From Poplar Galleries to Manganese

Sept-Oct 2012

Interesting Find on Eastern Cottonwood
I had a call from an arborist recently who observed significant mortality and dieback in a stand of 80-year old Populus deltoides in Eastern Ontario. During tree removal, abundant larval tunnels were found in the cambium just under the thick bark. 

If you’ve ever seen emerald ash borer or bronze birch borer larval galleries, the hair on the back of your neck would be standing up when you looked at these poplar logs. The galleries on these eastern cottonwood trees closely resemble those of a metallic wood-boring beetle (a Buprestid) in the same genus as EAB and BBB. In fact, it looks very much like Agrilus liragus, the bronze poplar borer.  

The trees in this park were under a lot of stress from dry, sandy compacted soils (increased foot traffic over the decades). And yes, even though it’s sand, it can still compact and become quite impenetrable (I have done field measurements on compacted sand to look at water movement in nursery production systems). In the literature it states that bronze poplar borer is not usually a pest unless the trees are being compromised by poor growing conditions. The arborist has started aerating the soil to alleviate some of the compaction stress in an effort to help save the few remaining trees. Infested trees are being removed and destroyed and the site will be monitored for continued signs of dieback. 

This pest has been detected across Canada and in several US states. The bronze poplar borer is not a common pest and I am not sure about its distribution in southern Ontario. If you are seeing the characteristic larval galleries on dead or dying eastern cottonwood, please drop me a line.  

Manganese and Plant Disease
Every once in a while I come across really interesting resources for ornamental plant production and maintenance. I recently ordered the book Mineral Nutrition and Plant Disease and it was an excellent find. In my winter workshops, I talk about soil chemistry and fertility in the landscape and how important these factors are for our ornamental plants. Some of the trends we notice after looking at several soil test results are high pH and the poor availability of particular micronutrients.

What we have often noticed is that the plant available form of manganese (Mn) is often quite low because of high soil pH and soil microbiology. Manganese may be present in the soil, but the high pH keeps it in a form that is not available for uptake by plants. Since manganese is a factor in photosynthesis and the synthesis of chlorophyll (the green pigment in plant tissue), manganese deficiency often results in interveinal chlorosis and leaf necrosis. It can be quite severe on some pH sensitive trees like red oak and paper birch.  

Manganese & Plant Health  
Resisting Disease. Manganese is also a crucial element in a plant’s ability to resist disease. Manganese deficiency adversely affects photosynthesis, root growth and reduces the synthesis of secondary compounds that play an important role in fighting off diseases and other pests. Researchers have noticed the correlation between deficient levels of Mn and increased levels of diseases in crops for years. Supplementing has shown promising results. For instance, foliar application of Mn has been used to reduce the incidence of powdery mildew on wheat. In this age of increasing plant pests and decreasing pesticide use, this is all very interesting.  

Soil Chemistry. It is very difficult to lower soil pH and many of you probably know this all to well from frustrating experience. Most soils are very good at buffering against changes in the soil chemistry. So why not just add some of the deficient nutrient and call it a day? Well, if you apply manganese to a high pH soil, it will quickly change into a form that is unavailable to plants. And if you apply it to the foliage, some of it will get into the foliage, but foliar uptake is really only effective during leaf emergence and manganese doesn’t seem to move very well from the leaves to the rest of the plant. (Here’s the part where I would blow my bangs away from my forehead – if I had bangs). If you were to buy the chelated of manganese form (e.g. water soluble) and apply it to the soil, then it will be accessible for root uptake in the plant available form. This is a labour intensive method but it works. And in the realm of the tree care industry, I think the economics are there for this to be a valuable service.  

I think we probably only understand a small fraction of the science behind the whole soil chemistry/biology/physical complex. It seems as though each time I learn something new, I have about 17 questions after I have a chance to digest it. 

Perhaps it is a little short-sighted to believe that there is a magic bullet like the application of one single nutrient that we can apply to solve all of the plant problems at a site. After all, soil pH affects microbial organisms which both affect the availability of manganese, which affects the uptake of iron, copper and calcium, which in turn affects cell wall integrity, which affects resistance to pests and so on. These factors are all interrelated. But, increased plant health has been observed with the application of deficient nutrients such as manganese in some cropping situations. So, perhaps it is just our understanding of how it all works that is deficient. 


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