Many growers plant river birch in our area to avoid bronze birch borer problems seen on European white birch. With the right site, some supplemental care, and some luck, the river birch trees will thrive. As you might expect, there can be problems, and this year has been no exception. We are seeing birches with chlorosis problems, branch dieback, and even death in a few cases.
To get the most out of any plant species, you need to place it on the right site. According to Dr. Michael Dirr, a well-respected plantsman and author, river birch is best adapted to moist soils. He says it will survive in drier soils. It also does best in a slightly acidic soil, with 6.5 or below preferred. Chlorosis develops in high pH situations.
We have seen recent cases where birch trees grow normally in the spring, then all of the leaves on a branch or two quit growing, wither, and die. Eventually the affected branches die too. Often the tree has yellow-green leaves with branch tip death.
There are many diseases of birch, but few cause the dieback symptoms described. Leaf blights will not cause such injury, nor will viruses or wood rots. Potential pathogens involved in this decline are the canker fungi (Botryosphaeria, Nectria, Physalospora, Diaporthe, and many others) and a dieback disease caused by a fungus named Melanconium. The dieback disease is actually closely related to canker problems. In all cases the tree is infected when under stress. Melanconium dieback is known to cause a progressive dieback of upper branches, especially following periods of drought. We certainly had drought conditions last summer, fall, and winter to set up the decline process. The canker fungi could also infect trees predisposed by drought, injuries, flooding, borers, and so on. Usually more than one factor is involved, and it is impossible to determine which one started the decline.
Birches in the Midwest are also very likely to develop chlorosis, because so many of our landscape soils have a high pH. The iron or manganese in the soil are bound at high pH levels and cannot be absorbed by the plant. Foliage appears yellowed with green veins. Branches may die from the tips back, or entire branches may die. The tree canopy becomes progressively thinned.
Each birch in decline will be different, because the stress will vary with each tree and on the particular site. There is no current disease epidemic on birch. In most cases it appears that early drought stress compounded by high pH soils has probably stressed these trees, predisposing them to infection by canker and dieback fungi. There will be no easy cure. Remove dead limbs to avoid problems with wood rot. Water the trees in periods of drought stress, providing at least an inch of water per week. Consider using a shredded bark mulch over the root system to maintain a more uniform soil moisture. Test the soil and find out what pH level the tree is growing in so that you can determine whether an acidic fertilizer is needed. Look for cankered areas on the wood and remove these where possible. Last, do some research to find out the particular needs of the birch species you have planted. If planting a new birch, do the research first. Consult Report on Plant Disease 603, Iron Chlorosis of Woody Plants: Cause and Control, for details on how to manage this problem. The publication is available in Illinois Extension offices or on the Web at http://www.ag.uiuc.edu/%7Evista/horticul.htm.