A Heavy Metal Cure
for Dreadful Birch
Mouse ear (leaf curl, little leaf, squirrel ear) has been a problem
in container-grown river birch (Betula nigra) since the 1970s.
I started working on the problem in 1996 when it first showed up at
a nursery in south Georgia. Mouse ear has been noticed in several
southeastern states as well as in Minnesota, Ohio, Oregon and
Wisconsin. The problem has caused considerable economic impact in
the southeast with some growers saying that they will never grow
river birch again. I am happy to report that the problem has been
solved! Mouse ear is caused by a deficiency of nickel!
The disorder is easy
to detect in nurseries as the plants don't grow well and appear to
have been "sheared" into their stunted form. The leaves are small,
wrinkled often darker green in color, commonly cupped and necrotic
margins. Chlorosis between the veins is generally lacking on
symptomatic leaves. New growth also has shortened internodes which
gives a witches-broom appearance.
Plant pathogens, eriophyid mites and
sulfonyl urea herbicides have been suggested over the years as
possible causes of mouse ear but could not be detected or confirmed.
I have seen problems on plants from tissue-culture liners to large
containers. Health of the root system appears to be important. The
problem appears to be correlated with plants that have been in
containers for too long and are rootbound. The problem occurs in
plants grown using the pot-in-pot system. Water stress also appears
to be related to the problem. One very good grower I work with often
has the problem on his border rows near the road where the
sprinklers do not provide enough overlap to adequately water the
plants. This generally occurs later in the summer when the roots
have filled the container. Containers on border rows are also
exposed to increased solar radiation.
I have seen the problem on Dura-Hear,
Heritage and seedlings from several sources. The problem occurs on
plants fertilized with controlled release fertilizers and in plants
receiving fertigation. Symptoms have been seen on plants with high
and low soluble salt readings, high and low substrate pH, with or
without healthy root systems, and early, mid and late season. Plants
may appear fine in the fall and have the problem when growth resumes
in the spring. Strangely, some plants may only show the problem on
one branch, or only certain plants in a block may show the [problem.
Symptoms may be uniform thru an entire block or quite often appear
randomly. Taking cuttings from container grown plants which are on
the verge of deficiency versus cuttings from plants growing i the
field probably contributes to the random appearance of symptoms.
I have only encountered mouse ear on
river birch in the field three items and twice it was associated
with excessive calcium and high pH. The third find was ion a plant
growing next to a pond (roots in the water) that had been treated
with copper for years. River birch is tolerant of high pH soils and
in native areas of Ohio only found on soils with a pH of 5.5 or
less. Plants with the problems that are transplanted into the field
generally grow out of the problem within one season. Plants growing
in containers which root into the native soil also do not express
the problem or grow out of the disorder.
After trying numerous micronutrients
over the years, I applied nickel sulfate to river birch trees with
sever mouse ear at a commercial nursery in June of 2003 and one week
later certain treatments could be picked out of the block of trees
because they were producing normal leaves. At 16 days after
treatment, up to two inches of new growth had occurred on certain
treatments. At 30 days after treatment with nickel, shoot length
increased 60 percent, leaf area increased 83 percent, and leaf dry
mass increased 81 percent compared to non-treated river birch rowing
in #15 containers. River birch trees showing improved growth also
had an increase in foliar nickel concentrations compared with the
mouse-eared controls. Trees with nickel concentrations in the
foliage of 5ppm or higher really have symptoms whereas trees with
<2ppm foliar nickel are often symptomatic. Several studies looking
at different rates, sources and formulation of nickel have since
been conducted. Not much is known about the physiological
requirements of nickel. It is required by certain monocots to
complete their life cycle and is required by the urease enzyme to
convert urea to ammonia. Nickel is common in low concentrations in
mineral soils. A majority of the literature deals with Ni toxicity
in the environment, with fewer papers discussing the positive side
effects of Ni as a beneficial plant nutrient. Nickel my improve
nitrogen use efficiency in certain groups of plants. Being a heavy
metal, my research has focused on developing the safest methods of
application and the lowest use rates possible. On trees with severe
symptoms, 100ppm nickel is the minimal dose required to reverse the
problem. Lower rates may be useful for prevention. Keeping root
systems healthy the the pH down (5.0-5.5) may be beneficial. Nickel
is taken up by fine feeder roots and decreasing the pH increases the
availability of nickel. Reducing water and heart stress should also
help minimize mouse ear.
Nickel was recognized as a fertilizer
in 2004 and NIPAN, LLC has registered a nickel lignosulfonate
product known as Advance NiTM. For more information on mouse ear and
river birch, see the 2004 proceedings of the SNA Research Conference
or the March 2005 issue of the Journal of Environmental
Horticulture. For product information visit
NIPAN
