• TABLE OF CONTENTS
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 Introduction
 Papaya apical necrosis
 Corynespora leaf spot
 Root knot
 Figures






Title: Some common diseases of papaya in Florida
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Permanent Link: http://ufdc.ufl.edu/UF00066899/00001
 Material Information
Title: Some common diseases of papaya in Florida
Series Title: Plant Pathology Fact Sheet PP-35
Physical Description: Book
Language: English
Creator: Pernezny, Ken
Litz, R. E.
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Department of Plant Pathology -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Forida
Publication Date: 1985
 Subjects
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066899
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
    Introduction
        Page 1
    Papaya apical necrosis
        Page 2
    Corynespora leaf spot
        Page 3
    Root knot
        Page 4
    Figures
        Page 5
        Page 6
        Page 7
Full Text



Plant Pathology Fact Sheet


Some Common Diseases of Papaya in Florida

Ken Pernezny and R. E. Litz, Professor of Plant Pathology, Everglades Research
and Education Center, Belle Glade and Professor, Horticultural Science, Tropical
Research and Education Center, Homestead, Florida. 1985; Revised November
1999
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean


Introduction

Papaya is a large herbaceous tropical
plant grown for its melon-like fruit. The papaya
is frost-sensitive, and is grown only in south-
ern Florida, with commercial production cen-
tered in Dade County where frost damage is
least likely.

Diseases are the most important limit-
ing production problems of papaya. Although
the plant is classified botanically as a peren-
nial, virus diseases have reduced the effective
crop life to 1-2 years.

This fact sheet describes the symptoms
of several important Florida papaya diseases
and recommendations for control. Since few
pesticides are registered by the Environmental
Protection Agency for use on papaya, and the
plant is prone to damage by some chemical
sprays, consult the University of Florida exten-
sion service for current fungicide recommen-
dations.

Papaya Ringspot

Papaya ringspot, caused by papaya
ringspot virus, is the most important disease
of the crop in Florida, severely limiting the po-
tential for commercial production. Sometimes
this virus infects cucurbits. Earliest symptoms


in papaya appear as yellowing and vein-clear-
ing of younger leaves. A prominent yellow mot-
tling of the leaves follows. One or more lobes
of infected leaves may become severely dis-
torted and narrow (Fig. 1). Dark green streaks
may develop on the petioles and stems.

The disease has been named because of
the striking symptoms that appear on fruit.
These consist of circles and C-shaped markings
that are darker green than the background fruit
color (Fig. 2). Later, these markings may become
gray and crusty in texture.

Infected plants exhibit growth reduction.
There is reduced fruitset, and quality, espe-
cially flavor, is adversely affected.

Papaya ringspot virus can be transmit-
ted mechanically and by grafting. However, it
is thought that aphid transmission is the most
important mechanism for disease spread in the
field.

Until recently, little could be done to ef-
fectively control this disease. Attempts to re-
duce disease levels by applying aphicides (in-
secticides) have not been successful. Cultural
controls, such as roguing infected plants and
physically isolating papaya orchards, have also
been ineffective. However, good sources of
field resistance have been identified by scien-
tists at the Tropical Research and Education
Center at Homestead, with potential for im-


PP-35







proved varieties for producers.


Papaya Apical Necrosis

Papaya apical necrosis is a relatively
new virus problem in Florida. It is caused by a
newly discovered virus (papaya apical necro-
sis virus or droopy necrosis virus) that is quite
different from papaya ringspot virus.

The first symptoms are drooping and
downward cupping of leaves in the upper part
of the plant. Blades of the youngest leaves are
pale yellow, sharply cupped downward, and
do not expand properly (Fig. 3). Petioles are
short and stiff, and stem internodes become
very short.

The disease usually increases in sever-
ity in the winter. Leaves often develop marginal
necrosis, and those near the stem tip begin to
drop. The tip of the stem usually dies. The dis-
ease progresses down the stem until the entire
plant is dead.

Neither the vector nor an alternative host
for this virus has been identified. Since field in-
cidence is low at present, the only practical
means of control are to rogue diseased plants
and isolate papaya plantings.

Anthracnose

Anthracnose, caused by Colletotrichum
gloeosporiodes, is an important fungus disease
that primarily affects papaya fruit. The disease
symptoms begin as small, water-soaked spots
on ripening fruit. As the spots develop, they
become sunken, turn brown or black, and may
enlarge to 2 in. (5 cm) in diameter (Fig. 4). The
fungus may produce a pink mass of spores in
the middle of these older spots. The pathogen
can grow into the fruit, resulting in softening of
the tissue and an off-flavor of the pulp.

Infection of older petioles may occur, but
this is thought to be important only as a source


of the fungus for further fruit infection.

Because anthracnose is such a poten-
tially damaging disease, it is important to be-
gin an effective fungicide spray program at the
beginning of fruit set and to continue at appro-
priate intervals while the plants are producing
fruit.

Powdery Mildew

Powdery mildew is caused by the fun-
gus Oidium caricae. The disease is easily recog-
nized by the presence of a white, superficial
growth on leaf surfaces. The disease begins as
tiny, light yellow spots on the lower surface of
leaves. As the spots enlarge, a white, powdery
growth, the actual body and spores of the fun-
gus, appears on lower leaf surfaces. At this time
pale yellow areas appear on the upper leaf sur-
face at the infection sites. In advanced stages,
white fungal growth will develop on the up-
per leaf surface of leaf lesions (Fig. 5).

The great masses of spores produced on
infected leaves are readily spread by wind cur-
rents to healthy plants. Year-round production
of papaya permits uninterrupted reproduction
of the fungus and continuous presence of the
disease in an active state.

Fruit infection has not been observed in
Florida, but has been reported in other papaya-
growing areas. No chemical control has been
approved by the Environmental Protection
Agency.


Phytophthora Blight

Phytophthora blight can be one of the
most devastating diseases of papaya. Complete
loss has been seen in some Dade County or-
chards. Phytophthora parasitica is thought to be
the causal agent, but more research is needed
on the identification of the fungal species. The
fungus causes a wide range of damage, includ-







ing damping-off, root rot, stem rot with girdling,
and fruit rot.

Damping-off is the rapid wilting and
death of very young plants. This occurs in pro-
duction houses and in the field shortly after
transplanting.

Spots on the stems of established plants
begin as water-soaked lesions, especially at
fruit and branch scars. These areas can enlarge
and even girdle the tree, resulting in wilt and
death of the top of the plant.

Root infection can be very severe. Un-
fortunately, the first indication of major root in-
fection often is rapid browning and wilting of
the trees, followed by total collapse of the plants
within days.

Fruit infection is the most obvious as-
pect of the disease and potentially very impor-
tant economically, because of the possibility of
carry-over to the market. Water-soaked spots
are again the first evidence of infection. Dis-
eased fruit then become covered with a charac-
teristic mass of whitish fungal growth (Fig. 6).
Fruits eventually shrivel and fall to the ground,
where they serve as an important source of
Phytophthora inoculum for root infection.

Phytophthora blight is a wet weather
disease. The spores of the fungus are spread
by wind and splashing rain. The spores must
have free water to germinate, producing large
numbers of smaller swimming spores that in-
vade healthy tissue.

Fungicide sprays, as discussed for an-
thracnose, can be used for Phytophthora blight
control.

Corynespora Leaf Spot

Corynespora leaf spot is a recently de-
scribed disease of Florida papaya, caused by
the fungus Corynespora cassiicola. It is a disease


primarily of the leaf blade but will occasion-
ally occur on petioles and male flower stalks. It
has not been observed on fruit or stems.

Older leaves are most likely to be af-
fected. Symptoms are first evident as small,
yellow areas. Fully developed spots have a
small 0.1 in. (2 mm) brown center, with a promi-
nent, yellow halo 0.2-0.4 in. (4-8 mm) (Fig. 7).
When leaf spots are examined closely, one may
observe faint, concentric rings (Fig. 8).

C. cassiicola forms spores on both upper
and lower leaf surfaces, but masses of spores
are most evident on the latter. Examination of
lower leaf surfaces with a hand lens for a dark
growth of the fungus body and spores is im-
portant for diagnosis of this disease.

Heavy infection results in premature de-
foliation with losses in yield and possibly fruit
quality.

Corynespora leaf spot is controlled very
well by periodic applications of fungicide. No
specific registrations exist for the disease. How-
ever, applications of fungicide as recom-
mended for anthracnose control have also pro-
vided excellent control of Corynespora leaf
spot.

Yellow Strap Leaf

Yellow strap leaf is a recently reported
and unusual disease of Florida papayas. It is
caused by an organism that does not actually
parasitize the host. The fungus Aspergillis wentii,
a common soil inhabitant, produces a toxin,
which is absorbed through the papaya roots.
Within the plant, the toxin interferes with nor-
mal protein metabolism, resulting in severe
plant damage.

The first symptom of yellow strap leaf
is a yellowing of new leaves, especially along
the leaf margins. Leaf blades become narrow
and, in severe cases, appear strap-like. Tips of







leaf lobes are hooked downward or laterally
toward the midrib, and are thicker than normal.
Petioles and internodes become very short, of-
ten giving the trees a bushy appearance (Fig.
9). Female flowers do not develop normally and
do not set fruit. Male flowers are stunted to-
ward the stem tip and produce no pollen.

Plants with yellow strap leaf are usually
in more or less circular "hot spots" in the field,
with the most severely affected plants in the
center of the affected areas.

Yellow strap leaf is a serious problem
in hot, rainy weather. Papayas may show dra-
matic recovery during the dry season, but
symptoms can recur when conditions become
favorable.

At present there is no practical control
for yellow strap leaf. Pasteurization of soil has
controlled the problem in small-pot, green-
house studies, but, of course, this is not practi-
cal in the field. One might expect fumigation to
work, but the disease occurs in plantings on fu-
migated beds as well as in plantings on level,
unfumigated land.

Root Knot

Root knot is caused by Meloidogyne
nematodes (tiny soil-inhabiting worms). The
larvae of these nematodes can travel short dis-
tances in soil, finding and attacking papaya
roots, usually near the tips. When female lar-
vae feed near the water-conducting core of the


roots, the plant cells increase in number and
size until readily visible swellings, called galls
or "knots" are formed (Fig. 10). To confirm a
diagnosis, one can cut into the galls and observe
pearly looking, pear-shaped female nematodes
embedded in the tissue.

Leaves of papayas that are affected by
root knot nematodes appear pale green or
slightly yellow and are generally unthrifty.
They may be more sensitive than normal to
water stress. Fruits are smaller than normal and
more likely to have an off-flavor.

Although papayas, especially young
plants, can be severely damaged by root knot
nematodes, recent experiments at The Tropical
Agricultural Research and Education Center in
Homestead suggest that the crop may not be a
preferred host for reproduction of the worms.

To control root knot nematode prob-
lems, avoid heavily infested land if possible.
It is recommended that papayas grown in
land likely to be infested, for example, "old"
vegetable land, should be set in raised,
mulched beds that have been fumigated.
Consult with the county agent or the Florida
Cooperative Extension Service Nematode
Control Guide for specific fumigant recom-
mendations. The effects of root knot can be
partially alleviated by maintaining plants at
optimum water and nutrition levels.



































Figure 1. Distortion and narrowing of pa-
paya leaf due to papaya ringspot virus infec-
tion.


Figure 2. Rings and C-shaped markings on
papaya ringspot virus-infected fruit.


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Figure 3. Apical necrosis of papaya.


Figure 4. Advanced anthracnose infection of
papaya fruit.


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Figure 5. Powdery mildew of papaya, with
the characteristic growth of the white fungus
on leaves.


Figure 7. Corynespora leaf spot; note promi-
nent yellow haloes around spots.


Figure 6. Phytophthora blight-the fruit
infection phase in the field.


Figure 8. Close-up of Corynespora leaf spot,
showing faint, concentric rings.





























Figure 9. Bushy, stunted appearance of
papaya plant affected by yellow strap leaf.


Figure 10. Root knot galls, incited by
Meloidogyne incognita in papaya roots.




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