Pumpkin Diseases. Roger
CUCURBITS
Watermelon - Citrullus vulgaris Schrad.
Cantaloupe - Cucumis melo L.
Cucumber - C. sativus L.
Winter squash - Cucurbita maxima Dcne.
Pumpkin - C. pepo L.
Summer squash - C. pepo var. melopepo (L.) Adef.
These crops are grown throughout the United States during warm weather; all
are frost sensitive. Watermelon and cantaloupe are most frost sensitive and
are concentrated in warmer areas. There are approximately 13,000 A of
watermelon and 4,000 A of cucumber in commercial production in Mississippi.
DISEASE - Bacterial wilt (cucumber, cantaloupe, squash and pumpkin).
Cause - Erwinia tracheiphila (E. F. Sm.) Holland. Bacterium.
Symptoms - In initial stages of infection the foliage becomes dull green in
appearance. Leaves, and eventaully entire branches, lose turgor and wilt
irreversibly. Bacterial exudate oozes from cut stems, and strands can be
drawn out 2 cm or more in length. Bacterial streaming may be observed from
cut stems immersed in water. These are diagnostic characteristics of the
disease.
Disease Cycle - This gram-negative, peritrichous flagellated rod
overwinters in and is vectored by striped (Acalymma vittata Fab.) and
spotted (Diabrotica undecimpuncata Barber) cucumber beetles. Invasion takes
place through wounds caused by the beetles, and the bacteria progress
through tracheal elements. The bacterium has a low heat tolerance.
Control - Insecticide application for beetle control.
DISEASE - Downy mildew (cucumber, cantaloupe and watermelon).
Cause - Pseudoperonospora cubensis (Berk. and Curt.) Rostow. Phycomycete.
Symptoms - Pale green areas on the foliage change to yellow angular spots
bounded by leaf veins. The abaxial leaf surface is covered with a fruiting
layer of the fungus under humid environmental conditions. Occasionally,
lesions range in color from white to almost black. Infected leaves may die,
and the disease may progress outward until the entire vine is killed.
Entire plants may be stunted and may die completely.
Disease Cycle - The purple fungal layer on the abaxial leaf surface
consists of sporanagia which function as conidia. These may be spread by
northerly winds which transport the organism long distances from
overwintering sites in warm areas. Free water is necessary for completion
of the life cycle. Dew periods at least 5-6 hr in duration, combined with
air temperature of 22-27 C and increasing wind velocity,
are necessary for spore release for development of epiphytotics. Spore
release peaks around midday, and the fungus enters the plant through
stomates.
Control - Resistant varieties.
- Carbamate fungicides.
DISEASE - Gummy stem blight (cucumber, cantaloupe and watermelon).
Cause - Mycosphaerella melonis (Pass.) Ch. and Walk. Ascomycete.
Symptoms - Spots on leaves and cankers on petioles and stems become pale
brown or gray and exude a gummy material. Stem cankers are found on vines
near the center of the hill. Green or yellow spots appear initially on the
fruit but later turn gray to brown and enlarge to indefinite size. Fruit
spots may also produce a gummy exudate. Black fruiting bodies appear in
spots on stems and fruit.
Disease Cycle - The pathogen lives between crops in diseased plant refuse
and in or on the seed. Penetration is direct, but the fungus may enter
through wounds. The pathogen may be disseminated on infected fruit. The
gummy exudate consists of spores which are spread by water.
Control - Disease free seed.
- Crop rotation (3 yr).
- Carbamate fungicides.
DISEASE - Scab (cucumber, cantaloupe and squash).
Cause - Cladosporium cucumerinum Ell. and Arth. Fungi Imperfecti.
Symptoms - The first symptom on the foliage is a slightly water-soaked or
pale green area. These areas are numerous and become brownish in color by
midsummer. Sporulation on lesions may be evident. Fruit is most susceptible
when young. Lesions on the fruit are gray, slightly sunken, and about 3 mm
in diameter. These lesions grow darker and become more sunken with age.
Infected fruit may be distorted and spots may have a gummy exudate.
Disease Cycle - The fungus overwinters in plant debris and on cucumber
seed. The source of inoculum in melons is unknown. Spores are disseminated
on insects, tools, and are blown long distances. Penetration is through
stomates and directly into fruit. The fungus is very sensitive to moisture
and temperature changes and is most active in cool, foggy weather.
Temperatures above 26 C cause a decrease in activity of the fungus.
Control - Disease free seed.
- Crop rotation.
- Resistant varieties.
- Carbamate and chlorinated hydrocarbon fungicides.
DISEASE - Fusarium wilt (cucumber, cantaloupe and watermelon).
Cause - Fusarium oxysporum f. sp. cucumerinum Owen on cucumber; F.
oxysporum f. sp. Melonis (Leach and Curr.) Snyd. and Hans. on cantaloupe;
and F. oxysporum f. sp.
niveum (E. F. Sm.) Snyd. and Hans. on watermelon. Fungi Imperfecti.
Symptoms - General symptoms include rotting of seed in the soil, damping
off of seedlings, and wilting of vines in mature plants. F. oxysporum f.
sp. cucumerinum causes vessels to become prominent as bleached white lines.
F. oxysporum f. sp. melonis incites necrotic streaks up to 0.5 m on stems
from which salmon-pink sporulation develops. A brownish, gummy exudate may
form, and vessels may turn orange-red following colonization. F. oxysporum
f. sp. niveum may girdle the hypocotyl by inciting a soft watery rot.
Necrotic lesions may develop on roots. Vascular browning is incited by all
three form species. Cottony sporulating mycelium may be found growing on
dead tissue.
Disease Cycle - All three form species are soilborne and attack seed,
seedlings, and roots. The fungus spreads from roots through the vascular
system as microconidia reaching all parts of the plant. The fungus is
persistent in the soil and is spread long distances on infected seed. Once
a host is infected, the fungus increases dramatically. Both macroconidia
and chlamydospores are produced on the surface of tissue killed by the
organism.
Control - Resistant varieties.
DISEASE - Anthracnose (cucumber, cantaloupe and watermelon).
Cause - Colletotrichum lagenarium (Pass.) Ell. and Halst. Fungi Imperfecti.
Symptoms - Spots on leaves begin as small yellowish or water-soaked areas
that enlarge rapidly and turn brown on cantaloupe or cucumber and black on
watermelon. Petioles may be attacked causing defoliation. Elongate lesions
similar to those on leaves are present on stems; these lesions, together
with destruction of the foliage, may kill the whole vine. Dark, sunken,
circular spots develop on the fruit. When the fruit pedicel is attacked,
young fruits darken, shrivel, and die resulting in fields with vines having
small black fruit.
On larger fruit the spots can be of considerable size and pinkish spores
masses are often conspicuous. Fruiting bodies appear in distinct zones on
fruit and are diagnostic of the disease. Cankers on fruit do not penetrate
edible flesh, but melons with a large number of lesions may be bitter.
These melons are also likely to be destroyed by soft rot organisms.
Disease Cycle - No disease is more destructive to cucurbits than
anthracnose; it has been known for more than 100 years. The fungus
overwinters on plant debris and on or in seed. Spores are spread by water,
and the disease is favored by rainy weather. The fungus penetrates the host
directly.
Control - Disease free seed.
- Resistant varieties.
- Crop rotation.
- Carbamate fungicides.
DISEASE - Root knot.
Cause - Meloidogyne spp. Root-knot nematodes.
Symptoms - The aboveground symptoms are not distinctive, and the disease is
often diagnosed incorrectly. Stunting, general unthriftness, and sometimes
death of plants will occur. Knots or galls are formed on the roots.
Disease Cycle - The root knot disease was first recorded on cucumber in
England in 1855. The mature female root knot nematode is a sedentary
endoparasite. Eggs deposited by the mature female will persist in the soil
for several years. Yound second stage larvae enter roots from the soil and
remain there until maturity with the production of new egg masses. Local
and long distance spread occurs on infested plant debris, soil, tools, and
machinery. The disease is more severe on sandy soils.
Control - Resistant varieties.
- Crop rotation.
- Soil fumigation or sterilization.
DISEASE - Mosaic.
Cause - Cucumber Mosaic Virus (CMV) - the type member of the Cucumovirus
Group. This virus occurs on all cucurbits listed.
Squash Mosaic Virus - a member of the Comovirus Group. Watermelon is
hypersensitive.
Watermelon Mosaic Virus - member of the Potyvirus Group. Occurs on all
cucurbits listed.
Symptoms - Symptoms on leaves vary from mild chlorosis to bold mosaic
mottling. Leaf distortion occurs in at least two forms. Blisters, or
enations, may occur sometimes associated only with leaf veins; leaf apices
may form long, narrow, twisted projections or "shoestrings". Leaves can be
stunted and, as the disease progresses, begin to curl downward. Fruits are
often covered with wart-like projections. In later stages of CMV infection
of cucumber, fruits are often produced which have a smooth greenish-white
color. This symptom is responsible for the term "white pickle" which was
the original name given to the disease.
Disease Cycle - CMV and SqMV may be seed transmitted. Watermelon mosaic
virus and CMV are both aphid transmitted while squash mosaic virus is
transmitted by beetles; all three viruses are mechanically transmissible.
Overwintering apparently occurs in wild perennial hosts.
Control - Sanitation (in the home garden rougue all infected plants).
- Elimination of overwintering hosts (applies particularly to the case of
CMV on celery in Florida).
- Use of resistant varieties (especially cucumber varieties resistant to
CMV).
BACK TO THE TOP!
Back to: Diseases of Crops
---------------------------------------------------------------------
To sign-off this list, send email to majordomo@mallorn.com with the
message text UNSUBSCRIBE PUMPKINS
