Q. My basil
plants seem to be dying. The leaves started getting yellow spots
that turned brown, and then the entire leaf died. I have been
growing basil for many years and have never had a problem. Can you
tell me what is going on?
sample submitted with this question had all the signs and symptoms
of basil downy mildew. This is a relatively new disease in
the United States, first identified in Florida in 2007. It has been
reported in a handful of states every year since then, from the East
Coast to Western states. It has also been identified in Canada and
Basil Downy Mildew Symptoms
first show angular yellow leaf spots that are delineated by the
major veins. The entire leaf often turns yellow, with brown
(necrotic) areas and irregular black spots. Affected leaves die
and drop prematurely, and affected plants look sickly. These
symptoms may be mistaken for sunburn or a nutrient deficiency.
What sets basil downy mildew apart from other problems are the
purplish-gray spores that develop on the underside of affected
leaves. The disease starts low on the plant and works its way
Spread of Mildew Pathogen
is caused by Peronospora belbahrii, a fungus-like organism classified as a
water mold or oomycete. Some of the most virulent plant
pathogens belong to this group of organisms, including late
blight of potatoes and tomatoes and sudden oak death. The causal
organism is carried as spores by wind and on seeds, transplants
and fresh leaves, although it may not be visible under cool, dry
conditions. The disease flourishes in warm, humid conditions and
can spread through a planting rapidly.
Mildew Prevention & Control
plants frequently and immediately remove any with suspicious
symptoms. Be sure to grow basil in full sun, and allow space
around individual plants to permit good air circulation. If
possible, use drip irrigation rather than overhead watering to
keep leaves dry. Home gardeners can also use potassium
bicarbonate as a preventative fungicide application.
This is a
relatively non-toxic fungicide sold under trade names such as
Bi-Carb, Green Cure and Remedy. Be sure to follow label
directions regarding frequency of applications and use the
shortest recommended intervals during wet weather. Potassium
bicarbonate is unlikely to cure severely infected plants. They
should be removed from the garden ASAP.
Resistant Sweet Basil Varieties
all sweet basil (Ocimum basilicum) varieties seem to be
particularly susceptible to basil downy mildew. Other types of
basil have more resistance, including lime, lemon (‘Lemon,’
‘Lemon Mrs. Burns’ and ‘Sweet Dani Lemon Basil’), Thai (‘Queenette’)
and spice types (‘Spice,’ ‘Blue Spice’ and ‘Cinnamon’). Red-leaf
varieties also seem more resistant, including ‘Red Leaf’ and
Be sure to
purchase fresh seed rather than saving your own, and carefully
examine all transplants before putting them out in the garden.
Although basil downy mildew spores may not overwinter in our
climate, be sure to rotate crops and avoid growing basil in the
same spot where you had trouble with it this year.
Since I just
wrote about impatiens downy mildew, I want to point out that the
downy mildew that affects impatiens does not infect basil, nor
does basil downy mildew infect impatiens.
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