AUGUSTA, Maine - Maple trees failing to leaf out properly throughout the state including southwest Maine have been frequently reported this spring and summer, according to the Maine Forest Service's most recent newsletter.
All species of maple seem to be affected, experts say.
Several field visits to address this issue have resulted in common findings: no fungal, bacterial or viral disease has been seen to be associated with this. Further, insects have not been seen to be associated with the issue; only in a few cases what appears to be heavy pear thrips damage was noted (this damage is characterized by small, malformed leaves with tattering of various parts of the inner and marginal parts of leaves).
However, since thrips damage occurs very early in spring, after which the thrips transition to an underground part of their life cycle, this has not been confirmed. So, in the absence of insect or disease evidence, there are a few theories about what may be happening that center on environmental conditions that could, alone or in concert, be causing the observed symptoms.
The growing season of 2016 was historically dry. This may have led to inadequate moisture for normal tree growth and health maintenance including varying degrees of fine root dieback, Forest Service officials note. This may have led to poor bud development (buds for the 2017 growing season formed in 2016) and even dieback of branches in the upper crown. Other symptoms include late foliage, small foliage, no foliage on some parts of trees, discolored foliage and heavy seeding.
Abnormal/incomplete leaf development can be caused by several things from lack of resources to herbicide injury. In the cases seen this year, the cause is likely environmental and related to the 2016 drought. Trees affected by this may only partially leaf out this year, they may push out a late flush of small leaves, they may suffer various degrees of branch dieback and some may die completely.
Discolored foliage is likely the result of the prolonged cold and wet soils this spring leading to nutrient deficiencies causing maple leaves to be yellowish or purplish. This effect may be enhanced on more alkaline (less acidic) soils. The needed nutrients are actually present in adequate amounts in the soil, but due to inundated and cold soil, the chemical processes on the cellular level needed for the roots to absorb enough crucial nutrients for normal leaf development occur too slowly.
Officials assessing the conditions for now are taking a "wait and see"
approach. While some maples will certainly die, others may recover and leaf out fully next spring. In this meantime, efforts to support tree health can be considered such as supplemental watering during times of drought and fertilization. However, excessive watering and fertilization too late in the summer (typically not after mid-July) could lead to late flushes of growth that may not harden off soon enough in fall and be vulnerable to freeze damage, further stressing trees.
The effects of not fully leafing out on next winter and spring sugaring season were not addressed in the report.