Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Each year we see sweet corn field fields with stand and plant vigor issues even in corn planted later in the season when soils were warm. There can be many causes for stand loss and weak seedlings: surface compaction and crusting, soil insects, soil diseases affecting seeds or seedlings, wet soils, fertilizer injury, deep planting, and herbicide injury are just a few examples.
Corn seedlings depend on the seed for food to grow for several weeks after emergence until sufficient leaf area has been produced and nodal roots have become established. Sweet corn is more susceptible stand loss and poor vigor problems than field corn because the seed has less food reserves. If you dig up low vigor seedlings and kernels are disintegrated and there is darkening at the mesocotyl attachment this means that the seeds deteriorated prematurely and the full content of the food reserves in the seed were not available for seedling development leading to the stand and vigor issues.
Seed deterioration and/or poor vigor seedlings can be due to diseases that cause seed rots, seedling blights and/or root rots. Fungal disease organisms such as Pythium, Fusarium, Rhizoctonia, Aspergillus, and Penicillium are common in soils and many can even be carried on seeds.
While most of these seed diseases are problems in cold and wet soils, Penicillium is a common problem in warmer soils. Penicillium can survive in the soil and can also be seed borne. Plants infected with Penicillium will be stunted and off-color and seeding roots and mesocotyls will show discoloration below ground. Blue-green mold may evident on or in the seed remnant.
Fungicide seed treatments are critical to control seedling diseases and a systemic fungicide such as difenoconazole (a component of Dividend Extreme) will be necessary for diseases such as Penicillium that can be seed borne.
Poor vigor can also result from poorer quality seed. Work with seed suppliers to obtain their best seed lots and the largest seed sizes. Avoid old seeds and obtain varieties that known for good seedling vigor.