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Alternaria
leaf spot
Pathogen - Alternaria tenuis
Symptoms - Alternaria leaf spot is characterized by small
(less than 1/8 inch wide) spots which are initially water-soaked.
These spots turn reddish-brown, may reach 1/4 inch in diameter and
are roughly circular with a yellow border. Spots generally do not
merge.
Control - Alternaria leaf spot can be controlled through
a variety of methods. Total elimination of water on leaves is sufficient
to completely control Alternaria leaf spot of many plants. Chlorothalonil
(Daconil 2787), mancozeb (Dithane M-45 and Fore), and iprodione
(Chipco 26019 50WP) are effective for controlling this disease and
are currently labeled for use on geranium in Florida. |
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Botrytis
blight
Pathogen - Botrytis cinerea
Symptoms - Spots usually appear on the leaf underside, on
petioles near the pot rim or in contact with the potting medium.
Wounded areas are especially susceptible to infection with Botrytis.
A small, water-soaked spot can rapidly enlarge and cover the entire
leaf. Sporulation on necrotic leaves or flowers appears as a powdery
grayish-green mass.
Control - Watch for Botrytis when the following conditions
occur - low light, high humidity, poor air circulation and warm
days with cool nights. Fungicides such as vinclozolin (Ornalin)
and iprodione (Chipco 26019) are registered for use on geranium
and effectively control Botrytis blight. |
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Pseudomonas
leaf spot
Pathogen - Pseudomonas cichorii
Symptoms - Spots are water-soaked and turn dark green to
black. Sometimes under severe conditions, centers of spots drop
out and give the plants the appearance they have been attacked by
insects. When plants are allowed to dry out the spots turn tan and
become papery.
Control - Avoid overhead watering as much as possible to
reduce conditions for infection and spread of the pathogen. Preventive
applications of a copper bactericide may aid in disease control
but none are completely effective. Keep in mind that other crops
are hosts of this pathogen (chrysanthemum, schefflera, aroids, lettuce
and celery to name a few) and it can spread easily from one crop
to another. |
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Rhizopus
blight
Pathogen - Rhizopus stolonifer
Symptoms - Symptoms occur very rapidly and can kill large
portions of the plant or a seedling flat in less than 1 week. The
black spores of the pathogen form in the dead stems and leaves within
a day of first symptom expression. Rhizopus blight is most common
when conditions are very hot and light levels are high. Heat stress
appears to predispose plants to this disease.
Control - There are no fungicides which are available for
this pathogen on geranium. When weather conditions are optimum,
plants should be examined daily and discarded as soon as they are
found to reduce spread of spores to adjacent plants. |
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Rust
Pathogen - Puccinia pelargonii-zonalis
Symptoms - Rust disease is easily recognized because of the
reddish-brown pustules which occur on leaf undersides. The spots
first appear white or light yellow when viewed from the upper surface.
Most rust pustules form on the undersides of leaves but some can
be found on leaf upper sides as well as petioles and stems. Sometimes
the pustules form in concentric rings or zones.
Control - Rust diseases are inhibited by frequent rainfall
or irrigation and high temperatures. Fungicides for rust control
on geraniums in Florida include chlorothalonil (Daconil 2787), ferbam
(Carbamate), triadimefon (Bayleton) and mancozeb (Dithane M-45 and
Fore). |
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Xanthomonas
campestris pv. pelargonii
Symptoms - Leaf infections start as small water-soaked spots
which are tan and circular to slightly irregular in shape. Occasionally
spots are bordered by a yellow halo and under some conditions, they
merge and affect large segments of the leaves creating wedge-shaped
dead areas. Large areas may be blighted and shoot dieback can occur
as well. Systemic infections usually result in plant death since
the plants ability to transport water and nutrients is destroyed.
Control - Growers must rely on use of pathogen-free certified
plants as well as a vigorous sanitation and scouting program. Ivy
geraniums usually do not develop severe symptoms and can have latent
or unexpressed infections thus acting as a source of inoculum. Be
sure that all of your geraniums remain symptom free by scouting
them weekly. |
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Edema
Symptoms - Small blister-like spots appear on leaf undersides.
Sometimes they are clustered around the outer portions of the leaf.
These blisters can become corky and sometimes result in leaf drop.
Leaves appear yellow from above when edema is severe.
Control - Edema occurs when a water imbalance causes more
water to be moved into the leaves than can be utilized. This water
collects and causes the blisters described above. Edema is most
common during the spring and fall when soil and air temperatures
differ substantially. Provide plants with optimal conditions and
reduce water applications to limit development of edema. |
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Bleach
phytotoxicity
Symptoms - White sunken areas form all over leaf surfaces.
These spots do not enlarge and appear usually within a day or two
of using bleach in the greenhouse. This is more common in the winter
when the structures are closed tightly to retain heat. Bleach phytotoxicity
can occur by direct splashing of the solution onto plants or as
an aerosol.
Control - Never use a bleach solution in a tightly closed
greenhouse and be careful to apply only to floors or benches well
away from plants. If bleach must be used during the winter be sure
to vent greenhouses afterward to reduce expose to fumes. |
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Mancozeb
phytotoxicity
Symptoms - Leaf margins turn yellow and then brown within
1 week of a soil drench with mancozeb or maneb. Root death results
in severe stunting.
Control - Apply all pesticides at recommended rates only
to the labeled site (foliar or drench) and at recommended intervals.
Many pesticides are safe when applied to the appropriate site but
cause severe damage when applied inaccurately. |
