Alan
McNab
Professor of Plant Pathology |
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| Tomatoes:
Leaf Spots and Blights |
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Leaf Spots and Blights, caused by several different
agents, include early blight, late blight, Septoria leaf
spot, gray leaf spot and bacterial spot.
Early Blight
Late Blight
Septoria Leaf Spot
Gray Leaf Spot
Bacterial Spot |
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Early
Blight, caused by the fungus Alternaria solani,
can affect seedlings but generally is observed on older plants.
On seedlings, dark spots develop on cotyledon leaves, stems,
and true leaves. Spotted cotyledon leaves may be killed,
and spotted stems may be girdled. On established plants,
dark brown spots with dark concentric rings develop first
on oldest leaves. Spotted leaves may die prematurely, resulting
in substantial early defoliation, fruit sunscald, and poor
fruit color (see the section on Tomato Fruit Rots for fruit
symptoms). The disease-causing fungus overwinters in residue
from diseased plants, where it can persist for at least one
year. The fungus also is seed-borne and can be introduced
on seed and on transplants. Disease occurs under a wide range
of weather conditions. It is promoted by heavy dews and rainfall
and is severe on plants of poor vigor. |
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Late
Blight, caused by the fungus Phytophthora infestans,
affects both tomatoes and potatoes. On tomatoes, symptoms
appear on foliage and fruit. Irregular greasy-appearing grayish
areas develop on leaves. These areas expand rapidly during
moist conditions and a white downy mold appears at the margin
of the affected area on the lower surface of leaves. If the
white fungus growth is not observed, leaves with suspicious
spots can be put into a polyethylene bag containing a moist
paper towel (to supply moisture) and held for one day to
promote appearance of this diagnostic sign. (Fruit symptoms
are described in the section on tomato fruit rots.) The disease-causingfungus
overwinters in southern frost-free areas, on winter-grown
tomatoes and potatoes, and in northern areas in potato cull
piles and in potato "seed." It may be introduced to tomato
fields on transplants or may be wind-borne from diseased
potato and tomato plants in nearby fields. Disease development
is promoted by cool wet conditions. |
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Septoria
Leaf Spot, caused by the fungus Septoria lycopersici,
is characterized by numerous small gray circular leaf spots
with dark borders. A few black pin-point-size pycnidia (fungus
reproductive structures) may be seen within the spot. The
disease-causing fungus survives in residue from diseased
plants, and on or in seed. Wet weather favors fungus growth,
spread, and subsequent disease development. |
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Gray
Leaf Spot is caused by the fungus Stemphylium solani.
Only leaves are affected and the oldest are affected first.
Numerous small dark brown spots appear and extend through
to the undersurface of the leaf. These spots may enlarge
to 1/8 inch in diameter. As spots enlarge they have a grayish
brown glazed appearance. The centers of the spots often crack
and finally the centers fall out entirely. When spots are
numerous, affected leaves turn yellow and eventually wither
and drop. When plants are severely affected, all leaves except
those near the tips may be killed and few fruit are produced.
Spots hardly ever form on stems. The fungus overwinters on
depris from diseased plants and can persist in soil for several
years. The disease, favored by warm moist conditions, is
more prevalent in southern states than in northern areas. |
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Bacterial
Spot, caused by the bacterium Xanthomonas vesicatoria,
affects tomatoes and peppers. Symptoms include small dark
greasy appearing spots on leaves and stems. Blossom infections
may cause blossom or fruit abortion. (Fruit symptoms are
diagnostic and are described in the section on Tomato Fruit
Rots.) Disease-causing bacteria may survive for one year
in residue from diseased plants. The bacteria are seed-borne
and therefore the disease can get started and be spread on
transplants. Wet weather promotes bacterial growth; splashing
rain favors spread; and driving rain assists entrance of
bacteria into plants. |
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