With most Midwest soybeans approaching flowering, seedling diseases may be far from growers’ minds. But after heavy rains last week, parts of many picture-perfect bean fields are suddenly wilting and shriveling, said University of Illinois plant pathologist Nathan Kleczewski. A handful of diseases are likely to blame, including Phytophthora, Rhizoctonia and Pythium — and some fields are showing more than one, he said.
“A lot of people assume this is Phytophthora, because of the rain, and some fields do have it, but a lot of the samples we’re getting have multiple pathogens present,” he said.
Another potential cause is compaction, which has produced shallow root systems that could be drowning in wet soils, he added.
A ONE-TWO PUNCH
Kleczewski said some of the sickly soybean plants in Illinois are showing a dual infection from both Rhizoctonia and Phytophthora.
Since these two diseases favor nearly opposite conditions — Rhizoctonia likes hot, dry conditions for development, whereas Phytophthora thrives in wet soils — some growers are left scratching their head, he noted.
“We had basically two seasons so far with our soybeans,” he said. “We had our dry, hot season, and then we had that recent deluge of water. And what you end up with is kind of a one-two punch to the plant.”
Rhizoctonia infections likely took hold earlier in the season and damaged the soybean’s root system. “That fungus was chewing away on the roots and compromising that root system and causing cankers to develop, so plants likely lost a lot of their initial roots,” he explained. “So they compensate by forming adventitious roots — these shallow, weak little root systems.”
Then along came heavy rains, and Phytophthora entered the picture. “That deluge of rain inundated these fragile root systems, and then it doesn’t take much infection to start to break down those plants,” Kleczewski said.
Other potential culprits of wilted soybean plants right now are Pythium and Fusarium, he added.
Keep in mind that compacted soils can also produce wilting or sickly plants, as they limit root development and make it hard for a plant to handle either dry or saturated conditions.
The best way to know what is ailing your particular field is to dig up the affected plants, examine them and consider sending them to a plant diagnostic lab.
Here’s a brief review of the most likely culprits:
RHIZOCTONIA: Look for a soil-level canker on the stem, extending only about an inch or so upward, Kleczewski, said. “They will be somewhat sunken lesions, kind of red in color,” he said. The stem will not be soft, as rhizoctonia is a “dry rot,” he added.
PHYTOPHTHORA: Normally, early season infections of this disease create wet, mushy roots that slough off easily on your fingers and cause emergence problems. At this time of year, symptoms will likely be soft, mushy stems with black or brown discoloration starting at the base and spreading steadily upward along the stem.
PYTHIUM: Look for stunted roots with “rat-tailing,” a condition where the outer root tissue sloughs off easily, leaving behind a stringy, white core of tissue, Kleczewski said.
FUSARIUM: Though more of an opportunistic disease that invades a previously injured plant, Fusarium could also be the cause of a wilting soybean, Kleczewski noted. Look for brownish-red and mushy roots, with decaying internal tissue, he said.
COMPACTION: Roots from soybean plants in compacted soils will branch out sharply at 90-degree angles just above the compaction layer. These shallow root systems can cause wilting in both hot and wet conditions, Kleczewski noted. In drier weather, the roots will struggle to reach water, and in wet soils, they will be too saturated to function properly.
At this point in the season, there is no way to treat seedling diseases, but growers should scout anyway, Kleczewski said. Knowing which of these problems is at work in your fields is important to planning variety selection and seed treatments for future soybean fields.
For example, Pythium and Phytophthora are oomycetes, which are not true fungi and thus don’t respond to many common fungicides used in seed treatments. They require the use of active ingredients such as metalaxyl, mefanoxam and ethaboxam, which in turn don’t work against fungus-based diseases like Fusarium and Rhizoctonia, Kleczewski explained.
Soybean varieties with genetic resistance to Phytophthora are also available, he added.