It’s that time of year when powdery mildew gets to our African violets. This is a virtually unavoidable fungal disease because the spores are always in the air. It is possible, though to minimise how the problem affects your plants. When the leaves and flowers are affected by a white powdery substance as in the photographs, you will know that powdery mildew is to blame. It always shows up more on dark flowers. Many other plants both indoors and outside are also affected by powdery mildew.
In African violets it shows up on the leaves and flowers when the nights are cool and the days are warm. You are also more likely to see it when the humidity level is relatively low. Stagnant air also seems to set up the right conditions, so you could improve ventilation perhaps with a fan. There are some plants that certainly seem more prone to powdery mildew and should be watched carefully.
Whatever else you might do, it is best to treat promptly when it appears because leaves may otherwise become marked and flowers destroyed.
If you can improve the humidity level do so, even if it is a matter of putting susceptible plants in a terrarium-like clear container while powdery mildew conditions persist.
Once the problem appears, you need some options for treating it. There are numerous proprietary fungicides that can be used but since powdery mildew is only a problem when the conditions foster it, you might resist using chemical pesticides despite the fact that it is often easier to use a prepared product.
Simply brush off the mildew with a wet brush, cleaning the brush in methylated spirits after finishing and allowing it to dry.
Wash the leaves with mildly warm running water (only in the early part of the day). Blot left-over water away from the centre of the plant.
Fill a mist sprayer with plain hot water and mist the plants until the water begins to run off. Again don’t leave puddles of water on the leaves, especially in the centres.
Spray with 1 teaspoon of laundry bleach in a litre of water (kills the fungus spores). Numerous African violet growers use this control.
Spray with 1 teaspoon of bicarbonate soda in 1 litre of warm water. This changes the pH of the leaf surface which kills the fungus. Some say to use with a few drops of liquid soap, others recommend adding half a teaspoon of insecticidal soap. Plain or with soap, this also is a tried and true recipe.
Spray with a solution of milk and water. The strength recommended varies between 10% solution and 40%. I have not used this myself but I imagine you would need to experiment a bit. The natural enzymes set up a condition where the mildew cannot thrive, It used to be said that the milk must be full cream, but now the theory is that the proteins do the trick. Make you own choice whether you use full-cream or skimmed! I have not tried this because I would fear a sour milk odour, but others do recommend it and say odour is not a problem.
You can combine the milk solution and the bicarbonate soda and use that as a spray, although I am not sure why you would if each is effective alone.
Spray 1 teaspoon vinegar in 1 litre water. Said to change the pH of the leaves and kill the fungus.
Sulphur preparations, including powdered sulphur, has long been recommended in the past, but beware of residue on the leaves which is not really acceptable in African violets. If you brush the leaves with powdered sulphur it will at least temporarily get rid of the problem. You must keep on brushing until all the sulphur is gone too – a tedious task at best. It used to be suggested that dishes of powdered sulphur placed around near the plants would help control powdery mildew. How much help it gives I don’t know, but it is easy to do and does no harm and I still do it myself. I often notice that small gnats and other nameless flying things commit suicide in the sulphur, which is interesting.
It is said that Cinnamon is useful to defeat fungi. I think it would have to be sprinkled on the leaves, but one quails at the effort involved in removing it from the hairy leaves afterwards. As bad as with sulphur.
Numerous other organic type controls have been suggested. A simple Google search would no doubt bring up heaps of them. It is useful to remember, though that apparently different species of fungi cause the powdery mildew in different plants. What works for one type of plant may not (or may!) work with African violets. You may even have your own “pet” organic treatment for plants with powdery mildew. If so, try it on a violet or two.
Some that have been mentioned are:
Compost tea, made from soaking compost in water and straining the liquid out. It is said to harbour beneficial organisms.
Garlic – grind two whole bulbs in half a litre of hot water, strain and add a little liquid soap .Use about 2-3 teaspoons per litre of water. I would worry about odour, once again.
Remember none of these sprays are guaranteed to solve the problem once and for all. Spray every part of the plant including the stems and under the leaves. And you should always try out on a few plants before going ahead to spray your whole collection. You may need to spray each week.
Still want to use a “proper” bought spray? Try Neem oil at recommended dilution.
Just prefer a “real” fungicide? For convenience in use and good results with African violets I think one of the best around at the moment is Yates Fungus Gun.
Remember that powdery mildew in African violets is a short term but recurring problem. When conditions change all will be well – until next time.
Copyright - Ruth Coulson & The African Violet Association of Australia (2018)
This article appeared in the May 2018 issue of "The African Violet", the quarterly newsmagazine of The African Violet Association of Australia. It is reproduced here for information. Please do not copy this copyrighted article, but link to it freely.