Some years, early season stand establishment problems are widespread, and in some cases, severe. The weather pattern during and immediately after planting is a major factor contributing to those problems. Corn, which begins to germinate before periods of cold or wet weather in April or early May tends to show damage from saturated soils, cold soil temperatures, frost injury, herbicide injury, nitrogen deficiencies, seed decay and seedling blights. In some fields the seed decay and seedling blight may progress into crown decay resulting in even more severe stunting and yellowing of plants. If weather patterns are favorable for germination and emergence of corn and not as favorable for development of corn seed and seedling diseases, there will be a substantial reduction in seed decay and seedling blight problems in corn.
Corn planting is later than normal because of usually wet conditions across most of the state. The unusual fluctuations in air temperatures (near record highs one weekend followed by lows the next weekend) and soil temperatures further impacted corn germination and emergence, as well as seedling vigor. Conditions which delay seedling development and emergence give seed decay and seedling blight fungi more of an opportunity to attack developing corn seedlings. This may be a year when seed decay and seedling blight are more widespread and damaging than usual.
Seed decay and seedling blights of corn are generally caused by soil-inhabiting fungi such as species of Pythium, Fusarium, Diplodia, Rhizoctonia and Penicillium. These fungi may rot the seed prior to germination or cause preemergence or postemergence seedling blight. Affected seeds are usually discolored and soft and may be overgrown with fungi. Rotted seed may be difficult to find because they decompose very rapidly and because soil adheres fairly tightly to the decomposing seed.
With preemergence seedling blights, the seed germinates but the seedlings are killed before they emerge from the soil. The coleoptile and primary roots are usually discolored and have a wet, rotted appearance. With postemergence seedling blights, the seedlings emerge through the soil surface before developing symptoms. Seedlings tend to yellow, wilt and die. Discolored, sunken lesions are usually evident on the mesocotyl. Eventually the mesocotyl becomes soft and water soaked. The root system is usually poorly developed, and roots are discolored, water soaked and slough off. If the primary root system and mesocotyl are severely affected before the nodal or permanent root system has developed, the plants have little chance of surviving.
Most of the fungi which cause seed decay and seedling blight of corn may also contribute to decay of the permanent root system and crown rot of young plants. Tips of the permanent root system may be 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 but may be more evident in internal tissues if the crown is split open. The internal crown tissues may be discolored ranging from light pink to light brown or dark brown to black and the texture may be very soft and spongy. 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.
The Pythium, Fusarium, Diplodia, Rhizoctonia and Penicillium species which cause seed decay, seedling blight and crown decay are common in soils throughout the state. If conditions are favorable for germination and emergence, these fungi may not have the opportunity to invade seed, germinating seed or young seedlings so seed decay, seedling blights and crown rot will not be significant problems. On the other hand, conditions that are not favorable for germination and emergence, give these soil fungi more time to attack the seed and developing plants.
Numerous other factors also contribute to early season corn establishment problems. Insect damage, nutrient imbalances, herbicide injury, soil conditions and environmental factors, especially saturated soil conditions and oxygen deprivation, may also cause or contribute to early season corn establishment problems. Corn seedling blights are more severe in wet soils, in low lying areas in a field or in soils that have been compacted or remain wet for an extended period of time. Low soil temperatures (50-55°F) and wet soil conditions especially favor Pythium seed decay and seedling blight. Disease severity is also affected by planting depth, soil type, seed quality, mechanical injury to seed, soil crusting, herbicide injury or other factors which delay germination and emergence of corn.
Planting high quality seed into a good seedbed when soil temperatures are above 50F will help minimize these early season problems. Virtually all field corn seed comes with a fungicide seed treatment. Hopper box treatments can be used to supplement the existing seed treatment.
Unfortunately, there are no controls for seed decay, seedling blights and crown decay in corn at this point. When evaluating corn stands this season, it is important to check several plants to determine the extent of damage to the initial root systems, the mesocotyls and the permanent root systems. It can also be helpful to split the lower stem and crown open on several plants to check for crown decay. 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 : missouri.edu