Can you give me some suggestions on how to treat a Nandina infected with powdery mildew?
Apparently, powdery mildew on Nandina is becoming a common problem in our
area, as the article linked here indicates.
While this fungal disease is unsightly, it generally does not kill
affected plants. Sometimes improving air circulation around the plant (by
pruning congested growth) can help, and making sure to practice good
sanitation by picking up fallen leaves affected by the mildew is also
important.
There is an interesting idea in this Science News article on
using milk powder in water as a spray to control the disease.
Several organic gardening sources recommend a baking soda spray. The
Organic Gardener’s Handbook of Natural Insect and Disease Control by
Barbara Ellis (Rodale Press, 1996) recommends dissolving 1 teaspoon of
baking soda in 1 quart of warm water. You can add up to a teaspoon of
dish soap to make the solution stick to the leaves more effectively.
Here is another source with slightly different recommendations, from the
National Sustainable Agriculture Information Service.
More from University of California, Davis. Excerpt:
Shade and moderate temperatures favor most powdery mildews. Locate plants
in sunny areas as much as possible, provide good air circulation, and
avoid excess fertilizer. A good alternative is to use slow-release
fertilizer. Overhead sprinkling may actually reduce the spread of powdery
mildew because it washes spores off the plant; also, if spores land in
water, they die. The best time to irrigate is in mid-morning so that the
plants dry rapidly, reducing the likelihood of infections by other fungi,
such as the ones that cause rust or black spot infections on roses. As
new shoots begin to develop on perennial plants, watch closely for the
appearance of powdery mildew.