Getting to the Root of Soybean Seedling Diseases
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This report could have been titled "healthy roots for healthy plants." Of course, this is only part of the story, since healthy roots start with healthy seeds. Regardless, the first groups of diseases that strike soybeans are seed rots and seedling diseases. Furthermore, not only do they affect emergence and stand count, but they also can cripple plants throughout the growing season. These diseases kill seeds and seedlings, reduce plant stands, and can significantly reduce yields. Yield losses are often difficult to determine, since many different factors impact yields throughout the season between the seedling stage and maturity. Additional difficulty in determining impacts of seedling diseases comes from the fact that they occur sporadically from year to year, location to location, and from one part of a field to another. This apparent inconsistency arises in large part from the dramatic effects that environmental variables have on the incidence and severity of seed and seedling diseases. Temperature, soil water content, tillage, and other soil factors are of particular importance.
Pathogens and Seedling Diseases
A variety of different common fungal and "fungal-like" pathogens cause seed and seedling diseases. Most of the pathogens that cause these problems inhabit the soil and exist there before seed is planted. Examples of these are Fusarium, Phytophthora, Pythium, and Rhizoctonia. In addition, some pathogens such as Colletotrichum (the cause of Anthracnose) can originate from infected seed.
These diverse fungal pathogens are favored by different environmental conditions. Phytophthora sojae and Pythium species infect most readily in very wet to saturated soil conditions, whereas Fusarium and Rhizoctonia prefer drier soils. Different temperatures are also favorable for infection by the various pathogens. Many Pythium species prefer cool soil (50 to 60°F), although some species prefer warmer temperatures. Thus, Pythium is often the first pathogen that causes problems after planting. Phytophthora prefers slightly warmer soils (60 to 76°F), and Rhizoctonia tends to be most active in warm soils over 74°F.
These pathogens also interact differently with soybean cultivars. The tan-brown, soft-rot symptoms on roots and stems caused by Pythium and Phytophthora are very similar—in fact, they are virtually indistinguishable without laboratory tests—whereas the reddish-brown, often sunken lesions caused by Rhizoctonia are easier to recognize. The symptoms associated with Fusarium damping-off are light to dark brown lesions on roots. Soybean cultivars are not available with high levels of resistance to Pythium, Fusarium, or Rhizoctonia; however, soybean cultivars are available with resistance to Phytophthora.
Focus on Phytophthora
Phytophthora rot is often considered to be among the five most damaging soybean diseases across the north central region of the U.S. The impact of this disease in Illinois varies from year to year and among locations, depending greatly on the occurrence of wet weather conditions early in the growing season. Phytophthora rot of soybean is caused by Phytophthora sojae. Unlike Pythium, which probably is most damaging to seed and seedlings, Phytophthora commonly attacks and kills soybeans from planting to harvest.
In addition to improving drainage in fields where possible, there are two keys to managing Phytophthora rot of soybean. First, high-quality soybean seed should be used that has resistance to Phytophthora. In most areas, cultivars with specific race resistance should be used. Specific resistance is controlled by major resistance genes such as Rps1a, Rps1c or Rps1k (Rps stands for Resistance to Phytophthora sojae). In addition to Rps resistance, cultivars with high partial resistance should be selected when possible. Second, fungicidal seed treatments are recommended as good insurance, especially in poorly drained and no-till areas that have a history of problems with seed and seedling rot. Seed treatments such as metalaxyl (Apron FL and Allegiance FL) and mefenoxam (Apron XL) can be effective against Phytophthora in the first few weeks after planting.
Although resistant cultivars have been the key to managing Phytophthora rot in Illinois, this disease remains significant in part because not all types of resistance are equally effective in all areas, depending on the races or pathotypes of P. sojae present in field soils. In many other Midwestern states, populations of Phytophthora have been reported that can defeat most or all major resistance (Rps) genes in soybean.
A project in Illinois is focused on determining if Phytophthora populations in Illinois are developing the ability to defeat available resistance genes. The results from this research will help producers and seed dealers select soybean cultivars with appropriate Phytophthora resistance for Illinois, and will be of value for breeders developing soybean cultivars with Phytophthora resistance best suited for Illinois.
Approximately 170 isolates of Phytophthora sojae were obtained from Illinois soybean fields. We isolated Phytophthora from 38 percent of the soybean fields tested. This suggests that Phytophthora sojae is widespread in Illinois.
We determined the pathotypes of 45 isolates of Phytophthora based on pathogenicity to the major resistance genes (Rps1a, Rps1c, & Rps1k) available in commercial soybean cultivars in Illinois. Soybean seedlings were inoculated in controlled greenhouse tests. The results indicate that under conditions favorable for Phytophthora rot, soybean cultivars with Rps1a are not effective in many areas of Illinois, the cultivars with Rps1c will be effective in more areas, and cultivars with Rps1k should be effective in most areas of the state. However, in northwestern Illinois, aggressive populations of P. sojae were found that defeat all the available resistance genes readily available in soybean cultivars widely sold in Illinois. We are continuing work to improve our understanding of the distribution of the aggressive isolates that defeat Rps1a, Ic, and Ik in Illinois.
Seed Treatments for Managing Diseases
Seed treatments can significantly improve plant stands and yields under some conditions. Seed and seedling diseases tend to be most severe when soil is cool and wet during or after planting, when soybeans are planted early (for example before April 28), when low seeding rates are used, under no-till conditions, and in low-lying fields that are frequently wet. Any of these conditions could warrant the use of seed treatments.
Not all seed treatments are equally effective against the different pathogens that cause seed and seedling diseases. Two general types of seed treatment fungicides are available for control of these pathogens. One type is effective against the water-mold pathogens Phytophthora and Pythium, and the other group is most effective against the other fungal pathogens. Allegiance-FL, Apron-FL, and ApronXL are systemic compounds that are effective for control of Pythium and Phytophthora, whereas Rival, Maxim-4FS, captan, and several other products protect against Fusarium, Rhizoctonia, and other fungal pathogens. For broad-spectrum control of different pathogens, some of these products are applied together. For example, Rival and Allegiance are applied together, carboxin is combined with PCNB and/or metalaxyl, and ApronXL and Maxim are often combined. Azoxystrobin is a new systemic fungicidal seed treatment that recently came on the market and may be available soon as a seed treatment for control of Rhizoctonia and other fungi on soybean.
Some of the common fungicidal compounds for control of seed and seedling rot diseases of soybean are listed in Table 2. All of these compounds can be effective, but they are not equally effective under different conditions and against all pathogens, as noted above. The mention of trade names does not imply endorsement of these products by the University of Illinois, nor does it suggest that other products are not equally effective. Additional information can be obtained in the current edition of the Illinois Agricultural Pest Management Handbook and from chemical companies and dealers.
Seed treatment fungicides can increase emergence, plant stand, and yield. The stand increases may not correlate with increased yield because soybean plants readily compensate for lower stand counts by increasing the numbers of branches and pods per plant. In several studies, seed treatments have resulted in yield increases of 5 to 10 percent. The benefits gained from the use of seed treatments will vary, depending in large part on soil conditions and weather for two to three weeks after planting.
Concluding Remarks
Seed and seedling diseases take a significant toll on soybean crops each year, and in severe cases, can kill well over 50 percent of seedling stands within two to four weeks after planting. Alternatively, they may take a hidden toll, with the full impact difficult to determine. They can kill 5 to 10 percent of plants in scattered patterns that may escape notice in fields, or they may reduce the vigor and function of root systems. Fortunately, management strategies are available to minimize losses from soybean seedling diseases, and research is underway to gain new information about these diseases that should lead to improved management and reduced losses.