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Chapter 4 - Disease Control, No. 11, Summer 1987

Seeding Date Controls Pythium in Winter Wheat

Roger Veseth

If Pythium root rot could be completely controlled, yields of winter wheat in the Northwest annual-cropped regions, receiving 16 inches or more annual precipitation, would commonly be 10 to 20 and sometimes 30 to 40 bushels/acre greater with the same water and fertilizer. That is the conclusion of STEEP researcher R. James Cook, USDA-ARS plant pathologist at Washington State University in Pullman. Although numerous control measures are being pursued, Cook states that the most effective control available is to seed relatively early into warmer, drier soils, conditions which are not favorable to the disease.

Cook and other researchers have been involved in Pythium research in the Northwest for more than 12 years. The research has been supported in part by the Washington Wheat Commission and the O. A. Vogel Wheat Research Fund.

Pythium Impact and Survival

To understand the importance of seeding date, growers need a basic understanding of the disease and what factors influence plant infection, Cook points out that Pythium root rot has largely gone unrecognized as a yield limiting disease in the Northwest. It occurs so widely and uniformly that affected wheat is often mistaken for ''typical" healthy wheat. It also appears that Pythium root rot has commonly been misdiagnosed as winter injury, poor soil fertility or toxicity from crop residue. Using soil fumigants and other soil treatments as research tools to reduce or eliminate the Pythium population in the soil, Cook has been able to quantify the damage caused by the disease.

Pythium is a parasitic water-mold fungus which persists in the soil, roots and crop residue as thick-walled spores called oospores. Like most soilborne pathogens of wheat, Pythium is largely confined to the top 4 to 6 inches of soil. It thrives in cool, wet conditions. Consequently, lateseeded winter wheat is most severely affected.

Pythium has been shown to limit winter wheat yields regardless of the tillage system used. However, greater amounts of surface residue cool the soil and help maintain a higher soil water content near the surface; conditions which favor the pathogen. Chaff and straw, as well as roots of weeds and volunteer plants, are also important food sources for Pythium. Consequently, Pythium can be a significant disease of wheat under reduced tillage systems with greater amounts of surface residue, particularly with continuous wheat or wheat after barley. Control strategies are therefore especially important under these conditions.

Worldwide, 19 different Pythium species are parasitic to wheat. In the Northwest. Cook has found that Pythium ultimum and Pythium irregulare are the two dominant species of over ten identified, although several others are commonly present. Pythium ultimum is most pathogenic on germinating seeds and emerging seedlings of wheat, peas and lentils, whereas Pythium irregulare is most pathogenic on barley.

Importance of Early Infection

Cook has demonstrated through several experiments that early infection of the wheat seed embryo during the first 1 to 2 days after planting is the first major event in Pythium attack on wheat. Once inside the embryo, Pythium is in an ideal position to extract nutrients as they are moved from the seed reserve (endosperm) to the young seedling tissue. However, the embryo infections generally are not lethal and typically only a few percent of the seeds are lost to seed decay. Seed decay may be higher in high residue seedbeds or if the wheat seed has been in storage for 2 to 3 years or more. If plant growth continues, embryo infection can account for the stunting of seedlings typically apparent by the 1- to 2-leaf stage, eventually resulting in reduced yields.

One experiment that shows the importance of early Pythium infection of the embryo consisted of pregerminating wheat seeds in a steam-pasteurized (Pythium-free) soil for 48 hours and then transplanting the germinated seeds into natural Pythium-infested soil. The transplanted seedlings produced vigorous healthy plants indistinguishable from those grown entirely in pasteurized soil. In contrast seedlings grown entirely in Pythium-infested soil exhibited typical symptoms of reduced and delayed emergence, twisted early leaves and stunted growth. These symptoms are typical of early Pythium infection.

With support of the O. A. Vogel Wheat Research Fund. Cook and his associates have been researching the relative importance of different Pythium species and the factors influencing Pythium infection, In recent greenhouse studies, they found that, under similar conditions and inoculum densities, Pythium ultimum infected up to 75 to 80 percent of the embryos of germinating wheat seeds compared to only 10 to 15 percent by Pythium irregulare and two other common Pythium species. Maximum embryo infection occurred within 20 growing degree days in silt loam soils with 200 or more oospores per gram of soil (about 100 growing degree days are required between winter wheat seeding and emergence). Soil water content was discovered to be a major infection-limiting factor. Maximum infection occurred at near saturated conditions ( – 100 mbar matrix potential) in a Palouse silt loam soil. Virtually no infection occurred when the soil water content was reduced to slightly below field capacity (– 500 mbar).

Besides the mixture of Pythium species, the total population of Pythium also varies. Through extensive sampling, Cook has found that most wheat-field soils in the Pacific Northwest contain a minimum of 250 to 300 oospores of Pythium species per gram of soil and some contain up to 2,000 per gram.

Whether embryo infection occurs at germination, Cook points out that some wheat roots in infested soil invariably contact oospores and lose root hairs and rootlets. The loss of root absorptive capacity leads to nutrient-deficiency symptoms in the leaves, plant or tiller stunting and reduced tillering. Moreover, seedlings that enter the winter with damaged primary roots (which develop from the seed) are slow to resume growth and develop in the spring.

Importance of Seeding Date

A 1986 field study by Cook near Pullman shows the influence that seeding date can have on the impact of Pythium on yield. Hill81 winter wheat was sown on three seeding dates: September 16, October 3 and October 14. Half of the experimental plots were treated with a soil fumigant to eliminate Pythium. On the unfumigated plots, yields decreased with progressively later seeding dates. On fumigated plots, however, all three seeding dates resulted in the same yield, 85 bushels.

Most importantly, the nonfumigated and fumigated plots yielded exactly the same with the September 16 seeding date; the yield increase to fumigation occurred only with the October seedings. Cook points out that this confirms that Pythium damage to winter wheat can be minimized or eliminated with earlier seeding into warmer, drier soils. The results also indicate that the well-known yield depression, associated with later seeding dates, may be partially the result of diseases caused by soilborne pathogens favored by cool wet conditions.

Integrated Management Strategies

Because of the unique complexes of diseases caused by soilborne pathogens in different Northwest wheat producing areas, growers need to develop a management strategy to control the diseases encountered in their particular area. One management option may reduce one disease but increase another. For example, the practice of delayed seeding, to reduce the potential for Cephalosporium stripe and Pseudocercosporella (strawbreaker) foot rot, increases the potential for Pythium root rot. An integrated approach, which focuses on all the major soilborne diseases present, will be most effective.

The researchers have developed an integrated management strategy to control Pythium and three other major soilborne diseases in the intermediate and higher precipitation areas of the inland Northwest. The following strategy is designed mainly for the annually-cropped region of northern Idaho, eastern Washington and northeastern Oregon receiving 16 inches or more annual precipitation. The strategy applies to both conventional tillage and conservation tillage systems.

1. 3-Year Rotation — Two years out of winter wheat and winter barley controls Cephalosporium stripe. Control of winter-annual grassy weeds, which serve as host to the pathogen, is important to the success of the rotation. This rotation may also help reduce the risk of strawbreaker foot rot.

2. Non-Host Crop for Take-All – To control take-all disease, the crop preceding winter wheat should be a non-host of the take-all fungus (peas, lentils, oats, rapeseed, etc.) or fallow. One year out of wheat or barley controls the disease, provided downy brome, quackgrass and other weed hosts are controlled.

3. Earlier Seeding Date for Pythium — The best control for Pythium root rot of winter wheat is to seed relatively early; for example, mid-to-late September instead of mid-October in the Pullman area. Under a 3-year rotation, seeding this early will not significantly increase Cephalosporium stripe, which would probably be the case in a 2-year rotation. If seeding is delayed, Cook suggests using current year seed for improved tolerance to embryo infection by Pythium. In delayed seeding, a fungicide seed treatment such as Apron (metalaxyl) may also provide some additional control, Cook's research shows that the best response to Apron seed treatment is with wheat no-till seeded into wheat or barley stubble in the higher precipitation areas. Pythium irregulare is least sensitive to Apron seed treatment and may be more active than Pythium ultimum at soil temperatures below 40F. The availability Of adequate mineral nutrition also increases plant resistance or tolerance to Pythium, take-all and other soilborne diseases. Fertilizer, especially phosphorus, should be readily available to the plant roots during early growth stages.

4. Consider a Fungicide for Foot Rot — The winter wheat should be closely monitored for strawbreaker foot rot in the spring and a fungicide applied if needed. Wheat seeded early in a 3-year rotation will likely have lush fall growth which is ideal for strawbreaker foot rot. No-till or minimum tillage seeding of winter wheat may also reduce the incidence of foot rot.

Cook suggests some second-choice alternatives to this 4-point management strategy. If a 3-year rotation is not possible, the risk of Cephalosporium stripe can be somewhat reduced by delaying the seeding date and using the most resistant variety available. Lewjain has generally been the most resistant variety while Stephens is the most susceptible. With later seeding, the use of new seed and a fungicide seed protectant such as Apron are particularly important to reduce Pythium damage, though some damage will still likely occur.

In the drier winter wheat-fallow region (14-inch annual precipitation or less) Cook states that Pythium root rot also occurs on wheat, but its control generally does not result in greater yields. His research shows that even the Pythium-infected wheat usually yields to the limit of the available water. In contrast to the higher precipitation areas, where disease is often more yield-limiting than water, yields in low precipitation areas are more limited by water than diseases.

To simplify information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.


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