus-like organisms that are favoured by wet conditions and can cause crown, collar and root rot on fruit trees. Between the 3th of June and the 2nd of July in 2013 we received 11.2″ of rain. If trees were predisposed to DWB injury on M-9, it may have been a period in some sites for Phytophthora blight to spread and move into surface wounds, such as those created by DWB. We have seen this in the Champlain and Mid-Hudson Valley regions this season with associated die back and crown, collar and root rotting on M-9.

The American plum borer larva (APB), Euzophera semifuneralis (Walker), are also found on apple in habitats similar to those preferred by the dogwood borer. Borers can be monitored by checking under tree guards in the spring to locate active infestations as guards applied to newly planted trees encourage adult egg laying and larval presence. The larvae size of the two species are similar. DWB larva are white to cream-colored, with one row of crochets on the abdominal prolegs, while the APB larva is dusky purple to gray in color, and has two rows of crochets on the prolegs. Pheromone traps can be used to estimate the timing of peak flight in the Northeast. Adult emergence begins in early June and continues into early September, peaking in mid July. Traps should be placed at 4 feet above the ground for optimum captures.

If DWB larva are found in burrknots of young trees, trunk applications of Lorsban should be applied using a course directed application to tree trunks at the earliest window to reduce the present larval population and prevent further infestation by newly emerging adult egg laying as they take to the wing over the next few weeks.