by: Karen Ripley, DNR Forest Health Program Manager
In addition to a continuous race between progress with the lawn mower and a jungle of wet grass, our wet spring has allowed many tree foliage pathogens to flourish. While always present to some extent, when damp weather conditions and susceptible foliage occur simultaneously, a huge irruption of disease symptoms can occur. A May 31 walk around my Vashon Island neighborhood provided opportunities to view several interesting cases (on poplar, madrone and shore pine) and clear insight into why tree foliage infections are frequently known as “Mother’s Day Diseases.”
Decades ago, an original farm that’s now been divided and developed into many ownership lots had Lombardy poplar (Populus nigra var. likely nigra) lining several of its roads and boundaries. Dozens of these mature trees remain, a few in lines, a few alone. This year much of the foliage and even some new shoots on those trees have dried and curled to a brown gray color. Many damaged leaves had fallen, leaving mostly bare trees with only a few green leaves here and there at the end of May. The spores of fungal pathogens must have been present in decaying leaves, beneath the trees. Released in damp weather, these spores are prone to drying out and are best able to penetrate succulent spring foliage that hasn’t developed waxy coatings or hardened cell walls. The 2012 weather must have provided a perfect timing opportunity with abundant spores present just as the new growth was coming out in April. And it got hammered.
Incidentally, last year (2011) was a very severe year for foliage diseases of black cottonwood (Populus trichocarpa) in Kittitas and Chelan counties. Many I-90 travelers wondered why all the trees “were suddenly dying”. They weren’t dying. They’d been severely hit by foliage diseases (diagnosed as Septoria spp. and/or Venturia spp. by U.S. Forest Service pathologists). Local observers say the trees do not appear to be affected in 2012. But, my neighborhood poplars must have had the right convergence of spores and foliage this year. Yikes! They look terrible! Within the next few weeks, all but the weakest trees are likely to produce another generation of leaves. They will likely be smaller in size and less abundant, but will provide an opportunity for photosynthesis and will see the trees through until 2013.
Pacific madrone (aka madrone), Arbutus menziesii, is also susceptible to many foliage pathogens. But the infection cycle of this “broadleaf evergreen” is a little different from the deciduous poplars described above. Think about it. The grey-brown infected leaves shown in the photo were not youthful and succulent in spring 2012 when the poplar leaves were getting infected. They were about a year old. However, madrone foliage is very easily injured by frost and winter cold. It’s that injury which makes the leaves vulnerable to early spring foliage pathogens. Infected leaves darken rapidly, wilt and dry out. This is the second year of extremely high levels of “madrone leaf blight” in the Puget Sound area.
Dr. Marianne Elliott of WSU Puyallup provides information about madrone leaf blight on a WSU website. She’s currently seeking reports of severely damaged madrone, but after the pale flowers and bright green 2012 foliage have emerged in May, I’m sure those reports have dropped off. The madrones with new foliage just don’t look as starkly awful as they did a few weeks ago. So, take a closer look at your local madrones!
The third photo depicts twigs of a shore pine (lodgepole) (Pinus contorta). Although native to western Washington, this specific tree is of uncertain origin. As part of the attempt to afforest our scotch broom and blackberry covered land, I rescued several small, unwanted pines from a friend’s North Seattle yard about 20 years ago. All died but this one, and it’s never thrived. Frequent or chronic bouts with foliage pathogens reduce the number of years of foliage held by a tree and commonly indicate an “off site” conifer, growing away from the suitable climate range its parents developed in. On this tree, the 2010 needles (green) seem to have escaped injury. The 2011 needles are almost all diseased. Typical of conifer foliage diseases, these needles are discolored in a non-uniform way along each needle and not every needle is affected the same amount. Depending on the specific pathogen, sometimes the damaged needles will also be marked by small fruiting bodies or burst blisters where spores have emerged.
Usually conifer foliage pathogens have a longer infection and disease cycle than those that affect broadleaf trees. The brownish 2011 needles are likely emitting fungus spores now. The 2012 needles, just starting to emerge at the end of May, are vulnerable and may become infected, but will likely remain on the tree for another year as the fungus grows inside the needle. My guess is that in 2013, the branch tips will look a lot like they do now. However, there will be a gap where the 2011 needles (now brown) will have fallen off. No wonder this tree doesn’t thrive … it’s only holding one to two years of needles, when a healthy tree would be holding four to seven years of needles.
So, if you thought a walk around the yard near Mother’s Day would only provide a chance to admire peak-bloom lilacs and rhododendrons, you may have been missing foliage disease activity on many trees. Take a closer look now. The 2012 spring weather conditions have created a viewing opportunity that’s been hard to ignore.