- Earlier planting, reduced tillage and increased use of cover crops are management practices that have inadvertently increased potential for Pythium occurrence.
- Improving soil drainage and planting into warmer soil temperatures narrows Pythium’s critical early infection period.
- Vayantis® fungicide seed treatment offers broader protection across Pythium species.
Among the most important seed and seedling diseases in U.S. corn and soybean production are those caused by Pythium species.1 Due to seedling establishment challenges, vigor reduction and the associated loss of plant stand caused by Pythium species, it is counted among the most economically impactful of the top 3 corn and soybean pathogens.1,2 Many of the same Pythium species cause significant damage to both corn and soybeans.3,4
Figure 1. Corn plant stand thriftiness and population reduced in southeast Iowa by Pythium infection under conservation tillage system.Soybean and corn yield losses to Pythium are predictably highest in years where cool and wet conditions persist.5 Although most often associated with cool soils, Pythium species vary in their optimum temperature for growth, with the dominant species in any given area differing across a field during the crop calendar. Complexes of Pythium species, as opposed to single Pythium species, are most commonly found in fields. Pythium is most often the first pathogen active in the Midwest during a growing season as it prefers cooler soils, relative to other plant pathogens.1,6 It also requires free soil water for oospores to germinate and produce mycelium or sporangia (spore cases) which then release mobile zoospores capable of plant infection.1,6
Why has the threat of Pythium infection increased in U.S. corn and soybean production in recent years? Several factors promote Pythium infection, but cooler, wetter conditions are likely the most important factors at play due to many corn and soybean farmers planting earlier. Recent university extension research shows that planting earlier provides greater access to longer maturity and higher yielding corn hybrids and soybean varieties, providing increased final yields while avoiding fall frosts.2,9 While these factors have encouraged earlier planting, average farm size has also increased significantly over the last 50 years. This has made earlier planting a necessity in order to achieve more farm acres being planted within the ideal planting date window so that vulnerable flowering periods avoid heat and drought stress in later summer months. Despite the positives of early planting, this practice also puts corn and soybean seeds and seedlings at risk of Pythium infection due to the cool, wet conditions of the earlier planting time.
Additionally, over the last 3 decades, soil tillage practices have consistently been moving toward reduced field trips with more plant residue left on the soil to promote increased soil protection from erosion. Reduced till and no-till both slow soil temperature increases as compared to traditional full tillage (Figure 1).7,8 The same is true of soil moisture. Reducing tillage tends to increase early season moisture, leading to longer periods of time that soils remain cool and damp, ideal conditions for Pythium.7,8
In recent years, cover crop adoption has also increased. Planting prior to terminating cover crops can sometimes happen due to spring weather. In these situations, cover crops have been suspected of acting as a bridge crop for disease establishment, intensifying damage from diseases like Pythium in corn.
Symptoms
Pythium species can cause seed rot, preemergence damping off disease, root rot, seedling blight and postemergence damping off. The symptoms most commonly associated with Pythium field infection are general loss of early seedling vigor and plant stand.3
Figure 2. Postemergence damping off of soybeans caused by Pythium speciesIn corn, plant stand loss is most often associated with yield loss proportional to the stand loss. In soybeans, stand loss is less directly correlated to yield loss as soybean plants are able to compensate because of their physiology and multiple fruiting positions on the plant. The leading soilborne fungi that causes corn seed rot and decay of roots are Pythium species.2,6 Pythium root rot is found in all soybean and corn producing regions of the U.S. These crops are attacked not by one species, but a complex of Pythium species. Soil temperature and moisture are primary factors influencing infection and largely dictate which Pythium species predominate and how disease-causing they are.
Infected seeds often have cracked seed coats and are soft and rotted with a foul odor. Within the cooler end of the temperature range for Pythium, seeds are slower to germinate and seedling establishment time is proportionally longer.1,6 This allows greater infection opportunity, increasing stand infection and potential for stand loss in both corn and soybeans.
Figure 3. Corn plant lost to Pythium postemergence damping off.Seriously infected seedlings exhibit visible lesions and root system discoloration.1,6 Proportionate to infection, some seedlings may not emerge and establish a stand, something that is called preemergence damping off. However, the plants that do establish stand are not out of danger. Soybean infections can occur on the upper hypocotyl (early stem). Within a few days, depending on level of infection and environmental conditions, they may collapse and die, which is referred to as postemergence damping off (Figure 2). Pythium lesions can range in size from so small that they are not detectable by the human eye to large, easily visible areas. They may be found on hypocotyls and cotyledons (early stem and leaves).1
Corn seedlings infected with Pythium that do emerge often have visible lesions and root discoloration.6 Emerged, infected corn seedlings often exhibit variable leaf color, from paler yellow to darker blue-green colors as seen in Figure 1. Depending on the growing environment (temperature and moisture levels), as well as the level of infection, some seedlings may grow out of the infection while more seriously infected plants are lost to postemergence damping off (Figure 3). If plants don’t ultimately die, they will often have much smaller, less developed root systems that continue with discolored rotting regions. Depending on temperature cycling (between warmer and cooler temperatures) and the soil moisture regime, these weakened plants may yet succumb to Pythium through the V3-V4 growth stages. Conditions that promote rapid germination and seedling stand establishment are advantageous to avoiding serious Pythium infection and associated stand losses and yield losses.1,6
Disease Cycle
Pythium species that cause corn and soybean disease are soil dwellers and overwinter in the soil and on plant residue as oospores.1,6 Survival without live plant tissue can occur for many years as oospores (resting sexual spores) get nourishment from dead or decaying organisms.6 Under favorable conditions, oospores germinate and produce mycelia or sporangia which produce and then release zoospores. Both mycelia and zoospores can infect germinating and developing corn and soybean seedlings.1,6 Disease severity is largely governed by the initial amount of Pythium inoculum, susceptible host age and environmental parameters during infection.
Soil temperature and moisture are the principal environmental elements influencing Pythium species infection ability. Free water within the soil is required for zoospore release and for movement towards plant infection. Pythium species may be organized by the temperature range ideal for infection, which is reported to be between 50-70°F.6
Management
Little to no plant genetic source differences have been reported for Pythium species resistance through plant breeding.1,6 Due to the fact that many plants provide host capabilities for Pythium species survival, crop rotation has little impact within cropping systems. Cultural practices increasing the rate of germination and seedling establishment often also reduce Pythium infection opportunity. Improving soil drainage and planting into warmer soil temperatures narrows the critical early infection period. If no free water is available for zoospores to infect plants, even if cooler temperatures are present, Pythium infection is predictably reduced. Planting high quality seed free of chips and cracks has been shown to reduce Pythium infection as well.1,6
Using a fungicide-containing seed treatment, such as one that includes metalaxyl, is the most commonly used practice to combat Pythium species across crops. The commercialization of ethaboxam occurred several years ago with predictable protection improvement. An exciting development from Syngenta Seedcare is the introduction of a novel mode of action in the molecule picarbutrazox, marketed as Vayantis® and registered by the U.S. EPA. Vayantis offers broader Pythium protection across species than either metalaxyl or ethaboxam. University extension plant pathologists increasingly recommend using 2 modes of action against Pythium.
Reducing seedling stress and promoting practices that increase early soybean and corn growth and development rates appear correlated to reduction in early season seed rots, damping off and seedling blights, including those caused by Pythium species.2,5 Early season herbicide applications, cool soil temperatures, extremely high or low soil pH levels, deficient soil fertility levels and soil compaction have all been linked to increased early season disease.1,6
By protecting seedlings from Pythium infection, seedling germination, early plant growth and seedling development are also protected, leading to more robust root mass accumulation and increased end of season yield potential.
Contact your Golden Harvest® Seed Advisor or agronomist for more information regarding Pythium control in corn and soybeans.
References:
1Hartman, G., J.Sinclair and J. Rupe, eds. 1999. Compendium of Soybean Diseases, 4th Edn. Am Phytopath Soc, St. Paul, Minnesota.
2 Sprague, G. and J. Dudley, eds. 1988. Corn and Corn Improvement. 3rd edition. Am Soc of Agron. Madison, Wisconsin.
3 Broders, K., P. Lipps, P. Paul and A. Dorrance. 2007. Characterization of Pythium spp. associated with corn and soybean seed and seedling disease in Ohio. Plant Dis. 91:727-735.
4 Chilvers, M., A. Rojas, J. Jacobs, A. Robertson and R. Matthiesen-Andersen. 2013. Pythium, seedling disease of soybean and more. Integrated Crop Management Conference – Iowa State University 2013. Ames, Iowa.
5 Matthiesen, R., A. Ahmad and A. Robertson. 2016. Temperature affects aggressiveness and fungicide sensitivity of four Pythium spp. that cause soybean and corn damping off in Iowa. Plant Disease 100: 583-591.
6 White, D. ed. 1999. Compendium of Corn Diseases, 3rd Edn. Am Phytopath Society, St. Paul, Minnesota.
7 Van Doren, D., G. Triplett and J. Henry. 1976. Influence of long term tillage, crop rotation and soil type combinations on corn yield. Soil Sci Soc Am J 40:100-105.
8 Wade, T., R. Claasen and S. Wallander. 2015. Conservation-practice adoption rates vary widely by crop and region. Economic Information Bulletin Number 147. USDA Economic Research Service.
9 Wilcox, J.R. ed. 1987. Soybeans: improvement, production and uses. 2nd Edn. Am Soc of Agron. Madison, Wisconsin.
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