As plans develop for planting corn and soybeans in April and May, it is be beneficial to consider the first wave of diseases that may attack: seed rots and seedling diseases. These diseases can kill seeds and seedlings, reduce plant stands, and significantly reduce yields. This article will briefly cover some key points on the pathogens that cause these diseases, conditions favoring these diseases, and options for management.|
Seed and seedling rots (also called pre- and postemergence damping-off) of corn and soybeans are caused by a number of different soilborne or seedborne fungi. Corn "gets by" relatively easily compared to soybeans because Pythium is the only common seed and seedling rot pathogen that is known to cause widespread problems in Illinois. Seed and seedling rots of soybeans in Illinois, however, are caused by several different pathogens. The most common and destructive in Illinois are considered to be Pythium, Phytophthora, and Rhizoctonia. Pythium and Phytophthora are "water-loving" fungal-like organisms. The same species of Pythium can infect corn and soybeans, but Phytophthora only infects soybeans.
These pathogens have different optimal conditions for infecting plants. Pythium and Phytophthora infect most readily in very wet to saturated soil conditions, whereas Rhizoctonia prefers moist but not saturated soils. The more striking difference among these pathogens is in the optimal temperature for infection. Pythium prefers cool soil (50 to 60°F), Phytophthora prefers slightly warmer soils (60 to 76°F), and Rhizoctonia has most activity in warm soils (over 75°F). Thus, the first pathogen to watch for after planting in most years is Pythium. The soft-rot symptoms caused by Pythium and Phytophthora are very similar and cannot be distinguished without laboratory examination, while the reddish-brown lesions caused by Rhizoctonia are easier to recognize.
This bit of background on these pathogens provides reasons why planting date, soil-moisture management, and selective use of fungicidal seed treatments are important for management of the diseases they cause. Plant resistance is also critical for control of Phytophthora diseases, but there is no resistance available for Pythium or Rhizoctonia.
What should you do to control these diseases, and are seed treatments worth the cost? Corn is often not as sensitive to seed and seedling rots as soybeans, so planting corn seed treated with fungicides such as Captan + Allegiance FL or Maxim 4FS + Apron XL (Table 1) at the recommended time when soil temperatures are rising may be enough to keep this pathogen in check. Keep in mind that problems with Pythium seed and seedling rot of any crop will be more severe when the soil is cool and wet. Control of these soybean diseases is a bit more complicated. First and foremost, plant good-quality soybean seed that has specific resistance to Phytophthora. Look for soybean diseases with specific Phytophthora resistance genes such as Rps1a, Rps1c, or Rps1k. Second, plant in warm, dry soil if possible. Since this is often not possible, waiting until May 1 to plant and/or using fungicidal seed treatments are recommended as good insurance. If you lose much of your stand and need to replant with the same variety into the same location, and the soil has not become appreciably drier or warmer, it is recommended that you treat your seed prior to replanting. For soybeans, Allegiance FL and Apron XL should control Pythium and Phytophthora for 10 to 14 days after planting, and Rival, Maxim 4FS, and captan should protect against Rhizoctonia and other pathogens for a similar period.
In trials conducted in no-till plots in summer 2000 at Urbana, Illinois, Dr. Wayne Pedersen and his research group reported that fungicidal seed treatments increased soybean yields 5 to 6 bushels per acre and corn yields 10 to 14 bushels per acre. Recommended fungicides for control of seed and seedling rot diseases of corn and soybeans are shown in Table 1, and additional information can be found in the Illinois Agricultural Pest Management Handbook, which is available for purchase or on the Internet at http://www.ag.uiuc.edu/~vista/abstracts/aiapm2k.html
The control strategies outlined here have been shown to be effective in many cases, but much damage frequently still occurs in Illinois from seed and seedling rots. What does the future hold for improved management of these diseases? Improved control will be based in part on enhanced understanding of the pathogens and diseases. Research is under way at the University of Illinois to gain more answers about these pathogens, diseases, and their interactions with other factors. Future improvements may also come in the way of new seed treatments. New fungicidal chemical seed treatments are being developed, and they will be tested in Illinois when they become available. Biological seed treatments, both fungi and bacteria, are also being investigated at various universities and private companies. They have potential advantages in having longer efficacy than currently available seed treatment chemicals and could prove to be safe for soybeans used as animal feed. To date, biological controls for soybean have proved to be inconsistent and often perform poorly under conditions of high disease pressure. Much work is in progress that may yield benefits for improved control of seed and seedling rot diseases. Until new controls are developed, we must continue to rely on management by planting high-quality seed in soil as warm, dry, and well-drained as possible and by using fungicidal seed treatments.--Dean Malvick