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ISSUE 7  JUNE 17, 1999

 

WOOD MULCHING IS GOOD MOST OF THE TIME.....

    Wood mulch storage techniques can significantly affect product quality. Wood chips and bark
nuggets have long been used as mulches in landscapes. The benefits of using wood by-products as
landscape mulch are many. Organic mulches stabilize soil, moderate soil temperature, retard weed
development, reduce evaporation of soil moisture, and enhance the aesthetic qualities of landscapes.
However, there is a significant, albeit rare, problem that may occur when using previously stockpiled
wood by-products for mulching. This problem is illustrated in the following actual landscaper experience.

    A landscape contractor in the eastern U.S. delivered a load of mulch to a residential client on a
Saturday. On Sunday, the homeowner spread the mulch around spring bulbs, perennials, and shrubs
in a mixed border. Within one-half hour, many of the plants began to wilt. By Monday, the foliage of
tulips, Phlox, Astilbe, and violets had turned white or light tan as if bleach had been thrown over the
plants. Turf adjacent to the mulch also turned pale yellow, but Rhododendrons and daffodils in the
border appeared unaffected.

    Plant response was so rapid that herbicide contamination of the bark mulch was first suspected as
the cause. However, a sample of the mulch product emitted a pungent odor and was warm and caustic
to the touch. A pH test verified the extreme acidity of the dark brown, mulch material.

    A couple of researchers in 1966 found that under certain conditions, stockpiled bark, wood chips,
and sawdust could produce volatile organic acids. This occurs when these wood by-products are piled
so high that moist materials deep within the pile become compacted and heat up to high temperatures.
This in turn, results in the exclusion of oxygen and leads to anaerobic fermentation of the wood
carbohydrates. The products of this fermentation include low molecular weight organic acids such
as acetic, propionic, and butyric acid.

    Buildup of these organic acids can significantly lower the pH of wood mulches, creating a "sour mulch".
A study of the changes that take place in piles of bark showed a drop of pH 4.6 to values as low 2.6
to 3.3, and the height of the bark piles was 20 feet. Andrew Baker of the Forest Products Research
Lab in Wisconsin, has stated that wood chips and bark composted in piles higher than ten feet are
prone to the type of anaerobic fermentation described earlier.

    Baker recommends that wood by-products be piled in compost windrows less than ten feet high
and be turned frequently to encourage aerobic decomposition. Others suggest that going no higher than
four feet in height for the windrows would yield better results.

    Once the mulch has soured, the problem may be corrected by spreading it out in a shallow layer and
watering heavily to leach the caustic chemicals. Adding large amounts of limestone will also help to restore
the mulch to neutrality making it then safely useable.

    In this particular case discussed at the start of this report, the homeowner was able to salvage plants
injured by the sour mulch by pulling the mulch back and drenching the area around the plants with water.
Within a few weeks all of the affected perennial plants and the turfgrass recovered and flowered again
the following spring. With the increase in the regulations prohibiting disposal of yard wastes and wood
products in landfills, wood recycling and composting businesses are growing. Be sure the source of any
wood by-product mulch you might use has followed proper storage standards described in this article.

    This is also of personal interest to me as I have been fooled in the past by such symptoms described
herein. I tried making it an insect problem, a root rot, plants not adapted to the site (partially true!)
herbicide residue, and possibly over-fertilization. But of course, nothing would fit! These symptoms will
be most common of course, in the early part of the growing season typically when people will mulch their
plants.

Ron Smith
NDSU Extension Horticulturist and Turfgrass Specialist
Department of Plant Sciences

 

THINGS TO LOOK OUT FOR THIS SUMMER WITH WOODY PLANTS

    As our summer bears down on our prairie environment, the plant material is going to be assaulted
by various fungi, bacteria, viruses, and plant-destroying insects. Monitoring the valued plants in your
landscape through the season will help prevent catching you unprepared. There are two basic diseases
of trees and shrubs that show up just about every growing season on many plants in our yards. One is
Verticillium wilt and the other, the already widely publicized leaf spots/anthracnose.

    With Verticillium wilt, we can most commonly expect to find it hitting ash, catalpa, elm, Kentucky
coffee tree, maple, and viburnum. This is a fungus that lives in the soil and forms resting structures which
sprout when roots of susceptible plants grow nearby. The fungus penetrates intact or wounded roots
and forms spores which move up into the plant with the flow of water. Toxins or direct infections kill
water conducting vessels. Newly planted plants and those under stress are most vulnerable to infection.

The Symptoms
    One type of wilt progresses slowly causing yellow-green foliage, marginal browning of leaves,
stunted growth, smaller than normal leaves and shoot dieback. A second type causes severe
symptoms such as shriveled leaves, early fall color, loss of foliage, and dieback of branches or entire
plants. Affected branches may show green-brown streaking of the sapwood.

What To Do
    Vigorously growing plants can often compartmentalize the infection within a few branches. Water
plants weekly during hot, dry, periods. Maintain adequate nutrient levels with a balanced fertilizer if
needed. Prune dead or dying branches to maintain appearance of affected plants. Replace plants with
resistant ones such as: birch, pine, spruce, yew, juniper, hackberry, walnut, arborvitae, apple, or linden.
Available fungicides show only inconsistent results, but research proceeds.

Conditions Favoring Anthracnose
    With leaf spots/anthracnose, the fungi overwinter on the fallen leaf litter and twig surfaces. Spores
are spread by rain splash and wind , and infect emerging leaves and succulent shoots during cool, wet
conditions. The cycle can repeat itself as long as conditions remain wet. In this way, epidemics can
develop. Dry weather conditions interrupt the epidemic.

What To Look For
    Symptoms include brown spots along the veins, discrete or blotchy spots anywhere on the leaf,
distorted leaves and early leaf loss. Usually margins of dead areas are dark colored, while the interior is
lighter. Dead spots on immature leaves constrict leaf expansion, causing leaves to appear crinkled and
curled. Small fruiting bodies often develop in dead tissue.

What To Do
    Maintaining tree vitality is critical. Collect and dispose of infected leaves in the fall or early spring
to reduce inoculum. Fungicides are not needed on trees with good vigor, but may benefit stressed or
newly transplanted trees. If wet conditions prevail, apply fungicides to protect new leaves and repeat
1-2 more times at 7-14 day intervals. At this time of year Daconil 2787 is recommended for control
on emerging growth. Try also, to select some varieties of trees reputed to be resistant.

References: Diseases of Trees and Shrubs, Sinclair, Lyon and Johnson, Cornell University Press;
PP697 (revised) Deciduous Tree Diseases, Stack and Lamey

Ron Smith
NDSU Extension Horticulturist and Turfgrass Specialist
Department of Plant Sciences

 

TREES

APPLE SCAB

    Apple scab has recently been showing up around the state. The apple scab fungus, Venturia inaequalis,
infects leaves, flowers, fruits, and/or shoots of apple, crabapple, and mountain-ash. Subpopulations of V.
inaequalis
may infect one host species, but not affect others.

    Apple scab infections of apple and crabapple initially appear as water-soaked lesions on leaves and
fruit. Small olive-green spots enlarge into circular, velvety lesions. Secondary lesions on leaves tend to
follow veins, while on fruit, secondary lesions appear as small spots surrounding the larger initial infections.
Heavy leaf infections can cause extensive defoliation. Multiple years of defoliation can weaken trees
causing them to become more susceptible to winter injury, insects, and other pathogens. Fruit infections
result in uneven growth, cracks, and corky lesions.

    Mountain-ash infected by V. inaequalis generally show leaf symptoms similar to infections of apple leaves.
This may result in premature defoliation. Some mountain-ash trees in central North Dakota were nearly 50%
defoliated by this disease last summer.

    Apple scab can be managed by selecting resistant cultivars, practicing good sanitation, and/or properly
applying fungicides. Before purchasing apples or crabapples, look for cultivars which have good resistance
to both apple scab and fireblight. Since V. inaequalis overwinters in fallen leaves, cleaning up leaves in
the fall reduces the potential for primary infection in the spring. Many fungicides are labeled for treatment
of apple scab, including captan or benomyl plus captan. Since infections can occur throughout much of
the growing season, commercial orchards will often repeat treatments every 10 to 14 days when apple
scab has been a problem. Always follow pesticide labels.

Marcus Jackson
Extension Forester
mjackson@ndsuext.nodak.edu


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