R. James Cook

USDA, Agricultural Research Service

Root Disease and Biological Control Research Unit

367 Johnson Hall, WSU

Pullman, Washington 99164-6430

Achieving the full production capability of a crop such as wheat or barley, with maximum fertilizer-use efficiency and ability of the crop to compete with weeds, depends critically on healthy, fully functional roots to support and sustain the plants through all stages of their growth and development. The health of the root system can determine seedling vigor, the amount of tillering by each plant, the size of the heads, and of course the amount of grain fill as the crop matures. Each plant depends on its roots to find and absorb water and nutrients. In the case of relatively immobile nutrients such phosphorus, which diffuse through soil too slowly to meet the needs of the crop the roots must be healthy enough to grow to the nutrients. A dense root system that literally "takes possession" of the soil and applied fertilizers also leaves less openings in the soil for the establishment of weeds.

Growers wishing to plant wheat or barley directly into the stubble of wheat or barley must be aware of several soil borne pathogens responsible for poor root health and hence the all-too-common, disappointingly poor performance of wheat and barley in this kind of management system. Crops managed in this way can appear "under fertilized," poorly tillered, stunted or uneven in height, and produce small heads. Often the plants are poorest in the old "combine row," which can now be explained on the basis of volunteer plants and grass weeds in this row having served as sources of inoculum of root pathogens. This general problem was formerly and incorrectly attributed to putative "phytotoxins" thought to be liberated from decomposing straw. To my knowledge, all research on the toxin theory to explain this problem has now been discontinued virtually worldwide, and scientists working on this problem are now more appropriately focusing on the root pathogens.

The root pathogens of wheat and barley diagnosed thus far are fungi specialized in their ability to parasitize and eventually rot roots. The most specialized of these is the wheat take-all fungus, Gaeumannomyces graminis var. tritici, which can attack the roots of wheat, barley, quackgrass, downey brome, cheat, and certain other grasses closely related to wheat and barley. Wheat take-all fungus cannot attack oats, although a special strain, Gaeumannomyces graminis var.avenae, can attack oats. The least specialized are the Pythium spp., which invade the germinating seeds, root tips, and small (fine) lateral roots of many kinds of plants. The parts of the root system destroyed by this group of fungi are especially essential if the root system is to fully explore the total volume of soil containing phosphorus and other immobile nutrients Rhizoctonia solani AG8 is intermediate in specialization it thrives on barley and wheat but can also attack canola (or oil seed rape), peas and lentils commonly used in rotations with wheat or barley in the Pacific Northwest.

The wheat take-all fungus, Pythium spp., and R. solani AG8 all are favored by cool, wet soil conditions. This is important in understanding why these particular root pathogens are favored by leaving crop residue on the soil surface. Take-all and Pythium spp. are common and widely recognized in the Great Plains. Thus far, R. solani AG8 has been found in the Northwest, including in Montana, but not in North Dakota or Manitoba.

The two most familiar root pathogens of wheat and barley in the Northern Great Plains are Cochliobolus sativus and Fusarium culmorum, C. sativus, like the wheat take-all fungus, is specialized for wheat barley, and certain closely related grasses. F. culmorum attacks oats as well as wheat, barley, a wide range of grasses, and even some unrelated crop plants such as asparagus and possibly clovers. C. sativus and F. culmorum are causes of the root disease known as common root rot and are more important in drier soil environments and when their host plants are under water stress. This information is useful in understanding why common root rot may not always be favored by leaving crop residue on the soil surface (discussed below).

The root pathogens of wheat and barley were probable, in our soils, especially the prairie soils, long before farming began in North America. They probably lived on the roots of native grasses. Those with the ability to also live on the roots of small grains have now become important as root pathogens of our small grain crops as these kinds of plants replaced the native grasses as the dominant vegetation on this land. These same root pathogens are not important in prairie soils planted to soybeans or other crops with roots that these fungi do not or cannot attack.

All root diseases of wheat and barley are favored by planting wheat and barley year after year, or at least every other year, on the same fields. In addition, take-all, Pythium root rot, and Rhizoctonia root rot, being favored by cool, moist soil, gain an additional advantage when harvest residues are left on or mulched into the soil surface. In other words, straw left on the soil surface helps to maintain the soil in an ideal condition for the activity of these three root pathogens. It is important to recognize that the root pathogens live mainly in the top 4-6 inches of soil--the first layer to dry and warm up after a rain or snow melt but less likely to dry and warm up if covered with straw.

With take-all, for example, removing the straw by burning in a no-till experimental planting of winter wheat after winter wheat near Pullman, Washington, reduced the incidence of disease, but burning the straw and then returning fresh straw to cover the burned-over soil surface with fresh straw resulted in the same high incidence of take-all as in the nonburned plots. This work shows that burning had no lasting effect on the take-all fungus in the soil.

The highest potential for root diseases is continuous no-till (zero-till) wheat or barley. The less the soil is disturbed, the more firmly entrenched and longer lived are the pathogens in the old roots and crowns left in the field after harvest. Gradually, these root pathogens become displaced or robbed of their food supply by the decomposer microorganisms in the soil. Slowly, the root pathogens lose assets as their home (stubble and old roots) decomposes, but then --enter volunteer plants and grass weeds. The pathogens grow from their foodbase of decomposing and fast-disappearing roots and crown tissues of one crop onto the young roots of volunteer and grass seedlings. They now reverse the trend of disappearing assets and begin to accumulate assets. When these volunteer plants and weeds are then sprayed with glyphosate, the little natural resistance they have to the pathogens begins to break down, and you might say that the pathogens in them become "millionaires" almost overnight. Enter the next crop planted directly into this unsanitary mess. Every place where volunteer plants and grass weeds once thrived and were sprayed just ahead of planting can show up later as a thin stand of stunted, chlorotic plants--caused by root diseases.

Common root rot being favored by dry soil conditions and crop water stress tends not to become more important on wheat or barley grown with maximum residue on the soil surface. However, I must emphasize the word "tends" because there can be exceptions. C. sativus forms its spores on infected stubble, and leaving stubble standing can contribute to later production of spores by this fungus. If the amount of inoculum rather than environment is the limiting factor to common root rot, then more inoculum associated with surface residues has the potential of resulting in more common root rot with conservation tillage.

The most sure way to control the root diseases of wheat and barley, regardless of tillage system, is to use crop rotation. Unfortunately these pathogens can live up to two years, sometimes longer, in soil in the absence of the roots of wheat or barley. Soils in the northern Great Plains and the inland Pacific Northwest tend to be too cold when wet enough for rapid breakdown of crop residue harboring the pathogens, or too dry when warm enough for rapid breakdown of these residues. Given enough time, however, the "soil will sanitize itself."

The safest way is to use a three-year rotation of wheat one year and crops not related to wheat the other two years. If barley is used with wheat in a three-year rotation, the barley should follow the wheat and the nonhost crop (e.g. lentils, corn, canola, sunflowers) should follow the barley. In barley, the take-all fungus, as well as Rhizoctonia and Pythium, remain confined almost entirely to the roots. In wheat, the take-all fungus and occasionally Rhizoctonia can establish in the tiller bases (crowns). Root tissues in soil decompose faster than tiller bases. Thus, a three-year rotation of wheat-barley-nonhost provides the necessary two years for the disappearance of wheat tiller bases containing root pathogens and the necessary one year for disappearance of barley roots containing root pathogens. Under most circumstances, three-year crop rotations are so effective in root disease control that other disease control strategies such as through tillage or residue management become redundant.

Without benefit of a three-year rotation, e.g. if wheat or barley is grown every year in the same field or alternated with chem fallow, root disease management becomes more difficult but is still possible to some degree or another. Several practices have shown positive results and should be considered as part of a total management program.

1. Control volunteer and grass weeds early, and wait at least 10-14 days after treatment with glyphosate before planting. This "spray-wait" procedure allows time for the root pathogens to reach their peak and then decline as the young roots occupied by these pathogens are eliminated by the decomposer microorganisms in soil.
2. Loosen the soil in the seed row at the time of planting. There is a growing body of scientific evidence that pathogens such as R. solani can be managed by use of tines or shanks ahead of disk openers, or by use of a hoe-type opener. It is important, however, to loosen (disturb) the soil 2-3 inches below the depth of seeding, precisely where the seedling roots will grow first and are vulnerable to attack.
3. Place fertilizer and especially phosphorus in the seed row directly below seed. Phosphorus can be placed with the seed but nitrogen must be 2-3 inches below the seed. This kind of placement is necessary where root disease are important so that even diseased roots can access the nutrient. With good root disease control, fertilizer placement is less critical.

Seed treatment chemicals can provide some control of root diseases, e.g. imazalil for common root rot, Apron for Pythium, Baytan for take-all, and PCNB for Rhizoctonia root rot. However, these chemicals serve largely to protect the germinating seed and young seedling and do little to control root rot.

Some hard red spring wheat varieties have modest resistance to common root rot, but no wheat or barley varieties have resistance to take-all, Rhizoctonia root rot or Pythium root rot. Several laboratories, including my own, are investigating the potential for biological control with bacteria introduced on the seed, which establish in the root zone where they inhibit the targeted pathogen(s). This work shows promise but is not yet at a stage for use on a large scale.