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New Insights into Management of Soilborne Crop Pathogens Under Direct Seeding: PythiumR. James
Cook, Endowed Chair in Wheat Research, Departments of Plant Pathology
and In our book, Wheat Health Management, Roger Veseth and I refer to Pythium root rot as the "common cold" of wheat (Cook and Veseth, 1991). For that matter, Pythium root rot is the "common cold" of all plants. Except for the defense systems built into plants against this disease, it would become the "pneumonia" and there could be no plants (Vijayan et al, 1998). This is because Pythium species are ubiquitous inhabitants of the top few inches of virtually all soils where they obtain their nutrients though a combination of parasitic and saprophytic activities. As parasites they mainly infect germinating seeds and root tips and strip off fine rootlets and root hairs of plants, although some also infect leaves. As saprophytes, they are the first to colonize fresh plant material added to soil, such as plowed down green manure and bright unweathered wheat straw-plant materials not already fully occupied by other microbial inhabitants (Cook et al., 1990). Pythium species are pioneers, even among plant parasites; they infect the newest most juvenile tissues. Based on work more than a decade ago, 10 species of Pythium were identified in Inland Northwest fields as parasites of wheat and other crops grown in this region (Chamswarng and Cook, 1985). Tim Paulitz is now reopening this area of investigation and will be using new molecular methods to identify and distinguish between species and to quantify the amount of Pythium inoculum in soil and plants. Symptoms of Pythium Damage in the Field Because of the ubiquitous presence of Pythium species in soil, virtually all plants are exposed to infection by one or more species starting with seed germination in the soil and continuing on the roots through plant development to adult status and maturity. Only when the soil drains and dries to a moisture content less than field capacity does Pythium activity as a seedling and root parasite begin to subside or halt. Pythium species survive in soil as thick-walled spores that can be counted using an old technique known as dilution plating of infested soil on a selective medium. Essentially all wheat field soils tested so far in the Inland Northwest have counts of 200 Pythium propagules per gram of soil, and the average propagule count is 350-400 per gram, nearly all of which is in the top 4-6 inches of soil (Cook et al, 1990). Some eastern Washington fields in a standard winter wheat/spring barley/pea 3-year rotation have registered 1000 propagules of Pythium per gram of soil. Independent studies indicate that at 200 Pythium propagules per gram of soil is above the threshold needed to saturate the available infection sites on plants so that the amount of damage is entirely a function of the soil environment and management of this disease must focus on plant protection or helping the plant tolerate or recover from infections. Wheat seeds planted without protection from Apron, Thiram, or other appropriate fungicide applied as a seed treatment become infected within the first 24-48 hours after planting into moist soil. The infection occurs in the embryo where the parasite is then positioned to obtain nourishment from sugars mobilized from the endosperm (Hering et al., 1987). Seedling emergence still occurs, but the seedlings may be spindly, stunted, or the first true leaf to form on the seedling will be twisted, cupped, or unusually short. These symptoms can be explained on the basis of a combination of embryo damage and starvation of the seedling owing to the use of endosperm sugars by Pythium. Low seedling vigor because of Pythium infection in the seedling stage results in less tillering, retardation of plant development, and less yield, although test weight may be greater because of water left unused by the smaller crop. With peas, garbonzo beans, corn, or other large seeded crops, seeds planted without protection against Pythium are likely to rot in the soil without producing a seedling, or the seedling may emerge but then die. There are many names given to this phase of Pythium attack on germinating seeds, including seed rot, preemergence seed decay, damping off, and seedling blight. Seed treatments are covered in a subsequent section of this paper; suffice to say here that with seed treatment, this kind of Pythium damage to crops is largely prevented. Recognition of Pythium root rot based on symptoms is difficult. One problem is that the root tissues infected, especially fine rootlets and root hairs, are quickly converted to spores or other biomass of the pathogen, leaving little or nothing left for diagnosis. Washed roots are easier to examine than roots still covered with soil, but this usually also removes all remains of Pythium-infected roots. For wheat and barley, this can produce what I call picture-wire roots-where the main roots, stripped of laterals and hairs, resemble the fine wire used to hang pictures. Like many investigators before me, I have used soil fumigation as a research tool to reveal what wheat and barley plants look like without the early damage to seedlings and continued root pruning caused by Pythium (Cook et al., 1987). Extensive studies going back 50 years when soil fumigants came into use after World War II have consistently pointed to control of ubiquitous Pythium as one of the major reasons for the universal increased growth and yield response of crops to soil fumigation. Wheat grown in fumigated soil in the high-precipitation zones of eastern Washington, for example, produces 20-25% more tillers, heads 2-3 days earlier, stands 2-3 inches taller when at full adult-plant height, and is more uniform in height compared to wheat grown in adjacent plots with soil left untreated. Drenching the soil with metalaxyl, the active ingredient of Apron, has produced the same increased grown response noted with soil fumigation (Cook et al., 1980), providing further evidence for the kind of symptoms produced by Pythium on wheat in the field. Not surprisingly, because of the ubiquitous presence of Pythium species in soil, every plant in nontreated soil is affected by Pythium more or less the same so that, without the comparison of how healthy plants should look, we accept plants with Pythium damage as normal "healthy" plants. Soil Conditions Favorable to Pythium Pythium species become active only when soil moisture is at or exceeds field capacity. Research by Allmaras and others (1988) showed at Pendleton that the tillage pan 4-6 inches deep becomes an impediment to water infiltration during periods of rain or snow melt, which then results in periods of saturated soil and more Pythium damage. From this conclusive research, we can speculate that the improved drainage of soil after a few years of direct seeding will lower the risk of Pythium root rot. Not surprisingly, the Pythium species adapted to Northwest soils are mostly low-temperature species. The Pythium species in warmer parts of the world are adapted to high soil temperatures. Of two common species of Pythium in Northwest wheat field soils, P. ultimum is active down at soil temperatures down 10 C (50 F) and P. irregulare is active at soil temperatures down of 5 C (41 F). While the cool-season crops grown in this region are also adapted to these low temperatures, some defenses needed to prevent the "common cold" from becoming "pneumonia" (Vijiyan et al., 1998) are thought to be compromised, thereby resulting in predisposition of the crop to greater Pythium damage at temperatures below 40 F. The favoring effects of cold wet soil on Pythium damage are one reason for the increased growth and yield response of direct-seeded wheat and barley to stubble burning. Bare black soil warms and dries faster, which favors the crop over the pathogen. While necessary in some situations, the goal of direct seeding and certainly the goal of the research programs in support of direct seeding must be to manage root diseases without depending on stubble burning. Research done by graduate student Ryo Fukui in the early 1990s used a laboratory test to precisely measure the soil physical and chemical factors most favorable to Pythium damage (Fukui et al., 1994). In this system, he maintained soil water at the ideal level for Pythium so as to then measure other factors limiting to parasitism by Pythium species. Two soil factors stood out as favorable to Pythium in addition to wet soil. These were high clay content and low soil pH. We speculate that both of these soil factors favor Pythium indirectly rather than directly. For example, high clay content also means that the soil holds more water when at field capacity or higher. More water on a soil volume basis also then means greater diffusivity of seed and root exudates to distances outward in the soil to reach and stimulate more germination of Pythium spores sitting dormant in the soil. The low pH was shown to have a suppressive effect on microorganisms that otherwise were competitors of Pythium in the soil. When Fukui added antibiotics to soil to suppress competitors, Pythium was then just as active in soils at higher pH values. These results fit with field observations that point to Pythium damage as a problem on wheat, barley, and the grain legumes mainly in far eastern and southeastern Washington and adjacent northern Idaho where clay contents of the soils and annual precipitation are highest and soil pH is generally lowest. Crop Rotation Because all plants are hosts of Pythium species, crop rotation is of little or no use in management of the diseases caused by these soilborne pathogens. Only periods without plants, such as bare fallow, are likely to provide the kind of break needed to lower the potential for damage on a subsequent crop. This is of little use in the Northwest since soil conditions are already least favorable in areas where fallow is practiced, owing to the relatively low clay content, dry conditions, and higher pH values of these soil. Further, as pointed out above, the amount of Pythium in our soils is commonly double the threshold inoculum needed for maximum damage, and achieving a 50% or greater reduction in Pythium load in the soil is highly unlikely during a 12-13 month fallow. The dormant spores of Pythium can last in soil for considerable periods of time. We have some evidence that different crops favor different Pythium species, so that rotation of crops also rotates the species of Pythium available for infection of the next crop. Research by graduate student David Ingram indicates that P. irregulare thrives on barley, while P. ultimum thrives on peas, and that both of these species do more or less equally well on wheat (Ingram and Cook, 1990). Thus, a 2-year wheat/pea rotation could be expected to select for P. ultimum while a wheat/barley/pea rotation could be expected to select for both P. ultimum and P. irregulare. There is also evidence from the scientific literature that Pythium species compete with each other and that some weaker parasites can preempt infection by stronger parasites. Sorting this out becomes very complex. Value of Current Year (Fresh) Seed One of the biggest breakthroughs for limiting the effects of Pythium damage on wheat and possibly also barley was the discovery of the value of current year or fresh seed. As seed ages, cells in the seed coat and other tissues surrounding and within the seed die. In addition to reduced seed reserves, the contents of these dead cells becomes fodder for attraction of Pythium infection when these seeds are then planted into natural soil (Hering et al., 1987). Seed may show high percent strong germ in a laboratory test but still perform poorly in soil, especially if planted directly into cold wet soil protected from drying by surface residues. The Northwest seed industry has done an outstanding job of responding to this problem and most growers know the importance of fresh or no older than 1-year-old seed when planting into high-risk seedbeds. Seed Treatments The introduction Apron as a seed treatment product in the 1980s represented a major advance for control of the many manifestations represented by Pythium attack of germinating seeds. The benefits of Apron in the seed treatment mix are usually apparent as better stands and greater seedling vigor. Apron is now a component of both Raxil XT, sold by Gustafson Inc, and Dividend, sold by Syngenta Crop Protection. Metalaxyl, the generic name of the fungicide in Apron, is actually a two-component chemical where only one component (mefanoxam, sold under the trade name Apron XL) is the ingredient active against Pythium. The latest Syngenta product, Dividend XL, is a mixture of Dividend plus Apron XL The data in Table 1 shows the combined results of seed treatment tests with direct seeded spring and winter wheat conducted over several years and locations. These results indicate that, over the long term, and on average, growers can expect 2-4 bu/A more yield in response to any of the seed treatments that include Apron, Apron XL, or the older Thiram for Pythium control. This response is small compared with the 13-15 bu/A increase in yield achieved in these same field tests with best root disease control using soil fumigation. Seed treatments provide protection of the germinating seed and seedling but do not protect wheat against Pythium root rot. The modest but real (roughly 5%) increase in yield is mostly or entirely a reflection the importance of enhanced seedling health and vigor to final crop yield.
The current seed treatments also provide protection against other seed and seedling infecting pathogens, most notably Rhizoctonia species. Pythium and Rhizoctonia are very different organisms yet they occupy similar habitats and compete for similar substrates in soil and plant seeds and roots. Research has shown that control of only Pythium can result in increased damage from Rhizoctonia, and conversely, control of only Rhizoctonia can result in increased damage from Pythium. For this reason, our seed treatments are actually mixtures aimed at both of these pathogens. Rhizoctonia root rot is covered in the paper in these proceedings by Tim Paulitz. Fertilizer Placement Placement of fertilizer directly under the seed, or below and slightly to one side of the seed at the time of planting offers one of the best management tools for all root diseases but especially Pythium root rot. Roots without laterals or striped of hairs cannot reach far for phosphorus and other relatively immobile nutrients. Our method of fertilization in fields where Pythium root rot and other root diseases are important requires that the nutrients be made easily accessible to the roots rather than expecting the roots to grow to the nutrients. The benefits of fertilizer placement have been demonstrated using soil fumigation. In soil with root diseases controlled by fumigation, yields of spring barley and spring wheat were the same whether the fertilizer is banded below or below and 6 inches to one side of the seed, but in adjacent plots of nonfumigated soil, yields dropped when roots had to reach more than 2-3 inches to one side of the seed row to access a band of fertilizer (Cook et al., 2000). This is why, when using a two pass system of direct seeding, where nitrogen is applied in one pass, possibly in the fall for spring seeding, and planting is done with a second pass, that some starter be applied at the time of planting. Placement of phosphorus in the seed row may be just as useful as banding. Conclusions: Pythium damages crops either though infection of the embryos of germinating seed or through destruction of fine rootlets and root hairs. Best protection of germinating seeds is achieved with seed treatments that include Apron or Apron XL Older products such as Thiram are also effective. Seed treatments improve seedling vigor and can produce, on average and across the Inland Northwest, an additional 2-4 bu/A. However, seed treatments do not provide protection against the continuing attack of roots by Pythium. The other important tool for wheat and barley is to use fresh seed, or seed no older than 1-year, so as to minimize susceptibility and maximize ability of the seedling to tolerate Pythium infection. Pythium species are most active as seedling and root pathogens in soils with moisture contents at field capacity or above, and in soils high in clay content and low in soil pH. For this reason, the damage to crops caused by these soilborne pathogens occurs mainly in far eastern and southeastern Washington and adjacent Idaho. Soils in these areas have, on average, about double the amount of spore load to produce maximum damage, and therefore the amount of damage is a reflection of soil environment. The ubiquitous and uniform presence of Pythium species in Inland PNW soils, especially in the higher rainfall areas with high clay and low pH soils, can explain why, for wheat and barley, the chronically poor tillering, stunting, and delayed maturity caused by this pathogen is so uniform in the fields that we have come to accept crops with Pythium-incited damage as the normal "healthy" crops. Cold wet trashy seedbeds typical of direct-seed systems will tend to favor greater damage caused by Pythium. This is because of the favorable effects of low temperature and high soil moisture on Pythium and possibly also the stimulatory effects of fresh wheat straw on Pythium as a saprophyte in soil. On the positive side, we can expect that improved soil drainage in response to structural changes in soil with the transition from conventional to no tillage will greatly help to limit Pythium damage. Also, the use of direct-seed drills that place fertilizer directly under the seed, or that include starter fertilizer below or with the seed, is helping the crops tolerate Pythium root rot, through making nutrients more readily accessible to diseased roots. Literature Cited: Allmaras, R.R., Kraft, J.M. and Miller, D.E. 1988. Effects of soil compaction and incorporated crop residue on root health. Annu. Rev. Phythopathology 26:219-243. Chamswarng, C. and R. J. Cook. 1985. Identification and comparative pathogenicity of Pythium species from wheat roots and wheat-field soils in the Pacific Northwest. Phytopathology 75:821-827. Cook, R. J. and R. J. Veseth. 1991. Wheat Health Management. APS Press, St. Paul, MN. 151 pp. Cook, R. J., C. Chamswarng, and W.-h. Tang. 1990. Influence of wheat chaff and tillage on Pythium populations and Pythium damage to wheat. Soil Biol. Biochem. 22:939-947. Cook, R. J., B. H. Ownley, H. Zhang, and D. Vakoch. 2000. Influence of paired-row spacing and fertilizer placement on yield and root diseases of direct-seeded wheat. Crop Science. 40:1079-1087. Cook, R. J., J. W. Sitton, and W. A. Haglund. 1987. Influence of soil treatments on growth and yield of wheat and implications for control of Pythium root rot. Phytopathology 77:1172-1198. Cook, R. J., Sitton, J. W. and Waldher, J. T. 1980 Evidence for Pythium as a pathogen of direct?drilled wheat in the Pacific Northwest. Plant Disease 64:1061?1066. Fukui, R., Campbell, G. S. and Cook, R. J. 1994. Factors influencing the incidence of embryo infection by Pythium spp. during germination of wheat seeds in soils. Phytopathology 84:695-702. Hering, T. F., R. J. Cook, and W. -h. Tang. 1987. Infection of wheat embryos by Pythium species during seed germination and the influence of seed age and soil matric potential. Phytopathology 77:1104-1108. Ingram, D. M., and R. J. Cook. 1990. Pathogenicity of four Pythium species to wheat, barley, peas, and lentils. Plant Pathology 39:110-117. Vijayan, P., Shockey, J., Lévesque, C.A., Cook, R.J., and Browse, J. 1998. A role for jasmonate in pathogen defense of Arabidopsis. Proc. Natl. Acad. Sci., 95:7209-7214. |
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