In July, we have an opportunity to review the first half of the soybean growing season. Soybeans have moved into the reproductive stages and we can go back and look at the earlier stages, trying to gauge the potential of the crop and what may have taken us to this point.
The Soy Envoy team held a conference call recently to discuss what was happening around the state in soybeans. The topic quickly came around to the influx of seedling diseases that occurred during the first part of June. I wasn’t able to come up with any exact figures, but the University of Illinois Plant Clinic was inundated with samples of soybean tissue to assay for various seedling diseases during late May and early June.
Many of the samples came back positive for phytophthora, but there were also cases of rhizoctonia, pythium, and fusarium. These reports were scattered all over the state, from south to north and from west to east. Severity ranged from scattered plants in a field to serious outbreaks that required replanting. Several questions were raised about why these pathogens showed up when they did, which in many cases was later than usually expected. As seedling diseases, we normally expect them to manifest themselves soon after emergence and in many cases they don’t show up until close to beginning bloom.
One thing discussed and agreed upon was that another stress factor is often involved when seedling diseases are found. Compaction, dry soil, wet soil and other stressors could be associated with the disease infection. One of my colleagues observed that these stressful conditions make the soybean plant less able to fend off the pathogens and this may explain the close association. Even the preparation of the plant to begin blooming can be a somewhat stressful time, contributing to disease infections given favorable disease conditions.
Another part of the discussion, and one that has been in publications over the past month, is why seed treatments didn’t protect the soybeans from these infections. This question has multiple answers, all of which are probably combined to some extent.
- Dosage. There has to be a high-enough amount of the correct fungicide to effectively control the given pathogen. Colleague Kris Ehler talks about the amount of “real estate” on the seed surface with respect to getting the right dosage of each product. https://www.ilsoyadvisor.com/on-farm/ilsoyadvisor/understanding-your-%E2%80%9Cseed-real-estate%E2%80%9D
- Soybean Growth. Most seed treatments protect an area adjacent to the seed. As the soybean grows, roots elongate outside this zone of protection, coming into contact with pathogens that can then develop throughout the plant.
- Time. As time goes by, the efficacy of the product will decrease, eventually running out altogether. Soybeans can then become infected by pathogens if conditions are conducive to disease development.
As our call concluded, we discussed how Soypocalypse 2018 might impact soybean yield in Illinois. In areas where the infection was scattered, chances are yield impact will be negligible. In those places where replant was necessary, yield impact really depends on conditions following the replant.
Will there be a repeat of Soypocalypse 2018? As with any disease infection, the three legs of the Disease Triangle have to be present, so there’s really no way of predicting. Remembering every growing season is different, we will just have to be aware and hope we can avoid Soypocalypse 2019.