> Volume 72 (January/February 2005, pages 8 - 11)
Powdery Mildew in Begonia
by Antoon Hoefnagel
Powdery mildew (Oidium
begoniae) is next to botrytis one of the most
important fungus diseases in pot plants.
Plants infected with
powdery mildew are easily recognizable through
the presence of a white layer of fungus
threads, usually on the upper surface of the
leaves or flowers.
This layer is easily
removed with one's fingers because powdery
mildew only grows superficially.
Severe damage to the
plant is easily detectable, because it may
also result in diminished growth.
In the U.S. problems
with powdery mildew also arise in Poinsettias.
Powdery mildew also
leads to problems in Gerberas and cut roses.
In different plant
families not all species have the same
sensitivity, sometimes there are varieties
that have a level of resistance to powdery
There are also
differences in resistance to powdery mildew
within the Begonia family.
If this resistance could
be included next to the magnificent leaf
colours and forms and the beauty of the
flowers, begonias should give much more
pleasure to all of us.
Powdery mildew is caused
by the fungus Oidium begoniae and belongs to
the section Ascomycetes. Oidium begoniae
creates generative and vegetative spores. On
the basis of these spores we are able to
determinate the family of the fungus. The
fungus is called oidium if the spores are
vegetative. In general powdery mildew can only
infect a specific plant species. This means
for example that the powdery mildew on a
Saintpaulia does not infect a Begonia.
On the other hand it
means that the powdery mildew of a Begonia
cannot infect a Saintpaulia. Fungus families,
that cause powdery mildew are obligatory
parasites . The syndrome of mildew can vary by
species and by cultivar. The reaction of the
plant to the fungus has influence on this
syndrome. Microsphaera begoniae creates
smooth white spots on one Begonia cultivar,
while it creates an irregular shape of mildew
on another one. The powdery mildew created by
M. begoniae on Begonias infects leaves, but
infection of the flowers is also possible.
A mildew colony begins
with a few traces on the leaf surface. Out of
these traces grow hyphal threads which branch
off several times. Special cells (haustories)
grow out of these hyphal threads through the
leaf surface and settle in a surface cell of
the leaf (epidermis cell). The fungus receives
nutrition from this epidermis cell of the
plant. These hyphal threads continue to grow
and multiply regularly. A colony can be
visible within days. This depends on which
mildew species is involved. This is also
called the latency period. From the hyphal
threads grow new spores, on these spores new
spores are produced. Under the microscope this
is visible as a chain of spores.
When the spores are
mature they can easily come loose and can be
spread by the wind. When the spores come down
on a susceptible leaf they can germinate and
form a new colony. This cycle can be completed
within days if the circumstances are
favourable. This cycle is the vegetative cycle
of the fungus. There is also generative cycle
, but this is rarely involved in the
development of a disease in horticulture.
Infection by mildew is
highly dependent on temperature. Most powdery
mildews have an optimum temperature of 21°C (Celio
& Hausbeck, 1998). The germination of
spores is less efficient at higher
temperatures. Also the growth of hyphal
threads will be curbed at higher temperatures.
There is no infection if the temperature is
above 30 ,(C. Research has found that the
maximum temperature is 30 °C for
appresoriumformation, while the colonies die
at a temperature of 32°C (Quinn & Powell,
1982). Mildew will not die at lower
temperatures, but its growth will decrease.
Although mildew is not dependent on liquid,
humidity can be important for the infection.
The fungal spores contain 70% liquid, so they
do not need water from the outside to
germinate. (Agrios, 1997). So infection is
possible at low humidity, in contrast with
other fungi like botrytis and rust, these need
high humidity for germination and infection.
Never the less some research shows that high
humidity promotes infection of mildew.
influences spore formation and the release of
spores. Sudden changes in humidity can result
in release of the spores. Finally there is
also literature describing infection in
combination with light (Quinn & Powell,
1982). Light can affect the release of spores,
because maturation is dependent on light. In
contrast, light can have an effect on the
lifespan of the spores. Increased radiation
results in a lower survival rate of the
spores. The effect of climate on epidemic
mildew is complex; so it is difficult to give
an unambiguous advice to change it so that you
can control infection. It must be recommended
to prevent big fluctuations in the climatic
Between cultivars within
one plant species there can be differences in
sensibility to mildew. By hybridisation the
resistance can be crossed into new cultivars.
Through on the years various publications have
been issued about the sensibility to mildew in
plants. (De Gelder & vd Wurff, 1994;
Strider, 1976; Strider, 1980). Resistance
breeding is still insufficiently used for
improvement in the florist sector.
In practice the mildew
in potting plants is controlled by the use of
chemical fungicides, or such as sulphur
Next to chemicals, it is
possible to take growing measurements to avoid
the spreading of infection. You could think of
removing the dead plants or plant parts,
avoiding draft spots in the greenhouse and
realizing a climate with no big fluctuations.
Research shows that
silicium provokes thicker cell walls,
especially in the epidermis of the leaf. With
this treatment plants will resist mildew
infection better. There are several examples
with 'plant enhancers'; at first sight they
give good results. The only disadvantage is
that the plant have to be sprayed at regular
intervals; and therefore the plants will be
more wet and that is not to recommended.