Extension Plant Pathologist
The "Dreaded" Root Rots Of Soybeans
Finally, much of Minnesota is experiencing more than one warm, sunny day in a row. The soybeans, and other crops, were stuck in a slow growth mode for the last month due to the rainy, overcast conditions. But now the crops should start developing more rapidly. Fortunately, Pythium root rot is no longer a concern since it likes cool wet soils and preys on newly germinated soybean seeds and small seedling. However, Fusarium, Phytophthora, and Rhizoctonia root rots are still a threat to soybeans. The warm conditions contributing to the rapid growth of soybeans may actually result in more noticeable root rot symptoms. Now would be a good time to dig seedling in wet, poorly drained areas of fields to look for root lesions caused by the fungal pathogens. Lesions on roots that appear red, reddish brown, or brown are most likely caused by Fusarium or Phytophthora (see Figure 1). Reddish brown lesions that occur on the stem near the ground level are probably caused by Rhizoctonia (see Figure 2). Watch stands in those areas for stand loss. There is not much you can do this year about the diseases. It is probably too late to benefit from a replant if stand loss has occurred. It would be worth while to identify what root rot diseases predominate in your fields and plan for managing them in the future.
Figure 1. Fusarium root rot on soybeans.
Figure 2. Rhizoctonia root rot on soybean. Note the sunken lesion on Stem.
Managing root rots include improving soil drainage and reducing compaction. Fungicide seed treatments offer some protection for soybean seed and seedlings early in the season. Currently, there are no resistant varieties available for Fusarium or Rhizoctonia but there are for Phytophthora. Soybean varieties with Phytophthora resistance posses at least one Rps resistance gene. They are for example Rps 1a, 1b, 1c, 1k, 2, 3, and 6. Last summer Dr. Jim Kurle, plant pathologist at the University of Minnesota, with assistance from the Minnesota Department of Agriculture randomly sampled soybean fields throughout the state to determine the races of the Phytophthora pathogen. They identified 17 races of Phytophthora, up substantially from the five races identified in the last survey conducted in 1986. Some of the new races are able to overcome all the currently available resistance genes used in soybean varieties. If you are using a Phytophthora resistant variety and the disease still is appearing in the field, you may want to switch to a variety with a different Rps gene than the one you have been using. Infected soybeans can be tested for race determination at the disease clinic at the University of Minnesota for a fee.
How did the Soybean Cyst Nematode fair with all the Water?
Just fine thank you. Nematodes are aquatic animals so if you hoped that the rainy conditions and saturated soils were killing them think again. The nematode absorbs oxygen through its cuticles. As long as the water is aerobic, that is contains oxygen, the nematode will do fine. The SCN lifecycle, from egg hatching to females producing new eggs, typically takes four weeks to complete. However, because of the cooler conditions this spring the time period may be longer. If your plan on scouting your soybean fields for SCN by digging roots to find the females on the roots in suspect areas in fields, wait a little longer. Females will probably need at least another week to develop on the roots. If you wait, females will be more abundant and easily to see. Also, since soybean growth has been slow, root growth will be limited for nematodes to infect and females to develop. Waiting a few more weeks would be best. After all there is nothing you can do this year to manage them in the current soybean crop. Management planning can be done this fall.
Bacterial Spots on Soybeans
Bacterial leaf spots have been appeared on unifoliate or trifoliate leaves of soybeans because of the constant wet conditions and high relative humidity. The bacteria actually live on the surface of the soybean leaf and take advantage of the cool temperatures and moisture. These early bacterial infections will probably not persist and will not cause economic problems. Hot dry weather will check bacterial growth. Typically, bacterial leaf spots are seen later in the season on soybean plants approaching maturity.
Pictures and further discussion of soybean diseases can be found at our web site www.soybeans.umn.edu. Check it out.
Corn seedlings had been coming into the plant disease clinic with rotted mesocotyls caused by either Pythium and/or Fusarium root rots. We are now probably past that concern.
Some early season leaf diseases have developed on corn plants this year because of the wet conditions. Leaf spot caused by Anthracnose may be appearing on lower leaves (see Figure 3). The lesions first appear as water-soaked small oval spots, enlarge to 15 mm in length, and then coalesce into irregular shape areas that blight leaves. The lesions are tan to brown bordered by a thin reddish-purple region. The potential for economic loss from anthracnose usually occurs later in the season when it causes leaf lesions and stalk rots. Stalk rots can cause early senescence and may lower yields or may cause corn to fall before harvested.
Another disease in corn that may show up this year is Holcus Spot (See Figure 4). It is a collection of small, usually circular to elliptical white spots on the tip region of the corn leaf. Holcus Spot is caused by bacteria that take advantage of free water on leaves. The disease is usually no more than a blemish on the leaves, doesn't spread, and doesn't cause economic damage to corn.
Figure 3. Anthracnose leaf lesions on a corn leaf.
Figure 4. Holcus spot on corn.