NC State Watermelon Disease Handbook
Watermelon Disease Descriptions
General
Watermelons are susceptible to several diseases that attack the
roots, foliage, and fruit. The most common diseases of watermelon
are anthracnose, fusarium wilt, , downy mildew, gummy stem blight,
root-knot nematode and virus diseases. Also, cercospora leaf spot,
powdery mildew, bacterial fruit blotch, damping-off, and root
rots/vine declines have been reported as a problem. Consult your
local agricultural Extension Service for identification and management
of watermelon diseases and the latest recommendations on the use
of chemicals. Disease control is essential in the production of
high quality watermelons. A preventive program that combines the
use of cultural practices, genetic resistance, and chemical control
as needed usually provides the best results. Cultural practices
are useful for limiting the establishment, spread, and survival
of pathogens that cause watermelon diseases. Many of the fungal,
bacterial, and nematode pathogens survive in old crop debris and
in soil. Fields should be rotated with non-cucurbit crops for
at least three years to reduce pathogen levels.Grass crops are
ideal for rotations where nematodes are a problem.Diseases such
as anthracnose, bacterial fruit blotch, gummy stem blight, and
Fusarium wilt are known to be carried on seed. This can lead to
rapid disease development and spread in greenhouse transplant
production and to the introduction of diseases into fields. Purchase
seed from reputable sources, and apply a fungicide seed treatment
prior to planting.Most foliar diseases are spread by water-splash
or are favored by long periods of leaf wetness. Use drip irrigation
or avoid frequent sprinkler irrigation with small amounts of water.
Finally, use tillage practices that promote the rapid decomposition
of old vines and melons soon after harvest. The use of disease-resistant
varieties is an economical means of controlling diseases. Several
varieties have resistance to Fusarium wilt. Some varieties also
are resistant to anthracnose.Management of foliar diseases such
as anthracnose, downy mildew, Cercospora leaf spot, and gummy
stem blight may require fungicide sprays. Consult North Carolina
Agricultural Chemicals Manual for a listing of fungicides approved
for use on watermelon
Around the world, over 10 viruses are known to be a problem in
watermelon production. Watermelon mosaic virus(WMV-2), papaya
ringspot virus-watermelon strain (PRSV-W, formerly known as watermelon
mosaic virus-1), and zucchini yellow mosaic (ZYMV) are the most
seriously important watermelon virus diseases in the USA..Virus
diseases are destructive to the watermelon crop, and are difficult
to control. The major control strategies involve insecticides
to eliminate the insect vectors, herbicides to remove alternate
hosts, or genetic resistance. The most economical method for control
of virus diseases is genetic resistance.
Anthracnose
Anthracnose is a very common fungal disease and is caused by
the seed-borne fungus Colletotrichum lagenarium. Most cucurbits
are susceptible. Most commercial watermelon varieties have some
resistance. Depending on weather, leaf and fruit spotting may
occur on young plants, especially in late plantings. Appears first
on crown leaves as small, brown-black spots. During damp weather,
orange-pink masses of spores develop in centers of larger leaf
spots. During dry weather the spore masses turn gray. Mostly the
youngest leaves are attacked. On fruit the fungus causes sunken
spots on the rind. Symptoms may not be visible at harvest but
may develop in transit or at the market. Small anthracnose spots
can be confused with pimples disease.CONTROL: A combination of
seed treatment, crop rotation and fungicide applications are necessary
for control on susceptible varieties. Follow general management
practices; start spray schedule at first appearance. Vines should
be thoroughly covered with fungicide spray. Avoid soaking seed
before planting. It is advisable to grow watermelons, cucumbers,
squash or pumpkins only once every five years on the same land.
Downy Mildew
Downy Mildew is caused by the air-borne fungus Pseudoperonospora
cubensis. Disease is serious on late season cantaloupes, pumpkins,
squash, and watermelons. The fungus over-winters in Florida, and
each year the spores are blown northward and arrive in North Carolina
from May to August, if at all. Disease is most severe during periods
of mild temperatures and rainy season. Typically its arrival coincides
with summer rains. CONTROL: Resistant cultivars; start spray schedule
at first appearance.Follow a protective spray program during periods
favorable for infection and spread. Begins as small yellowish
areas on the leaves. Early in the morning when moisture has been
present overnight, young spots may appear as water-soaked, somewhat
angular areas on the lower side. As the lesions enlarge in a somewhat
angular manner they change from yellow to brown. On the lower
side, purplish spore masses may be seen, and several infected
leaves may die in 10-14 days. Older leaves become infected first.
Downy mildew does not attack stems or fruit.CONTROL: Follow general
management practices; start spray schedule at first appearance.
Fusarium Wilt
Fusarium wilt is caused by Fusarium oxysporium f. sp. niveum.
The fungus may survive several years in soil. The disease
is favored by warm, and sandy soils like those in watermelon-growing
areas.Susceptible varieties are killed at any stage of growth.
If inoculum levels of Fusarium are high, seedlings may wilt in
the field.Foliage of infected plants turn yellow and wilt.Plants
die soon after symptoms are observed.On the dying vine's stem,
a pinkish white cottony growth can appear near ground level. The
fungus spreads outward from infected plants.There are three races
of the wilt fungus. Genetic resistance has been incorporated into
many of the newer cultivars for two of the more commonly occurring
viruses. The third race has only been found in a few isolated
areas of the world. There is no source of resistance to this race.
CONTROL: Wilt losses are managed by following a five year rotation
program with non related plants and use of resistant varieties.
Gummy Stem Blight
Gummy Stem Blight is a warm weather southern disease. It is caused
by seed borned fungus Didymella bryoniae. All cucurbits
are susceptible, and depending on the weather, stem and leafspotting
may occur on young plants, especially in late plantings.Early
symptoms are round black, wrinkled spots on young leaves and dark
sunken areas on stems. Early infection usually develops from contaminated
seed.Many times it produces brownish circular spots which rapidly
expand into large tan lesions of round, indefinite size. The lesions
may exhibit brown gummy exudate on the surface and stems may be
girdled. In prolonged rainy periods most of the foliage and vine
may rapidly collapse. Fruit lesions appear as small water-soaked
areas, similar to anthracnose, which rapidly enlarge to indefinite
size. Also, a brown streak may appear in the blossom end of the
fruit. Although this disease may attack young tissue, it is primarily
a disease of older tissue. CONTROL: Follow general management
practices; start spray schedule at first appearance.
Root-Knot Nematode
Root-Knot is a nematode diseased that is caused by Meloidogyne
spp. are usually present in agricultural fields and affect
many crops. Nematode damage is visible as stunted yellowed plants.
The plants develop symptoms of plant stress and when the population
is very high, the plants die. Examination of the roots reveals
small to moderately sized galls. The galls will be scattered over
the root system. Normally nematode damage is restricted to a small
area of the total field. CONTROL: Promptly disk all crops after
harvest. Soil fumigation with a soil nematicide will reduce the
soil population.Rotations with corn, sorghum or pasture grasses
will also reduce the root knot population. The practice of summer
fallow will reduce the nematode population as well.
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