Sickly Soybean Plants are Infected Rhizoctonia and Phytophthora

Views:1     Author:Site Editor     Publish Time: 2018-06-20      Origin:Site

After heavy rains last week, parts of many picture-perfect bean fields are suddenly wilting and shriveling owning to without waterproof shade net, such as most Midwest soybeans approaching flowering, a handful of diseases are likely to blame, including Phytophthora, Rhizoctonia and Pythium -- and some fields are showing more than one.


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. Another potential cause is compaction, which has produced shallow root systems that could be drowning in wet soils, he added.


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.

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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. 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. As long as using of the ground cover can avoid the occurrence.


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. They will be somewhat sunken lesions, kind of red in color. The stem will not be soft, as rhizoctonia is a dry rot.


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.


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.


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. 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. Knowing which of these problems is at work in your fields is important to planning variety selection and seed treatments for future soybean fields. So first choice is anti insect net Soybean varieties with genetic resistance to Phytophthora are also available.


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