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Chapter 4 - Disease Control, No. 1, October-November 1984

Management for Take-all and Pythium Root Rot Control

Roger Veseth

Take-all and Pythium root rot are the two main root diseases of wheat in the Palouse region. Both diseases are favored by recropping wheat after wheat or barley, especially under reduced tillage. Development of controls for these two diseases is the purpose of a STEEP research project under the direction of R. James Cook, USDA-ARS plant pathologist at Pullman, WA. This project is part of an overall research effort to improve the root health of wheat and barley grown under intensive management systems.

Take-all is mainly a problem in irrigated wheat, although in recent years the disease has caused significant losses in annual cropped no-till wheat in the Palouse. Symptoms include blackened roots, In severely diseased plants, the lower 1 to 2 inches of the stem may also be blackened by the fungus. Plants infected early may be stunted and appear nutrient deficient because the damaged roots are inefficient in uptake of nutrients. When the fungus rots the crown and lower stem, the plant dies from lack of water and will have "white heads. " The take-all fungus survives in the crop residue and is especially successful when crowns of infected plants are left undisturbed, as with notill. The fungus can grow from this large residue food base directly into the plants of the succeeding wheat crop.

The best control for take-all is crop rotation. Do not grow spring or winter wheat after spring wheat, winter wheat or barley. One crop year out of wheat or barley, such as fallow or pea, effectively controls take-all. Barley is susceptible to take-all infection but has not shown significant yield losses, Tillage is an option for take-all suppression only if a rotation is not followed; but the best option is crop rotation.

The severity of take-all eventually decreases with prolonged cereal recropping (take-all decline), because of a natural biological control by other soil microbes. This. usually takes 1 to 3 years after the first year of serious take-all infection. Unfortunately, noncereal rotation crops break this cycle so that when wheat is again grown, severe take-all will occur, and another 1 to 3 years will be needed for the disease to decline again.

Cook and David Weller, also a USDA-ARS plant pathologist at Pullman, have been working to develop this natural biological control. Root-colonizing bacteria antagonistic to take-all were obtained from soils where take-all had declined and applied as a seed treatment at planting time. Their research has shown that the seed treatment will give 10 to 25 percent yield increases about 50 percent of the time. Private industry has recently started developing commercial seed treatments with the antagonistic bacteria for suppression of take-all.

Research on other management options considered in take-all control includes the application of ammonium, chloride and phosphorus fertilizers. Ammonium-based nitrogen fertilizer, particularly with N-Serve to keep it in the ammonium form longer, has been shown to suppress take-all. The increased acidic soil environment created by the ammonium fertilizer is not conducive to the take-all pathogen. The success of ammonium fertilizer in take-all suppression is best in slightly acid soils (pH 5.5 to 6.5), with little or no effect in alkaline soils or acid soils that had been recently limed.

Research by Oregon State University in the Willamette Valley has shown that addition of potassium chloride with the seed at planting has significantly suppressed take-all. The scientists believe this is also a result of a more acid soil environment created by the chloride. Broadcast or banded applications of ammonium chloride also look promising for take-all suppression.

Good phosphorus fertility can also give some control of take-all. Cook has found that in fields where take-all is the important yield-limiting factor, additions of about 40 pounds per acre PzO~ with or banded below the seed has consistently increased wheat yields 5 to 10 bushels per acre.

Pythium root rot is a problem mainly on late-seeded winter wheat. The fungus lives as spores in the soil. It may thin a stand of wheat by causing seed decay or seedling blight before or shortly after emergence. Pythiwn also damages the fine rootlets (feeder roots) of the plant resulting in spindly plants with shortened and distorted leaves, uneven height, fewer tillers and smaller heads. The plants also appear poorly nourished. Cook points out it is now clear from the research results over the past 10 years that the 15 to 25 percent yield response in soil fumigation trials is due mainly to Pythium control. This illustrates the important impact that Pythium has on wheat production in this region.

Pythium is more active when straw and chaff are left on the soil surface (as with no-till), or mixed with the top few inches of soil (as with minimum tillage). The fungus is favored as a pathogen by cool, wet soils, which is typical of soils with surface residues. It uses nutrients from straw and chaff as a food source. In the soil fumigation studies where Pythium was eliminated, winter wheat performed equally well under no-till and conventional tillage.

Work on Pythium control has involved a search for chemical and biological controls. Several compounds have been found to protect against seed rot and seedling blight phases of this disease. The compound must be systemic, however, or must somehow move or be placed into the root zone beneath the seed to be effective against the root damage caused by Pythiwn. Unfortunately, many Pythium species are involved in this root-rot complex and a compound may be effective against some but not all species. Currently 10 Pythiwn species have been identified as part of the seed-rot or root-rot complexes. The compound metalaxyl as a seed treatment controls about half of the Pythium species which makeup a large percentage of the total Pythium population in some fields of the Palouse. Metalaxyl is marketed under the trade name Apron, now a currently registered seed treatment for Pythium control. In crop rotations where wheat follows other cereals, especially with no-till or minimum tillage, yield increases of 5 to 10 bushels per acre have been obtained with Apron seed treatment.

The effect of crop rotation on Pythium control is uncertain,and it is currently under investigation. Preliminary evidence indicates Pythium is less damaging in a 3-year rotation than a 2-year rotation or continuous wheat.

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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