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Chapter 4 -Disease Control, No. 10, Spring 1987

Seedling Test for Pythium

Roger Veseth

A seedling test to select winter wheat breeding lines for resistance to Pythium root rot is being developed by R, James Cook, USDA-ARS plant pathologist and STEEP researcher at Washington State University in Pullman. After 1 year, Cook is hopeful that the test will help accelerate the selection and release of Pythium-resistant cereal varieties.

Impact of Pythium

Research by Cook and associates over the past 12 years has shown that Pythium root rot can be a major production constraint for wheat and barley in Northwest cropland receiving at least 16 inches annual precipitation. Where Pythium populations in the soil were experimentally reduced or nearly eliminated, wheat yield typically increased 15 to 20 percent and sometimes up to 50 percent.

Pythium is a parasitic water-mold fungus favored by cool, wet conditions. Consequently, late-seeded winter wheat is most severely impacted. Early-seeded spring wheat and spring barley can also be damaged if cool, wet soil conditions prevail after planting. Pythium has been shown to limit yields regardless of the tillage system used. However, since this pathogen is favored by cool wet soils, and it benefits from chaff and straw which serves as a food source, Pythiwn can be an important problem under tillage systems with larger amounts of surface residue. Where the Pythium population was experimentally reduced, wheat yields under no-till have equaled or exceeded yields under clean conventional tillage. Consequently, Pythium resistant cereal varieties could significantly increase the adoption of conservation tillage systems.

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 step in Pythium infection of 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 actual seed decay is generally only a few percent, but may be higher in high residue seedbeds, If growth continues, Pythium infection can account for the stunting of seedlings typically apparent by the 1- to 2-leaf stage, and eventually reduced yields.

One experiment that shows the importance of early Pythium infection 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 on pasteurized soil. In contrast seedlings grown entirely on Pythium-infested soil exhibited typical symptoms of reduced and delayed emergence, twisted early leaves and stunted growth.

Seedling Test

Because of the importance of early infection, a Pythium resistance test must begin at germination and extend through early growth stages. Cook's seedling test consists of planting wheat seeds in paired soil treatments in the greenhouse. One treatment is steam-pasteurized (Pythium-free) soil. The other treatment is natural (Pythium-infested) soil with 1 percent (wt/wt) wheat chaff added as a high energy food base to stimulate Pythium attack. The test is conducted at 59F in moist soil to further favor Pythium infection. Three growth factors are compared: (1) emergence rate and percentage; (2) length of the first true leaf at 2 to 3 weeks; and (3) plant height after 2 to 3 weeks.

The first true leaf appears to be a particularly sensitive indicator of embryo infection at germination and subsequent damage to the first seminal root (radicle), which develops from the seed. Total leaf height within the first few weeks is also significantly shorter for Pythium-infected plants.

To compare wheat responses, an index is calculated by dividing the results of each parameter measured in Pythium-infested treatments by the results of the same parameters measured for the wheat in pasteurized Pythium-free soil, and then averaging the three values. An index of 100 indicates that the wheat emerged as well and that the seedlings were as large in the chaff-amended natural soil as in the pasteurized soil. This index system was developed by J. W. Sitten, who works with Cook and is responsible for the wheat seedling test. About 900 winter wheat lines have been screened so far and of these, 8 to 10 have produced indices of about 100.

These findings indicate that resistance may exist to early Pythium infection. The best lines were predominantly from northern European countries such as England, France and the Netherlands. These outstanding selections from the 1986 greenhouse tests were planted under Pythium-infested field conditions last fall for further evaluation.

Seed Age and Quality

Cook's research demonstrates that seed age and quality also influence resistance to Pythium infection. Old seed (3 to 5 years old) was typically more vulnerable to Pythium damage than current-year seed. The greatest Pythium damage occurred with seedlots 4 and 5 years old, but even l-year-old seedlots produced fewer seedlings and sustained greater seedling damage than did current-year seed.

Storage conditions may affect the impact of seed age in Pythium resistance. Cook found that 1979 Daws seed stored at a constant 41F for 7 years was less susceptible to Pythium damage than the same seed lot age stored in a shed without heat in the winter or cooling in the summer.

To avoid variability in Pythium resistance because of seed age or storage environment, all wheat screened for seeding resistance to Pythium are tested from current-year seed. Cook plans to continue the research for 2 more years correlating seedling test data with growth response and yield in field trials. If the seedling test continues to be a good indicator of Pythium resistance, he hopes that it will be expanded and incorporated into the standard wheat breeding program.


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