Plant Pathology Fact Sheet
Alternaria Leaf Spot of Cucurbits
Tom Kucharek, Professor, Plant Pathology Department, Institute of Food and
Agricultural Sciences, University of Florida. 1985; Revised. January 2000
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean
Cause and Symptoms
Alternaria leaf spot of cucurbits is
caused by the fungus Alternaria cucumerina.
Cantaloupes and watermelons are particularly
susceptible in Florida, but squash, cucumbers,
gherkins, balsam apple and other cucurbits are
susceptible to some degree, probably with va-
rietal differences within crop types. Plant vigor
and fruit sugars are reduced if this disease is
Alternaria leaf spots of cucurbits usually
appear on the oldest leaves first (Fig. 1) and later
spread to the newer leaves towards the tips of
the vine. Leaf spots begin as bright to pale yel-
low or tan flecks, 1/50 to 1/16 inch (0.5 to 1.5
mm) in diameter on the upper leaf surface.
These small spots may be surrounded by light
green or yellow halos. Also at this early stage,
these flecks may be surrounded by a greasy
(water soaked) appearance due to cell wall
breakdown within the leaf by the fungus (Figs.
2 and 3). In time, the spots enlarge to 3/8 to 3/
4 inch (1 to 2 cm) in diameter (Figs. 2 and 4).
The older spots are somewhat circular-to ir-
regularly-lobed and are light brown-black in
color. Older spots may or may not have con-
centric-rings.The darker bands or portions
within a spot contain spores that are 1/115 to
1/195 of an inch long and these spores are dis-
persed by wind, primarily. Individual spots be-
come brittle or blister-like and will tear and
appear ragged within the darkened tissue (Figs.
2 and 3). If Alternaria leaf spot becomes severe,
leaf curling, defoliation (leaf loss), premature
ripening, lower yields, lower fruit sugar and
fruit deformity (especially on cucumber) are the
ultimate results. Also, defoliation predisposes
fruit to sunburn.
Alternaria leaf spot can occur on the
same leaf as leaf lesions of gummy stem blight
(see PP Fact Sheet No. 27) and be similar in ap-
pearance. Also, downy mildew (see PP Fact
Sheet No. 2) and Cercospora leaf spots can oc-
cur simultaneously with Alternaria leaf spot,
but these leaf diseases are usually distinguish-
able from Afternaria leaf spot. In cantaloupe,
young Alternaria lesions can resemble downy
mildew. If adequate control measures for these
diseases are not initiated early in the season,
the grower could have epidemics of all four
diseases on the leaves simultaneously.
Fruit infection is not common but can
occur if the leaf phase of Alternaria leaf spot is
not controlled, especially on melons other than
watermelon. Overripe and sunscalded fruit are
most susceptible. Sunken spots up to 1 inch (2.5
cm) or slightly larger that have a green-black
color are typical of the fruit rot stage in the field.
Later, during storage or transit, these spots may
enlarge to 2 inches (5 cm) or more in size. Inter-
nal decay of fruit is tough and dry but if the
rotted area progresses, the fruit tissue becomes
moist and spongy. Some of the fruit rot attrib-
uted to A. cucumerina may be caused by another
species, A. tennis. Cladosporium fruit rot also
has symptoms similar to Alternaria fruit rots.
Seedling blight caused by this fungus
has been reported but is considered of mini-
mal consequence at the present time in Florida,
but it may be a problem in containerized trans-
plants-grown in pasteurized soil.
Alternaria cucumerina can survive in or on
crop debris, cucurbit weeds (balsam apple),
volunteer cucurbits and seed. Leaf debris left
on the soil surface or buried 6 to 9 inches (15 to
23 cm) deep is capable of regenerating spore
production for at least 8 1/2 months. However,
infected leaf debris on the soil surface would
be a more likely source of spores because of its
exposure to wind. Where cucurbit crops are
planted in succession to achieve multiple har-
vesting dates, leaf spots upwind in the older
plantings can serve as another source of spores
for the younger plantings.
Wetting and drying is conducive for
spore formation and spore release, respectively.
Wind is a common method by which spores are
dispersed but spore movement by rain splash
and mechanical vectors probably occurs to
some extent. If a spore lands on a cucurbit leaf
and is wetted by water, it germinates and pro-
duces special structures by which the fungus
penetrates the tissue. Spores are capable of ger-
mination, even if exposed to dry periods for as
long as 8 months above freezing temperatures.
Within 3 to 12 days after penetration of leaf tis-
sue, leaf spots are capable of producing a new
"crop" of spores. Variation in the incubation
period is probably related to factors such as
temperature, variety, and moisture conditions.
Infection can occur from 410 to 950F (50 to 350C)
with the optimum temperature for infection
being 680F (200C). Disease will progress most
rapidly as nighttime temperatures approach the
optimum and will continue at a progressively
slower rate as nighttime temperatures increas-
ingly depart from the optimum. A diurnal cycle
from 680 to 900F (200 to 320C) is ideal for Alter-
naria leaf spot of cucurbits.
The length of time leaf wetness occurs
influences the amount of penetration. Some
penetration can be initiated with 2 to 8 hours of
leaf wetness, but when leaves are wet for 10 to
24 hours, the number of penetrations is in-
creased drastically. Rain frequency and length
of dew periods are more influential than
amounts of rain.
While Alternaria leaf spot of cucurbits
is more likely to be a problem on older plants
from flowering to harvest time, younger plants,
including seedlings, can have leaf spots even
in the cotyledons (seed leaves). The few leaf
spots on these younger plants are the most
likely source of inocula (spores) for a severe
mid- to late-season epidemic. Older plants that
are stressed from nutrient deficiency (especially
nitrogen or "minor" elements), inadequate soil
pH (6.0 to 6.5 is best), nematode damage, high
yield potential or other factors could be more
susceptible to Alternaria leaf spot.
Effective control is dependent upon us-
ing a sequence of individual control measures,
all of which are designed to reduce leaf spot
early and thereby slow epidemic development.
Treat seed or use seed treated with a
broad spectrum fungicide. This will reduce in-
ocula originating with seed.
Destroy volunteer cucurbit crop plants
and weeds that are capable of harboring spores
of this disease.
Lime and fertilize the soil so that the crop
is not in stress. Stressed plants, due to lack of
nutrients or an imbalance in nutrients, are more
susceptible. Also, inadequate foliage cover can
result in sunburn of fruit which, in turn, is more
susceptible to Alternaria fruit rot.
Use crop rotation where cucurbit crops
are not planted on the same land in successive
years. This practice will reduce other diseases
and nematodes as well.
Plant spring to early summer crops as
early as possible to minimize exposure to dis-
ease-favorable weather that occurs with warmer
nighttime temperatures and increased fre-
quency of thunderstorms.
Begin and maintain a spray program
with fungicides when nighttime temperatures
exceed 600F (150C), especially when leaf wet-
ness is lengthened by frequent rains, long dew
periods, or overhead irrigation. Crops planted
in the fall in North Florida may require spray-
ing soon after emergence but a spring crop in
the same location may not have to be sprayed
until shortly before the vines begin to "run".
Cucurbit crops planted during winter months
in South Florida are prone to infection for most
of the season except during prolonged cold pe-
riods. Spray intervals of 7 to 10 days are ad-
equate with protectant fungicides but intervals
may need to be shortened to 5 to 7 days during
ideal weather conditions, especially as new
plant growth becomes exposed. Your County
Extension Office has publications, periodically
updated, which list available fungicides for use
on cucurbits and other crops.
Plow old crop debris below the soil sur-
face as soon after harvest as possible to reduce
available spore numbers for the next season.
This tillage practice will contribute significantly
to disease control.
Figure 1. Alternaria leaf spot in older water- Figure 2. Variation of Alternaria leaf spots in
melon leaves, watermelon leaves.
Figure 3. Alternaria leaf spot in cantaloupe
Figure 4. Zonate appearance of Alternaria
leaf spot in watermelon leaf.