Cause and symptoms

Title: Downy mildew of cucurbits
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066851/00001
 Material Information
Title: Downy mildew of cucurbits
Series Title: Plant Pathology Fact Sheet PP-2
Physical Description: Book
Language: English
Creator: Kucharek, Tom
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Department of Plant Pathology -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1979
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066851
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 51334362

Table of Contents
    Cause and symptoms
        Page 1
        Page 2
        Page 3
Full Text

Plant Pathology Fact Sheet

Downy Mildew of Cucurbits

Tom Kucharek, Professor, Extension Plant Pathologist, Plant Pathology Dept.
1979, Revised Nov. 2000
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Cause and Symptoms

Downy mildew of cucurbits, caused by the fun-
gus Pseudoperonospora cubensis, is found annually
on squash, cucumbers, pumpkins, muskmelons, and
other cucurbits grown in all areas of Florida. This same
disease occurs on watermelons each year in the south-
ern half of the peninsula but, in the northern part of the
state, downy mildew on watermelons is severe some
years but not others. Although we consider downy mil-
dew of all cucurbits to be caused by the same species,
P. cubensis, strains within the species seem to exist. For
example, it is not uncommon to see squash, cantaloupe,
and cucumber severely diseased by downy mildew
whereas watermelons nearby show no signs of this dis-
ease. P. cubensis can kill plants if plants are severely
infected early. Downy mildew can reduce yield, fruit
quality, and harvesting time. It does so by leaf infections
which impair necessary food production in the plant. Late
season infections often signal an excuse to buyers for
reduced prices because of alleged reduced sugar con-
tent in the fruit.

Leaf symptoms can be used to diagnose downy
mildew in the field in some cases. On cucurbits other
than watermelon, small yellowish areas occur on the
upper leaf surface (Figures 1 & 2). Later, a more bril-
liant yellow coloration occurs with the internal part of
the lesion turning brown (Figure 3). Usually the spots
will be angular as they are somewhat restricted by the
small leaf veins. When the leaves are wet, a downy
white-gray-light blue fungus growth can be seen on the
underside of individual spots (lesions). On watermel-

ons, yellow leaf spots may be angular (Figure 4) or
non-angular, and they will later turn brown to black in
color. Often on watermelons an exaggerated upward
leaf curling will occur (Figure 4).

Spores of this fungus are produced primarily on
the underside of the leaf within the downy growth men-
tioned earlier. Spores are easily dispersed by wind from
one leaf spot to another leaf in your planting or to an-
other planting nearby. Spore movement occurs prima-
rily during late morning to midday. After a spore lands
on a leaf and when the leaf is wet, the spore germinates
and penetrates the leaf tissue. Within four to seven days,
new lesions are produced as a result of infection. Thus,
new sites with more spores are produced. As this cycle
continues, an epidemic occurs and control becomes in-
creasingly difficult.


Control of downy mildew on cucurbits is
achieved primarily by the use of resistant varieties and/
or fungicide spray programs. Fungicide sprays are rec-
ommended for all cucurbits. However, resistant variet-
ies are currently available, particularly in cucumber, and
allow for fewer spray applications. Squash, pumpkin,
cantaloupe, and non-resistant cucumber varieties are very
susceptible and should be sprayed every five to seven
days. Contact your county Extension agent about rec-
ommended fungicides. Spray programs for downy mil-
dew on any cucurbit are most effective when initiated
prior to the first sign of disease because once downy
mildew occurs in a planting, it becomes increasingly dif-
ficult for fungicides to control downy mildew.


Several variables influence the severity of downy
mildew. Resistant varieties, as mentioned earlier for cu-
cumbers, minimize downy mildew. If you plant cucurbits
near established cucurbit fields with downy mildew, a
spray program should be started as soon as the first true
leaves are present, because spores from the older field
will be blown into the younger field. When nighttime tem-
peratures are between 55 and 750 F. and relative hu-
midity is above 90%, conditions are ideal for infection.
Thus fall and spring plantings in South Florida generally
have more downy mildew than a winter planting. Fur-
thermore, spring and fall plantings may be infected as
early as the appearance of the first true leaves. In North
Florida, cooler nighttime temperatures in the spring of-
ten delay the onset of downy mildew epidemics until
flowering on squash and cucumbers.


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Figure 2. Early symptoms of downy mildew
on cucumber leaf.

I i


Figure 1. Downy mildew spots on
upperside of pumpkin leaf.

Figure 3. Advanced symptoms on cucumber

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Figure 4. Leaf spots and curling on water-
melon leaf caused by downy mildew.



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