Plant Pathology Fact Sheet
Peanut Leaf Spot and Rust
Tom Kucharek, Professor, Extension Plant Pathologist, 1979, Revised Nov.2000
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean
Peanut Leaf Spot
Each year, peanut leaf spot is the most prevalent
peanut disease in Florida. It causes defoliation, and thus,
can cause yield reductions of over 75 percent when not
controlled and less than five percent when a total control
program is utilized. Currently, it is estimated that peanut
leaf spot is causing an annual 10-20 percent loss in
Florida. Not too many years ago statewide losses
attributable to peanut leaf spot exceeded 50 percent.
Actually, two leaf spot diseases occur but
together they are called peanut leaf spot. Early peanut
leaf spot, caused by the fungus Cercospora
arachidicola, usually is the first to occur. It is
characterized by a round, brown-red spot and may have
a yellow halo (Figure 1). Late leafspot is caused by a
related fungus, Cercosporidium personatum, and is
characterized by a somewhat round spot that is black
on the underside of the leaflet and it may or may not
have a yellow halo (Figure 2). Lesions (spots) of either
leaf spot disease may be found in leaflets, petioles, stems
and pegs. However, lesions are not found in petioles,
stems or pegs until later in the season or after numerous
lesions are found on leaflets. Occasionally, chemical bums
from insecticides or cracking time herbicides cause dark
spots that may be confused with leaf spot.
Microscopic spores which are produced on the
surface of the lesions are disseminated by wind, rain or
irrigation. When the leaf, petiole or stem surface is wet,
the spores germinate and penetrate the tissue. Within 10
to 14 days after these infections occur, new lesions with
more spores are produced. Leaf spot causes premature
leaf drop. Fallen leaves with lesions will provide primary
inoculum (spores) for the next season if peanuts are
planted in the same or adjoining fields.
Control of Leaf Spot
Control of leaf spot is achieved by reducing
spore production. This can be achieved by utilizing
several control measures. No one control measure by
itself has the capability of reducing spore production
adequately. The following control measures when used
together are effective for controlling peanut leaf spot. 1)
Use crop rotation. It is best not to have peanuts in the
same field more than once every four or more years, but
if land is not available, do your best not to plant peanuts
following peanuts. 2) Earlier plantings tend to reduce
leaf spot. 3) Begin a fungicide spray program no later
than 35 days after planting. Where peanuts follow
peanuts in a rotation, beginning a spray program when
the plants are 30 days old is advisable. 4) Spray intervals
of 10 to 14 days are adequate if other control measures
are used. Ten day intervals should be used when your
spray program or some other variable has allowed a
build-up of leaf spot. 5) Use fungicides recommended
by your county Extension agent. See Extension Plant
Pathology Mimeo No. 12 for updated recommendations.
6) Sprays are more effective than dusts. 7) Use a
spreader sticker if the label of the fungicide states that it
is necessary. 8) Use the maximum rate of fungicide
allowable on the label where peanuts follow peanuts or
where the first spray is delayed beyond 44 to 50 days
or where spray intervals exceed 14 days. 9) Where
ground rigs are used, make sure thorough coverage along
the center of the row is achieved. 10) Where 20 to 30
gallons of water per acre are used, pressures of 50 to
300 psi have performed well. Using higher water rates
have performed best during dry years. 11) Where it is
apparent that weather conditions such as a hurricane or
stalled front will delay harvest, continue to apply fungicide
sprays to minimize leaf spot increases at the end of the
season. 12) Control weeds, as they will trap the spray.
13) Some varieties have modest levels of resistance to
Peanut rust does not occur in each field each
year. In the Florida panhandle, rust may occur in
scattered fields, usually no earlier than August. In the
Marion-Levy-Alachua County area, it has typically been
found earlier. Peanut rust is caused by the fungus
Puccinia arachidis which produces orange to brown,
raised pustules primarily on the undersides of leaflets
(Figure 3). Petioles, stems and pegs also have pustules.
At a distance, infected plants will appear yellowish at
first. As the disease progresses, leaf drop occurs and
"burned out" areas occur in the field.
Control of Rust
Use crop rotation and destroy volunteers.
Numerous fungicide sprays are available and should
be used when rust is first found in a peanut field.
Again, see Extension Plant Pathology Mimeo No. 12
in your county Extension agent's office for the
recommended fungicides. Fungicide sprays with sulfur,
are effective against rust. Some varieties with
resistance are available.
Figure 1. Early leaf spot on leaflets and petiole.
SIji '; .
. .- "
Figure 2. Late leaf spot on leaflets and peti-
Figure 3. Rust on peanut leaflets.