Topical Gardening Tips
Question:
Late in the summer, I began noticing white, dusty blotches on the
leaves of several herbs, which I fear is powdery mildew. It starts
on the bee balm and then spreads to the rosemary. I eat these
herbs, so I don’t want to use chemicals to control the problem. Is
there anything else I can do?
Answer:
Older strains of bee balm (Monarda didyma) are famous for
contracting serious cases of powdery mildew. Five types of fungi
can cause powdery mildew on various plants, and even scientists
have trouble telling the culprits apart. The two very similar
strains of powdery mildew fungi that attack cucumbers and squash
also can infect bee balm, basil, sage and rosemary.
Powdery mildew fungi have characteristics that help them prevail
in late summer, when plants often are too tired to defend
themselves. While most fungi only can spread when leaf surfaces are
damp, powdery mildew fungi can blow about on humid winds, land on a
dry leaf, and promptly gain access by melting cell walls with
special enzymes. If they are successful, an outbreak is visible in
less than a week. Left uncontrolled, spores and tiny threads of the
fungi quickly spread to nearby leaves. Powdery mildew seriously
weakens its victims, but it seldom kills them.
The good news is that powdery mildew is easier than ever to
manage, thanks to the availability of mildew-resistant monardas and
a novel cure of water and milk. In 1999, a Brazilian scientist
reported success controlling powdery mildew in cucumbers by
spraying infected plants with a mixture of milk and water — a folk
remedy that dates back at least 200 years. Since then, gardeners
and organic farmers from Australia to Arizona have begun using milk
to control numerous fungal diseases. Last winter, I stopped powdery
mildew in its tracks by spraying an infected rosemary plant once a
week with a mixture of 1/4 cup fat-free organic milk and 3/4 cup
water. Within a month, the mildew was gone.
Several theories might explain milk’s potency as a natural
fungicide. When exposed to sunlight, a protein in milk (and other
dairy products) might produce oxygen radicals, much as in a weak
form of hydrogen peroxide. At the same time, salts in the milk help
dehydrate the fungi. Milk treatment also provides calcium and
potassium, which stimulate plants’ immune systems.
The mixture I made was strong. You could see good results with
only 1 tablespoon of milk per cup of water. You can use whole,
low-fat or fat-free milk. Begin the milk therapy as soon as you see
powdery patches on your plants.
Preventing plant disease problems always is easier than curing
them, so move your mildew-prone bee balm away from your other
herbs. Good air circulation can help, so thin out one-fourth of the
branches in early summer.
Better yet, replace your old bee balm with a variety that
resists powdery mildew. Among robust 4-foot-tall varieties that
have resisted this disease in multiyear trials are bright pink
‘Marshall’s Delight’, dazzling red ‘Gardenview Scarlet’ and deep
purple ‘Violet Queen’. If you want a more compact variety,
15-inch-tall pink ‘Petite Delight’ is an excellent choice.
But even your resistant varieties can contract the disease if
you overfertilize or plant in a site with more than a half-day of
shade; both factors can lead to soft, floppy growth that encourages
powdery mildew. Spores can survive winter in plant debris, so rake
up old foliage and replace old mulch in early winter, after plants
become dormant.
No garden can be made completely free of powdery mildew. Weeds
serve as alternate hosts, and wind can carry spores into your
garden. You can, however, demote the disease from a recurrent
problem to a minor nuisance by growing resistant varieties; making
sure your plants get plenty of sunshine and fresh air; and moving
in when needed with a spray bottle loaded with milk and water.
Barbara Pleasant is a contributing editor to The Herb Companion
and author of several books about gardening, including The Whole
Herb (Square One, 2004).