With the 2018 harvest of soybeans delayed beyond the ideal window of time, the opportunity for diseases to infect seed pods and in some instances, to the seed itself, was greatly increased.
Across the state, seed suppliers have reported that this year’s crops of seed soybean are frequently testing positive for the Diaporthe fungus (Phomopsis seed decay), which is resulting in lower than normal germination rates of seed. Seed decay is characterized by cracked, shriveled seed with white chalk-colored mold on the seed surface.
The large amounts of rain that occurred throughout August and September set up the year’s soybean crop to be at a disadvantage to the Diaporthe fungus, which typically infects soybean pods between the R5 (early seeding stage) and R6 (fully seeded pods) growth stages. This is important because only infections initiated in the pods can infect seeds and cause seed decay.
Infected seed will have a lower probability of germination in the following season, when planted. If soil conditions are wetter and cooler than normal, this could drastically impact both the survival and stand count of plants. Diminished seed quality and reduced seed vigor, germination and emergence are all consequences of seed decay. Seed decay can also reduce seed test weight and oil content.
Many dealers will want to have their seed treated with a fungicide to increase the chances of germination and prevent seedling diseases in 2019. The Iowa State University Seed Lab is able to test the germination rate of seeds to verify if and what percentage of seeds are infected with the Diaporthe fungus. Depending on the germination rate and incidence of infection, the use of a seed treatment may be warranted.
According to past Iowa State University research, appropriate seed treatments can increase germination rates by 10-15 percent. Given the progress and development of fungicides, germination rates could be further improved, given the right mix.
However, winter storage of seed can also increase germination rates, due to the fact that under dry storage conditions, the mycelium of the fungus will die, improving the seed quality. A suggested practice is to dry-store low germ-seeds over winter and re-test the seed in February.
It is recommended to not use seed lots with more than 20 percent Diaporthe infection because severely infected or moldy seed will fail to germinate even after being treated.
Although fungicide applications may reduce disease and improve seed quality, yield may not be impacted.