> Volume 67 (March/April 2000, pages 54 - 56)
by Mike Stevens
In the previous issue we published two
articles where growers from the Northem
Hemisphere gave details of the problems they
had experienced with mildew during their
recent summer season. As it will not be too
long before similar problems may well beset us
here in the south and we have not dealt in
depth with this for some time, I would like to
examine some of the aspects relating to it.
Powdery Mildew is a fungal disease which
attacks a wide variety of omamental plants
amongst which is the begonia. The damage due
to infection of the fungi can be slight to
severe, can affect some plants and not others
and be worse in some seasons than others.
Usually the first symptoms of the disease
are a white to pale-grey coloured fungus
growth similar to small white stars which
appears on the leaves, stems or flowers. Young
plants and those under stress are usually more
severely damaged than older healthy ones.
In order for mildew to occur there are
three factors which must be present and this
is true of all diseases. Firstly, we must have
the disease-causing organism, secondly a
suitable plant to be infected and finally the
right conditions to allow any attack to be
The white mildew on the plant surface is
actually comprised of the threads (mycelium)
and asexually produced spores (conidia) of the
powdery mildew fungus. These spores are wind
blown to other parts of the same plant or
other plants of the same species. Powdery
Mildew is quite host specific so, for example,
mildew of a petunia will not spread to a
begonia and vice versa. Also they are obligate
parasites, meaning they can only grow on
living tissue. Some Powdery Mildew fungi
survive the winter as colonies of mycelium but
may switch over to sexual production in the
autumn, producing brown to black specks amid
the old mycelium on the dying leaf or other
part of the plant. These are able to survive
the winter and in the spring release another
type of spore to start the cycle all over
again. It is therefore important to ensure
that all dead dry foliage is removed from your
A suitable plant
As mentioned above there must, in addition
to the organism, be a susceptible host. Some
begonias are more susceptible than others to
mildew. In particular are B. sutherlandii,
B. gracilis var. martiana, and B.
dregei amongst the species and B.
'Avalanche' and B. 'Lou Anne' among the
tuberous hybrids. On the other hand there are
many begonias which seem to be impiune to this
disease, even those grown among infected
plants. There are a number of reasons for this
-- genetic makeup of the plant, its physical
characteristics such as hairy leaves that
prevent the spore touching the actual leaf
surface and seemingly some having the ability
to kill the fungus which attacks them.
The right conditions
The third contributing factor in any mildew
attack is, of course, the environment in which
the plant is being grown, therefore both the
weather and the time of year will have a
bearing on this.
Generally speaking mildew becomes more
prevalent in the autumn when high humidity is
rapidly converted to moisture by the onset of
cooler evenings. Moisture is the precursor for
mildew for without it the spore would not
germinate. Only the very thinnest film of
moisture, which settles on the plant from out
of the atmosphere, is necessary for this to
occur and powdery mildew will germinate on
leaves that are damp for just one hour.
The temperature is also an important player
in this formula. As I mentioned cooler
evenings in autumn allow water vapour in a
highly humid atmosphere to condense thus
producing the conditions ripe for gen-nination.
Wan-ner areas, which often have a high
humidity are no less susceptible when there is
just a slight drop in the temperature.
How are we best able to control mildew?
In my opinion prevention is far better than
trying to cure a later problem so by not
growing those plants known to be highly
susceptible to mildew you will reduce the
chance of spreading the infection.
A second point to consider is adopting a
preventative spray program from the beginning
of the season, before you have any mildew. We
will look at what to use shortly.
The final environmental consideration is
where you actually grow your plants. If you
use a glass house then you MUST give adequate
ventilation and of course control the
humidity. Shade houses have the advantage of
better natural ventilation. In both, plants
should have some space between them. Jai-nming
your plants close together will reduce the all
important air flow around them thus increasing
the risk. Those plants growing in the open
garden are of course at the risk of the
elements but good spacing here will also
reduce the chances of infection. Always spray
your plants in the morning and when watering
endeavour not to wet the foliage, otherwise
again do it in the early part of the day.