By Laura Sweets, Plant pathologist
University of Missouri
The May 5 issue of the Integrated Pest & Crop Management Newsletter contained an article entitled "Seed Decay and Seedling Blights of Corn." That article focused more on seed decay and early seedling blight. Recent questions concern larger seedlings and plants which are growing slowly because of cool, wet conditions.
This article describes in more detail seedling blight and crown rot symptoms. The long spell of unusually wet and cool weather has resulted in poor stands, uneven stands and yellow, stunted plants or dead plants in fields in some areas of the state. There are numerous factors that that have contributed to these poor stands and poor plant vigor. The cool, wet weather has been a major factor, but other contributing factors may include herbicide injury, insect damage, nutrient deficiencies and seed decay or seedling blights. It is difficult to impossible to determine which problem occurred first, what percent of damage is due to each problem and which problem finally killed the corn.
While symptoms of many of these problems may be evident on the above-ground portions of the plants, to diagnose seed decay, seedling blights and crown rot, it is important to dig up plants and examine the seed, root systems and mesocotyl. Symptoms that may be evident include the following:
Seed Decay: seed rotted prior to germination or just as germinating. Affected seeds are discolored, soft and may be overgrown with fungi or putrid from bacterial decay. Rotted seed may be difficult to find because they decompose very rapidly and because soil adheres fairly tightly to the decomposing seed.
Decay of Initial Root System and Mesocotyl: initial root system may be poorly developed and roots discolored and deteriorated. Mesocotyl also may show brown to blackish-brown discoloration and be soft and water soaked. In some cases, the mesocotyl is more extensively rotted perhaps to the point of rotting completely through.
If the initial or primary root system and mesocotyl are severely affected before the nodal or permanent root system has developed, the plant has little chance of surviving.
Decay of Permanent Root System and Lower Crown of Plant: some plants show little decay of the initial root system or mesocotyl, but the permanent or nodal root system and base of the crown of the plant show discoloration and deterioration. Tips of the permanent root system are water soaked and discolored with the outer layers sloughing off. The base of the crown on the young plant is discolored and soft. This discoloration may be evident on the outside of the plant and may also show up in internal tissues if the crown is split open. Severely affected plants are not likely to survive. Less severely affected plants may survive but may remain stunted and low in vigor throughout the rest of the season.
Outlook: unfortunately, there are no controls for seed decay, seedling blights and crown decay in corn at this point. Warm, drier conditions that would favor corn growth would be the best thing right now. With good growing conditions, marginally affected plants might recover and take off. If stressful conditions continue, marginally affected plants may continue to decline and more plants may show symptoms. Also, although warm, drier conditions would be helpful; hot, dry conditions, especially with drying winds would not be helpful. Warm temperatures with drying winds could stress plants with poor root systems causing them to wilt, turn gray-green to brown in color and even die.
SOURCE: University of Missouri.