Seedling disease is one of the major causes of stand reduction in Iowa every spring. A reduced stand can result in extra labor and materials for replanting, or yield reduction. By properly identifying seedling disease, you have the best chance of correcting the problem in the future. It also is important to know which fungi cause a seedling disease because different fungicides are effective in controlling different fungi.
Several fungi can cause seedling disease either before or after emergence. When seed fails to emerge because of fungal attack, the disease is called seed rot or pre-emergence damping-off. Pythium and Phytophthora are two major fungi that cause pre-emergence damping-off in Iowa. When fungi attack seed before germination, seed rot occurs. Seed that is dead before germination will be soft, rotted, and may have soil adhering to it. When fungi attack seed after germiniation, the seed may fail to emerge or the dead plants will have a short, discolored root.
When plants are killed at the seedling stage (after emergence), the disease is called seedling rot, seedling blight, or post-emergence damping-off. Diseased plants may stand singly or in small circular groups, particularly in low spots in the field, or they may be scattered over an entire field. The fungi Pythium, Phytophthora, Rhizoctonia, and Fusarium cause seedling blight in Iowa.
The symptoms of seedling blight by Pythium and Phytophthora are very similar and generally cannot be distinguished without further laboratory tests. When seedling blight occurs, dead seedlings are visible on the ground, and diseased plants are easily pulled from the soil because of rotted roots. Infected plants dead before the true leaf stage will have a rotted appearance (see Photo 1). If leaves are present, the leaves of infected seedlings first will be a gray-green color and then turn brown. A few days later, plants die and have a rotted appearance.
Seedling blight by Phytophthora can be differentiated from Pythium at the V2 growth stage or later. Plants infected by Phytophthora have a brown discoloration that extends from the root up the stem (see Photo 2). Soybean planted in cold, wet soil is most subject to infection by Pythium. If disease occurs in warm conditions, it is more likely caused by Phytophthora.
Rhizoctonia and Fusarium also can cause seedling diseases. Seedling diseases by these fungi are different from those caused by Pythium and Phytophthora. Seedling blight by Rhizoctonia normally appears as the weather becomes warm. Unlike Pythium/Phytophthora damping-off, stem discoloration by Rhizoctonia is usually limited to the cortical layer of the main root and hypocotyl. Infected stems remain firm and dry. Typical symptoms are localized brown to reddish brown lesions on the hypocotyl (see Photo 3) and a lower stem that does not extend above the soil line. The reddish brown color is a good symptom to aid in diagnosing the disease. Compared with other fungi, Fusarium is a minor pathogen in Iowa, causing about 10 percent of the seedling disease problems each year.