Plant Pathology Fact Sheet
Early Blight on Tomatoes and Potatoes
Tom Kucharek, Professor and Extension Plant Pathologist 1979, Revised Novem-
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean
Cause and Symptoms
Early blight is a fungal disease that
causes yield loss each year on all varieties of
tomatoes and potatoes. Eggplant and pepper
are also susceptible but early blight has not
been a significant problem on these crop spe-
cies in Florida. Early blight is caused by the
fungus, Alternaria solani. This fungus is capable
of overwintering on tomato and potato volun-
teers or non-decomposed debris from previ-
ously diseased tomato or potato plants. Black
nightshade is also reported to be susceptible.
Spores of the fungus are formed on debris, or
current crop species, when temperatures of 60-
90F (75-85F is optimum) occur provided wet
weather is present. Spores, dislodged by wind
or rain, land on susceptible host tissue and ger-
minate when the tissue is wet and penetrate
leaf, stem, petiole, or fruit tissue. Potato tubers
are also susceptible. Within 7 to 14 days, de-
pending on numerous weather variables and
host vigor, symptoms will appear and a new
generation of spores is formed on this diseased
tissue. With each new generation, the epidemic
spreads and becomes increasingly difficult to
Leaf symptoms begin as pinpoint-size
brown to black spots, usually on the older
leaves. These lesions expand in size up to one-
half inch across, remaining brown, with or with-
out yellowing surrounding the spot (Figures 1
and 2). Concentric rings are usually seen within
the enlarged spots. Similar spots may occur on
stems (Figure 3) and if the plant is in the seed-
ling stage, the spot will girdle the stem, often
killing the plant. Symptoms in tomato fruit are
usually found associated with the stem end and
shoulder and may expand in size. Fruit symp-
toms include a sunken, greenish-brown-black
spot with concentric rings (Figure 4).
Control of early blight is best achieved
by using several techniques together. Cultural
controls will allow the fungicide to do a better
job as cultural controls reduce the amount of
initial inoculum (spores). 1) Use crop rotation
where possible. 2) Use disease-free tomato
transplants or disease-free seed pieces for po-
tatoes. 3) Destroy volunteer tomato and potato
plants in and around the field. 4) Adjacent fields
planted to potatoes or tomatoes the previous
season should have been plowed down imme-
diately after harvest. 5) Maintain host vigor via
adequate fertilization. Less vigorous plants are
more susceptible to early blight than vigorous
plants. 6) Begin a fungicide spray program at
first sign of disease or before, based on your
experience in your particular area. Maintain
spray applications on a 5 to 14 day interval
throughout the growing season. Use the shorter
intervals if rainfall is frequent or where history
of early blight has been severe or when tem-
peratures from 75-85F prevail. Also, if your
spray program started after disease buildup
occurred, shorter intervals would be appropri-
ate. Use fungicides recommended by your
county Extension agent. These materials
change from time to time and are not being
mentioned in this publication.
Figure 1. Early blight lesions on potato leaves.
Figure 2. Early blight lesions on potato
Figure 3. Early blight lesions on tomato
leaves, stems and fruit.
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