powdery mildew, boytritis
How often do you apply Bordeaux mix in the winter? Had a bad case of everything last year.
Apply sulphur or copper based fungicides at the first signs of disease.
Spray all parts of the plants throughly and repeat every 7 to 10 days-up until harvest.
Work on prevention by planting disease resistant varieties. Open and prune plants to help with air circulation. Remove and dispose of diseased foliage.
Use tools that have been cleaned with alcohol. Water in the am hours so leaves can dry and use soaker hoses.
Use slow release organic fertilizers to avoid excess nitrogen.
Destroy all remains of plant material after harvest, never compost diseased plant material.
Zone 9 and all of landscape involved; garden, citrus grove, birds of paradise, rose,sagos and cactus and succulents, not day lilies. I did get items for my own recipe. nothing says how often for heavy infestation.
Treatment would depend on the type of plants and location-growing zone.
You should apply the chosen treatment according to the packaging directions.
Here are some links with more information on Powdery Mildew.