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ISSUE 15  AUGUST 12, 1999


TREES

WOOLLY ELM APHIDS ON JUNEBERRY ROOTS

    An important insect pest of juneberry is the woolly elm aphid (Eriosoma americanum). This aphid
can be found in the soil, feeding on juneberry roots during the summer. Oftentimes, woolly elm aphid
injury is improperly diagnosed as poor stock, bad planting technique, deficient maintenance, or
inadequate soils.

    Woolly elm aphids inject leaf-curling fluids, as they feed on the sap of American elm leaves, in early
May. During late spring or early summer, winged adults fly to juneberry plants and give birth to live,
orangish-yellow nymphs on the undersides of leaves. These nymphs migrate to juneberry roots and
multiply rapidly. Colonies of woolly elm aphids may be found on roots from July throughout the remainder
of summer. In late summer and early fall, another generation of winged adults moves from the soil, back to
American elm trees, where they lay eggs in bark crevices to overwinter. As American elm leaves begin to
unfold in the spring, nymphs hatch from the eggs and the cycle is repeated.

    Juneberries appear to be most susceptible to woolly elm aphids when plants are two or three years old
and when they are grown in exposed sites. Early symptoms, of an aphid infestation, include sparse foliage,
early fall coloration, and a lack of vigor. Dead plants may be pulled from the ground with little resistance.
Digging down 2" to 5" below the soil surface near affected, living plants, will reveal woolly or waxy,
bluish-white masses of aphids on the roots.

    Although acephate soil injections are registered in Canada, no chemical treatment has been labeled for
this pest on juneberry in the United States. A number of alternative methods for managing woolly elm aphids
in juneberry are under investigation; however, none have been proven effective. These methods include the
use of yellow plastic mulches, entomopathogenic nematodes, diatomaceous earth, and identification of resistant
cultivars. Since the woolly elm aphid requires American elm to complete its life cycle, we may see reduced
numbers of this pest where American elms continue to be lost to Dutch elm disease.

Marcus Jackson
Extension Forester
mjackson@ndsuext.nodak.edu

 

HORTICULTURAL THINGS TO BE ON THE LOOKOUT FOR DURING AUGUST

    August often shows the accumulation of the stresses of the previous spring and summer months.
This is the time when chronic disease and insect problems begin to take visible toll on the plant materials
in the home landscape. Consequently, this is a good time to survey the property taking notes as to which
plants have branch die-back, leaf spots, premature yellowing, powdery mildew, and cankers. It may
be time to consider replacing some of the more disease prone plant material with some resistant varieties.

Disease Resistance:
    Disease resistance is not disease immunity. It simply means that plants have adapted or had bred into
them, the ability to resist the effects of pathogens and still produce a crop (adequate foliage, flowers, or
fruit) to complete their seasonal cycle. Resistant plants have biochemical or structural mechanisms that
either prevent infection or limit its impact. This is one of the reasons to maintain good plant vitality. A plant
in good health can maximize its ability to respond to disease, infections and to protect itself. Chronic disease
problems are best managed by replacing the susceptible plants with varieties or cultivars of the same species
that are resistant or selecting other kinds of plants that are immune or resistant to the disease.

Three Common Examples:
    Powdery mildew fungi grow superficially over both green shoots and leaf surfaces, appearing as a dusty,
gray to white coating on infected plants. Infected immature leaves become yellowed, puckered, and stunted.
Many powdery mildew fungi begin to form fruiting bodies in the late summer and overwinter on fallen leaves.
Others overwinter by infecting buds. The fungus resumes growth as the buds open the following spring.
Generally, the damage caused by powdery mildew is minor and does not warrant control, but it can cause
plants to look unsightly. Where the fungus overwinters on fallen leaves, inoculum can be reduced by collecting
and destroying the leaves. Provide adequate air circulation and avoid planting in shaded areas. Applications
of anti-transpirants, a baking soda solution, or fungicides are effective when begun just as symptoms of
powdery mildew begin to show. These products must be reapplied to maintain the protection. Resistant
varieties of many of the host plants are available; this is the easiest way to reduce incidence of the disease.

    The leaves, fruit and green stems of apples and crabapples are susceptible to apple scab infections. The
first symptoms on leaves are spots that turn olive green to dark gray and develop a velvety appearance. As
the season progresses, the fungus repeatedly reproduces itself on these spots, infecting the new foliage over
the entire season. Severe infection may cause early defoliation, often during the month of August. Prune when
conditions are dry to increase air circulation and promote rapid drying of foliage. Remove and destroy infected
leaves in the vicinity of the tree during and after the growing season to reduce spores available for infection.
To maintain an attractive looking tree throughout the growing season, apply fungicides at bud break, petal fall
and two or more additional times at 7 to 10 day intervals. An easier and more effective approach would be
to plant resistant cultivars to the apple scab fungus.

    Black spot on roses has been very common this year because of the weather conditions. It causes round,
black spots on infected leaves and fruit. Infected first year canes have lesions that are reddish colored initially
and then turn black. Eventually, the spotted leaves drop prematurely from the plant, which weakens roses
so they are more prone to winter damage. This is a fungus that overwinters on the fallen leaves and canes.
Rainy spring weather disperses the fungus and provides wet conditions necessary to infect the roses. This
cycle is repeated several times during the growing season. Good air circulation and irrigation practices that
minimize water getting on the foliage will help to suppress the disease. Repeated fungicide applications are
needed to maintain protection of the new foliage from infection. There are a number of attractive varieties
of rose with resistance to black spot and powdery mildew that are much easier to maintain than susceptible
ones.

    Finally, another common malady that typically shows at this time of year is sooty mold. These are saprophytic
fungi that are common on pine, elm, linden, and maple, although they can show up on apples trees and other
species as well.

    The fungi grow and reproduce on the honeydew produced by the excrement of soft scale, aphids, mealy
bugs, and leaf hoppers that are feeding on the foliage and branches. Sometimes the mold develops as a result
of the organic exudates that secrete from leaf tip openings The black crusty growth can cause yellowing of
leaves when sunlight is obscured.

    Sooty mold is unsightly, but it does virtually no harm to the plant. Presence of sooty mold may be an
indication of a heavy infestation of some of the above mentioned insects, that have more of a potential to
cause damage. Proper identification of the insect should be determined and control considerations made.

Ronald C. Smith
Department of Plant Sciences
ronsmith@ndsuext.nodak.edu


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