Mildews are a group of fungal infections that plants can develop, usually affecting the leaves and stems but rarely flowers and fruits. The most common forms are Powdery mildew and Downy mildew. In short Powdery mildew creates a powdery coat on plants while Downey mildew creates discolour blotches on the upper leaf surface while a white or grey or purple mould forms below.
There are some fungi that will focus specifically on one plant host such as Rose powdery mildew or Onion downy mildew. Fortunately, geraniums do not, so far, seem to be host to any specific mildew creating fungi but are not immune to being infected by Powdery mildew. The pratense species and pratense hybrids are the most likely to be inflicted. Primarily you are most likely to notice a white powdery coating on the leaves in a heavy infection leaves and stems can become stunted or distorted.
Prevention and Cure
A healthy well-fed plant is less likely to be susceptible to infection. Creating a regular feeding plan will encourage a strong, healthy plant that is less likely to become infected.
If you have an infected plant removal of infected leaves and shoots in the early stages of infection will help slow the spread of the disease but avoid removing all leaves as this may stress the plant, reducing vigour and the plants ability to photosynthesise. If your plant is infected avoid feeding with anything high in nitrogen. Nitrogen encourages leafy growth and mildews will colonise young lush, growth very quickly.
Do not compost infected leaves. The spores of some strains of mildew are very resilient and will overwinter on your compost heap ready to continue their lifecycle the following year.
Remove fallen leaves from around the base of the plant so the resting structures can’t overwinter on them releasing fungal spores into the air in the spring.
Other preventions include:
- Increase air circulation around plants, plant away from walls and fences. Don’t plant too densely.
- Plants that suffer from drought stress are more likely to be infected. Ensure your plants have good drainage but are watered regularly especially in dry periods.
- Water in the early hours of the day.
- Avoid watering the leaves.
There are some chemical controls available to home gardeners such as Provanto Fungus Fighter Plus or Scotts Fungus Clear Ultra. Different chemical controls will have a different active ingredient and are likely to have different application instructions and possible side affect’s on beneficial insects. Please read instructions thoroughly before use.
Disposal – Good garden hygiene https://www.rhs.org.uk/advice/profile?pid=612
Further reading on Powdery mildew: https://www.rhs.org.uk/advice/profile?pid=253
Further information on Downy mildew: https://www.rhs.org.uk/advice/profile?pid=683