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Chapter 4 - Disease Control, No. 8, Summer 1988

Seed Age Affects Pythium Tolerance

Roger Veseth

Traditionally, wheat growers in the Pacific Northwest have preferred seeding 1-or 2-year-old winter wheat seed to avoid dormancy problems of new seed, Dormancy of new seed, however, is mainly a problem under warm soil conditions such as commonly encountered with August September seedings in the wheat-fallow area of eastern Washington higher precipitation, annual-cropping areas of the Inland Northwest, where Pythium root rot is most important, growers typically seed in late September and October. By this time, soils are usually cooler and wetter, and dormancy with new seed is no longer a problem.

Recent research has shown that the age of winter wheat seed may have a significant impact on the seedling tolerance to infection by the Pythium fungus. The research was conducted last fall and winter by R. James Cook, USDA-ARS plant pathologist and STEEP researcher at Pullman, WA, and T. H. Hering, a visiting plant pathologist from England. Cook suggests that growers consider using high quality, current-year seed if seeding dates are late September or later, when cool, wet soils are often encountered.

Background on Pythium

Research by Cook and other plant pathologists over the past 10 years has shown that Pythium root rot can be a major production constraint for winter wheat in Northwest cropland areas receiving at least 16 inches annual precipitation. Where Pythium populations in the soil were experimentally reduced or nearly eliminated, yield typically increased 15 to 20 percent and sometimes up to 50 percent.

Pythium is a parasitic, water-mold fungus which favors cool, wet soil conditions. It often attacks wheat shortly after planting by infecting the embryo of the seed as germination begins. The germinating embryo of the seed is ideal tissue for Pythium infection because it is young, growing tissue relatively unprotected by secondary cell wall thickening or cuticle development.

Once in 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. Actual seed decay is generally only a few percent. If growth continues, the Pythium infection can account for the stunting of wheat seedlings typically apparent by the 1- to 2-leaf stage. Generally, seed-reserve nutrients and energy should be adequate for normal seedling development through this early stage.

The embryo and region of root-shoot transition is also a source of hormones essential to normal seedling development, Any hormonal disturbance can affect seedling growth. This may account for the twisting and cupping of the first true leaf on seedlings infected with Pythium.

The Research

Seed lots of Daws, Nugaines and Stephens winter wheat varieties, each harvested in 1985, 1984, 1983 and 1981, were selected for greenhouse studies. Seeds were sown in pasteurized (Pythium-free) soil and pasteurized soil reinfested with two Pythium species, ultimum and irregulare, to common natural soil levels. These two species have been found to be responsible for a high percentage of the early seedling damage, although more than 10 Pythium species have been identified in the region.

All seed lots were certified as 95 to 99 percent "strong sprout" in a 7-day blotter germination test by the Washington State University Seed Laboratory. In the Pythium-free (pasteurized) soil treatment, 94 to 100 percent of all seed lots emerged within 7 days (about 95 growing degree days) after sowing. In contrast, there was an inverse relationship between seed age and seedling emergence in the Pythium-infested soil. The 1985 and 1984 seed had the highest percent emergence, whereas the 1983 and 1981 seed had the lowest. The trend was most evident for Nugaines where emergence progressively decreased with increasing seed age from 1985 to 1981 (Fig. 1).

Fig. 1. Relationship between age of Nugaines winter wheat seed, the blotter germination test (percent strong sprout) and ability of the seedlings to emerge in a Pythium-free (pasteurized) vs. Pythium-infested soil.

Impact of Seed Age

Cook points out that as seed ages, the rate of germination slows and seedling vigor is reduced. Seeds are known to become "leaky" with age as cell membranes degenerate. This allows loss of nutrients and the seed's energy reserves, particularly when it absorbs water at planting in the soil. This leakage from the seed attracts parasites such as the Pythium fungi. Pythium may also have easier entry into the seed as the cell membranes break down with age.

Cook found that even though emergence was good for all seed lots in the Pythium-free soil, seedling height generally decreased with increasing age. This suggests that seed reserves may become progressively more limited as the seed ages.

Treatment of 1981 Nugaines seed with seed protectant fungicides resulted in seedling emergence in Pythium infested soil similar to that of 1985 seed without seed fungicides. Plant height from 1981 seed was still slightly lower than the 1985 seed, however. Cook points out that this is further evidence that the failure of older seeds to emerge is because of pythium infections, probably embryo infections. This finding also indicates the importance of using a seed treatment fungicide if older seed is to be planted at later seeding dates. However, the results reveal just as clearly that using new seed with or without a seed treatment fungicide may provide as good or better control of Pythium damage than old seed with a fungicide.

Factors other than seed age can also affect the vulnerability of the seed and seedling to Pythium. For example, for some unknown reason, the percent emergence of Daws seed was lower for 1983 seed than for 1981 seed. This was possibly due to the seed storage conditions. Preliminary results of research by Cook indicate that seed stored under continuously cool conditions have better seedling vigor and Pythium tolerance than seed stored under warm or fluctuating warm-cool conditions. Consequently, grain storage environment can also be important in management of Pythium and other soilborne diseases.


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