Cause and symptoms

Title: Gummy stem blight of cucurbits
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066891/00001
 Material Information
Title: Gummy stem blight of cucurbits
Physical Description: Book
Creator: Kucharek, Tom
Publisher: Gainesville : Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1983
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066891
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 - 51340802

Table of Contents
    Cause and symptoms
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
Full Text

Plant Pathology Fact Sheet

Gummy Stem Blight (GSB) of Cucurbits

Tom Kucharek and Norm Schenck, Professor and Extension Plant Pathologist
and Retired Professor, respectively; Department of Plant Pathology, University
of Florida, Gainesville, 32611. 1983; Revised Oct. 1999
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Cause and Symptoms

Gummy stem blight (black rot) is caused
by the fungus Mycosphaerella citrullina. In
Florida, watermelons are infected annually.
Cucumbers, muskmelons, cantaloupes and
other melons and citrons are infected fre-
quently. Squash, pumpkins, gourds, chayote,
balsam pear, other members of the cucurbit
family and opuntia (a member of the cactus fam-
ily) may become infected with gummy stem
blight. Cucurbit plants may be infected at any
growth stage from seedlings to mature vines
with fruit. Infection and symptoms can occur
on all plant parts except roots. Winter squash
types (butternut, hubbard etc.) are likely to ex-
hibit symptoms in the fruit only or in older

Symptoms in seedlings occur as light to
dark brown spots on the seed leaves (Fig. 1)
cotyledonss) or as a light to dark brown to black
colored, sometimes gummy, lesions on the
main stem (Fig. 2). Prior to the occurrence of
tissue yellowing chlorosiss) or browning (ne-
crosis), the same tissue may appear
watersoaked. Wilting, followed by death of the
young plant, can occur. Main stem lesions en-
large and slowly girdle the main stem result-
ing in a red-brown-black canker that cracks and
may exude a red to amber gummy substance
(Fig. 3). Vine wilting is usually a late symptom
(Fig. 4). Individual lesions on vine stems can
also occur (Fig. 5) and they may exude a gummy

substance. Embedded in older diseased tissue,
small, clear white (when young) to black (when
old), pimple-like structures (pycnidia) will
swell and release clear to white tendrils of
spores if the tissue is wetted with water. A hand
lens or stereoscope may be used to see this
phenomenon easily. Generally, this disease
progresses from the central stem of the plant to
growing tips. Because other plant disorders can
cause exudation of a gummy substance,
"gummyness" should not be relied upon en-
tirely for diagnosis of gummy stem blight. An-
thracnose (another fungus disease) and inad-
equate liming can cause stem lesions and gum-

Leaf spots are variable in shape, red-
brown in color and initially associated with
portions of leaves that retain moisture for long
periods of time, such as veinal areas and leaf
margins (Fig. 6). Pycnidia (mentioned earlier)
will form in leaf lesions also.

Fruit rot in watermelons usually is not a
problem if vine tissues are not diseased from
gummy stem blight. Lesions in fruit of water-
melon, cucumber and muskmelon are first oval
to circular and greasy-green in color. Later these
lesions will become dark brown-black with coa-
lescing of individual lesions. Older lesions will
appear depressed in the center. Internally, the
rind will become dark brown to black and
cracked. (Figs. 7 & 8). Butternut squash fruit can
be infected with the vines being healthy. A dark


yellow to brown, crusty appearing lesion oc-
curs on large areas of the butternut fruit (Fig.

The fungus causing gummy stem blight
can be seedborne or it can survive in weeds
such as citron, balsam pear and other volun-
teer cucurbit plants or on organic debris from
previous infected cucurbits. The gummy stem
blight fungus produces two spore stages, a
sexually produced spore ascosporee) and an
asexually produced spore (pycnidiospore).
Both spore types are produced within enclosed
structures. The ascospore is more apt to serve
as primary inoculum as it is windborne and can
be disseminated from field to field. The
pycnidiospore (formed in pycnidia mentioned
earlier) functions in secondary spread prima-
rily. Pycnidiospores are released in a mucilagi-
nous (gummy) substance that make them more
adaptable for short distance spread by splash-
ing water. Often growers remark about a dis-
ease occurring "overnight." What they are ac-
tually seeing are the results of secondary
spread, which is more difficult to control than
primary spread simply because of increased
spore numbers with increased diseased tissue.

Moisture and temperature influence the
infection process and spore production. A mois-
ture film from dew, rain or irrigation is neces-
sary for spore germination. The optimum tem-
perature for infection is 61 to 750F. Nighttime
temperatures and moisture conditions are ideal
during much of the crop growing season in
Florida. After a spore germinates on a suscep-
tible host, the fungus penetrates the plant tis-
sue and with optimum temperatures, symp-
toms can appear in 7 to 12 days. Wounding of
host tissue promotes infection. Harvest wounds
on fruit can be a point of entry. The fungus is
capable of growing from 40 to 900F but tem-
peratures below 450 F retard fungal growth.
Thus storage or holding of cucurbit fruit should
be done using temperatures near 45-500F.
Avoidance of chilling damage necessitates a

compromise in storage temperature. Canta-
loupes can be held or stored safely at slightly
lower temperatures.


Gummy stem blight can be controlled if
the grower utilizes several control measures in
a sequence. First and foremost, the primary
source of inoculum (spores) in seed and the
field must be reduced. Seed can be infected
without expressing symptoms. Because no cer-
tification program is established, the grower
should purchase seed from companies that pro-
duce seed fields where gummy stem blight is
kept under control. Seed from healthy fruit will
be free from gummy stem blight. Do not use
transplants that have gummy stem blight or
other diseases.

The second source of primary inoculum,
and the most important, is on organic debris
from previous cucurbit crops. As soon as a cu-
curbit crop is harvested, the decaying debris
from that crop should be disced and plowed
under with a mold board plow, thereby reduc-
ing inoculum. Wild citrons, balsam pear or vol-
unteer cucurbits are a third source of inoculum.
Crop rotation and destruction of weed hosts are
important for gummy stem blight control. The
more growers that utilize these cultural tactics,
the more effective will be your control efforts
by reducing primary sources of inocula.
Varietal differences in susceptibility to
gummy stem blight exist in cucumbers. It is
noteworthy that gummy stem blight on water-
melons became a more serious problem in
Florida about the time the Charleston Grey va-
riety was released.

Finally, multiple applications of fungi-
cides are necessary to control gummy stem
blight. Current recommendations are avail-
able from your county Extension office. Begin
your fungicide program prior to the first sign
of gummy stem blight. In south Florida
(below Tampa), the spray program may need

to be initiated soon after emergence. In other
areas of the state, fungicide spray programs
can be initiated when the vines begin to
"run." However, in 1982 a severe epidemic of
gummy stem blight occurred on seedlings in

Figure 1. GSB lesions in watermelon seed
leaves cotyledonss).
'.'.. i
,":" .. E.

central Florida where rainfall was frequent
and thus the grower should monitor his
fields. Gummy stem blight will be more
severe in "wet" years.

Figure 2. Early GSB lesion in watermelon
crown tissue.


rrl -u
I 4
~ *:

Figure 3. Late GSB lesion in watermelon

Figure 4. Vine wilting from GSB in water-

Figure 5. GSB lesion in cantaloupe vine. Figure 6. GSB lesions in watermelon leaves.


Figure 7. GSB lesion in watermelon fruit.

Figure 8. GSB lesions (young and old) in
honeydew melon fruit.

- Y. "

Figure 9. GSB lesions in butternut squash

Ak -*


University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs