What's Up With That?!

Birch Abnormal Growth Syndrome (BAGS) aka. Mouse Ear Disorder

Carl Rosen, Extension Soil Scientist and Karl Foord, Extension Educator


The strange leaf symptoms on this river birch tree, taken on August 7 (Photo 1, left) were diagnosed as birch abnormal growth syndrome or BAGS. New leaves are severely stunted and take on a mouse ear appearance. For many years the cause of this disorder was a mystery, but it is now known to be due to a deficiency of nickel.

Photo 1 (left): River Birch 'Summer Cascade' at planting time showing symptoms of nickel deficiency (BAGS). Photo taken August 7, Karl Foord.

Nickel is an element only recently shown to be essential for plant growth and is required in very small amounts. Almost all soils have enough nickel to support plant growth, but under some conditions, nickel deficiency can still occur. The mouse ear symptoms on this river birch were first seen when growing in a peat-based container mix, and were initially misdiagnosed as bud damage from a late frost. However, after the tree was transplanted into the soil in mid-May, the symptoms, after continuing for the next few months, have now begun to appear normal.

Based on research conducted at the University of Minnesota and in other areas of the country, BAGS almost exclusively occurs on river birch when grown in peat-based media and can be corrected by soil or foliar applications of nickel.

Photo 2 (right): Close up of 'Summer Cascade' river birch leaves with BAGS. Photo taken August 7, Karl Foord

Research has also shown that when soil is added to the peat media (20-30% by volume), the nickel deficiency symptoms will not occur, suggesting that there is enough nickel in the added soil to meet the nickel requirements of the plant.
In cases where the symptoms are most severe, an analysis of the peat has shown excessively high levels of zinc. These high levels of zinc in the peat likely accentuate the nickel deficiency. Therefore, adding soil to the peat mix may help by 1)alleviating BAGS symptoms by adding the needed nickel,  and 2) by tying up or diluting some of the excessive zinc in the peat.

Photo 3 (left): Up close. Mouse ear symptoms of nickel deficiency on peat-based media. Carl Rosen.

As shown by the picture taken on September 14 (Photo 4, below), the tree has nearly recovered from its mouse ear symptoms and is expected to make a complete recovery once the roots have fully established into the native soil. 

In general, BAGS has been a problem most apparent to the nursery industry, as trees showing the symptoms are usually not sold. However, if the problem does occur in containers, it can be corrected with nickel applications or by transplanting to a medium containing at least 20% soil. Soils in Minnesota have enough nickel to support plant growth, therefore nickel application to river birch growing in the landscape is not necessary.

Photo 4 (left): Recovery of river birch 'Summer Cascade' from BAGS.  Photo taken September 14, Karl Foord.