Think twice about planting soybeans in cold ground

By Anne Dorrance, Ohio State University Extension plant pathologist (from the OSU CORN Newsletter)

Phomopsis infected soybean seed. Photo by OSU Extension.

One of these weeks it will actually warm up and we will get to plant the 2011 crop! If you start getting anxious and want to put soybeans into cold soil, you may want to rethink this option. A very nice study was just published by Iowa State Researcher, Leonor Leandro, which compared inoculations of soybean seed with the sudden death syndrome pathogen, Fusarium virguliforme. Seeds which were inoculated at the day of planting developed symptoms at all of the temperatures tested. Seedlings that were three and seven days old developed more root rot and greater severity of foliar symptoms at cooler temperatures (62 and 73) than those inoculated at warmer temperatures (82).

Their conclusions were that soybean seeds are more vulnerable to infection than seedlings, but seedlings grown in cold soils are also vulnerable. For Ohio producers that must manage sudden death syndrome, this study indicates that it may be best to wait until the soils are warmer and plant at optimum conditions for seed germination.

Lots of products are going onto soybeans to provide protection from many soil borne pathogens as well as insects.

Replanting is one of the major reasons to consider using a seed treatment fungicide. In addition to fields that have a history of poor stands, those that are in continuous soybeans and no-till may also benefit from a seed treatment. All of these conditions favor the build-up of inoculum for soil borne pathogens. Replanting of both soybean and corn have become issues in Ohio over the past eight production seasons.

Based on surveys from problem fields in the state over the past decade, the spectrum is quite diverse, and includes several water molds (Phytophthora and Pythium) as well as Fusarium graminearum (same fungus that causes head scab, corn ear and stalk rot), Rhizoctonia, and charcoal rot. Seed treatments can be very effective for protecting seedlings when environmental conditions are very conducive to seedling infection. However, not all seed treatment products protect against all of the different seedling pathogens. Product active ingredients and rates of application are all key components for protecting seedlings when the correct environmental conditions occur for seedling diseases to develop.

Phytophthora sojae is limiting factor for soybeans on poorly drained soils. This is managed primarily by resistance genes. But in Ohio, Rps1a is no longer effective in any field, while Rps1c and Rps1k may provide protection in approximately 40% of the fields. Partial resistance for P. sojae is now more important as a management tool in which under high disease pressure, some roots do become infected but that infection is limited and the plants produce good yields. This is also known in the industry as field resistance or tolerance. Seed should be treated with either metalaxyl or mefenoxam, but the rates of both of these fungicides are different due to the amount of the primary active ingredient. In addition, for both products, at the low labeled rates, substantial infection occurs. At the mid-rate, some disease does develop but at the high rates, not any infection occurred in greenhouse inoculations. We have also shown that in very high disease situations, the highest rate of both products protects stand and yield of highly susceptible and moderately susceptible varieties.

There are now a number of strobilurin compounds labeled for seed treatments. These fungicides impact P. sojae by reducing the size of the colony growth on agar assays and there is also some protection against root infection, but it is not a complete control. Thus, these compounds provide limited efficacy towards P. sojae.

Pythium spp. are becoming more and more prevalent as seedling pathogens of both corn and soybeans. Numerous species have been identified that contribute to stand loss and many of these can cause seed rot of both corn and soybeans. Planting date is no longer an option to avoid these pathogens as for each planting temperature there is a Pythium spp. that is a seed pathogen. The seed treatments for Pythium are longer and include: Captan, mefenoxam, metalaxyl, azoxystrobin, pyraclostrobin, and trifloxystrobin all provide control for Pythium, but not for all species. Based on our surveys there are isolates of several Pythium spp. that are resistant to mefenoxam/metalaxyl, or strobilurins, or mefenoxam/metalaxyl and strobilurin. Fields with a long history of replant issues may see the most benefit from using treatments that have metalaxyl or mefenoxam combined with a strobilurin in the mix.

Fusarium graminearum association with soybean seed and seedling decay has only recently been recognized. For this, the number of spores (inoculum) present can make a big difference. The more residue (corn with Gibberella stalk rot or scabby wheat) the greater the potential for infection. Seed Treatment compounds include: Captan and fludioxonil are very effective in controlling seed infections caused by F. graminearum in both corn and soybean. The strobilurins have limited efficacy towards this pathogen based on seed assays. There is concern for both fludioxonil and strobilurin compounds that isolates will become resistant to these chemistries.

Rhizoctonia solani is another soil borne pathogen that can infect both corn and soybeans. R. solani causes a brick red lesion to form on the roots and hypocotyls. Seed Treatments include the following: Previously, Rival fungicide (a combination of PCNB, TBZ, and captan) was very effective as a seed treatment for Rhizoctonia solani, but it is no longer available. There are several products which are currently labeled: fludioxonil, ipconazole, carboxin + PCNB, and strobilurin compounds. There is limited data from Ohio on these compounds as the tests are quite severe and in greenhouse studies none of the fungicides provide complete control. Rhizoctonia can infect throughout the season, thus combinations of host resistance and seed treatment are needed where this pathogen is a consistent problem.

Phomopsis seed rot is caused by several different fungi that are part of a complex. This is a seed pathogen, and severely infected seed is chalky white in color and does not germinate. Fludioxonil is one of the primary treatments. In our lab, the strobilurin fungicides were compared in a rolled towel assay and pyraclostrobin treated seed also provided protection in seed lots with 35% level of Phompsis infection.

Overall use high quality, healthy seed, plant into soil that is optimum for seed germination and growth as well as plant to optimum depth and moisture levels. Remember, don’t beat the rain and have one of those 3 and 4-inch dumpers that will saturate the field for days. If your seed is not protected you will most likely have to replant that field.

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