Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
This year has been challenging for early planted sweet corn and a number of fields have reduced stands and poor seedling growth. There are many causes for poor sweet corn stands and low vigor in emerging seedlings. The following is a list of possibilities from my observations over the years:
● Often farmers are pushing the limits and are planting sweet corn too early. While field corn will start to germinate at 50°F, many types of sweet corn need much warmer soils. This is especially true of supersweets and other shrunken types which perform best at soil temperatures 65°F or higher.
● One the obvious issue is early planting in cold soils. Sweet corn that takes more than 10 days to emerge is at great risk of injury due to insects and diseases as seed treatments dissipate. It is also at risk to damage from soil applied herbicides due to prolonged exposure of the mesocotyl to the chemicals.
● Seedling blights can be an issue, especially in overly wet soils. A recent article on seedling blights from the Iowa State Integrated Pest Management News relating to field corn applies well to sweet corn (http://www.extension.iastate.edu/CropNews/2009/0519robertson.htm). The following are some excerpts:
“Survival of young corn seedlings depends on a healthy kernel and mesocotyl which should remain firm and white through at least growth stage V6. Damage to the kernel or mesocotyl prior to establishment of the nodal root system can result in stunted, weak or dead seedlings. A developing corn seedling relies on the kernel endosperm for nourishment until the nodal root system has fully developed, usually around the 6-leaf stage. Thus the mesocotyl acts as the “pipeline” for translocation of nutrients from the kernel and seminal roots to the seedling stalk and leaf tissues.”
“Seedling diseases of corn (seed rots, seedling blights and/or root rots) are caused by numerous fungi including Pythium, Fusarium, Rhizoctonia, Aspergillus, Penicillium, and Trichoderma, all of which are common inhabitants of soils. In addition, these fungi also can be seed-borne in corn, except Pythium. Seedling susceptibility to infection increases the longer the seed sits in the ground, and the more stress germinating corn undergoes. Corn germinates well at soil temperatures above 68°F. When soil temperatures are below 55°F, germination is greatly retarded. Thus seedling disease often is more severe in early planted or no-till/reduced tillage fields because of cool soil temperatures.”
“Typical below ground symptoms associated with seedling disease include rotting seed and brown discoloration (rotting) of the mesocotyl and seminal roots. It is sometimes possible to determine in the lab which fungus is the culprit, however this information is not crucial since management options are the same for all seedling disease: plant high quality fungicide-treated seed, plant when soil temperatures are above 50°F, and ensure planting depth is not too deep.”
● A number of times, I have found that stand issues are related to poor vigor in seed lots. Ideally, a cold germination test should be run on all seed lots to be used for very early plantings. Never used year old seed – this is especially critical for early plantings.
● Soil compaction and crusting over will lead to delayed emergence. The coleoptile and mesocotyl will be thickened and may “snake” or “corkscrew” below ground. Often you will also see the corn seedling leafing out underground in these conditions. Rotary hoes can be used to break up the crust in severe cases. If seedlings are underground for extended periods or leaf out underground, they may never resume normal growth. Sidewall compaction in the seed slot due to smearing when planting in soils that are too wet will restrict early root growth to the slot and cause stunting.
● Waterlogging and compaction will lead to low oxygen conditions. In these conditions, the seedling root system will be intact but small and the corn shoot will also be smaller than normal with poor color. Seedlings will be thin and week. Low oxygen restricts the mobilization and movement of reserves from the seed. Seedlings can sustain direct injury as cells in the mesocotyl and coleoptile die from oxygen starvation. This will appear as a water soaked area and the seedling will eventually collapse.
● Insect, slug, and bird damage is often a cause of poor stands. Seed corn maggots and wireworms can feed on the seed directly causing stand losses. Grubs feed on seedling roots causing stunting. Wireworms and certain grubs will also feed on the mesocotyl, causing seedlings to collapse. Cutworms will eat seedlings at the ground level. Slugs can feed heavily on emerging seedlings and are especially damaging when seed slots are open (due to planting in wet soils). Birds can pull out seedlings and eat the seed. In larger seedlings, stink bugs can pierce the growing point of emerged plants, killing the main shoot.
● Problems with planter fertilizer applicators may lead to fertilizer being placed too close to the seed resulting in salt damage to seedlings and reduced stands.
● I have often seen pH and nutrient deficiencies lead to dead spots or stunted seedlings in sweet corn. If the pH of the soil drops below 5.2, corn will often emerge and then die. This may be due to aluminum toxicity or to severe magnesium deficiencies induced by the low pH. In addition, roots will not grow in soils with this low of pH. High pH (above 6.5) can lead to chlorotic seedlings and stunting in some soils, due to manganese and/or zinc deficiency.
● Finally, I have seen sweet corn fields with dead areas and poor stands due to certain species of nematodes. In troubleshooting, this is another possibility to consider, especially in very sandy soils.