Dealing with Downy Mildew
The combination of heat and rain alternating with cooler temperatures throughout most of South Africa has caused an outbreak of the fungus disease, Downy Mildew, among impatiens.
According to Kirchhoffs Marlaen Straathof, the first symptoms of downy mildew are usually pale green leaves. If you turn the leaf over, the underside may show a sparse or dense white layer covering the entire leaf surface.
The spores are spread in a number of ways; by air currents, by infected plants and by water splash – in other words, normal garden watering.
The first line of defence is a preventative spraying programme; in much the same way as rose growers routinely spray their roses against black spot, says Straathof.
Fungicides with a copper base such as Coppercount and Virikop are very effective. Dithane WG, which has a different active ingredient, can also be used.
The Margaret Roberts Organic Fungicide is also effective, although it does not specifically list Downy Mildew in its information leaflet. These fungicides should also act against botrytis.
All these are contact fungicides that coat the leaves and prevent the spores from entering the leaves. However, for them to be effective they must be used with a sticker like Ludwig’s Spray Stay to keep the fungicide on the leaves.
Use a spray pump and make sure the undersides of leaves are drenched, to the point where the spray is dripping off the plant.
When to spray
During the current weather conditions plants should be sprayed weekly or in the case of the Margaret Roberts Organic Fungicide, every five days. Should there be persistent, ongoing rain; the plants should be sprayed the moment the weather clears.
Young plants that have just been bought from the garden centre should be sprayed within two weeks of planting out. If it is raining when they are planted out they can be sprayed immediately with a weaker solution (30ml Coppercount to 10 litres water). This weaker solution will not burn the leaves.
To prevent the mildew building up a resistance to the sprays, Kirchhoffs recommends alternating the sprays; for example, one week with Coppercount and the next week with Dithane or Virikop.
The recommended dosages should be strictly followed says Straathof. If the dose is made too weak the mildew will develop a resistance against it and if it is too strong it will just be a waste.
Tips for keeping plants healthy
Spraying on its own will not solve the problem if there are other environmental stresses. According to Kirchhoffs gardeners should look out for the following problems:
Keep plants dry as possible without stressing them. Water plants in the morning so their foliage dries off and the environment around them is dry as well.
Be careful about watering on humid days. Re-set your automatic irrigation system during rainy periods.
Air cannot circulate where plants are too close together and this encourages humidity. Space bedding plants like impatiens, begonia, pansies, and violas at least 20cm apart.
The healthiest plants are those in areas where there are correct light levels. Plants sold in reputable garden centres will include information about their light requirements and this should be followed for greatest success. Move plants if they are in the wrong position.
Scout for pests and disease
Do regular checks on your plants and watch out for signs of something wrong or unusual. Spraying stands more chance of being effective if it is done during the first signs of disease.
Remove dead or sick plants immediately and put them in the dust bin, not on the compost heap. Refill the beds with different varieties of plants. For instance if the impatiens have died replace them with bedding begonias, torenia, bedding salvia or coleus.
Keep plants well fed but do not overfeed. Liquid plant foods like Margaret Roberts Organic Supercharger, Nitrosol or Seagro do not have such high nitrogen content. Vigorosa can be used but at half strength.