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Soybean Seed Treatments and Control of Seed and Seedling Diseases

April 5, 2002
Numerous questions come in each year from Illinois growers concerning soybean seed treatments. Common questions often relate to which diseases and pathogens are controlled by seed treatments, which conditions favor these diseases and suggest that seed treatments would be beneficial, and which seed treatments are available and effective. This article will provide information on seed treatments and address these questions.

Seed treatments can protect against fungal and fungal-like pathogens that attack seeds and seedlings. Seed can be rotted, seedlings can be killed (damping-off), plant stands may be reduced, and yields can be significantly reduced by several different pathogens. Most of the pathogens that cause these problems are soilborne and are in the soil before the seed is planted. Common examples of these are Pythium, Phytophthora, Rhizoctonia, and Fusarium. Some of the pathogens that infect and damage seeds and seedlings can originate from low-quality, infected seed. Examples are the fungi Colletotrichum (causes anthracnose of soybean) and Phomopsis (causes Phomopsis seed decay).

Seed treatments will often provide a benefit under common spring field conditions. The seed and seedling diseases are typically most severe when soil is cool and wet during or after planting, when soybeans are planted early (for example, before May 1), 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.

Particular pathogens tend to infect under different conditions. The most common and destructive seed and seedling pathogens in Illinois are Pythium, Phytophthora sojae, and Rhizoctonia. Fusarium can also be common. Pythium and Phytophthora are water-loving fungal-like organisms. Some species of Pythium can infect soybean and corn, but Phytophthora only infects soybean. Pythium and Phytophthora infect most readily in very wet to saturated soil conditions, whereas Rhizoctonia prefers moist but not saturated soils. These pathogens also differ in their favored temperatures for infection. Many species of Pythium prefer cool soil (50­60F), although some species prefer warmer conditions. Phytophthora prefers slightly warmer soils (60­76F), and Rhizoctonia is most active in warm soils over 74F. Thus, the first pathogen that often causes problems after planting is Pythium. The soft-rot, tan symptoms on roots and stems caused by Pythium and Phytophthora are very similar and cannot be distinguished without laboratory testing, while 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. Many soybean varieties are available with high levels of resistance to Phytophthora (Rps genes 1a, 1c, and 1K); however, varieties are not available with high levels of resistance to Pythium, Fusarium, or Rhizoctonia.

Not all seed treatments are equally effective against these different seed and seed-rotting pathogens. In simple terms, there are two groups of fungicides to consider for control of these pathogens. One group is highly effective against the water mold pathogens Pythium and Phytophthora, and the other group is most effective against the other fungal pathogens. The systemic compounds Allegiance-FL, Apron-FL, and ApronXL are most effective for control of Pythium and Phytophthora, whereas Rival, Maxim-4FS, captan and several other products protect against Rhizoctonia and other pathogens. Azoxystrobin is a new systemic fungicidal seed treatment that recently came on the market and should be widely available soon for control of Rhizoctonia. For full-spectrum control of different pathogens, some of these products are often applied together. For example, Rival and Allegiance are often applied together, carboxin is combined with PCNB and metalaxyl, and the product ApronMaxx RTA is a mixture of ApronXL and Maxim.

A partial list of fungicides for control of seed and seedling rot diseases of soybean is shown in Table 2. These fungicides are listed here for basic information only. All of them can be effective, but they are not all equally effective under different conditions and against different pathogens, as noted previously. Additional information can be found in the Illinois Agricultural Pest Management Handbook, which is available for purchase from University of Illinois Extension. 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. Consult dealers and chemical companies for the latest information on these and other seed treatment products. Always read and follow the current product label before using a pesticide. The U.S. Federal Seed Act requires that containers with treated seeds should be labeled with the following information: "This seed has been treated with (common chemical names of active ingredients) and fungicide(s). Do not use for food, feed, or oil purposes." If there are concerns about the effects of fungicidal seed treatments on rhizobial N-fixing nodulation, the manufacturer of the seed treatment product should be contacted. Although seed treatments generally have minimal or no impact on nodulation, specific formulations and recommendations have been prepared for some products to minimize any effect of the product on nodulation.

The bottom line is whether soybean seed treatments provide significant benefits. Seed treatments will often result in increased emergence, plant stand, and yield. The stand increases often will not correlate with increased yield because soybean plants can compensate for lower stand counts by increasing numbers of branches and pods per plant. In several studies, seed treatments have resulted in yield increases of around 10%. Benefits from the use of seed treatments will vary depending in large part on weather conditions for 2 to 3 weeks after planting. In summary, many effective seed treatment fungicides are available for soybeans, and they can often reduce the damage that frequently occurs in Illinois due to seed and seedling diseases.--Dean Malvick

Author: Dean Malvick

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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