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Chapter 4 - Disease Control, No.3, April-May 1985

Pythium Root Rot: Limiting Pacific Northwest Wheat Yields

Roger Veseth

Pythium root rot has been shown to be an important limiting factor in winter wheat production in the Pacific Northwest and other regions of the country. Its impact has gone largely unnoticed, however, because it affects the overall root health and does not cause distinctive plant symptoms as do other diseases such as Cephalosporium stripe or leaf rust. Because Pythium damages root tips, branch roots and the fine root hairs, the plant stress has often been mistakenly attributed to low nutrient availability, straw toxicity, cold soils or other environmental factors.

Through the use of soil fumigants as a research tool, STEEP researcher R, James Cook, USDA-ARS plant pathologist at Pullman, WA, has been able to diagnose and determine the effect of Pythium root rot. After 10 years of research, Cook has shown that the 15 to 25 percent wheat yield response with soil fumigation trials is largely due to Pythium control. His work shows that the infection begins very early in the life of the plant, probably even before the seed germinates and certainly before the seedling emerges.

Near Pullman, winter wheat seeded conventionally in mid-October 1983 yielded 90 bushels/acre in a 2-year wheat-pea rotation. Where 75 to 90 percent of the Pythium population was eliminated by either Telone C or chloropicrin soil fumigants, yields were increased to 110 bushels/acre. In a 3-year wheat-spring barley-pea rotation, with similar seeding date and conventional tillage, winter wheat yielded 109 bushels/acre with no soil treatment and 129 bushels/acre with soil fumigation. A major factor leading to the 20 bushels/acre yield increase in both the 2-year and 3-year rotations was an earlier, more vigorous spring growth and significantly more tillering of the wheat where Pythium had been eliminated. The 19 bushels/acre yield difference between the 2-year and 3-year rotations (90 to 109 bushels/acre) on nonfumigated sites was attributed in part to a higher incidence of Cephalosporium stripe and strawbreaker foot rot (Pseudocercosporella) in the 2-year rotation, and in part to more soil water available at the 3-year rotation site.

Research by Cook in 1984 showed that soil treatments which reduced Pythium inoculum levels in the top 2 inches of soil increased winter wheat yields from 85 to 98 bushels/acre. This illustrates the potential for early infection when sufficient inoculum is present in the seed zone, Besides root damage, Pythium fungi can also reduce stands by attacking the seed as it is germinating, causing seed decay or seedling blight before or immediately after emergence. Winter wheat seedlings weakened by Pythium can result in greater loss of plants from winterkill and slower renewed growth in the spring. With heavy plant infection levels, the first 1 to 3 tillers may be skipped because of the plant stress, resulting in lower yields. The first few tillers are generally the highest producing.

Pythium root rot has been shown to limit wheat yields regardless of the tillage system used. However, since the fungus is favored as a pathogen by cool wet soils and it benefits from chaff and straw as a food source, Pythium can be especially important in higher surface residue systems. Where the Pythium population was reduced with soil fumigation, wheat yields under no-till equaled or exceeded yields with clean conventional tillage. Cook points out that, where Pythium is controlled, the yield potential is actually higher under no-till than under conventional tillage because of the increased soil water storage efficiency with surface residue.

The presence of Pythium in Pacific Northwest cropland and its importance on wheat yield has been proven. Cook's research is now focusing primarily on developing control alternatives. Four control approaches will be explored: chemical seed treatments, low rates of deep banded soil fumigants with the grain drill, biological controls and varietal resistance.

Chemical Seed Treatments

The chemical metalaxyl effectively controls about half of the 10 Pythium species presently identified in soil of this region. Fortunately, it appears that the species controlled by metalaxyl are the more prevalent and damaging species. Metalaxyl is marketed as a seed treatment for Pythium control under the trade name Apron, Apron has given an average yield increase of 5 to 10 bushels/acre and the response is most consistent where wheat follows wheat or barley in the rotation. Metalaxyl is also effective as a soil treatment but it is not economical for small grain production,

Low Rates of Banded Soil Fumigants

Using soil fumigants to eliminate Pythium has been a very valuable research tool and was not intended as a method to control the disease for small grains in the field. However, a new approach to the use of soil fumigants in grain production was initiated by Cook and Bill Haglund, WSU plant pathologist, on spring wheat in 1984 near Pullman. A very low rate of chloropicrin (1 gal/acre) was injected with the fertilizer in a deep band through a no-till drill at planting time. A 5 X 15 inch paired-row spacing was used with the fertilizer-fumigant band 4 to 5 inches deep between the pair of seed rows 5 inches apart. The treatment eliminated about 50 percent of the Pythium inoculum from the top 6 inches of soil between each 5 inch pair of rows. Yield was increased by an average of 9 bushels/acre — 55 bushels/acre compared to the check at 46 bushels/acre.

Biological Controls

Research on biological control of Pythium has shown some promise. In 1984, USDA-ARS plant pathologist Dave Weller and Cook found two strains of rhizobacteria inhibitory to Pythium that increased wheat yield. But this occurred only when the soil was amended with lime to reduce soil acidity. Soil pH was raised from 5.6 to about 6.0. An 8 bushels/acre average yield increase was found with both the rhizobacteria and lime, but response with the lime or rhizobacteria alone was not significant.

Plant Resistance

Selection of wheat varieties showing resistance to Pythium will initially be made through greenhouse studies using soil naturally infested with Pythium. The same soil, steam treated at 140F for 30 minutes to eliminate Pythium spores, will be used to produce healthy seedlings of each-variety for comparison. This will enable rapid evaluation of a large number of varieties with close control over Pythium and other microbial populations.

These four control approaches will be the primary focus of Cook's future Pythium research, but other management influences such as crop rotation and fertilizer placement will also be included. The development of management options for Pythium root rot will increase wheat yield potential under both conservation tillage and conventional tillage in the Pacific Northwest.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the 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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