have just about dried up and died this year. They started growing
well enough, but about a month ago I started noticing yellowish
spots on the leaves. Then the leaves started to dry up. Before I
knew it, the plants were completely dead. I know they get powdery mildew,
but I did not notice the typical powdery appearance on the leaves.
It did not seem like the wilt they get from cucumber beetles,
either. What else could it be?
A. There are a
number of foliar diseases that we see on cucurbits
(an all-encompassing term for cucumbers, melons, pumpkins,
squash, etc.), including powdery mildew. It usually manifests
itself as a grayish-white coating on the upper leaf surface. As
it progresses, the leaves do appear to "dry up" and die.
However, from your description and from reports from commercial
growers, it is more likely that downy mildew is the cause for
the quick death of your cucumbers. Although downy mildew occurs
sporadically in our area, it was has been present this year due
to favorable weather conditions. Commercial growers throughout
the eastern and mid-Atlantic states have suffered crop loses
from this disease.
Mildew on Cukes
Downy mildew is a fungal disease that first appears as
pale green areas on the leaves that turn yellow as the disease
progresses. The spots take on an angular appearance because the
leaf veins delineate their spread, especially on cucumbers. The
spots turn brown as the leaf tissue dies. During humid weather,
you may see a purplish-grey downy growth on the underside of the
leaves, but you cannot always see this symptom. When the leaves
die, they often remain attached to the erect, still green
petioles (leaf stems). The infection usually starts near the
center of a planting and works its way out toward the ends of
the vines. New growth is stunted, and fruiting is reduced. Those
fruits that are produced are smaller than normal. When weather
conditions are conducive, downy mildew can severely damage a
cucurbit crop within a few days.
The causal organism, Pseudoperonospora cubensis does not
overwinter in our climate. It is not able to overwinter north of
the southernmost states and Mexico. Spores of the causal fungus
are blown northward as the growing season warms and conditions
become favorable for them to survive. It is favored by cool to
moderately warm temperatures (60 - 80 degrees F), although it
can flourish in hot weather as long as it is humid and/or wet.
It is most severe when we have heavy dew, fog or frequent rain.
Overhead irrigation that wets the foliage frequently can also
create favorable conditions for downy mildew to develop. Working
on or around infected plants, especially when they are wet, can
spread downy mildew. Simply brushing against an infected plant
can spread the disease; it is also spread on tools used around
Managing downy mildew successfully takes a multi-pronged
approach. Plant resistant varieties whenever they are available.
Cucumber varieties that are resistant to downy mildew include
slicers such as Fanfare, Marketmore 76, Dasher II, and Indy.
Picklers that have resistance include Calypso, Patio Pickle, and
Wellington. Burpless types that show resistance are Burpless 26,
Green Dragon, Sweet Slice and Orient Express. Cantaloupe
varieties that are listed as resistant include Allstar and
Optimum Growth is Helpful
It is also helpful to grow cucurbit crops as well as possible.
Have your soil tested, and follow the recommendations to adjust
soil pH and fertilize them for optimum vigor and productivity.
Be sure cucurbits are planted in full sun, and that they are
spaced properly to allow good air circulation around each
planting. Use drip irrigation or soaker hoses, if possible, to
avoid wetting the leaves more than necessary. If you must use
overhead irrigation, water during the day so that the leaves
will dry quickly. Avoid watering at night when the leaves will
stay wet all night, or first thing in the morning when the
leaves are already wet with dew. You can remove infected leaves,
but be careful not to spread downy mildew to healthy plants as
Control Measures for Mildew
Fungicide applications are the main way to control downy mildew.
They are best used preventatively, before the disease strikes.
Frankly, the fungicides that commercial vegetable growers use
are more effective than those home gardeners have access to. Daconil 2787 or Ortho Max Garden Disease Control (chlorothalonil)
is a standard fungicide labeled to control downy mildew on
cucurbits. Copper-based fungicides are often recommended for
organic gardeners, but must be used with care because they can
burn the leaves of cucurbits when applied during hot, humid
weather. Other options for organic gardeners include BioNeem (azadirachtin)
and Remedy (potassium bicarbonate.
Repeated applications of all of these products are necessary to
control downy mildew. Check the label of the product you are
using to determine the intervals between applications. Spray at
the shortest possible interval allowed by the label when weather
conditions are favorable to downy mildew.
Biofungicide for late blight on