The phone has been ringing a lot lately and my office is filled with samples of dead branches and partially eaten leaves. Spring is my favorite time of year, but it’s also the season when many “sick tree” problems become evident, from moisture stress to bark beetles, to defoliating insects. And this year is no exception.

At the top of the list are Douglas-fir branch and top dieback and outright mortality, followed by an apparent outbreak of the fruit tree leafroller in oaks. But there is plenty of other action to go around, including bark beetles in pines and white fir, among others.

 

It’s mostly about the weather

In the big picture, many trees are likely still suffering from the effects of 2013, the driest year on record here in the Rogue Valley. Medford received 8.97 inches of precipitation over the year, less than half the average value. To put this figure into perspective, areas that average less than 10” of precipitation in eastern Oregon typically don’t support tree cover at all!

 

2013 was the driest on record in Medford

Eastern Oregon’s Alvord desert gets less than 10 inches of rain per year on average.

December 2013 was notable in having a few days of extremely cold temperatures, which killed some trees and shrubs outright, and injured others. Very cold temperatures can injure sapwood and impede water transport, and damage may not show up until later in the season when water needs increase with warmer temperatures.

In 2014, we had slightly above normal precipitation, but it was the warmest year on record in the Rogue Valley, in terms of average temperature. We also experienced the hottest summer on record.   Warmer temperatures increased evapotranspiration from trees, which resulted in increased moisture stress, compounding the moisture deficit from 2013.

2014’s snowpack was about 50% of average in the Cascades, and this year of course was even worse. Thus, despite increased rainfall from last fall through this winter, we are still in the grips of a moderate to severe drought.

In a nutshell, it’s been very dry and hot over the past 2 years and many trees have been and continue to suffer from moisture stress. A chronic lack of water results in reduced growth and reduced production of defensive compounds that can ward off pests. That’s why drought-stressed trees are more vulnerable to “secondary” insects and diseases that wouldn’t affect more vigorous trees. Severe drought may also reduce tree water below critical levels, resulting in the death of branches or even whole trees. This ODF pest note does a great job of explaining the interactions of water stress and insects and diseases.

 

The culprits

Given our stressed, vulnerable trees, which specific insect and disease pests are doing trees in?  For Douglas-fir, the flatheaded fir borer is probably responsible killing many trees on marginal sites while various other insect pests may be responsible for branch dieback and top kill, especially on smaller, younger trees.  The Phomopsis canker is also likely implicated in top kill and mortality of smaller trees.

 

Too many trees in the wrong places

Most people in the woodlands and forestry worlds are familiar with the notion that fire exclusion over 50-100+ years has likely resulted in greater density of trees on many sites, leading to increased competition and stress for a limited supply of soil moisture. That makes trees a lot less resilient to drought when it inevitably occurs.

Along with increased density is the probable encroachment of Douglas-fir on marginal, lower elevation sites where it was probably much less common historically. Pines and oaks often do better on these sites and Douglas-fir can really struggle.

Dead and dying Douglas-fir on the slopes of Johns Peak, just went of Central Point. Not a great DF site.

What to do?

The above-mentioned ODF pest note summarizes it well, and I quote from that document below (I’ve made a few additions):

Most tree damage occurs on disturbed sites and is due to a combination of factors including soil conditions, tree species, and weather patterns. It is unlikely that stress will be alleviated by simply altering a single factor. Rather, improvement will come from an accumulation of many moderate changes to relieve stress and increase vigor.

• Prevent soil compaction caused by vehicle or animal traffic near trees. Livestock can compact surface soils and damage fine roots, most of which lie within a foot of the soil surface . Clay soils are especially vulnerable.
• Avoid direct damage to trees and roots by grazing animals or by machinery.
• Reduce competing vegetation via thinning and brushing. Favor trees that are more drought resistant. The research is clear that thinning in pine stands can improve tree resistance to bark beetle attack. It’s less clear if the same if true for Douglas-fir and the flatheaded fir borer, but the basic principle of improving tree vigor to increase pest resistance still holds.
• Apply mulch to maintain soil moisture (1-3 inches is usually sufficient).
• Irrigate landscape trees during dry weather. Apply water slowly over many hours so it penetrates to tree roots or use drip irrigation lines.
• Do not alter drainage patterns (ditches, ponds, etc.) near established trees.
• Plant trees that are well suited for the site; use local seed sources and species that are adapted to your soil types. On sites where Douglas-fir mortality is occurring it may be advisable to plant ponderosa pine or hardwoods.
• If insect larvae or branch/stem cankers are evident, prune and destroy affected branches to reduce the spread of these agents.

 

What about the oaks?

 Landowners have been calling in with reports about moth-eaten oak leaves and “dying trees.” The culprit has not yet been positively identified, but may be the fruit tree leafroller (Archips argyrospila).  The larvae of this small moth feed in spring on new leaves, resulting in leaves that are distorted and ragged in appearance.  Young leaves are rolled and tied with silk.  There is one generation per year.  If you are walking under oak trees you may run into small wiggling larvae dangling from threads.    There have been reports from Jackson and Josephine County this year, so the problem appears widespread.

In outbreak years trees may appear partly or completed defoliated, resulting in fears that “My oaks are dying!”

However, as alarming as it may look, tree mortality seldom if ever occurs from this pest.  If the pest turns out not to be the fruit tree leafroller, it is likely something similar, with similar effects.

Action needed: None.  Birds and other natural enemies of the insect will deplete its population and the infestation will run its course.  Trees will likely leaf out again.  Those who absolutely must do something could spray BT but in addition to being expensive and logistically challenging, is very unlikely to dampen population levels with such a widespread outbreak.

Reference: PP 16-19 in “A Field Guide to Insects and Diseases of California Oaks.”

http://www.suddenoakdeath.org/pdf/psw_gtr197.pdf