Figure 5. Phytophthora damping off
Phytophthora damping-off and root rot is caused by the fungus Phytophthora sojae, and is particularly a problem in low, poorly drained, clay soils. However, the disease can be encountered on a variety of soil types if the soil remains wet for several days soon after planting. Phytophthora can attack soybean plants at any stage of development and stands can be reduced by seed rotting and pre-emergence damping-off. Young plants can be killed soon after emergence. Brown, water-soaked stems and yellow, wilted leaves are the primary symptoms of post-emergence damping-off.
Phytophthora survives in the soil as thick-walled spore, called oospores. Early in the growing season, when the soil remains wet for several days (temperatures above 60 degrees F), these spores will germinate, producing a second type of spore called a sporangium. The sporangium then germinates to produce a third type of spore called a zoospore in large numbers. Zoospores are attracted to chemicals released by soybean roots and swim through a thin film of water until a root is encountered. The zoospores produce a hyphae (a threadlike structure) and infect the soybean root. The fungus grows in the roots and eventually into the plant stem. As the plant dies, oospores are formed by the fungus.
Figure 5. Phytophthora damping off
Pythium damping-off can be caused by several species of Pythium. Pre-emergence damping-off, seed decays, and root rots of soybean all can be caused by this group of fungi. Pythium rots can occur on soybeans at any stage of plant development but are much more prevalent on seedlings. Diseases caused by Pythium can occur over a wide range of temperatures, but are most common during cold periods (below 55 degrees F) with high soil moisture.
Seedlings infected with Pythium often fail to emerge. Stems of infected seedlings appear water-soaked and translucent above the soil line. Diseased areas later turn brown and appear shrunken; eventually, the stem and smaller roots decay and the seedlings die.
Pythium species survive also as oospores. During periods of high soil moisture, the resting spore germinates and infects seeds or young plants in much the same way as Phytophthora. Germinating soybean seeds release a wide variety nutrients or by-products that can stimulate the growth and attract spores of the fungus. Many different seed treatment materials will effectively prevent losses from Pythium seed rot and seedling blights.
Rhizoctonia seedling blight is caused by the fungus Rhizoctonia solani, and may occur at any time, but a protected cool period of low soil moisture followed by warming soil temperatures with a brief rainy period favors disease development. Seedlings under stress are more susceptible to Rhizoctonia seedling blight. Example of stress are herbicide injury or desiccation during early seedling growth. Seedlings and young plants are most often affected. Infected plant stems and older roots appear reddish brown and have sunken lesions. Stems may be girdled just above the soil line; tissue thus damaged may appear cracked or cankered. In dry, windy weather, severely infected plants wilt and die rapidly.
Rhizoctonia solani survives in the soil as sclerotia. The fungal mycelium body itself may also survive in soil or in association with old plant residue. Growth of the fungus in soil depends on soil nutrient supply, pH, moisture, and temperature. As the fungus grows in soil, it can encounter and infect germinating soybean seeds or young plants. Several seed treatment materials can control the seed rot phase of this disease, however, none are highly effective against the post-emergence or stem rot phase of the disease.
Phomopsis seed rot is a common seed disease in Ohio and is caused by the seed-borne fungi, Phomopsis longicolla, Diaporthe phaseolorum var. sojae, and D. phaseolorum var. caulivora. Infected seeds typically germinate poorly or not at all.
Figure 6. Phomopsis seed rot of soybean
Severely infected seeds appear shriveled, cracked, and elongated, and may be covered with a white, moldy growth. However, seeds may be infected and fail to show any symptoms at all. Infection of seeds is most common when warm, wet weather delays harvest. When moldy seeds are planted, seedling death results in poor stands. Generally, higher soil temperatures favors development of seed rot and seedling blight. Seed treatments can increase the germination rate up to 20 percent. Seed lots with less than 80 percent germination should not be used.
Recent research demonstrates that Sclerotinia white mold can be introduced into new fields on infested soybean seed. In addition, this fungus, Sclerotinia sclerotiorum, forms hard, black, small, irregular-shaped sclerotia both inside and outside of infected soybean plants. These sclerotia are harvested with the seed or returned to the soil with plant debris at harvest. Seed should be well cleaned to remove sclerotia and treated with appropriate fungicide seed treatment to eradicate the fungus from infested seed.