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Introduction
There are
numerous diseases that fall under the general description of powdery
mildew. They will be treated together here since most of these have
similar habits and similar management practices. Houseplants that are
commonly infected by powdery mildew include African violet and begonia.
Outdoors there are numerous plants that may be infected in any year, but
lilac, phlox, some rose varieties and fruit trees seem to be infected
every year. The actual injury to the plant varies greatly with the species
and even the variety attacked. For instance, lilacs are typically infected
late in the growing season and this does not usually injure the plant
seriously. Lilacs are able to survive year after year in spite of the
disease. On the other hand, begonia is seriously injured by even a mild
infection. The tissue under the fungal growth dies soon after infection
resulting in leaf drop and poor plant growth. Thorough management
practices must be implemented in this case. Among roses there are
varieties that are more seriously affected than others. If powdery mildew
has been a problem in the past, choose a variety that has some resistance
to the disease.
Symptoms
Figure 1: Powdery Mildew
on Rose. |
Figure
2: Cleistoflecia of the
fungus. | |
Powdery mildews appear as a dusty white to gray coating over leaf
surfaces or other plant parts (Figure
1). In most cases this fungal growth can be partially
removed by rubbing the leaves. It might be identified incorrectly as dust
accumulated on the leaves. Powdery mildew, however, will begin as discrete
circular, powdery white spots. As these spots expand they will coalesce,
producing a continuous matt of mildew (similar to dirt or dust). A plant
pathologist using a microscope can determine whether a fungus is present
anytime white areas are present.
Symptoms usually appear late in
the growing season on outdoor crops. The fungus is favored by high
relative humidity periods or site conditions that promote this type of
environment, such as close spacing of plants, densely growing plants, or
shade. Indoors, symptoms may occur at any time of year, but the rate of
spread and development will be affected by the relative humidity and
temperature.
Injury due to powdery mildews includes stunting and
distortion of leaves, buds, growing tips, and fruit. The fungus may cause
death of invaded tissue (begonia, for example). Yellowing of leaves and
death of tissue may result in premature leaf drop. Nutrients are removed
from the plant by the fungus during infection and may result in a general
decline in growth and vigor of the plant. The seriousness of the disease
will depend on the extent of the various types of injury.
Disease cycle
The fungi which cause powdery mildew are
spread by spores produced in the white patches. These spores are blown in
the wind to other parts of the plant or to other plants during the growing
season. Generally each species of fungus will be limited in the number of
plant species that can be attacked. For example the species of fungus
infecting lilacs will not cause powdery mildew on apples.
During
the winter the fungus survives on infected plant parts and in debris such
as fallen leaves. It may produce a resting structure known as
cleistothecia, which resist harsh winter conditions. These will appear as
small black dots within the white powdery patches (Figure 2). The next spring spores within
the cleistothecia (ascospores) are shot up into the air and carried by air
currents to leaves of plants. This is the general cycle for powdery
mildews of outdoor plants. With houseplants the overwintering stage is of
little significance. Depending on the environmental conditions indoors,
the fungus could continue to grow and spread during the entire year.
Management Strategies
For indoor plants disease management includes gathering up and
destroying fallen leaves, reducing relative humidity around plants, and
spraying with a fungicide. Use a product with label directions for use on
houseplants.
For outdoor plants, gather up fallen leaves in autumn
and destroy them. Where powdery mildew is a problem, resistant varieties
(if available) should be grown. During the growing season, begin fungicide
sprays when the first white patches are noticed. Repeat as indicated on
the product label during cool humid weather. Management for fruit trees
may be combined with the normal spray schedule and should begin at the
green tip bud stage for apples. Always read and follow pesticide label
directions carefully.
Effective fungicides include potassium
bicarbonate (Remedy), benomyl (Benlate), wettable sulfur, funginex
(Trifroine EC), folpet (Phaltan), cycloheximide (Actidione PM), Pipron.
Wettable sulfur and cycloheximide are known to cause injury to some
plants. Check the label for directions for the particular plant you are
treating, and cautions about sensitive plants. If benomyl does not seem to
be controlling the disease do not use it in the future. In some cases the
fungus is able to develop a resistance to this chemical.
There are
several effective fungicides available, but use on plants varies with each
product. Check label directions of specific products for more information.
Last updated, KLS, 8/99
This publication contains pesticide recommendations. Changes in
pesticide regulations occur constantly, some materials mentioned may no
longer be available, and some uses may no longer be legal. All pesticides
distributed, sold, and/or applied in New York State must be registered
with the New York State Department of Environmental Conservation (DEC).
Questions concerning the legality and/or registration status for pesticide
use in New York State should be directed to the appropriate Cornell
Cooperative Extension Specialist or your regional DEC office.
READ THE LABEL BEFORE APPLYING ANY PESTICIDE.
__________________________________________________________________________________
The Plant Disease Diagnostic Clinic at Cornell
University is located at 334 Plant Science Building, Ithaca, NY,
14853. Phone: 607-255-7850, Fax: 607-255-4471, Email: kls13@cornell.edu |
