Allelopathy—the inhibition of one organism by another through production of a toxic chemical—has been a common default explanation for the poor growth of crop plants when exposed to the decomposing plant residue, usually fresh plant residue. Allelopathy has also been targeted as a means to control weeds. The connection between symptoms of injury and the presence of decomposing plant residue, including the crop’s own residue, is usually quite clear. The effect is usually then confirmed by growing the sensitive plant in soil amended with the putative toxic plant residue and comparing that plant growth with growth in the same soil without the plant residue.

Hired by the U.S. Department of Agriculture’s Agricultural Research Service in 1965 to develop a research program on root diseases and soil-borne plant pathogens of wheat, and stationed at Washington State University in Pullman, I was confronted with a long-standing explanation that wheat straw was allelopathic to the growth of wheat. This explanation dates back to the introduction of stubble-mulch farming in the North American Great Plains in the wake of the “Dust Bowl”, also known as the “Dirty Thirties.” Growers adopting this new method of farming, where the stubble and other harvest residue was left on the soil surface or mixed into the top few inches of soil, soon noticed that yields of wheat were depressed compared to the traditional complete burial by plowing.

Two competing theories emerged to explain the effect: the straw was toxic or the microorganisms responsible for decomposition of the straw also tied up the available soil nitrogen, thereby starving the crop of nitrogen. When studies in greenhouse pots revealed that wheat was severely stunted when grown in soil amended with fresh chopped wheat straw, and that the stunting effect was not eliminated by adding more nitrogen, the toxin (allelopathy) theory was accepted and would remain the focus of study for the next the 30 years, from roughly the 1950s to the 1980s, including in Australia and the UK. While no toxin was ever isolated and shown to cause the problem, the allelopathy explanation has lived on and still comes up in grower meetings and discussions to this day.

The Injurious Organisms are in the Soil not in the Straw

Two studies conducted in two different fields separated by about 70 miles in eastern Washington, both direct-seeded (no-till), confirmed that wheat yields can be depressed significantly by straw on the soil surface, but showed further that the organism(s) responsible for the yield-depressing effect of straw are in the soil and not in the straw. The study conducted on a farm near Fairfield, WA is described in more detail below, since the results were the same in both experiments.

The field in which the study was done had been planted to wheat the previous year and the stubble and other harvest residue was burned to produce a bare black surface. An area within the field was then divided into four blocks (replicates), each large enough to accommodate six plots each 8 feet by 24 feet (2.4 x 7.3 m). Three plots in each block, picked randomly, were fumigated with methyl bromide under clear plastic tarp.  The other three plots in each block were left natural. Fresh bright wheat straw hauled to the site was also fumigated or left natural.

A few days later with the tarps removed, the entire experimental site was direct seeded into the bare black soil, eight rows spaced 12 inches (30 cm) and 24 feet (7.3 m) long in each plot, and immediately the plots were then covered or not covered with either fumigated or natural straw. The final layout included all combinations of fumigated soil covered with either fumigated or natural soil, natural soil covered with either fumigated or natural straw, and natural and fumigated soil left bare black. The amount of straw added was estimated at about 4 tons per acre. One plot in each block was left non-fumigated and bare black.

Yields of Winter Wheat – Fairfield, WA 1989

Each value the average of four replicate eight-row plots 24 feet long.

Data from: Cook, R. J. and Haglund, W. A.  Wheat yield depression associated with conservation tillage caused by root pathogens in the soil not phytotoxins from the straw.  Soil Biol. & Biochem. 23:1125-1132. 1991.

As shown in the table above, yields were lower and essentially the same in natural soil and fumigated covered with straw compared with natural soil left bare black. This confirms that yields of wheat after wheat can be depressed by straw on the soil surface. However, yields were 25% greater in response to soil fumigation plus added straw and, again, this increase was essentially the same whether the straw was fumigated or left natural. The highest average yield at 109 bu/A (7.57 mt/ha) occurred in plots left bare black and fumigated. The same or even more spectacular results were obtained in the experiment done near Pullman.

The take-all root disease in particular, but also Rhizoctonia root rot and Pythium root rot are common on wheat after wheat in eastern Washington, including at the site where the above experiment was conducted. All three root diseases are favored by cool moist conditions in the top few inches of soil. Removing the harvest residue by burning favors faster and greater drying and warming of the soil layer where these diseases develop, whereas covering the soil with a layer of straw would create soil conditions more favorable to these root diseases. Even soil fumigation does not completely eliminate these pathogens, the take-all pathogen in particular, which can account for the highest yield in fumigated soil left bare black.

Fresh Straw, Especially Fresh Chaff, Serves as a Stimulant for Pythium Root Rot

Pythium species, being such successful parasites of plant roots and rootlets, are ubiquitous in agricultural and garden soils if not all soils regularly colonized by roots. The investigators of toxic straw apparently had no awareness of this fact since their research, having been done with untreated soil taken from wheat fields, would have been done with Pythium present in the soil. Any study done to investigate the role of a food base such as fresh organic material on plant growth and development without accounting for a role of Pythium species will likely lead to an erroneous conclusion.

From left to right:  Natural soil no added chaff; pasteurized soil plus 1% chaff by weight; natural soil plus 1% chaff; and pasteurized soil.

Pythium spores are eliminated from soil by treatment (pasteurization) with moist heat at about 45 C (about 115 F) for 20 minutes. For my experimental system under controlled conditions in a greenhouse, I used fresh wheat chaff added at 1 gm dry chaff to 100 g air-dry soil (from any wheat field) distributed in small containers, planted to wheat and watered. Wheat chaff was claimed to be particularly “toxic” to young wheat plants and to account for the so-called “combine row effect” where wheat is stunted in areas where the chaff falls behind the combine at the time of harvest (before the days of better straw spreaders).

Indeed, wheat seedlings exposed to chaff added to soil were greatly stunted and sickly yellow compared to seedling of the same age and grown in the same soil and under the same conditions but without chaff. As expected if Pythium was involved, plants grew normally and were healthy in soil pasteurized before adding natural chaff. A separate study showed that the population of Pythium species increased many-fold in response to the chaff, presumably using the chaff as a source of sugars and food base. [see Cook, R. J., C. Chamswarng, and W.-h. Tang.  Influence of wheat chaff and tillage on Pythium populations and Pythium damage to wheat.  Soil Biol.  Biochem. 22:939-947.  1990.]

Metaxyl Fungicide Specific for Pythium and its Close Relatives Eliminates the Problem

One of my PhD students, Chiradai Chamswarng from Thialand, conducted an experiment with wheat grown to the tillering stage while exposed to 1% fresh wheat chaff added to the soil.  Rather than pasteurized soil, he drenched the soil with a water-soluble chemical known as metalaxyl specifically inhibitory to Pythium and its closest relatives. He then withdrew the root balls from the pots to expose the roots present at the interface between the soil and the inside edge of the container. As seen in the photo above, the roots were white and obviously healthy and the plants were of normal height grown in soil drenched with metalaxyl but were clearly damaged and missing and the plants stunted when grown in soil drenched with water only.

Quaker Oat Meal also a Stimulant of Pythium as a Pathogen

Just as the earlier work was carried out without awareness of the ubiquitous presence of Pythium species in agricultural and garden soils, the earlier work also looked only at components of wheat straw and not other plant materials that might have served as what, in experimental science, are called controls. For one control, I used Quaker oatmeal from our kitchen cupboard, added dry to soil just as dry chaff was added, at 1 gm per 100 gm of air dry soil planted to wheat and watered. The oatmeal was every bit as “injurious” to the young wheat plants as was chaff, for the same reason:  it served as a source of nutrients and food base for Pythium species.

The Oatmeal Study Works as Science Project for Students

My granddaughter, Jessica Brockmeyer–when seven years old and in the first grade–used the same experimental system I used for wheat chaff and oatmeal except she used soil from her mother’s garden sieved through quarter-inch hardware cloth to remove plant material and break up clods. She also used peas rather than wheat as her test plants, being that peas are super susceptible to damping off caused by Pythium. She did her experiment with pea seeds planted into small plastic drinking cups filled, respectively, with natural soil with or without oatmeal chopped in a coffee grinder, and soil pasteurized in a double boiler (microwave would have worked just as well or better) with or without oatmeal. She had four cups for each treatment and five pea seeds planted per cup.

Not a single pea seedling emerged from any of the four cups containing natural soil with added oatmeal (see C-1; labeled + organic matter above). In contrast, all five seedlings emerged from either heat-treated on natural soil with no added oatmeal. A few seedlings failed to emerge in pasteurized soil amended with oatmeal, presumably because of Pythium spores that escaped the heat treatment.  The seeds that did not germinate were mushy and encased in soil, typical of Pythium damping off.

The fact that this experiment using a garden soil produced the same results obtained with a wheat field soil confirms the ubiquitous presence of Pythium species in soil. This experiment should work with any soil subjected to normal plant vegetation.

A Parallel from Medicine

My father, Robert D. Cook (1906-1990), suffered most of his adult life with peptic ulcers. The diagnosis during his day was that ulcers were caused by stress and an acidic stomach. Accordingly, he was on a regular regiment of anti-acid products and was even hospitalized at one time.  Then two physicians in Australia discovered that peptic ulcers are caused by Helicobacter pylori and the treatments changed to antibiotics.

Accepting the disappointing performance of wheat when exposed to fresh wheat straw and especially chaff, or when grown under a wet blanket of straw, as allelopathy or nitrogen tie-up is like being treated for an allergy or nutrient deficiency when all the time the illness was due to an infection.

Just as treatments for peptic ulcers can now proceed on a scientific basis because of the correct diagnosis, so the yield depression of wheat caused by wheat straw can now proceeds and is proceeding on a scientific basis because of the correct diagnosis—root diseases not toxic straw.