In most every orchard visited over the past three years I’ve found a block or two of newly planted dwarf apple with severe dogwood borer injury. In many cases the infestation has led to severe injury including girdling of the tree and high percentages of tree decline. We are seeing increasing losses as the years go by.
In recent surveys throughout New York, dogwood borer, especially in Macoun, have been implicated in tree decline, often associated with phytophthora root, crown and collar rots. DWB and black stem borer, a newly emerging ambrosia beetle infesting trees undergoing excessive water stress, are often found together infesting the trunks of high density planting systems.
Stress: Given the past five years of hurricane and torrential rains followed by years of severe early and mid-season drought, I’m more convinced that the dwarfing series rootstocks in the northeast need to be managed with greater concern for irrigation prior to the expression of tree stress, consideration of herbicide selection to reduce canker injury to the trunk and yearly management for DWB larva.
Why is this happening now: Bottom line… New high density apple production systems using smaller rootstock with narrow trunk diameter produce trees that stress more easily. Given trunk diameter, they are more susceptible to herbicide injury, produce more rooting initials and flakes to foster DWB development. The addition of herbicide to a wound produced by DWB may increase uptake of the herbicide to the cambiam layer where DWB larva feed. A smaller rooting zone in times of drought or flood become insufficient to support relatively high crop loads. High density systems have become increasingly vulnerability to environmental extremes, and under moderate to severe stress, produce ethanol (ETOH) that acts as a cue for the female black stem borer beetle to locate and begin infestations. These beetles are forest dwellers, often moving from infested woodland edge hardwoods to apple trees under stress. Boring into the center of the tree they are unreachable by insecticide applications. The female provide fungal spores to developing larva within the gallery, feeding on the growing fungus within the tree. Some species of fungus were found to be detrimental to the tree, plugging up the cambiam layer, restricting nutrients to the tree during fruit development. The perfect storm of these combined conditions causes the tree to simply collapse.
The age of apple trees we have seen infested with these conditions range from Pink Lady on M9/Nic during year of planting to 8-9 year old Fuji on M.9 in full production.
To apply generalizations to this topic, I’ll begin with some foundations for present tree fruit planting systems. Horticultural studies conducted since the 1990’s have shown that high density apple production, utilizing full dwarfing rootstocks, planted on 3′ x 11′ spacing, employ 1200 to 2000 trees to the acre, capable of producing 1000 bu./A and up. Full dwarfing trees maintain a small diameter trunk and a confined rooting system, achieving and maintaining tree height from the third to fourth leaf at 10-12 feet in height.
Nic 29® (RN 29 cv.) is a Malling 9 type rootstock, with a more robust root system than Malling 9. Geneva® 11 (G 11), is a cross of M 26 x Robusta 5 hybrid, similar in vigor to EMLA 26 and should be supported. Trees are extremely precocious, productive and more resistant to wooly aphid then EMLA 26, somewhat resistant to fireblight and collar rot, resisting suckering.
The G.11 rootstock produces fewer root initials, however, flaking bark on this rootstock can also act as a location for borers. Although there are fewer DWB in this variety, they should not be overlooked during scouting. A PDF on ‘Characteristics of apple rootstock burrknot expression’ is a very helpful resource in rootstock selection.
Budagovsky 9 (B 9) is a cross between M 8 and Red Standard, a very hardy rootstock of Russian origin. B9 is full dwarf rootstock producing a tree with the same vigor as M 9, requiring support, extremely cold hardy, providing winter frost or damage and resistant to fireblight, collar rot, mildly resistant to powdery mildew and apple scab. Fruiting too early or too hard may create stunting and slow growth.
Malling 9 (M 9) rootstock selection is recommended to be planted in well-drained soil with structural support such as a trellis system. However, the M 9 is very drought sensitive in light soil and stresses easily from the year of planting on through its commercial lifespan, requiring adequate irrigation.
Injury to the trunk, such as herbicide injury giving rise to cankering and insect injury which reduces movement of nutrients to the canopy, as well as environmental conditions such as very wet soil or conditions of drought, cause trees to produce ethanol or ethyl alcohol (ETOH), which is a metabolic response to stress.
The M9 produces prolific above ground rooting initials which inhibit the movement of nutrients to the tree canopy. These rooting initials are course structures with facets that offer ideal habitat sites for dogwood borer adults egg laying and larval development. DWB will move from these sites during dry conditions following the cambium for mositure and nutrition to further partially or completely girdle the tree.
MORE BORER: High density systems carrying 1200 to 2000 trees per acre can carry very high populations of endemic dogwood borer, simply by the shear number of infestation sites utilized. We have found an average of 1 to 4 larva per 3-4″ tree caliper, in highly infested trees that have had successive infestations for three or more years. In our 2015 surveys, tree losses in highly infested blocks exceeded 10%, with increasing decline and loss on the near horizon.
Dogwood borer, Synanthedon scitula, (DWB) is very prevalent throughout New England and NY orchards with newly planted and young high-density slender spindle blocks plantings using dwarfing rootstock. Dogwood borer have caused considerable damage to young establishing trees. The very small diameter of the trunk coupled with very small root mass can not outgrow sustained larval feeding, especially in years of low soil moisture (Spring 2015) and summer drought (Summer 2015).
The dogwood borer is a common pest in the burr knots of apple trees on clonal rootstocks. The dwarfing rootstocks M.9, B9, and new Geneva series including G11, the bark of which have a tendency to split and flake or create burr knots which act as the substrate for the adult clear wing moth to lay eggs. These root initials provide ideal locations for the dogwood borer to lay eggs and for larva to burrow and feed upon.
Management: Simply put, highly dwarfing rootstock are very susceptible to drought, especially in light loam soils. In addition, it can’t be highlighted enough, that larva of the clear wing DWB moth can cause significant damage to cambium trunk tissue, adding to tree stress, subsequent yield and tree losses in high density orchards on M9 and G11 series stock if left unmanaged for multiple years.
Cultural Management: Undiluted white latex paint applied by brush to the lower trunk before egg laying will significantly reduce infestations. Painting trunks can reduce the attractiveness of trunks to DWB but in most cases will not completely prevent infestations, requiring yearly applications.
Remove Tree guards. Although meadow and pine vole injury is of concern, tree guards employed against mouse damage act as harborage for DWB and should not be left on longer than necessary.
Mating Disruption: If the revocation of Lorsban by EPA is imminent, then becoming familiar with the use of DWB mating disruption should be strongly considered as the backbone of future DWB management practices. We have not see DWB flight to date and as such, there is still time to consider this option in eastern Long Island, the Mid to Upper Hudson Valley and Lake Champlain fruit growing regions.
Prior to the adult emergence, DWB management could be employed using mating disruption by applying 150 ties per acre in year one followed by 100 per acre there after. Research conducted by Art Agnello and Dave Kain has shown good results in NY using this approach. “Assuming that efficacy of treatment with the pheromone is adequate, the ease of applying the pheromone dispensers, plus the fact that no special equipment is needed, and presumably its greater worker safety, may make this tactic attractive to growers looking for a good alternative approach.” Further information on dogwood borer mating disruption.
Chemical Control. Lorsban: Put aside all the press regarding the merger between Dow and DuPont, and future possibilities of the EPA canceling chlorpyrifos registration, and recognize the importance of this material for DWB management this season. A single application of Lorsban (chlorpyrifos) this season can still be made to tree fruit if you have not yet applied chlorpyrifos.
If chlorpyrifos (Lorsban or similar generics) is used in a pre-bloom foliar application, it cannot be used in a post bloom trunk application. A pre-bloom chlorpyrifos application made at early pink will have a considerable impact on San Jose scale (SJS), rosy apple aphid (RAA), emerging obliquebanded leafroller larvae (OBLR), mullen and tarnish plant bug (MPB & TPB), European apple sawfly (EAS) and white apple leafhopper (WALH). Most importantly, if bees are brought into a block in a season of cooler temperature and delayed petal fall of later varieties, a pink application provides increased management of early migrating plum curculio. This will reduce the pressure to remove bees from a mixed block while active pollinators continue to work king and secondary flowers. It also provides a bit of insurance if beekeepers are delayed in removing hives from mixed variety blocks.
A directed, coarse, trunk spray is the recommended method of application in blocks with active borer larva. Simply increasing the lower nozzles on the airblast sprayer will provide some control, but in the end, will offer less then stellar results.
Assail can be used in place of Lorsban in trunk applications if chlorpyrifos had been used in a pre-bloom application. Assail 30SG applied at 8 oz/acre has a 7 day PHI and 12 hour re-entry interval providing a moderate degree of efficacy YET requires multiple applications to be most effective.