Overview: Phomopsis seed decay is caused primarily by Phomopsis longicolla, but other Diaporthe/Phomopsis spp. also can cause seed decay.
Symptoms: Severely infected seeds are shriveled, elongated, and cracked and appear white and chalky. Seeds also may be infected and not show symptoms. Affected seeds usually do not germinate or are slow to germinate. Seed infection may cause pre- and postemergence dampening-off, and under severe conditions, stands can be reduced enough to lower yield.
The fungus initially colonizes seed coats followed by the cotyledons and plumules. Mycelium invades the ovule and developing seeds through the funiculus and hilum. Within the seed, the fungi colonize all tissues of the seed coat and cotyledons and eventually the radicle and plumule. DAS-ELISA is more specific and 100-fold more sensitive in detecting the fungus than indirect ELISA.
Causal Organisms: P. longicolla colonies on potato-dextrose agar are floccose, dense, and white with occasional greenish yellow areas. The undersides of cultures are colorless with large, black, spreading stromata.
Disease Cycle: Infested crop debris and soil are the major sources of primary inoculum. However, diseased seeds are an important factor in the long-range dissemination of the pathogen. Seed infection tends to be more severe when harvest is delayed, when early-maturing cultivars are grown, or when crops are grown in regions where warm, humid weather prevails at harvest time.
More seed decay occurs in plants that are deficient in potassium, infected with one or more viruses, such as soybean mosaic potyvirus, or heavily attacked by insects.
Management: Some resistance sources have been reported in the following plant introductions: 80837, 204331, 205907, 205908, 205912, 259539, 279088, 341249, 360835, 423903, 385942, 360841, 205089, and 219635.
Rotate soybean with maize or other nonlegume crop, and plow down infested residues where soil erosion is not a concern. Plant high-quality seeds relatively free of the pathogen or use a fungicide seed dressing if seed infection is above 15%.
If possible, planting should be timed so that maturation occurs during a dry period. A foliar fungicide can be used if the risk of seed infecion is high, according to the point or pod infection prediction systems. Fungicides applied to pods from mid-flowering (R3-R4) to the late pod stages (R6-R7) may lower the incidence of seed infection. It is also important to harvest soybean seeds promptly at maturity.