Home > Begonian > Volume 67 (March/April 2000, pages 54 - 56)

Mildew
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 successful.

The organism

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 site.

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.