Title: Florida plant disease management guide
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Permanent Link: http://ufdc.ufl.edu/UF00053871/00027
 Material Information
Title: Florida plant disease management guide
Alternate Title: Ornamentals and turf
Fruit and vegetables
General plant pathology, field crops and pasture grasses, fungicides, adjuvants and application techniques
Physical Description: v. : ; 28 cm.
Language: English
Creator: University of Florida -- Dept. of Plant Pathology
Florida Cooperative Extension Service
Publisher: The Extension
Place of Publication: Gainesville Fla
Frequency: annual
regular
 Subjects
Subject: Plant diseases -- Periodicals -- Florida   ( lcsh )
Pesticides -- Periodicals   ( lcsh )
 Notes
Statement of Responsibility: Plant Pathology Dept., University of Florida and Institute of Food and Agricultural Sciences, Florida Cooperative Extension, University of Florida.
Numbering Peculiarities: Issued in three volumes: v. 1, General plant pathology, field crops and pasture grasses, fungicides, adjuvants and application techniques; v. 2, Ornamentals and turf; v. 3, Fruit and vegetables.
General Note: Description based on: 1999-2000.
General Note: "SP-52"
 Record Information
Bibliographic ID: UF00053871
Volume ID: VID00027
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 44549741
lccn - 00229071
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Preceded by: Florida plant disease control guide

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PDMG-V3-55
U UNIVERSITY of
UF FLORIDA
IFAS Extension



2006 Florida Plant Disease Management Guide:

Watermelon1


Pamela Roberts and Tom Kucharek2


Specific Common Diseases

Alternaria Leaf Spot (Alternaria cucumerina)

Symptoms: The causal fungus infects the leaves
only. Lesions are usually round to irregular, dark
brown or black, and frequently occur with concentric
rings. Lesions of Altemaria leaf spot can often be
confused with young gummy stem blight leaf spots
because of the zonate appearance. The pathogen
over-seasons on old diseased plant debris. Spores are
readily dispersed by wind and rain. Spores can be
found in association with seed. See Plant Pathology
Fact Sheet PP-32.

Chemical control. See PPP-6.

Anthracnose (Colletotrichum orbiculare)

Symptoms: This fungus infects leaves, stems, and
fruits. It is seedbome and the disease may first appear
as a brown spot on seedling cotyledons. Leaf lesions
are angular or irregular, dark brown to black, and
usually with a narrow yellow border. Many lesions
characteristically develop on a single leaf thus
originating the term "measles" for this disease.


Lesions on the stems are usually deep and
elongate while those on the fruit are raised with
sunken centers. The pathogen is spread from plant to
plant in wind-driven rain and are carried by people
and machinery moving through the vines when they
are wet.

Cultural Controls: Plant only
anthracnose-resistant watermelon varieties. Deep
plow plant residue and practice crop rotation.

Chemical Controls: See PPP-6.

Angular Leaf Spot (Pseudomonas syringae
pv. lachrymans)

Symptoms: The bacterium affects both leaves
and fruit. On the leaves, lesions begin as small
water-soaked circular black spots surrounded by a
yellow halo. The center of the spots may become
white. As they enlarge, lesions become angular in
shape and may involve an entire lobe of even larger
portions of the leaf. This is a cool-weather pathogen.
The pathogen is seedbome and seedlings may exhibit
watersoaked lesions on the cotyledon while still in
the transplant house.


1. This document is PDMG-V3-55, one of a series of the Department of Plant Pathology, 2006 Florida Plant Disease Management Guide, Florida
Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date revised: December 2005. Please visit the EDIS
Web site at http://edis.ifas.ufl.edu.
2. P.D. Roberts, associate professor, Southwest Florida Research and Education Center, Immokalee, FL; T.A. Kucharek, professor emeritus, Plant Pathology
Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
Arrington, Dean






2006 Florida Plant Disease Management Guide: Watermelon 2


On fruits, lesions begin as small circular
water-soaked areas a few millimeters in diameter and
enlarge with age to cover larger portions of the melon
surface. Lesions do not penetrate deeply into the fruit.
On large lesions the cuticle ruptures and peels free
from the melon surface. Bacterial ooze may be
present.

Cultural Controls: Use disease-free seed.

Chemical Controls: See PPP-6.

Bacterial Fruit Blotch (Acidovorax avenae
subsp. citrulli)

Symptoms: Watermelon fruit develops a dark
water-soaked lesion on the top or sides of watermelon
fruit approximately 2 weeks before maturity or later.
As these lesions expand, they crack and develop a
brown, scaly appearance. Additional fruit breakdown
occurs. The bacterium causes small, angular or linear
necrotic lesions on true leaves. These lesions are
small and similar to those caused by downy mildew
or gummy stem blight. No significant leaf loss
occurs. Lesions in seed leaves become necrotic, but
they typically are water-soaked areas on the
underneath side first. The pathogen is seedbore.
Spread within a field is by windsplashed rain,
workers, or equipment. Wet conditions are favorable
for disease development.

Cultural Controls: Use pathogen-free, indexed
seed and healthy transplants.

Chemical Controls: See PPP-6.

Blossom-end Rot (Calcium Deficiency)

Symptoms: The blossom end of the fruit turns
pale green to brown to black. These affected areas
enlarge into sunken spots. The discolored fruit portion
will shrivel into a dry rot unless the tissue is invaded
by secondary organisms which often extend the area
of rot throughout the fruit.

This condition can result from a number of
situations that limit available calcium to the plant.
Poorly limed soils may suffer from a deficiency of
calcium in the soil. Calcareous soils seldom have this
problem. Dry soil can limit calcium availability to
roots regardless of the amount of calcium present in


the soil. Fluctuations in between wet and dry soil
conditions, even for short time periods, can result in
deficiency symptoms.

Cultural Controls: Follow a program of soil
testing and lime according to the soil test results.
Supplement erratic rainfall with irrigation during fruit
development to avoid blossom-end root development.
Foliar application of calcium may help existing
blossom-end rot problems.

Cercospora Leaf Spot (Cercospora citrullina)

Symptoms: The fungus only causes symptoms on
the leaves. Lesions are usually round, small, dark
brown to black, with white centers, and encircled
with a yellow halo that extends several millimeters
wide. The pathogen over-seasons on old debris.
Spores are readily wind-bome and rain splashed.

Chemical Controls: See PPP-6.

Damping-Off (Pythium spp., Fusarium spp.,
Rhizoctonia spp.)

Symptoms: Several soil-inhabiting fungi and
fungal-like organisms that are almost universal in
occurrence cause this disease affecting seedling
watermelons. These fungi infect portions of the plant
at or below the soil level, resulting in collapse and
death of the seedling. Conditions unfavorable for
rapid emergence of watermelons (cool, wet weather)
are usually most favorable for this disease. See Plant
Pathology Fact Sheet PP-1.

Chemical Controls: Plant only fungicide-treated
seed (most seed purchased is pre-treated). See PPP-6.

Downy Mildew (Pseudoperonospora
cubensis)

Symptoms: This fungus infects the leaves
primarily. Lesions first appear on the crown leaves as
yellow, mottled spots with indefinite borders. Older
lesions are dark brown, contrasting sharply with the
healthy tissue, and with only a small yellow border.
As several lesions coalesce, the leaf curls inward
toward the midrib and presents a "cupped hand"
appearance. Under favorable conditions, downy
mildew develops rapidly, resulting in a
"bumed-off' appearance over the entire field. The






2006 Florida Plant Disease Management Guide: Watermelon 3


pathogen is spread rapidly by means of airborne and
rain-splashed spores. The causal fungus overseasons
in southern Florida and thus the fungus moves
northward gradually by air currents as spring
watermelon planting proceeds northward. See Plant
Pathology Fact Sheet PP-2.

Chemical Controls: Fungicide applications will
effectively control the fungus if infection is
diagnosed at an early stage. See PPP-6.

Fusarium Wilt (Fusarium oxysporum f. sp.
niveum)

Symptoms: Infected plants develop a
progressively severe wilt syndrome. Sectoring of wilt
symptoms on one or more runners can occur. The
vascular tissue in the crown develops a light brown to
red discoloration. In severe cases, the entire root may
become dark brown and a soft rot develops near the
crown. The pathogen can be spread to new areas on
seed on in soil transported by equipment, drainage
water, and man. See Plant Pathology Circular 1025.

Cultural Controls: Plant watermelon varieties
with resistance to this disease. However, even with
resistant varieties, it is desirable to use new land or
have a maximum number of years between crops on
the same land. On land previously cropped in
watermelon, some wilting of plants can occur even
with resistant varieties and final thinning should be
delayed as long as possible to eliminate the greatest
number of wilt-susceptible plants before final stand is
established. Contamination of new fields with soil
from Fusarium-infested fields should be avoided.

Gummy Stem Blight (Didymella bryoniae)

Symptoms: This fungal pathogen can cause
damping-off, crown rot, leaf spot, stem canker, and
fruit rot of watermelon. Lesions in the cotyledons and
leaves are round or irregular, brown and with faint
concentric rings. Lesions on the crown and stem are
brown and usually turn white with age. Lesions on
fruit are brown soft, nearly circular, and up to 4-6
inches in diameter. Lesions in stems and fruit may
ooze of bleed an amber plant fluid: hence the name
"gummy" stem blight. The pathogen is seedbore,
spread by splashing rain from plant to plant, carried
long distances on wind currents, and


"over-seasons" on old plant debris. See Plant
Pathology Fact Sheet PP-27.

Cultural Controls: Use crop rotation. Turn vines
when foliage is dry.

Chemical Controls: Plant only fungicide-treated
seed. This disease requires fungicide applications to
obtain adequate control. See PPP-6.

Phytophthora Blight and Fruit Rot
(Phytophthora capsici)

Symptoms: Any part of the plant can be infected
by this fungus. However fruit symptoms are the most
commonly observed. When the roots of stems are
infected, wilting of the plant will occur. A whitish
mold may appear on the outside of the stem. The fruit
rot will appear as greasy blotches on the outer rind. A
whitish, mold is likely to be present on the greasy
tissue. This disease is most likely to occur during or
after periods of excessive rains where water remained
in the field.

Cultural Controls: Avoid fields known to have
had this disease because the fungus can survive for
many years in the soil. Provide for adequate drainage
of water.

Chemical Controls: See PPP-6.

Powdery Mildew (Sphaerotheca sp.)

Symptoms: Initially, older leaves have a
reddish-brown bronzed appearance. The reader must
realize that viral infections, chemical phytotoxicity
and certain nutrient deficiencies can cause similar
symptoms. At this point, microscopic examination is
required to discern if characteristic spores of powdery
mildew are present. At some point, obvious white,
powdery fungal growth may occur. This has been an
increasing problem for the past 5 years.

Cultural Control: None.

Chemical Controls: See PPP-6.

Rind Necrosis (Several bacterial species)

Symptoms: Characteristic symptom of this
disease is the development of light brown, dry corky
spots in the rind which may enlarge and merge to






2006 Florida Plant Disease Management Guide: Watermelon 4


form rather extensive necrotic areas that rarely
extend into the flesh. There are no external symptoms
of rind necrosis, but infected fruits appear to have
exceptionally tough rinds in the affected areas. The
means of dissemination of the pathogen is not known
nor is the etiology of the disease well understood, but
apparently infection is limited to the fruits.

Cultural Controls: Watermelon varieties differ
in the relative incidence and severity of rind necrosis.

Speckle or Moonspots (unknown cause)

Symptoms:This name refers to the development
of small white or yellow circular spots on the leaves
and fruits. The cause or causes are unknown but this
condition does not appear to be typical of an
infectious disease. It is heritable. There is no control
for this disease.

Tomato spotted wilt (Tomato spotted wilt
virus)

This disease has occurred on a few occasions in
watermelon in Florida, but an adequate range of
symptoms cannot be described. No controls are
available

Mosaic (Papaya ringspot virus type W,
Watermelon mosaic virus 2, Zucchini yellow
mosaic virus)

Symptoms: The most common symptom is
mottling of the leaf (alternate light and dark green
areas). However, one of more of the following
symptoms may also be associated with mosaic:
stunted growth, abnormal leaf shapes, shortened
intemodes, "bushy" and erect growth habit of the
runner tips, and mottled or bumpy appearance of the
fruit.

This disease is usually spread by winged aphids
during feeding. The aphids pick the virus up from
weed hosts such as the creeping cucumber or
melonette, Melothria pendula L., in south Florida or
alyce clover (Alysicarpus sp.) farther north in the
State. Virus transmission requires 9 seconds or less
of aphid feeding on watermelon. See Plant Pathology
Circular 1184.


Cultural Controls: No effective commercial
control but isolation of cucurbit plantings by use of
surrounding plantings of solanaceous crops (tomato,
potato, eggplant, pepper) might be helpful in delaying
initial infection. Elimination of wild hosts in the
vicinity of commercial plantings of watermelons and
other cucurbits is critical to the control of these
viruses.




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