Extension Communications
Extension 4-H Youth Building
Ames, Iowa 50011-3630
(515) 294-9915
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Extension Communications |
5/26/04
Contacts:
Alison Robertson, Plant Pathology, (515) 294-6708, alisonr@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Diseases That May Affect Flooded Corn and Soybean Seedlings
By Alison Robertson,
Plant Pathologist
Iowa State University Extension
The heavy rain, flooding and hail that has hit Iowa over the past few days has made some corn and soybean plants susceptible to seedling diseases.
First, it is important to determine if seedling problems are related to a disease or some other cause. Above ground symptoms of seedling diseases include failure to emerge, or emergence of plants with poor color, slow growth, and wilting and withering of the leaves, followed by the collapse of the plant. Symptoms are worse in areas that are wet, compacted or have heavier soil. Symptoms may occur on scattered plants or in small-to-large patches. Since other pests and problems mimic these symptoms, it is important to dig up seedlings and examine them for the symptoms described below, or for symptoms of herbicide injury or insect feeding.
Corn seedling diseases
Pythium and Fusarium are the most common fungi associated with seed rot and
seedling blight of corn. Other fungi that can cause seedling problems include
Rhizoctonia, Penicillium, Colletotrichum and Diplodia.
Pythium
Pythium infection is favored by wet, cool soils. Several species of Pythium are able to attack young seedlings after emergence. Seedlings tend to yellow,
wilt and die. Examination of the seedlings reveals dark, slimy lesions on
the roots or mesocotyl. Often the outer tissue of the root is infected and
may
peel off, revealing a white stele.
Fusarium
Fusarium symptoms tend to be tan-to-reddish brown leathery lesions that cause
the root or mesocotyl to shrivel. If the seedlings continue to contend with
wet conditions, Fusarium infections can persist and move into the crown and
stalk tissues. Normally, the crown tissue of healthy corn plants is slightly
darker than the surrounding tissue, but Fusarium infection usually results
in an even darker color. The fungus can cause substantial rotting of the
crown tissue which results in stunting, leaf discoloration, wilting, and
even death
of infected plants. Stressful growing conditions cause the plants to become
more susceptible to this type of infection.
If the weather improves, you can expect some of the infected plants will recover. However if the decay is severe, the plants may die later, since the decayed crown cannot support the moisture needs of the plant after the weather becomes warm and sunny. In this case, watch for plants that wilt and die suddenly. Crown infections can persist well into the season, and eventually result in stalk rot. Therefore, if you observe crown rot problems in a field, watch for stalk rot later.
Since Fusarium species are common in crop residue and soil, resistance to this type of infection is not available. However, there are a few practical control measures growers can follow. Fungicidal seed treatment will not persist long enough to help in this situation. If predisposing stresses can be identified, alleviating them will reduce losses to Fusarium. If plants are stunted but the crowns are not totally destroyed, cultivation may promote development of the nodal root system and help the plants recover.
Others
Rhizoctonia, Penicillium, Colletotrichum, and Diplodia all cause a general
darkening and reduction in size of the young root system and it is very difficult
or impossible to tell them apart in the field.
Anthracnose leaf blight
Anthracnose leaf blight (Colletotrichum graminicola) frequently accompanies
other seedling diseases and contributes to the death of a plant by killing
off what little leaf tissue it has left. This fungus causes brown or tan
oval leaf lesions with a dark brown border.
Soybean seed decay, pre- and post-emergence diseases
Wet soils are ideal for fungal root rot diseases in soybean fields and it is
not uncommon to see “damping-off” in areas larger than 50 acres.
In Iowa, seedling blight is most commonly caused by Pythium, Phytophthora,
Rhizoctonia and Fusarium. Disease problems associated with these pathogens
include seed decay and pre- and post-emergence.
Pythium and Phytophthora
Seedling blight symptoms caused by Pythium are very similar to those caused
by Phytophthora and laboratory tests are often required to separate the two.
Dead seedlings are visible on the ground when seedling blight occurs. Infected
plants that die before the true leaf stage will have a rotted appearance.
If leaves are present, they are a gray-green color and then they turn brown
and
die a few days later. Diseased plants are pulled easily from the soil because
roots are rotted. Pythium infections are associated with soybeans planted
in cold, wet soil. If seedling blight occurs in warm conditions, it is more
likely
to be caused by Phytophthora. In addition, the stems of young Phytophthora-infected
seedlings may show a brown discoloration that begins at the soil line and
extends up the stem.
Rhizoctonia
Rhizoctonia and Fusarium seedling disease fungi show slightly different symptoms
from those caused by Pythium and Phytophthora. Seedling blight by Rhizoctonia normally appears when warm weather begins to arrive. Unlike Pythium or Phytophthora damping-off, stem discoloration by Rhizoctonia usually is limited to the
cortical layer of the main root and the hypocotyl. Infected stems remain
firm. Typical
symptoms are localized brown to reddish-brown lesions that girdles the hypocotyl
and lower stem. The reddish-brown color is a good symptom to aid in diagnosing
the disease.
Fusarium
Compared with the other fungi, Fusarium is a minor problem in Iowa, causing
about 10 percent of seedling blight problems each year. Any factor that delays
germination and emergence favors the development of Fusarium seedling blight
and root rot. Brown to a dark purple brown or black lesions develop on the
taproot and may increase in size and girdle the taproot. The lower part of
the taproot and the lateral root system can be completely rotted.
Brown spot
Soybean plants with root rot are more susceptible to brown spot caused by
Septoria glycines. Practices that reduce plant stress and increase vigor
will help reduce
the chances of infection by brown spot. Plants, as young as the V2 growth
stage, can show brown spot symptoms. The fungus spreads in splashing rain
from the
soil to soybean plants. Symptoms of the disease are many irregular, chocolate
to blackish-brown spots on both upper and lower leaf surfaces. Adjacent lesions
frequently merge to form irregularly shaped blotches. Sometimes, brown spot
is mistaken as bacterial blight, however in early disease stages, lesions
of bacterial blight have a yellow halo, brown spot lesions do not.
We cannot predict the development of this disease so there are no remedial measures that can be taken. Plant growth is normally unaffected and soybean can outgrow the disease if the weather in midsummer is less wet. However, if the rain continues, the disease will progress rapidly from the lower leaves to the upper leaves.
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