U UNIVERSITY of
2007 Florida Plant Disease Management Guide: Squash1
Pam Roberts and Tom Kucharek2
Specific Common Diseases
Angular Leaf Spot (Pseudomonas syringae
Symptoms: This is not a common disease in
squash in Florida. Infections are found in the leaves,
stems, and fruit. Spots in the leaves are angular, and
water-soaked. Free moisture allows bacteria to ooze
from the spots which dry later leaving a white
residue. These spots of dead tissue will occasionally
drop away from the healthy tissue leaving holes in the
leaves. This is a cool weather disease.
The spots on the fruit are generally smaller and
nearly circular. The dead spots on the fruit turn white
and the tissue may crack open. Wet, cool seasons
favor this disease. The bacterium is seedborne and
dispersed by rain or irrigation water.
Cultural Controls: Plant disease-free seed.
Rotate land away from cucurbit crops. Do not work
diseased plants when they are wet.
Chemical Controls: See PPP-6.
Damping-off (Pythium spp. and Rhizoctonia
Symptoms: Seed fails to germinate due to rapid
colonization of seed by soilbore fungi. Excavated
seed will be rotted and soft often with evidence of
fungal mycelium. Young, newly emerged seedlings
often collapse at soil line and crown. The stems may
exhibit an obvious discoloration ranging in color
from a reddish-brown to black and may be dry or
mushy to the touch depending on the soil fungus
involved. See Plant Pathology Fact Sheet PP-1.
Cultural Controls: Avoid planting seed when
soil moisture, soil preparation, temperature or
planting depth do not favor rapid emergence. Plant in
well tilled soil where old crop debris had been
destroyed 30 days previously.
Chemical Controls: Use a fungicide seed
treatment. See PPP-6.
Downy Mildew (Pseudoperonospora
Symptoms: Symptoms appear on the foliage as
pale-green to yellow, angular spots, with gray-tinged
1. This document is PDMG-V3-49, one of a series of the 2007 Florida Plant Disease Management Guide, Department of Plant Pathology, Florida Cooperative
Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised December 2005. Reviewed January 2007. Please visit the
EDIS Web site at http://edis.ifas.ufl.edu.
2. Pam Roberts, associate professor, Plant Pathology Department, Southwest Florida Research and Education Center, Immokalee, FL; Tom 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
2007 Florida Plant Disease Management Guide: Squash 2
spore masses on the undersides of these spots.
Severely infected leaves become chlorotic, turn
brown, and shrivel. The fruits are rarely affected
directly, but fail to color properly and are usually
sunburned and tasteless. Spores are readily wind
dispersed. See Plant Pathology Fact Sheet PP-2.
Chemical Controls: Initiate a fungicide spray
program in advance of disease occurrence. See PPP-6.
Gummy Stem Blight (Didymella
Symptoms: This fungus can cause damping-off,
crown and stem rot, leaf spots and fruit rot on winter
squash. Infection can begin on seed leaves but
usually occurs on the older leaves close to the soil
line. Lesions are round to irregular, brown and
sometimes concentrically zoned. In Florida, this
disease is found in fruit as black surface lesions.
Stem or vine lesions are brown, often splitting
open and turning light colored with age. The black,
speck-like fruiting structures (pycnidia) can often be
seen in these stem or vine cankers. Lesion
enlargement may girdle the stem or vine causing
The pathogen can be seedbore but often
survives in previous crop debris. Spread is achieved
by rain splashing and strong winds. See Plant
Pathology Fact Sheet PP-27.
Chemical Controls: Plant only fungicide-treated
seed. Avoid planting in field with residual crop debris
still present. Apply fungicides as needed. See PPP-6.
Phytophthora Blight (Phytophthora capsici)
Symptoms: The disease can occur on the plant at
any stage causing damping-off, seedling blight, foliar
blight, and plant death preceded by wilting.
Symptoms on mature plants are seen as dark,
water-soaked areas in the crown. Leaf spots are
rapidly expanding, water-soaked lesions. Infection of
the plant, particularly summer squash, leads to rapid
death. Sunken, brown water-soaked areas appear in
infected fruit. A white growth may cover the lesion
and sporangia can be readily recovered. Sporangia
are rain-splashed dispersed or by moving infested soil
or contaminated equipment. Surface moisture is
required by the swimming zoospores for infections.
Standing water in fields is an ideal situation for
occurrence of this disease if inoculum is present is
Cultural Controls: Plant in well-drained soils
and avoid waterlogged conditions. Do not move
plants or equipment from infected fields to
non-infected fields. Avoid fields known to have had
this disease because the pathogen can survive for
many years in the soil.
Chemical Controls: Use a soil fumigant. See
Powdery Mildew (Oidium spp.ISphaerotheca
fuliginea or Erysiphe cichoracearum)
Symptoms: This disease affects the leaves and
stems, first appearing as round whitish spots on the
upper or lower leaf surfaces. The spots increase in
number and size, coalesce, and appear on the upper
surface as a white, powdery growth. Severely
affected leaves lose their normal dark-green color and
become pale yellow-green, then brown and shriveled.
Also, the young stems are killed. Fruits on infected
vines ripen prematurely, are of poor quality, and often
sunburn. Spores are readily wind-dispersed over long
Chemical Controls: Apply fungicides as needed.
Viruses (Cucumber mosaic virus, Papaya
ringspot virus Type W, Watermelon mosaic
virus 2, and Zucchini yellow mosaic virus)
Symptoms: Young infected plants may exhibit
prominent vein clearing, chlorotic spotting and a
mosaic on leaves. Older plants may exhibit stunting
with varying degrees of mottling, leaf blistering and
malformation and vein extension along leaf borders
depending on the strain of virus, age of infection and
possibly other factors.
Yellow squash varieties will exhibit varying
degrees of fruit greening in a striped or mottled
pattern, sometimes with raised yellow blisters.
Green-fruited squash may lighten or mottle in color
as well as blister. Fruit distortion can be severe across
2007 Florida Plant Disease Management Guide: Squash 3
This virus is spread by aphids from weed hosts
within Florida. Common weed hosts include the
creeping cucumber or melonette (Melothria pendula)
in south Florida and alyce clover (Alysicarpus sp.)
farther north in the State. Dayflower (Commelina sp.)
is a major host for cucumber mosaic virus. See Plant
Pathology Circular 1184.
Cultural Controls: Do not grow squash behind
or adjacent to other cucurbit crops since these viral
diseases affect all cucurbits. Isolation of squash fields
may limit aphid buildup from other crops and use of
noncrops (solanaceous crops) as buffer fields should
reduce field to field spread. Control weeds prior to
cropping. Use of JMS Stylet Oil on a schedule can
reduce losses to virus. See PPP-6 for use of JMS
Stylet Oil. Certain varieties of yellow summer squash
and zucchini squash have resistance to some of these
Wet Rot (Blossom Rot) (Choanephora
Symptoms: This disease affects the blossoms and
fruit. The infected part rapidly becomes covered with
a mass of whisker-like, white-stalked, black-headed
fruiting bodies of the causal fungus. The tissue
beneath this mass of fungus becomes water-soaked
and rotted. During dry periods, fruit may rot back
from the blossom-end without the characteristic
fungus growth present. See Plant Pathology Fact
Cultural Controls: Occurrence of blossom-end
rot may predispose fruits to invasion by this weak
pathogen. Use of fungicides in the control of other
diseases may aid in the control of wet rot. Minimize
crowding of plants and control weeds; these practices
enhance air circulation. In gardens, removing the
spent corolla (flower) after successful pollination will
control this fungal disease on those fruit.