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Vol. 16, No. 16
Article 7 of 9
September 1, 2006
Field Crop Disease Update for August 28, 2006
By Laura Sweets
Corn
There have been a few reports of charcoal rot in corn. Charcoal rot
may begin as a root rot and move into the lower internodes of the
stalks. Brown, water-soaked lesions develop on the roots. As the plant
matures, the fungus spreads into the lower internodes of the stalk,
causing premature ripening of the plant, shredding of pith tissues and
breaking of the stalks at the crown. The charcoal rot fungus,
Macrophomina phaseolina, produces small survival structures called
microsclerotia, which may be visible as small, black flecks just
beneath the stalk surface of on the vascular strands remaining in the
interior of the shredded stalks. The stalk and pith may have a silvery
gray to gray or bluish-gray cast from the buildup of microsclerotia in
these plant tissues.
The charcoal rot fungus is a pathogen that affects soybean and sorghum
plants as well as corn. The microsclerotia produced by Macrophomina
phaseolina are survival structures which enable the fungus to survive
in the soil for extended periods of time. Charcoal rot is severe in
hot, dry seasons. Soil temperatures near 90 degrees F are favorable
for disease development while low soil temperatures and high soil
moisture decrease disease development.
Soybean
Charcoal rot has also been found in both full season and double crop
soybeans. Symptoms typically begin to develop as plants move into
reproductive stages of growth. Infected plants are less vigorous and
have smaller leaves. Leaves may turn yellow and wilt. Leaves
eventually turn brown and have a dry appearance. The taproot and lower
stem develop a silvery gray to light-gray discoloration of the
epidermis (outer layer of the soybean stem). The epidermis may flake
away or shred away from the stem, giving the stems a tattered
appearance. Fine black specks or microsclerotia may be evident in
tissues below the epidermis and eventually in epidermal tissues.
Again, charcoal rot is favored by hot, dry weather.
There has also been another flush of bean pod mottle in some fields.
Fields which received rain over the last week or so and have had bean
leaf beetle may also be showing symptoms of bean pod mottle. Bean pod
mottle causes a green to yellow mottling of young leaves in the upper
canopy.
Laura Sweets
573-884-7307
