White dusty fungus on your plant leaves may be Mildew. Each species is susceptible to it’s own Mildew and either powdery or downy mildew it is still a pain.
Some plants suffer more than others and with some varieties it is seasonal. Michaelmas Daisies and Phlox seem to suffer late in the year whilst Gooseberries can suffer in spring.
Brassicas, Marrows, Cucumbers, Sweet Peas and even Potatoes can suffer badly.

Tips to Avoid Mildew

• Buy plants that have been bred to be mildew resistant. This is well worthwhile when selecting roses.
• Do not let plants get stressed particularly through lack of water. Mulch your roses after a good rain and keep the mulch topped up and give them plenty of water.
• Allow air to circulate around plants by careful pruning. Mildew thrives in a humid, still atmosphere.
• Prune out affected parts and do not compost affected leaves. Spray badly affected plants before winter to try stop spores over wintering.
• Do not over use a nitrogen fertiliser that will encourage sappy growth which is prone to attack.
• If moderate powdery mildew is present plant-based oils such as neem oil and jojoba oil can be used to reduce or eliminate the infection.
• Chemical treatment is controlled on edible plants but Rose Clear 3 and Systhane Fungus Fighter are good fungicide products but read the label.

Mildew 2011 update

• The dry spring and early summer has kept mildew down to a minimum in my garden this year – so far at least.
• The Flowering Currants and Honesty Lunaria annua are the only plants with signs of mildew and they are not worth treating with chemicals.
• Since mildew is plant specific I do not worry about composting old leaves.

 

Read Gardener’s Mildew Cures