NPIPM:Damping off and seed decay on soybean
Compiled by: Buyung Hadi, from the materials by: Dean Malvick and Lawrence Osborne
Causal Organisms
Damping-off and seed rot can be caused by infection of several different fungi. The more common causal pathogens are Fusarium, Rhizoctonia, Phytophthora and Pythium.
Symptoms and Signs
Typically, seedlings fail to emerge, or if they do emerge, the seedlings later die or are stunted. The symptoms of each fungal infection can be similar and difficult to distinguish in the field. Phytophthora infection may cause pre- or post-emergence damping-off. Infected seedlings may show tan brown discoloration with soft and decaying tissues. At the primary leaf stage (V1 developmental stage), infected stems appear bruised and soft, secondary roots are rotted, the leaves wilt and die. Pythium can cause seed decay before emergence and post-emergence damping off. Cottony mycelium may show at the base of Pythium-infected plants. The seedlings rot and die, producing soft-brownish discoloration, occasionally with fishy odor. Rhizoctonia may attack and kill seeds before emergence. When Rhizoctoniainfected seedlings emerge, firm rusty brown decay or sunken lesions on the root or lower stem may show up. The infection may be superficial, in which case no damage occurs on the plant. In other cases, the roots of infected plants may be rotted which cause plant stunting or death. Rhizoctonia root rot is commonly co-associated with iron chlorosis. Conditions that favor iron chlorosis are similar to those predisposing plants to Rhizoctonia infection. Fusarium can damage seeds and can stunt or kill seedlings. Fusarium- infected roots show light to dark brown lesions that may spread all over the root system.
Disease Impacts
Damping off and seed decay are common on soybean. These diseases reduce the stand. Losses can range from negligible to high.
Life Cycle and Epidemiology
Typically, damping off and seed decay occur when soils are wet, compacted and poorly drained, especially in the first few weeks after planting. Each fungal genus causing a seedling disease has a unique set of optimal conditions for infection and development. Phytophtora and Rhizoctonia infection and development are favored by wet and warm soils while Pythium infection is optimum in wet and cool soil conditions. Some of the Fusarium fungi grow best in dry and warm soil conditions while some others require cool and wet soil conditions. The pathogens causing damping off and seed decay may survive in the soil for an extended period of time in association with plant debris.
Management Approaches
Cultural Methods
In general these diseases of soybean can be reduced by planting good-quality seed in well drained, non-compacted soil. Conditions that favor rapid emergence will reduce the likelihood of soybean seedling diseases. Delaying planting until soil temperatures are above 55˚F and relatively dry can be beneficial in avoiding infection by pathogens causing seed decay and damping off. Crop rotation has been reported to be of some value in managing these diseases
Host Plant Resistance
Soybean varieties resistant to Phytophthora are available commercially. There are two types of resistance available, specific resistance (soybean varieties with Rps genes) and tolerance, or general resistance. Specific Rps genes (e.g. 1a, 1c, 1k) convey resistance to certain races of Phytophthora sojae. The combination of tolerance, Rps resistance and seed treatment (see below)can help manage Phytophthora infection in the field. Despite Pythium and Phytophthora both being water molds, Phytophthora tolerance and resistance will not convey any protection against Pythium. No resistant varieties are available against other pathogens causing seed decay and damping off. ===Chemical Control=== Fungicidal seed treatment may reduce seed and seedling diseases. Since individual causal pathogen response to fungicidal active ingredients vary considerably, seed treatments with two or more active ingredients should typically be used. Products containing mefenoxam or metalaxyl can be effective against Pythium and Phytophthora. Other seed treatment fungicides have had mixed success against Rhizoctonia and Fusarium. Products containing fludioxonil or a strobilurin product (azoxystrobin, trifloxystrobin or pyraclostrobin) can be effective against Fusarium and Rhizoctonia.
