Plant Pathology Fact Sheet
Rust Diseases of Several Legumes and Corn
Ken Pernezny and Tom Kucharek, Professor of Plant Pathology, Everglades Re-
search and Education Center, Belle Glade; and Professor and Extension Plant
Pathologist, Department of Plant Pathology, University of Florida,
Gainesville. 1987; Revised 1999.
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean
A number of related pathogens known
as the rust fungi cause diseases of several im-
portant vegetable crops in Florida. These patho-
gens, unlike the majority of plant pathogenic
fungi, are characterized by an absolute require-
ment for a compatible host for growth and re-
production. These fungi require living host tis-
sue for production of spores and all other as-
pects of their life cycles, except for spore ger-
mination. Growth of most of these rust fungi in
artificial culture in laboratories has not yet been
Rust fungi are also characterized by hav-
ing complex life cycles, especially in temper-
ate regions of the world (Fig. 1). Three or four
distinctly different stages of development (with
equally distinct spores) may occur during the
completion of a rust cycle. Some rust fungi re-
quire nonrelated host plants for completion of
the life cycle. Two of the rust fungus stages, the
spermatial and aecial, are associated with the
portion of the fungus life cycle in which mat-
ing types merge and reassociate genetic mate-
rial. The spermagonia are the structures where
mating actually occurs. Aecia are a direct re-
sult of the growth of the fungus after mating;
they are typified by the formation of orange
blisters on the host plant filled with aeciospores
that mature when the fungus ruptures the epi-
dermis of the host. Aeciospores are readily
transmitted by wind currents. The spermago-
nia and aecia usually are formed on a non-crop
plant, commonly referred to as an "alternate
host". The alternate host can serve as a reser-
voir of the pathogen in the absence of the crop
On crop plants in Florida and in other
tropical and subtropical areas, however, the
rust fungi often are found almost exclusively
in one part of the life cycle known as the "re-
peating stage" or uredial stage. In this stage
masses of spores called urediospores are pro-
duced by simple asexual reproduction. The
time between generations of these urediospores
can be as short as five days, and the spores are
easily transmitted by wind currents.
As the crop matures and tissue naturally
senesces, the fungus may form another type of
spore, called a teliospore. Teliospores give rise
to basidiospores which are capable of infecting
the alternate host. One is most likely to find
basidiospores in the fall in North Florida-grown
crops. Those parts of the life cycle other than
the urediospore stage are seen less commonly
in Florida. Another source of the rust fungi may
be rust urediospores borne on air currents from
distant sites in the Caribbean and South
America where the spores develop on infected
Due to frequent changes in use restric-
tions for pesticides, consult the University of
Florida Cooperative Extension Service for cur-
rent fungicide recommendations.
Rust, caused by the fungus Uromyces
phaseoli var typical, has been observed on most
types of snap beans and dry beans grown in
the state. Historically, this disease has been
particularly severe on pole beans. A closely
related fungus, Uromyces phaseoli var vignae,
causes a destructive disease of southern pea.
Rust does not seem to be a major problem in
lima bean production in Florida, although the
disease does occur.
The rust pustules characteristic of this
disease are usually evident on the bean leaves.
Occasionally, rust will appear on pods in a
given production area. Observations of rust on
bean stems are rare. Rust symptoms begin as
whitish, minute, slightly raised pustules. Later,
these pustules become the distinct, circular,
reddish-brown spots on the underside of leaves
that readily identify the disease (Fig. 2). The
characteristic color of the pustules is due to the
production of great masses of urediospores of
the fungus. If not properly controlled, pustules
may accumulate over much of the leaf surface
(Fig. 3) with subsequent premature leaf drop.
Similar symptoms appear somewhat later and
more sparingly on the upper surface of leaves
(Fig. 4). In very susceptible varieties, pustules
are often surrounded by striking, yellow haloes
(Fig. 5). In resistant reactions symptoms may
consist only of small necrotic flecks.
If a compound microscope is available,
the presence of rust can be definitely confirmed.
Urediospores taken from the pustules will ap-
pear round, brown, with distinct, short spines
on the surface (Fig. 6). U. phaseoli, unlike most
rust fungi, does not require an alternate host
for completion of its life cycle. All developmen-
tal stages occur on beans. In northern latitudes,
basidiospores are known to infect beans.
Bean rust is primarily found during the
cooler production months when heavy dews,
rather than actual driving rain, provide mois-
ture for spore germination and penetration of
host plants. In Dade County, rust usually first
appears in early January and becomes progres-
sively more severe through the end of the com-
mercial crop in April.
The optimum temperature for
urediospore germination is from 17.5 to 22.5C
(63.5-72.5 F). Germination still occurs as low
as 100C (500F) and as high as 250C (770F). No
germination occurs at 4C (390F) or at 27.50C
(81.50F). After infection, symptoms may be seen
in about five days and spores produced in an-
other five to ten days.
Variation in pustule size, prominence of
haloes, and other symptom patterns are due,
in part, to the many races of the bean rust fun-
gus 57 at last count. Each race is pathogenic
on a specific combination of bean varieties. If
varieties resistant to those specific races in a
given locale are planted, control of rust can be
very good. However, new races of the patho-
gen seem to appear almost as fast as new vari-
eties are released. As a result, most of the com-
monly grown bean and southern pea cultivars
are susceptible to one or more races of the rust
Prompt crop destruction after harvest is
very important in the control of rust. If fields
are abandoned after picking and not destroyed,
rust can continue to develop and serve as a
major source of inoculum for fields in full pro-
duction. Brown clouds made of literally mil-
lions of rust spores have been observed above
abandoned fields on gusts of wind. Such inocu-
lum loads can make it difficult to control rust
even with the most intensive spray schedule.
Currently, the most important method
for rust control is periodic application of
protectant fungicides. Aircraft or ground equip-
ment may be used, but the latter is much pre-
ferred because of superior coverage of the un-
derside of leaves and better penetration of the
spray into the plant canopy. Initiate the spray
program prior to the first sign of rust if rust is
an annual problem. Where rust is sporadic in
occurrence, begin the spray program at first sign
of the disease. Subsequent sprays may have to
be at 5 to 7 day intervals.
Sweet Corn Rust
Two rust diseases of sweet corn occur.
in Florida: common rust, caused by Puccinia
sorghi, and southern (polysora) rust, caused by
Puccinia polysora. The symptomology of these
diseases is similar but differences do occur.
Laboratory assistance is often needed to differ-
entiate these two rust diseases.
Symptoms vary on corn varieties due to
the large number of reaction types depending
on the specific strain of the rust fungus present.
Symptoms also vary with temperature and light
regimes. Common rust pustules may occur on
all above-ground plant parts but are especially
numerous on leaves. Pustules occur simulta-
neously and in about equal numbers on both
leaf surfaces. Individual pustules are circular
to elongate and golden brown to cinnamon
brown and become raised early in develop-
ment. The fungus membrane enclosing the com-
mon rust spores also ruptures early in devel-
opment, exposing the large masses of cinna-
mon-brown urediospores (Fig. 7). As disease
severity increases, pustules become more nu-
merous (Fig. 8) and result in yellowing and
death of leaves.
Common rust is a cool weather disease.
Urediospores of P. sorghi germinate from 4 to
300C (39-860F), but the optimum is from 15 to
170C (59-630F). Optimum temperature for for-
mation of infection structures is also near 150
C. Penetration into the host is via stomates (air
pores) on either leaf surface. Germination of
urediospores occurs within 1 to 6 hr, with in-
fection structures forming in 3 to 4 hr. In 24 to
48 hr light green to yellow flecks may be vis-
ible on foliage.
Generation time can be as short as five
days at temperatures from 25 to 300C (77-86F),
and as long as 16 days at 100C (500F). Pustule
formation is most rapid at 15 to 200C (59-68F).
If a microscope is available, one can distinguish
the common rust from southern rust by exami-
nation of the urediospores. Common rust
urediospores appear spherical to slightly ovoid
and are reddish brown.
Oxalis spp. serve as the alternate host for
P. sorghi. Although these weeds are important
in the common corn rust life cycle in Mexico,
their role in Florida is undefined. Spermago-
nia and aecia occur on Oxalis, and the
urediospores and basidia (when produced) are
found on all types of corn. Other grass hosts
for the urediospore stage include teosinte.
Southern rust can be a serious and de-
structive disease of sweet corn. Many hybrids
have no resistance. Yield losses of 45 percent
have been recorded, with up to 65 percent lodg-
ing of field plants. This disease is more likely
to be a problem on corn grown during warm
weather, such as late planted corn within a
double cropping system in north Florida.
Southern rust is characterized by light,
cinnamon-brown pustules confined for the most
part to the upper leaf surfaces. The fungus
membrane enclosing the southern rust
urediospores usually, but not always, remains
intact longer (Fig. 9) than in common rust.
Heavily infected leaves become generally yel-
low and dry.
Under the microscope the urediospores
of P. polysora appear distinctly different from
those of P. sorghi. The P. polysora urediospores
are definitely ovoid and are golden in color.
They also have fewer spines on the urediospore
surface than those of the common rust fungus.
Teliospores of P. polysora have been ger-
minated but no alternate host is known. Other
grass hosts include teosinte, plumegrass and
Urediospores of southern rust germinate
well at 27 to 280C (81-820F), and stomates are
readily penetrated at these temperatures. Six
or seven days after the fungus spores alight on
corn leaves, yellow spots can be seen. Pustules
can form in nine days at 27 to 280C but require
about 14 days at 240C (750F). Pustule develop-
ment ceases at 320C (900F). When dew periods
are about 16 hr, optimal infection has been re-
corded at 260C (790F), with some infection
found from 16 to 320C (61-90F). If the dew pe-
riod is reduced to 8 hr, infection may occur only
in the range of 24 to 280C (75-82 F). By the ninth
day, the host epidermis may rupture.
Another corn rust, caused by Physopella
zeae, occurs in Mexico, Central America, South
America, and the Caribbean islands. In 1969 and
1970 it was found in the United States at
Fairchild Tropical Gardens in Miami on
Figure 1. Schematic diagram of a "typical"
rust fungus life cycle.
Euchlaena perrennis (a wild corn relative) and
possibly Tripsicum spp. brought into the coun-
try for possible sources of resistance to corn
Helminthosporium leaf spots. The disease was
completely eradicated and has not been identi-
fied in the United States since that time.
Urediospore germination of P. zeae can
occur in 1 to 2 hr at 22 C (72 F). Infection struc-
tures can form within 5 hr of spore germina-
tion, followed by fungus penetration of the host
within 12 hr.
Sweet corn rusts can be controlled by
fungicide applications. However, in south
Florida, fungicide applications are dictated,
for the most part, by the potential severity of
leaf blights caused by Helminthosporium spp.
These sprays often also provide good control
of the rusts, accounting for the low incidence
of observations of severe rust infections in
commercial fields. Different degrees of resis-
tance to rusts probably exist in commercially
available sweet corn varieties.
Figure 2. Circular, reddish-brown pustules
characteristic of rust on underside of snap
J; -'-A 1"M ; Y
Figure 3. Heavy accumulation of rust pus-
tules on underside of sanp bean leaf from
field with inadequate spray program.
Figure 5. Large, bright haloes surround rust
pustules on very susceptible bean variety.
figure 4. Beginning rust pustule aevelop-
ment on upper leaf-sruface of pole bean.
Figure 6. Bean rust urediospores as seen
through a microscope at 400X magnification.
Figure 7.Common rust pustuhleon und-er-
side of sweet corn leaf. The fungus mem-
brane has ruptured relatively early revealing
the masses of cinnamom-brown
figure o. extensive common rust damage on
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Figure 9. Southern (polysora) rust on corn. The fungus membrane enclosing the
urediospores is still fairly intact.
urediospores is still fairly intact.