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Group Title: Circular
Title: Some diseases of vegetable and agronomic crops caused by fusarium in Florida
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Title: Some diseases of vegetable and agronomic crops caused by fusarium in Florida
Series Title: Circular
Physical Description: 10 p. : col. ill. ; 28 cm.
Language: English
Creator: Kucharek, Tom, 1939-
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1992
Subject: Fusarium   ( lcsh )
Vegetables -- Diseases and pests -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: Tom Kucharek ... et al..
General Note: Title from caption.
General Note: "April 1992."
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Full Text

I April 1992

Circular 1025

The Genus Fusarium contains many species
(Fusaria) of fungi that are commonly found in soil.
Some Fusaria are capable of causing wilts, crown
rots, root rots, or fruit rots. Others are opportunists
because they colonize plant tissues after some type
of stress debilitates the plant; that is, Fusaria do
not cause the initial damage in those situations.
Interestingly, many Fusaria that abound in the soil
are not capable of causing disease.

Fusarium wilts are caused by the species
Fusarium oxysporum. Infection of plants by other
Fusaria or other organisms can cause wilt, but dis-
eases called Fusarium wilts are caused only by F.
oxysporum. Within F. oxysporum, specificity exists
because some forms (form species) within the spe-
cies are capable of infecting certain plant species.
For example, F. oxysporum f. sp. lycopersici causes
Fusarium wilt in tomato and F. oxysporum f. sp.
niveum causes Fusarium wilt in watermelon. Fur-
ther complicating this situation is the existence of
pathogenic races within form species. One patho-
genic race may cause disease in certain varieties of
a crop species but not in other varieties. Other vari-
eties may be infected by other pathogenic races.
Form species and races of Fusaria are indistin-
guishable in appearance. For the remainder of this
publication, the generic term Fusarium wilt will be
used to identify the complex group of wilt diseases
caused by F. oxysporum and its form species and

Other species of Fusarium cause plant diseases.
Fusarium solani causes root rots, stem rots, crown
rots and fruit rots. Fusarium subglutinans infects
*Professor Extension Plant Pathologist, Plant Pathology
Dept., Gainesville; Professor of Plant Pathology, respectively,
Gulf Coast Research and Education Center Bradenton; Cen-
tral Florida Research and Education Center Leesburg; Cen-
tral Florida Research and Education Center Sanford; Institute
of Food and Agricultural Sciences (IFAS), University of Florida.

sugarcane, corn, sorghum, and some broad leaf
plants. Identification of the species of Fusarium
associated with a problem is of little benefit for the
grower unless specific control practices (e.g., resis-
tant varieties) are available for control of specific
species, form species, or pathogenic races.

Some Fusaria, including F. oxysporum and F.
solani, produce thick-walled spores called chlamy-
dospores. F. subglutinans does not. Chlamydos-
pores are capable of surviving in soils, soil debris,
or other substrates for more than 10 or 20 years.
Chlamydospores that survive in the soil germinate
and form tube-like structures hyphaee) that pen-
etrate roots or other plant parts. These spores can
be carried in or on seed, transplants, wooden field
stakes, clonal propagation stock, such as potato
seed piece tubers, and soil attached to equipment.

Fusarium spp. can also survive by colonizing
roots or stems of so-called non-host plants. For ex-
ample, roots of barley and nutsedge can serve as
colonization sites for the Fusarium wilt pathogen of
cotton. Brazilian pepper, cudweed, and carpet weed
will continually support populations of the fungus
that causes crown rot in tomato. The fungus that
causes Fusarium wilt in tomato has been found to
infect eggplant, mallow, amaranthus, crabgrass,
and ricegrass. The watermelon wilt pathogen has
been associated with roots of citron, bitter apple,
rice, peanut, and tomato. Thus, the production of
thick-walled spores and the ability of Fusarium
spp. to colonize alternate plant hosts provide
mechanisms for long-term survival in soil. CC

Wind-blown dispersal of sporeff Fusariurrnt~
may occur. For example, air-borne spores (copidi)
of the crown rot pathogen of tomato can redoptanrT
inate nearby fumigated sites in the field or the,
greenhouse. However, for most diseases caused by
Fusaria, soil-borne chlamydospores are generally
regarded as the primary source of inocula. "

U _


Some Diseases of Vegetable and Agronomic Crops

Caused by Fusarium in Florida

Tom Kucharek, John Paul Jones, Don Hopkins and Jim Strandberg*
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean

Figure 1. Wilting of tomato plants with Fusarium wilt.

Figure 2. Tomato stem with discolored xylem tissue.

Figure 4. Wilting of tomato plants with crown rot.

Figure 3. Fusarium crown rot of tomato.


Figure 5. Lower tomato stem with crown rot.

Figure 8. Inner watermelon tap root with Fusarium wilt.

Figure 6. Inner tomato stem with crown rot.

---- * -- -* .' --
: -- ..- -,..'

Figure 7. Wilting of watermelon plant with Fusarium wilt.

Stress factors such as drought, flooding, chemical
damage, mechanical damage, low soil pH, deposi-
tion of soil on stems while cultivating, or incorrect
usage of fertilizer may predispose plants to infec-
tion by Fusarium spp. Other pests such as fungi,
bacteria, nematodes, or insects may occur simulta-
neously in roots or stems or predispose plants to
infection by the Fusaria. For example, nematode
damage accentuates Fusarium wilt in cotton but
not in any notable way in tomatoes, watermelons,
or sweet potatoes. Also, lower stems of soybeans
and beans are commonly infected with Fusaria and
other fungi after hot soil is deposited on stems dur-
ing cultivation.

Root and stem diseases caused by Fusarium
debilitate plants by rotting (plant cell destruction)
root and stem tissue or by plugging the water-
conducting vessels (xylem) in roots or stems.
Fusarium wilts can be diagnosed tentatively by slic-
ing the stem in half lengthwise. The presence of two
dark bands of water-conducting tissues indicates
that the xylem vessels are plugged from infection. If
the stem is sliced perpendicular to the length of the
stem, the darkened tissue will often appear as a
circle. If the infection is one-sided in the plant, a
portion of the circular tissue will be darkened. The
blockage of the water and nutrient-conducting tis-
sue of the plant results in stunting, discoloration of
leaves (usually yellow, initially), and wilting. Yield
losses are greater and plants are more likely to die
if infection occurs while plants are young. Because
other pests and diseases can cause similar symp-
toms, you are advised to attain formal diagnoses
until you become familiar with symptoms and field
diagnostic procedures for the crops you produce.

Fusarium wilts

Fusarium wilt of tomato
Fusarium wilt of tomato (Fig. 1) would be more
common and devastating throughout Florida if re-
sistant varieties were not used. Resistance to
Fusarium wilt in tomato is complete; that is, all
plants within a variety are resistant to the desig-
nated races unless a seed or transplant mishap has
occurred. Currently, races 1, 2, and 3 exist in
Florida. If Fusarium wilt is present, slicing the
stem lengthwise into equal halves will reveal two
dark, orange-red, or brown streaks of darkened
water-conducting vessels (Fig. 2). Symptoms in-
clude lower leaf yellowing and then browning,
stunting, vascular discoloration, and plant death.
Leaf yellowing is often seen on one side of the plant
and frequently in leaflets on one side of the leaf.

The darkened vascular tissue often extends up to
the middle part of the stem and sometimes to the
top of the stem and into the vascular tissue of the
fruit. The optimum temperature for development
of Fusarium wilt and growth of the fungus is 82F
(280C). Growth of the fungus is reduced at tem-
peratures below 680F (200C) or above 930F (340C).
However, Fusarium wilt has occurred in Florida at
temperatures above 950F (350C).

The primary control for Fusarium wilt of tomato
is the combined use of crop rotation, resistant vari-
eties for the pathogenic races in your area, disease-
free transplants, and preplant fumigation with a
broad spectrum fumigant (e.g., methyl bromide +
chloropicrin, vapam, etc.). Reinfestation of fumi-
gated soil with non-fumigated soil and pond water
should be avoided. Transplant production areas
should be sanitary and should not be adjacent to
field production areas or equipment cleaning

Sterilizing used field stakes will reduce, not nec-
essarily eliminate, the amount of inoculum associ-
ated with the stakes that can infest fumigated soil.
When tomatoes are planted in a new field, it is
best to use new stakes because sterilization proce-
dures such as steam or chemicals may not elimi-
nate all of the inoculum. When planting tomatoes
in a field that contained tomatoes in the past, use
of sterilized stakes is preferred. However, with use
of unsterilized stakes, the crop may be near the
end of harvest before the wilt fungus causes any
noticeable problem.

Additional controls for Fusarium wilt of tomato
include adjusting pH, fertility and moisture of the
soil. The soil pH should be 6.5 or above coupled
with minimum usage of ammonium- containing
fertilizer. Avoid the use of excessive amounts of
nitrogen, phosphorus, and magnesium. Soil mois-
ture should be adequate for crop production but
not excessive. Use of drip or seepage irrigation
coupled with adequate drainage for heavy rains
will provide a proper soil moisture situation.
These adjustments to the soil will be most effective
for control of Fusarium wilt if the entire root sys-
tem is confined within the treated soil such as that
within a plastic mulch system.

This fungus can be seed borne. However this
source of inoculum is not common. The importance
of seed-borne inoculum relates to the potential for
introducing new races from other areas of the

Fusarium crown rot of tomatoes
Fusarium crown rot of tomatoes (Fig. 3) is be-
coming widespread in southern Florida on the
sandy, acidic soils and it has been found occasion-
ally on the high pH soils in Dade County. Symp-
toms of Fusarium crown rot are distinctly different
from those of Fusarium wilt of tomato. The bril-
liant yellowing of the foliage typical of Fusarium
wilt does not occur with crown rot. With crown
rot, the leaflets often have a marginal necrosis
(death). Infected plants often wilt during the day
and recover during the night (Fig. 4). With crown
rot, definite root and crown rots occur (Fig. 5).
Additionally, unlike Fusarium wilt, the vascular
discoloration is limited to the lower 12" of the stem
(Fig. 6). An infected plant rarely dies but infected
plants will be stunted. Even with severe vine
symptoms, some yield is attainable. This fungus is
a member of the F. oxysporum species. It grows
best from 500F (100C) to 680F (200C) which is
lower than the optimum for the fungus that causes
Fusarium wilt. The optimum temperature for dis-
ease development is 70'F (210C).

Control of crown rot is similar to that of
Fusarium wilt (e.g., crop rotation, sanitation, in-
crease soil pH, minimize use of ammoniacal nitro-
gen, and soil fumigation). Resistant varieties for
control of crown rot are currently available only
for greenhouse production. Crown rot is likely to
occur with a higher frequency where direct seeding
is used instead of healthy transplants and where
the soil contains high levels of chloride salts. The
utmost sanitation production scheme for trans-
plants in greenhouses should be used so that indi-
viduals or equipment used within or around the
transplant site do not become contaminated with
disease-causing organisms from the field. Consid-
erable information on sanitation within transplant
systems is available in Plant Protection Pointer
No. 25. Finally, transplants should be transported,
pulled, and set without tissue damage as damaged
tissues are likely to be sites for infection.

Fusarium wilt of watermelon
Fusarium wilt of watermelon is common in
Florida where resistant varieties are not used but
it occurs to some extent even when resistant vari-
eties are used. Resistance to Fusarium wilt in
watermelon varieties is not complete; that is, some
plants within the "resistant variety" may be sus-
ceptible. However, a susceptible variety contains a
higher percentage of susceptible plants than a re-
sistant variety. Three races (0, 1 & 2) exist and

resistance to all three is necessary. Currently, re-
sistance to race 2 has not yet been incorporated
into a commercial variety.

Fusarium wilt of watermelon usually occurs
without plant yellowing; usually the entire plant
wilts quickly (Fig. 7), becomes brown and dies.
Occasionally, wilting of vines on one side of the
plant occurs, particularly on older plants. Slicing
the tap root lengthwise into two equal halves will
reveal two streaks of vascular tissue that are dark
yellow-brown, orange brown or reddish brown
(Fig. 8). In Florida, Fusarium wilt is likely to
occur prior to fruit set. The optimum temperature
for infection is near 800F (270C) with little infec-
tion occurring above 860F (300C). This wilt patho-
gen has been found to be associated with seed but
the importance here would be the potential of
introducing a new race into the field from other
areas of the world.

The best control for Fusarium wilt of watermel-
ons is the use of resistant varieties coupled with
crop rotation. Where land is limiting for an ad-
equate rotation scheme on your farm, two alterna-
tives exist. First, you may lease land that has not
had watermelons for many years. Secondly, the
use of certain varieties (e.g., Crimson Sweet) on
the same land year after year, although not ad-
vised, may result in less Fusarium wilt than if
other varieties (e.g., Jubilee types) are planted.
Rotation includes the absence of watermelon for
five years or more. Also, some varieties of yellow
summer, zucchini, and scallop squash are suscep-
tible to this fungus and should be excluded from
the rotation.

Additional controls include the use of certified,
disease-free transplants and delayed thinning of
direct seeded stands. Delayed thinning permits a
better chance of culling susceptible plants rather
than resistant plants within the population. The
use of higher soil pH's and less ammoniacal nitro-
gen has not consistently reduced the amount of
wilt in watermelons as it has in tomatoes.

Fusarium wilt of cantaloupe
Fusarium wilt of cantaloupe has not been for-
mally identified in Florida. However, it is likely
that it has occurred in the panhandle region of
Florida. Symptoms are similar to other Fusarium
wilts. This wilt pathogen has been found on seed.
This disease is favored by cool temperatures be-
tween 65 to 77F (18 to 250C).

Figure 9. Fusarium wilt of tobacco.

Figure 11. Fusarium wilt-like symptoms in beans.

Figure 10. Fusarium wilt of cabbage seedlings.

Figure 12. Fusarium wilt-like symptoms in chickpeas.

Figure 15. Fusarium-infected peanuts.

~1.z 4.".

*"- :I~'. 4

r' :I!

Figure 13. Fusarium root rot (red root) in celery.

Figure 16. Onion seedlings infected with Fusarium.

Figure 14. Seedling blight of parsley caused by Fusarium.

Fusarium wilt of cucumber
Fusarium wilt of cucumber occurs occasionally in
Florida, usually where cucumbers are grown on the
same land year after year. Symptoms are similar to
that of Fusarium wilt of watermelon and canta-
loupe. Controls include crop rotation, and liming of
soil coupled with use of fertilizer with a minimum
amount of ammonium salts.

Fusarium wilt of cotton
Fusarium wilt of cotton occurs infrequently in
Florida. Prior to the use of varieties with resis-
tance this disease was more prevalent in Florida.
Infection of cotton by the Fusarium wilt pathogen is
highly dependent upon damage from nematodes but
does not necessarily occur when plants are infected
with nematodes. Symptoms include stunting, lower
leaf yellowing and browning, wilting (often seen
first at flowering), gradual reduction of plant vigor,
and possibly plant death. Vascular discoloration
may be dark brown to black. Controls include crop
rotation, suppression of nematodes and use of vari-
eties resistant to prevalent races.

Fusarium wilt of tobacco
Fusarium wilt of tobacco is present occasionally
in Florida. Symptoms include leaf yellowing and
wilting (Fig. 9). Leaf yellowing and wilting may
occur on one side of the plant but not the other, a
symptom not uncommon with Fusarium wilts. In-
jury from cultivation or nematodes will predispose
plants to infection. Calcium deficiency will accentu-
ate Fusarium wilt of tobacco. Fusarium wilt of to-
bacco will occur across a wide range of soil pH's, but
it is likely to be more severe at 7.0. This wilt fungus
can grow from 45 to 950F (7 to 350C) but grows best
between 77 to 860F (25 to 300C). This disease is
best controlled by crop rotation, resistant varieties
and reduction of nematode damage. Sweet potatoes
should not be used as a rotational crop as they are
also susceptible to this fungus.

Fusarium wilt of sweet potato
Fusarium wilt of sweet potato occurs occasionally
in Florida if resistant varieties are not used. Leaf
yellowing and browning of the oldest leaves during
vine elongation is a common symptom. Stunting
and eventually plant death may occur. Vascular
discoloration may be similar to that of other wilts
with two discolored bands being evident when the
lower stem above or below the soil surface is cut
lengthwise. However, if the wilt is one-sided, the

vascular discoloration may be only on one side of
the stem. Vascular discoloration will vary in color
from brown to purple. Infection can occur anytime
during the growth of the crop but infection is most
likely during or shortly after transplanting. Dam-
age to roots from pulling transplants or any other
factor favors infection. This fungus grows at all
temperatures that are favorable for crop production
but may be inhibited to some degree at soil tem-
peratures above 86 to 950F (30-350C). Control in-
cludes crop rotation, resistant varieties, selection
of healthy transplants, minimizing stress and
avoidance of planting when soil is cool.

Fusarium wilt of crucifers (cole crops)
Fusarium wilt of crucifers (cole crops) has oc-
curred sporadically in Florida in cabbage, collards
and radish but generally this disease has not im-
pacted upon crop production significantly. Other
crucifer species are also susceptible. Because little
is known about the degree of susceptibility of dif-
ferent crucifer varieties, except for cabbage, the
occurrence of this disease in other crucifer crops
may occur in the future. Symptoms include seed-
ling death (Fig. 10), stunting, stem curling, leaf
drying on the edges, yellowing of lower leaves,
dropping of leaves, bud formation on leafless
stems, vascular discoloration, and often plant
death. Black rot, a bacterial disease, can be con-
fused with Fusarium wilt because it causes black
veins in stems, roots and leaves. Fusarium wilt is
most likely to be a problem in plantings that are
initiated in the late summer to early fall or mid to
late spring. The fungus grows best at 80 to 90F
(27 to 320C) and is strongly inhibited below 61F
(16'C) and above 95'F (35C).

Control of Fusarium wilt in crucifers includes
use of crop rotation and disease-free transplants.
For cabbage, many resistant varieties are avail-
able. When cabbage or other crucifers are grown
without crop rotation, use of resistant varieties is

Fusarium wilt of soybeans
Fusarium wilt of soybeans has been found on
occasion in Florida but the extent of its incidence
and importance is unknown. Symptoms include
lower leaf yellowing, leaf drop, stunting, wilting in
mid season, and possibly plant death. Infection is
enhanced by cool temperature from 57F (140C) to
740F (23C). Thus, this disease might be a problem
if soybeans are planted too early (prior to mid May)

or if soils remain cool and wet into the normal
planting season. Wounding from cultivation, nema-
todes and herbicides enhance the development of
this disease. Resistance exists, but the extent of use
of this resistance by plant breeders is not known.

Fusarium wilts of beans
and chickpeas
Fusarium wilts of beans and chickpeas have
probably occurred in Florida based upon symptoms
(Figs. 11 & 12) but official documentation is not

Fusarium root and
lower stem rots
Fusaria are commonly associated with roots and
lower stems of unthrifty plants. As indicated earlier,
the presence of Fusarium spp. in roots or lower
stems does not indicate that the Fusaria are the
cause of the problem. However, primary or second-
ary invasion of plant tissue by Fusaria can cause
root debilitation which is usually followed by stunt-
ing, loss of green color in leaves, wilting, and other
decline symptoms. Some crops that are commonly
infected with Fusarium spp. in Florida include cel-
ery (red root, Fig. 13), parsley (Fig. 14), carrots, corn
(particularly field corn), sorghum, millet, peanuts
(Fig. 15), soybeans, snow peas (possibly a wilt type
of Fusarium), beans, southern peas, forage legumes,
onions, and some herbs. Probably, most plants
sustain some damage from Fusaria. Infection by
Fusaria often begins during the seedling stage of
the plant for direct-seeded crops (Fig. 16).
Fusarium-induced diseases of potatoes (Fig. 17)
would be more of a problem in Florida if certified
seed pieces were not used.

Cultural and chemical control practices present-
ed for Fusarium wilt diseases will reduce Fusarium
root rots. Also, recontamination of fumigated soil
should be avoided because fumigated soil has less
natural biological diversity. Soils with less microbio-
logical diversity may allow for a rapid increase of a
plant pathogen (Fig. 14). Fertility adjustments may
not be as effective for Fusarium root rots as they are
for Fusarium wilts. Additional controls include fun-
gicide seed treatments, destruction of all green ma-

nure 30 days or more prior to planting, preparation
of soil into a loose tilth for planting, avoidance of
deep setting of seeds or transplanting, use of
healthy, undamaged transplants, and care not to
deposit soil on stems when cultivating.

Fusarium fruit and flower rots
Infection of flower or fruit parts by Fusaria has
not been common in Florida but both occur. Pea-
nuts and corn are the crops that are most likely to
incur infection of fruit by Fusaria (Figs. 18 & 19).
Infection of peanut pods can be associated with
damage from soil insects and nematodes. Fruit
(ears) of field corn are commonly infected with
Fusaria, particularly if damage to the husk or silks
occurs from hail, insects, etc. Some Fusaria that
infect grain crops such as corn and wheat produce
toxins that can poison livestock.

Occasionally, fruit of vegetables may be infected
with Fusaria (Fig. 20). Control for such fruit rots
include avoidance of damage to fruit from insects
or other factors, trellising crops or use of plastic
mulch so fruit are not produced in direct contact
with the soil. The infection of flowers by Fusaria
is not a common problem but has been noted in to-
mato flowers infested with thrips, a small insect.

It is not possible to adequately generalize about
the complex range of symptoms, environmental
factors, and controls associated with Fusarium-
induced diseases for the many susceptible crops.
The reader is advised to make every effort to seek
professional advice on the importance of the
Fusaria found in association with a given situation.
However, numerous cultural and chemical controls
can be routinely incorporated during the produc-
tion of a crop so that the impact from Fusaria will
be minimized. As noted throughout this publica-
tion, successful control of Fusarium-induced
diseases begins with healthy seed, healthy seed
pieces, healthy transplants, and properly prepared
and rotated land. Changes of soil fertility, use of
resistant varieties, avoidance of plant stresses
(biological and physical), and cultural manipula-
tions can be used successfully in some situations.

Figure 17. Fusarium-infected potato tubers.

Figure 18. Peanut pods with Fusarium and other fungi.

Figure 19. Fusarium ear rot of field corn. Courtesy of Dr.
Johnny Crawford, University of Georgia.

Figure 20. Pumpkin fruit rot caused by Fusarium.

Director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June
30, 1914 Acts of Congress; and is authorized to provide research, educational information and other services only to individuals and institutions that
function without regard to race, color, sex, age, handicap or national origin. Single copies of extension publications (excluding 4-H and youth
publications) are available free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers
is available from C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing
this publication, editors should contact this address to determine availability. Printed 4/92.

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