In the areas of Illinois where corn has been planted and is growing, many of the seedlings appear yellow, stressed, and stunted. In most fields this is due to the obvious environmental stresses that have plagued Illinois recently: the frequent rain, wet soils, and cool temperatures. Stress caused by these conditions (if not excessive) should readily disappear when warm and drier conditions return. However, seed and seedling diseases may also cause the yellow and stunted appearance. Disease is likely to be a minor problem compared to environmental stresses in most areas, but it may be important in some fields. Although most seed corn in Illinois is treated with fungicides to reduce problems with seed and seedling diseases, these chemicals have a limited time of efficacy (probably in the range of 14 to 24 days after planting depending on soil conditions). In some cases the cool temperatures and saturated conditions may have created a situation where the seedlings are very small and susceptible and have minimal protection remaining from the seed treatments. This article briefly describes some of the pathogens and diseases that may affect seedling corn.|
Most corn seed and seedling diseases have several common generalized effects. Plants may fail to emerge due to seed rot or preemergence damping-off; growth may be slow, resulting in stunted plants; plants may appear yellow; wilting may occur; and plants may collapse due to postemergence damping-off. In some cases, infection may cause damage that persists into the growing season.
It can be difficult to distinguish environmental stress from some disease damage, but symptoms can help deduce whether pathogens are causing problems. Here is a list of symptoms that may indicate problems with corn seed or seedling disease in a field. Symptoms of seed and root infections include rotted seed; rotted seedlings (preemergence damping-off); leaf tip necrosis; stunting (sometimes with a mixture of short and tall plants); yellowing-reddening of older leaves; roots that are rotted, pruned-off, and discolored with firm or soft, brown-reddish to gray lesions or decay; poorly developed root systems; soft and discolored coeleoptile; leaf tip necrosis in streaks or patches; wilting seedlings; and discolored sunken and soft lesions on mesocotyl. Symptoms of foliar seedling infections include round to elliptical tan spots on leaves (holcus spot); necrotic, wavy necrotic streaks on leaves (Stewart's wilt); and oval lesions on seedling leaves (anthracnose).
A number of different pathogens can cause one or more of the symptoms of seed and seedling disease. Some of the more common genera of fungi or fungal-like organisms are Pythium, Fusarium, Rhizoctonia, Diplodia (Stenocarpella), Colletotrichum, and Penicillium. Their importance will depend in part on crop rotation, location, and soil conditions. Pythium is a widespread soil fungal-like pathogen that causes seed rot and seedling infections, and is favored by wet and cool soil conditions. Erwinia (Pantoea), which causes Stewart's wilt (see the article in a previous 2002 edition of this newsletter), and Pseudomonas, which causes holcus spot, are two bacterial pathogens that can infect corn seedlings. Nematodes also may affect corn seedlings in some areas (see the article in a previous 2002 edition of this newsletter).
Various conditions, factors, and cultural practices favor infection by these pathogens. The following list is not complete, but covers common problems. Favorable factors include poor seed quality (such as cracked seed and infected seed), soil temperatures at planting below 50 to 55 deg F, wet soil conditions, soil compaction, slow emergence and growth, fertilizer burn, improper use of pesticides, injury from herbicides, soil crusting, high temperatures (Penicillium seedling blight--not a problem yet this year!), high populations of flea beetles (Stewart's wilt), and sandy soils (nematodes). The obvious theme here is a combination of conditions that favor the pathogens and stress the corn seed and seedlings.
Fortunately, in most normal springs, seed and seedling diseases cause minimal damage in most fields in Illinois. Other than using fungicidal seed treatments, no special management tactics need be put into effect. Most management tactics are obvious from the list above: follow good agronomic practices and try to avoid those conditions and factors that favor seed and seedling diseases. In addition, crop rotation and practices that minimize corn residue may be of value. As noted, most seed corn in Illinois is treated with fungicides. The treatments used have distinct efficacies against various pathogens. The primary fungicidal seed treatments used are of two main groups. The first group targets Pythium; some examples are ApronXL, Allegiance, and Apron. The second group targets the true fungi (all other fungal genera listed above); two examples are Captan and Maxim. Other products may also be available that offer similar efficacy, and products are available as mixtures of the active ingredients from both of these fungicidal groups.--Dean Malvick