Issue No. 9, Article 5/May 26, 2006
"Rotten weather" or "rotting weather"--either phrase seems appropriate for the last week in portions of Illinois where conditions were cold, wet, and just plain dismal. Almost anyone in their right mind would say it's been rotten, and almost any pathologist would say that conditions have also been ideal for the rotting of seeds and seedlings. Chief among stand-reducing culprits is that cursed corn and soybean disease known as Pythium.
The seed and seedling rot called Pythium rot is actually caused by several species of fungi (pathologists prefer the term "water molds" or "algae") belonging to the genus Pythium.
Plants infected by Pythium spp. typically suffer a soft, wet rot of seed or seedling tissue. As plants are removed from the soil, rotted outer tissue often sloughs away, leaving a thin thread that barely holds less-damaged halves of the plant together. These damping-off culprits affect corn, beans, and other crops as well.
Pythium spp. belong to a larger grouping of organisms termed "oomycetes," to which organisms like Phytophthora sojae (the cause of soybean Phytophthora root rot) also belong. The various types of Pythium arise from hardy, thick-walled, overwintering spores termed "oospores." Given ideal conditions for oospore germination, these microscopic spheres give rise to structures termed "sporangium" or to structures termed "hyphae." Hyphae are strands of fungal hair (strand masses are termed "mycelia"). Hyphae in turn produce additional structures. Those structures may be male, termed "antheridium," and/or they may be female, termed "oogonium." Many fungi can only produce one of the sexual stages per each strand of fungal hair, but Pythium spp. (and a few similar organisms) can produce both structures from the same strand. The fusion of the male structure and the female structure results in a thick-walled, fertilized oogonium, with excellent overwintering capacity. Plant pathologists term this hardy, thick-walled, overwintering structure an "oospore." As noted earlier, the oospore may continue the cycle just described (i.e., it may produce more hyphae), or it may give rise to structures termed "sporangia." A sporangium looks like a small, microscopic sack resting atop a microscopic slender stalk. The sack is filled with zoospores.
I previously noted that oospores germinate given ideal conditions. Those conditions for Pythium tend to be 50°F to 60°F. Pathologists worry, then, about stand reductions caused by Pythium spp. when soil temperatures are in that range. Pythium's relative, Phytophthora sojae, becomes more prevalent in the upper 50s and low 60s. Pathologists also know that soils must be fairly saturated for Pythium seed and seedling rots to appear in the field. That requirement has everything to do with the zoospores noted in the previous paragraph. A zoospore is a small spore with a tail on the end and a small projection on the front. Upon emerging from a sporangium, the zoospore begins to flap its tail ("flagellum"), causing the zoospore to move. The small projection on the front of the spore acts much like the keel of a boat. In other words, the fluttering flagellum and the "keel" on the front of the zoospore allow it to maximize movement in a controlled direction as it swims through soil moisture (thus the propensity for Pythium and Phytophthora in wet conditions). Under a microscope, this movement appears random, yet zoospores are actually "attracted to 'amino acid' sources such as those on roots in the area of growth or wounds" (Introductory Plant Physiology--University of Nebraska). In other words, zoospores are attracted to the most vulnerable of subterranean tissues. Once the zoospore finds susceptible tissue, it encysts, or forms a hard covering about itself, and later forms a germ tube that penetrates the plant, resulting in infection and the disease we call Pythium.
Practices that manage seed and seedling exposure to the two key Pythium variables (cold and wet conditions) provide some of the most effective control for this type of rot. Adequate drainage is critical to avoiding problems with Pythium. A planting date that minimizes soybean exposure to sub-65°F soil temperatures also wards off the disease, and Phytophthora as well. Since no resistance exists to Pythium, mefenoxam or metalaxyl seed treatments (products such as Apron or Allegiance) may also be recommended, especially where early planting is a must. In beans, the first 10 to 14 days following planting provide a real window of opportunity for Pythium spp. to infect the plant. (Phytophthora resistance does not kick in until after the first two weeks, making that period critical for Phytophthora as well.) Information regarding general seed rots and seedling rot seed treatments is presented in Table 1.--Matt Montgomery