The 2007-2008 winter, and the early portions of the spring season, has brought substantial moisture into many parts of the state. With further rain forecasted throughout the state over the next few days and weeks, it is important to understand how early season soybean development may be compromised by several plant pathogenic fungi that cause seed rot, seedling mortality, and root and stem decay. Frequently seedling health is ignored because plant populations are acceptable and stem and leaves do not express symptoms during early vegetative growth. Phytophthora sojae, Pythium sp., and Rhizoctonia solani are believed to be most important seedling pathogens, but others, such as Phomopsis and Fusarium sp. are actively invading plants from growth stages VE to V4 (Table 1). Infection may cause chronic symptoms, or remain latent and cause symptoms of plant decline during the reproductive growth stages.

 

Table 1. Summary chart of the symptoms caused by common soybean pathogens found at emergence and early seedling growth stages, and the environmental conditions that favor disease development.

 

	Seed rot	Root rot	Stem lesions	Warm wet soil	Cool wet soil
Phytophthora	X	X	X	X
Pythium	X	X	X		X
Fusarium		X	X	X
Rhizoctonia	X	X	X	X

Poor stands may be the first indication of a pathogen problem. To determine if seedborne pathogens are affecting germination, take 100 seeds and wrap them in a moist paper towel. Keep the paper constantly moist and check to see if seeds have germinated in a few days. If seed rot is evident and/or a white mycelium is found on seeds, Phomopsis Seed Rot (Phomopsis longicola) may be a culprit (Figure 1A). Wrinkled seed indicates Phomopsis as well; however it is important to note a significant percent of “normal” seed may be infected although symptomless. These seed may rot during the germination or emergence phase (Figures 1B and 1C). Other potential fungi that may be observed are Fusarium sp., which are often pink or purple colored. 

 

 Figure 1A. Phomopsis seed rot may be characterized by chalky white seed. The seed in the low left corner is an example of very severe Phomopsis infection, typically not present in properly conditioned, seed and compared to properly conditioned seed. 1B. Wrinkled seed may also indicate infection by Phomopsis. 1C. While some infected seed may have obvious symptoms of Phomopsis, soybean seed that appears asymptomatic may also harbor significant Phomopsis infection as illustrated by the white mold growing from seed.   

 

 As seedlings emerge, roots and stems may become necrotic, causing rapid plant death. (Figure 2a). One of the most likely culprits at this stage is Phytophthora. Often, there can be an uneven distribution of mortality in the field, or limited to especially wet areas of the field (Figure 2b). Control tactics include use of fungicide treated seed or Phytophthora resistant varieties. In fields where resistant varieties have been planted but losses have still occurred, it is likely the pathogen has overcome the resistant gene(s) used. The Rps1a gene has limited effectiveness in Wisconsin, while the Rps1c is effective in about 75% of fields and Rps1k is effective in 99% of Wisconsin fields. There have been reported failures of the Rps1k gene in Wisconsin; therefore, it is necessary to get lab confirmation of the Phytophthora race before relying solely on a particular gene.

 

Figure 2. A) Recently emerged seedlings killed by Phytophthora. B) Seedling mortality caused by Phytophthora, showing the uneven distribution of the pathogen.

 

Other culprits during the early growing season may hinder soybean development. Carefully dig soybean roots, gently remove soil and examine soybean roots for evidence of lesions. Lesions on roots can be indicative of Pythium, Fusarium or Rhizoctonia root rots, especially in wet years (Figure 3A, 3B).    Often, these lesions are reddish brown and intensify in the hypocotyl area. In a dry year, lesions can be indicative of Macrophomina phaseolina or Charcoal rot. These lesions also appear reddish brown initially, but in severe seedling infections, plants may turn black and die, hence the term “charcoal rot”. Control all with appropriate seed treatments.

Figure 3. A) Seedling lesion caused by Pythium sp. Often, these lesions are reddish brown and appear most often at the hypocotyl of the seedling. B) Lesions casused by Rhizoctonia solani. 

 

Treating seeds with fungicides may be beneficial when planting in cold, wet soils, or in reduced till or no till fields. Additionally, a benefit may be obtained is seed germination is below 80% or a seed lot has low seed vigor. Seed that is mechanically unsound may be damaged further by the treatment process.

Follow label instructions for application rates and additional application instructions. Apply fungicide seed treatments before applying a Rhizobium inoculum and plant seed immediately. Consult the Soybean Plant Health website (www.plantpath.wisc.edu/soyhealth) or the 2008 Pest Management for Wisconsin Field Crops publication (A3646, or http://learningstore.uwex.edu/pdf/A3646.PDF).

 

Plant health assessment is important during the seedling phase. This information can be used to make adjustments in crop management in subsequent years, and may explain symptoms later in the season as well as disappointing yields.