Many parts of Australia are experiencing flooding, while West Australia is experiencing extremely hot weather with bushfires in the south, so looking after the garden is the least of their problems. My sincere sympathy to all those affected by these extreme weather events, many of whom are facing heart-breaking work to rebuild their lives.
For those of us who are fortunate enough to still have gardens, the prolonged wet conditions provide the perfect conditions for downy mildew to flourish. A set of three conditions (10:10:24) is necessary for downy mildew to establish – a minimum of 10 mm of rain, a temperature higher than 10 degrees Celsius, and foliage that stays wet for more than 24 hours. It is also more likely to affect plants that are stressed for some reason, and where there is poor air circulation. This group of fungal diseases produce pale green or yellow spots on the upper side of leaves, and white-grey furry patches on the underside of leaves of a wide range of plants, including cucurbits, the cabbage family, lettuce, onions, peas and grapes (starts as oily spots). Different species of the fungi infect different varieties of plants, so that downy mildew on your cauliflowers does not mean that other vegetables in your garden will be affected.
The best thing you can do is remove badly damaged foliage and dispose of it in a sealed plastic bag – compost it! Then give soil around affected plants a drink of seaweed extract tea at the strength advised on the label. A good potassium content in seaweeds strengthens plants cell walls helping plants to build resistance to diseases.
Spray remaining leaves with 100 mls milk in 900 mls water (to make 1 litre of spray), and add a good pinch of bi-carbonate of soda (baking soda) for every litre of spray. Full cream milk works best because the fat content helps the spray stick to the leaves, and full cream, organic milk is even better (according to some gardening gurus) – if you can spare it. Milk and bi-carb are not fungicides, but they produce unsuitable conditions on leaf surfaces for the fungi’s survival. Spray leaves early in the day, and repeat every four or five days until mildew clears.
Downy mildew tends to disappear as weather becomes drier because it needs constant humidity. However, powdery mildew needs high humidity, but not wet weather, to establish and will continue to flourish after weather becomes drier. Powdery mildew is caused by an entirely different genus of fungus and treatment for that disease can be found here.
P.S. When spraying – be careful to avoid these tiny fellows below – the adult and larva of the Fungus-eating Ladybird.