Alan
McNab
Professor of Plant Pathology |
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Root
Rot, is caused by a complex of several different soil-borne
fungi. Symptoms vary, depending on organisms present and on environmental
conditions. Root rot should be suspected whenever plants wilt and
die, leaves turn yellow or drop off, plants are stunted, or pods are
small with under-sized seed. Characteristic symptoms caused by four
major root rot fungi are described here.
Fusarium Dry Root Rot
Rhizoctonia Root Rot
Pythium Root Rot and Wilt
Thielaviopsis Black Root Rot |
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Root
Knot, caused by the Meliodogyne sp. of nematode,
appears as swellings on roots. The disease is discussed with tomato
diseases. |
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Bacterial
Blights of beans and lima beans are caused
by several specific kinds of bacteria, each causing a different disease.
Bacteria that cause the blights overwinter in the seed and in residues
of disease plants left in fields; syringae blight bacteria also overwinder
in several woody plants-including lilac, cherry, pear, polar, rose,
and forsythia. Bacteria are spread in splashing rain, by workers and
on impliments.
Halo Blight
Common Blight and Fuscous
Blight
Brown-Spot (Syringae) Blight |
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Angular
Leaf Spot, caused by the fungus Isariopsis
griseola, affects beans and lima beans. Leaf spots appear first on
the oldest leaves; the spots are dark brown to gray with a distinct
margin, are angular as delimited by leaf veins, and somtimes are covered
with a gray mold on the lower leaf surface. Severely spotted leaves
senesce prematurely and drop off. The fungus can survive for 2 years
in residue from diseased plants and in soil. Fungus spores can be
spread long distances on seed, and short distances by wind, splashing
water, insects, and implements. Disease is promoted by wet conditions
casued by frequent rainfall, poor drainage, and slow drying conditions. |
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Cercospora
Leaf Spots
are caused by the fungi Cercospora canescens
and C. cruenta. Leaf spots caused by C. canescens are circular to
slightly angular with a gray center and a reddish border. Lesions
on lima beans generally are smaller and have more intense red borders
than those on other beans.
Spots produced by C. cruenta occur on stems, leaves, and pods of mature
and senescent plants. Leaf spots are brown to rust-colored, irregular
in size and shape, angular, and form a checkerboard pattern. The fungus
appears as a dark fuzzy growth on the undersurface of the leaf. Tissue
at the center of leaf spots often drops out, producing a shothole
effect. |
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Anthracnose
of beans, caused by the fungus Colletotrichum lindemuthianum,
is characterized by black sunken lesions on pods, cotyledons, and
stems. Spots are about 1/2 inch in diameter and usually are covered
by a salmon-colored ooze during moist conditions. Pod spots are most
conspicuous. Veins on lower leaf sufaces commonly turn black. On lima
beans, spots resembling sooty mold develops on leaves and pods; spot
margins are diffuse, even on veins on the lower leaf surface. Presence
of a diffuse margin helps distinguish anthracnose from syringae blight
of limas. The anthracnose fungus overwinters in bean seed and in field
soil on residues from diseased plants. The fungus is spread by wind
or rain, aminals, workers, and implements. Cool wet weather promotes
disease development. |
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Powdery
Mildew, caused by the fungus Erysiphe polygoni,
begins as faint discolored leaf spots from which a diagnostic grayish
white (talcum-like) powdery growth spreads to all above-ground parts.
Young leaves are dwarfed and curled and may turn yellow and drop.
Pods are dwarfed and distorted. Powdery mildew usually develops on
mature plants late in the season. |
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Mosaic
symptoms include stunting of plants; mottling,
puckering, stunting, and vein-dying of leaves; dying-back of shoot
tips; and sometimes distortion and stunting of pods. Common Bean Mosaid
(BV-1), Bean Yellos Mosaic (BV-2), and Peanut Stunt (PSV) are three
bean virus diseases. BV-1 is seed-borne and overwinters in sweet white
clover. BV-2 and PSV are not seed borne. BV-2 overwinters in clovers
and in gladioli. PSV overwinters in sweet white clover. All three
viruses are spread from diseased plants to healthy plants by aphids,
by leaves rubbing against each other, and by workers handling healthy
plants after working with diseased plants. |
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Downy
Mildew of Lima Beans is caused by the fungus
Phytophthora phaseoli. Disease is conspicuous because of a white downy
mold on pods; mold seldom develops on leaves. Infected pods shrivel,
die, and turn black. Young shoots, flowers, and leaves also are affected,
and leaf veins may become purpli8sh and distorted. The downy mildew
fungus overwinters in diseased seed and in refuse from diseased plants.
In wet weather, the fungus grows, reproduces, and is spread rabidly
by wind. Susceptible plants can be destroyed in a few days. |
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Gray
Mold, Caused by the fungus Botrytis cinerea,
is chiefly a pod disease on beans. It is characterized by a distinctive
grayish powdery mold on bean pods. THe casual fungus is present on
most dead organic matter in fields. Here the fungus grows and produces
dry reproductive spores which are spread by wind. Wet conditions promote
disease development. |
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White
Mold, caused by the fungus Sclerotinia sclerotiorum,
is characterized by a diagnostic white cottony growth on pods and
stems. Small, hard, black, seed-like structures (sclerotia) often
form in the white mold growth. Rotted pods become wet and soft. Affected
plants often die. The causal fungus overwinters in residue form diseased
plants in and near bean fields. Fungus spores are produced during
wet spring weather and are carried to bean plants by wind. The fungus
gets its start on dying blossoms and on injured tissue. Prolonged
wet conditions promote disease development. |
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Rust,
caused by the fungus uromyces phaseoli typica,
is characterized by reddish dusty spots which occur mostly on lower
surfaces of leaves but also on pods. It is most common on mature plants.
The causal fungus overwinters on residue from diseased plants. Dusty
spores produced in leaf and pod spots can be spread long distances
by wind and short distances by movement of workers and implements. |
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Lima
Bean Pod Blight, caused by the fungus Diaporthe
phaseolorum, causes irregular brown patches on leaves. Later, small
black pycnidia (fungus reproductive structures) appear arranged in
concentric rings within leaf spots and on pods. In these pods, seeds
may not form; if they do they may be shriveled. The disease-causing
fungus overwinters on seeds and in residue from diseased plants. Sticky
spores are produced in the pycnidia on leaves and pods. Wet conditions
promote production and spread of spores and development of the disease |
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