In 2018 I received some images on my cell phone. These images were of red crown rot, a soybean disease I had encountered while I was the field crop pathologist at the University of Delaware. The images clearly showed distinctive brick red, pinhead sized “balls” aggregated on the lower stems of the plant. My immediate response to the individual who sent the images was “That’s red crown rot (RCR)- It’s typically a non-issue and routinely found in legume / soybean rotations. Probably just an anomaly.” Then I was sent a bevy of additional pictures of the affected field. It was clear that this was not an anomaly. I quickly checked with the UIUC diagnostic clinic and did a literature review, and surprisingly, Illinois had never reported red crown rot in Illinois. Intrigued, I went to the field, and it was suffering from widespread wilting and premature senescence of beans that I estimated covered approximately 25-30% of the field. When we compared infected to non-infected areas of fields, an average incidence (number of symptomatic plants of 100) of 28.5% was associated with a 39.6% yield loss. This was something to keep an eye on.
Since then, we have detected red crown rot in 12 fields across 6 counties in Illinois (Figure 1), and I continue to work with members of industry and individual growers to monitor and learn more about this disease and its potential impact in Illinois soybean production.
Here are some quick points on red crown rot in Illinois:
- We do not know how it became established here. We are finding it across a wide area, and where we are locating it, we are observing pronounced infection. This indicates that the disease probably has been here for several years. This is a soilborne disease, and most likely it has been spreading with equipment, and potentially with field runoff or even wildlife.
- This disease can look very similar to sudden death syndrome, and the two diseases share a lot in common. Both diseases are caused by soilborne fungal pathogens that survive in the absence of soybeans, infect soybean seedlings when conditions are wet after planting, and cause interveinal chlorosis of the foliage once pods set. The temperature optima differ slightly for the individual pathogens, with SDS favored by slightly cooler soil temperatures vs RCR. However, there is overlap in the temperatures needed to grow and infect. Therefore, there may be some situations where we have coinfection of the diseases. Both pathogens can produce a toxin around R3, which may accumulate in foliage and cause interveinal chlorosis, wilting, defoliation, and plant death.
- It is easy to misdiagnose RCR. There are other species of fungi (known generally as saprophytic fungi) that feed on dead/decaying soybean stems that have a pink/orange coloration, and several conditions can cause the lower portion of stems to develop a purple/red coloration indicative of RCR onset. These include physiological anomalies, Rhizoctonia, Phytophthora, stem canker, and damage by gall midge or grape colaspis.
- Management options right now are limited. There are no resistant cultivars that are known in commercial lineups sold in Illinois, mostly because this historically has been an isolated issue in warmer regions where soybeans are rotated with peanuts. Where the disease has been more problematic, such as Louisiana and Mississippi, a standard recommendation is to rotate to corn for at least a year and avoid early planting. Alfalfa also is a host, so be sure to avoid rotating soybean with alfalfa. We have done some preliminary research on seed treatments and shown that at least two of the more commonly used SDS seed treatments can reduce early onset RCR symptoms and seedling growth (click here for journal abstract). We currently do not know if seed treatments will reduce late season foliar symptomology and yield impacts. There are some plans for field scale assessments this season, which hopefully provides additional information for subsequent growing seasons.
All that being said, how do we identify RCR in our soybean fields?
The first thing to keep in mind is that this disease is favored by wet conditions around planting. We are likely to observe more RCR later in the season when we have persistent rains within the first 3 weeks or so after after emergence.
Second, look for fields with initial stand issues. RCR can cause a pre/post emergent damping off resulting in stand reductions early in the season. Typically, these are going to be associated with poorly drained areas of the field or areas with compaction issues. Unfortunately, there is nothing distinct at this stage that indicates RCR, but fields with stand issues early should be checked for RCR later in the season.
Next, fields suffering from RCR will show somewhat sporadic, patchy areas with interveinal chlorosis after R3/4 (Figure 2). If conditions are warm and wet, those plants may wilt and almost have a “melted” look to them. It is hard to describe this symptom, but once you have observed it you likely will be able to notice it in the future.
Look at affected areas and observe the lower stems of several plants. If it has been warm and wet you likely will observe a flat, pink to orange growth on the lower stems of many plants. These are simply other species of fungi feeding off the dying plant. Do not confuse these with the fruiting bodies of RCR which are distinct brick red pin head or smaller sized balls that are often aggregated at the lower stem, crown, and sometimes observed on roots in the upper soil layers (Figure 3). Often, the lower 2-3 inches of stem develops a deep purple to red color prior to the formation of fruiting bodies. Splitting the lower crown and stem may reveal a grey central pith. With SDS, that inner pith remains white, and the outer vasculature has a brown discoloration. Another way to distinguish RCR from SDS is that SDS produces blue spores on the roots and occasionally lower crown.
If you think you have RCR, the best thing to do is to send samples to your local diagnostic clinic. They will be able to make the detailed assessments needed to correctly confirm the presence of the disease. Let’s get our 2022 soybean season off and running!
Figure 1. Counties where we have confirmed red crown rot as of March 2022. The disease likely can be found in other counties but has not yet been reported.
Figure 2. Wilting and interveinal chlorosis due to red crown rot.