Rhizoctonia Root Rot Management Considerations

Chapter 4 -Disease Control, No. 14, Winter 1988

Roger Veseth

Cereal production in the Inland Northwest faces a new management challenge with the recent identification of Rhizoctonia root rot. The disease was first identified in a few wheat and barley fields across Oregon, Idaho and Washington in 1984. Since then, Rhizoctonia root rot has been found in many areas of the region, and a research program has been under development to work on this disease.

Rhizoctonia root rot appears to be caused by an indigenous soilborne fungus whose effects on cereal production have been overlooked in the past. Like other once-minor pest problems, however, this disease may also be increasing in response to subtle changes in crop rotation and management of crop residue and weeds over the past decade.

Although newly identified here, Rhizoctonia root rot has been documented in Australia for more than 60 years and is also known in several other countries. Preliminary indications suggest that the effects of this disease on cereals in the Inland Northwest may be greater in the lower and intermediate precipitation zones. However, the disease is favored by wet soil conditions and may also be a problem in irrigated cereals. It has been most commonly identified in recropped cereals under reduced tillage systems but has been found under conventional tillage as well. Consequently, the study of Rhizoctonia root rot and the search for management options have been added to the STEEP conservation farming research effort in the Northwest.

Several STEEP researchers have begun to focus considerable attention on this disease. Two of the plant pathologists leading this effort are R. James Cook with the USDA-Agricultural Research Service at Washington State University in Pullman and Richard Smiley with Oregon State University at the Columbia Basin Agricultural Research Center near Pendleton. They are coordinating their efforts and have an interdisciplinary research program under development with other researchers. The researchers emphasize that increased support will be needed to adequately address this disease problem.

To help with the early investigation of this “new” disease in the Inland Northwest, two world-authorities on Rhizoctonia were brought to Washington State University: Akira Ogoshi from Japan in 1986 and A. D, Rovira from Australia in 1987. Their assistance has been instrumental in helping to identi~ the Rhizoctonia strains present in the region as well as possible environmental and management factors associated with the disease.

Pathogens and Susceptible Hosts

Rhizoctonia root rot of cereals is caused by at least two Rhizoctonia species of fungus: Ogoshi identified these as R. solani (strain AG-8) and R. oryzae. Strains of R. solani, which cause Rhizoctonia root rot of beans, potatoes, sugarbeets and other crops, have also been identified in the Inland Northwest, but these are different from the strain or strains of R. solani that attack wheat and barley. However, it appears that the R. solani strain AG-8, which most severely affects wheat and barley, may also have some affect on peas, chickpeas, lentils and rapeseed.

Preliminary research by Cook and his group indicates the R. oryzae maybe even more pathogenic to wheat and barley than R. solani. This species also occurs as a multitude of strains pathogenic on different crops including rice and corn. Distribution of the different Rhizoctonia species and strains in the region is largely unknown. Some locations appear to have one dominant species or strain while others have a mixture. It is presently unknown if the different species and strains respond differently to management options.

Although diagnosed on both wheat and barley in the region, severe cases of the disease have been found more commonly in fields of spring barley than either spring or winter wheat. Based on Rovira’s research in Australia and preliminary field observations in the Inland Northwest, the researchers think that many of the grass weeds encountered in cereal production may also be hosts of the pathogen. Downy brome, jointed goatgrass and other major grass weeds may be particularly important in building or maintaining high levels of the pathogen, particularly between cereal crops. Some broadleaf weeds may also be hosts of the pathogens.

Other native and introduced grass species may also be Rhizoctonia hosts. Smiley and Dale Wilkins, USDA-ARS agricultural engineer at Pendleton, have initiated a study to determine methods for reconverting affected grasslands in the Conservation Reserve Program (CRP) back into cereal production in the future. The impact of Rhizoctonia root rot potential on cereals after the 10-year period of grass may be a concern.

Disease Symptoms and Survival

The researchers think that, like infection of cereals by Pythiwn species, infection by Rhizoctonia begins early in plant development, probably during seed germination and while the plants are still in the seedling stage. Root growth rates can be significantly affected within 3 weeks after seeding. The most visual diagnostic symptoms are brown sunken lesions which girdle and eventually sever the roots, typically leaving the roots as pinched-off, pointed brown stubs, commonly referred to as “spear points” (Fig. 1).

Severely infected plants can easily be pulled out of the soil because so many of the roots are severed, usually in the surface few inches of soil. The amount of root damage varies with the fungus strain, population and ”energy status” of the pathogen in the field. Plant symptoms may be confused with nutrient or water deficiencies because of the impaired root systems.

The disease ranges from a chronic or mild form of damage responsible for slight stunting and reduced tillering to a severe form that either kills plants between germination and maturity or prevents plant growth. In the chronic form, random plants are stunted and the disease often goes unnoticed. At this level, the disease mayor may not translate into reduced yield. The severe form of the disease, which generally results in significant yield losses, tends to occur in patches and has been known as “bare patch” or ‘crater disease” in other countries. Patches typically range in diameter from a few feet to more than 50 feet.

The disease effects on yield are difficult to measure because of the extreme range in disease severity encountered and the irregular size, shape and distribution of the patches. Smiley used a combination of aerial photography and field samples to estimate the effects of Rhizoctonia root rot on yield in two patch-affected fields in northeastern Oregon in 1987. One field was Dusty winter wheat and the other was Steptoe spring barley. The wheat field had been tilled twice with a chisel plow, cultivated once and rod-weeded four times in a stubble mulch tillage system under a wheat fallow rotation. The barley was recropped annually and was also under a stubble mulch tillage system.

Compared to adjacent non-stunted areas in the field, yields inside the patches were lower by 52 percent for wheat and 46 percent for barley. Patch-affected plants constituted at least 8 percent of the total area in wheat field and 17 percent in barley field. Assuming that Rhizoctonia root rot caused no yield loss in the non-stunted areas (which is unlikely), yield losses because of the patches were 4 percent (3.3 bu/acre) for wheat and 7,5 percent (200 lb/acre) for barley.

The most important food sources for the Rhizoctonia strains pathogenic on wheat and barley are the living roots of the many crop and weed species susceptible to their parasitic attack. In the absence of living roots, such as during a season of clean chemical or tillage fallow, the’ ‘energy status” of the fungus drops rapidly in soil. Cook indicates that the continued presence of living plant roots, or what he calls the “green bridge, ” appears to be vital to the build-up and maintenance of damage levels of Rhizoctonia. The fungus also lives, probably to a lesser degree, on crop residues and even organic in the soil.

Cook states that the chronic forms of Rhizoctonia root rot are probably the result of these fungi occurring in soil in low populations or in a state of near-starvation. However, like Pythiwn root rot, the chronic form of Rhizoctonia root rot can still occur and be so uniform and subtle that the appearance of the crop is accepted as normal.

Management Options

Crop Rotation

Severe cases of Rhizoctonia root rot, evident as patches, have largely been associated with recropping of wheat or barley after these cereals. The continual presence of the living roots of wheat, barley or other susceptible hosts, including the roots of volunteer and associated grass and broadleaf weed hosts in and between crop seasons, is thought to provide the high “energy status” needed by Rhizoctonia to cause severe disease. Although some alternate crops may be hosts for the pathogen, different herbicide options for weed control, tillage practices and variations in actual crop susceptibility to the pathogen may reduce the risk of this disease in cereals after alternate crops.

Preliminary field investigations in the Inland Northwest indicate that the most effective rotation control of Rhizoctonia root rot, regardless of tillage system, is a full season of summer fallow. However, to achieve the full benefit of this control, growth of volunteer cereals and weeds must be kept to an absolute minimum through chemical fallow, mechanical tillage or some combination of chemical and tillage weed control. This may be particularly important for the few weeks preceding planting.

Preplant Herbicide Timing

A potential management option for reducing the effect of Rhizoctonia root rot on a cereal crop is to lengthen the period of time between the application of a preplant herbicide, such as Roundup (glyphosate), or Bandmaster (glyphosate plus 2,4-D), to control volunteer and weeds and seeding the crop.

From research by Rovira in Australia, and limited field observations and research in the Inland Northwest, it appears that Rhizoctonia invades the roots of plants dying from a preplant herbicide applied shortly before planting and gains energy from that foodbase for attacking the next crop. As the plants become weaker because of the herbicide, Rhizoctonia more easily penetrates the roots and takes advantage of this food source.

Research evidence to support this in the Northwest was obtained from a field experiment near LaCrosse, WA in 1987 initiated by Alex Ogg, USDA-ARS weed scientist at WSU. Spring barley was seeded after winter wheat where the stubble, volunteer and weeds (mainly downy brome) were left untouched overwinter until the experiment began in late March. Glyphosate and 2,4-D amine alone or in two commercial mixtures as Bandmaster and Bandmaster II were applied either 12 or 3 days before seeding, the day of seeding or not at all. The entire area was then disked shallowly, rod-weeded and seeded with a conventional, double-disk drill in 1 day.

Rhizoctonia root rot infected the barley in all plots but was most severe in plots treated 3 days before planting with herbicides that contained glyphosate. Similar results would be expected from other herbicides that would kill volunteer and emerged grass weeds. Barley in plots treated 12 days before planting yielded 26 and 43 percent more, respectively, than barley in plots treated the day of planting or 3 days before planting.

Similar findings on spring barley were reported by Smiley in an experiment at the Columbia Basin Agricultural Research Center near Pendleton, OR. The previous crop was winter wheat and 2 years of spring barley preceded the winter wheat. Spring barley was no-till seeded on March 30. Roundup was applied to volunteer and weeds at six intervals before and after planting. The percentage of plants stunted by Rhizoctonia and the crop yields were measured. Only the Roundup treatment 3 days before seeding resulted in numbers of stunted plants and yield that were significantly different from the other treatment intervals (Table 1).

Cook explains that applying a preplant herbicide for control of emerged weeds and volunteer well in advance of seeding (if possible) is thought to help avoid the Rhizoctonia root rot in two ways: (1) the plants are smaller then, and therefore present a smaller potential root foodbase for Rhizoctonia, and (2) the fungal mass generated on dying or dead plants probably peaks and is declining again by the time the new crop is planted. He stresses that more research is needed on this management option since it is based only on limited field observations and 1 year of research in the Northwest.

Research in Australia indicates that some soil-active herbicides applied for weed control in wheat and barley may weaken the crop sufficiently to predispose roots to infection by Rhizoctonia. Preliminary observations in this region have been inadequate to properly evaluate those research findings under Northwest conditions. Smiley has two cooperative field studies underway to explore possible interactions between soil-active herbicides and Rhizoctonia root rot.

Tillage

In general, field observations and preliminary research in the Northwest indicate that the incidence of Rhizoctonia root rot decreases with increasing intensity of tillage. However, it is difficult to predict the effect of tillage method on disease severity and, more importantly, the influence of disease on crop yield. Some of the factors that influence the disease include the complicating interactions of herbicide timing, populations of volunteer plants and weeds, crop rotation, precipitation level and available water for the crop, density and distribution of inoculum of the fungus, timing of tillage and other factors.

Table 1. Influence of Roundup application date on percent of plants stunted by Rhizoctonia root rot and yield of no-till spring barley, 1987, Pendleton, OR (Smiley-OSU, Pendleton).

As with preplant herbicides, the effect of the tillage system on Rhizoctonia root rot maybe related, at least in part, to the’ ‘green bridge” of volunteer and weed roots which host the pathogen between crops. Field observation by Cook indicate that the preplant herbicide/tillage system selected for reseeding winterkilled winter wheat or winter barley fields strongly affects the potential for Rhizoctonia root rot.

For example, in Adams and western Whitman counties in Washington in 1986, severe Rhizoctonia root rot was documented where winter-damaged fields received an application of glyphosate 1 to 3 days before seeding spring barley. Some tillage was used before seeding, but it was shallow and mostly intended to smooth out the ridges left by the deep furrow drill used the previous fall. In addition to the fungus invading roots of herbicide-affected plants, it may also have increased its “energy status” on the roots of plants killed or weakened over winter. In general, the more intensive the tillage used to eliminate surviving plants and prepare a seedbed before reseeding in this two-county area, the milder the Rhizoctonia root rot. However, the higher disease severity associated with less tillage may have been more a response of application of glyphosate shortly before seeding than to the lack of intensive tillage.

Field research on the effects of tillage systems on Rhizoctonia root rot was conducted by Smiley and Wilkins in 1987 in a 10- to 14-inch precipitation zone near Pendleton and Holdman, OR (Table 2). Three tillage systems were evaluated on two fields of recrop winter wheat and winter barley where the previous crop was spring barley. All three treatments were seeded with a USDA-ARS experimental hoe-type drill, which placed the fertilizer 2 inches below the seed at a 14-inch seed row spacing. Although the percentage of plants stunted by Rhizoctonia are higher with no-till than the plow/disk treatment, yields were also higher. Yields in roto-tilled and no-till treatments were comparable. Low yields in the plow/disk treatments were believed to be the result of soil water loss from the tillage operations in this recrop system.

The researchers have numerous long-term tillage and rotation experiments underway to develop a better understanding of tillage-disease relationships. They will evaluate possible adjustments in primary and secondary tillage strategies to reduce the incidence of this disease without compromising the objective of conservation tillage systems.

Table 2. Yield and percent of plants stunted by Rhizoctonia root rot in winter wheat and winter barley recropped after spring barley under three tillage systems near Pendleton and Holdman, OR (Smiley and Wilkins, OSU and USDA-ARS, Pendleton).

Seed Treatment

No fungicidal seed treatments are registered for control of Rhizoctonia root rot. Smiley conducted 10 trials at four Oregon locations in 1987 to evaluate 13 combinations of commercial and experimental fungicidal seed treatment. Yields of recropped, no-till Stephens winter wheat and Steptoe barley (planted in both the fall and spring) were not influenced by the treatments as compared to the untreated seed. In addition, no consistent differences between plant growth or disease incidence were found among treatments.

Two experimental fungicides did show activity against R. solani in field trials by Cook in 1986, so there is still some possibility that chemical seed treatments could be developed to assist in the control of Rhizoctonia root rot. These and other non-registered fungicides are currently being evaluated in field and greenhouse studies. The potential for biological seed treatments is also being explored.

Control of Root Rot Mixtures — Cook points out that combinations of microbial or fungicidal seed treatments, cultural practices and plant resistance will have to be developed to control the mixtures of root diseases found in the region. His research has shown that Rhizoctonia and Pythium root rots often occur as mixtures in many fields in the intermediate and high rainfall areas of the Inland Northwest. Take-all is also a component of the mixture if wheat follows wheat or barley in areas with 20 to 24 inches of annual precipitation (18 to 20 inches if no tillage is used). Rhizoctonia and Pythium occur individually or in various mixtures in areas too dry for take-all or where peas, lentils or some other crop not susceptible to takeall is used in rotation with wheat or barley.

Cook’s research has shown that in fields where Pythium root rot is the dominant root disease, and soils at the time of seeding are cool, wet and favorable for Pythium, the application of a fungicide that is selective for Pythium (such as Apron) can produce a significant yield increase. However, where Pythium and Rhizoctonia root rots occur together, his research indicates that control of only Pythium with the Apron seed treatment can have a negative effect on growth and yield because of increased damage from Rhizoctonia. Conversely, he has found that experimental fungicides which specifically control Rhizoctonia and not Pythium can result in increased damage and yield loss from Pythium where these two diseases occur in mixtures.

Varietal Resistance

Cereal varieties with resistance or tolerance to Rhizoctonia have not been identified. Smiley and cooperating researchers initiated a study in fall 1987 to determine if relative differences in root rot tolerance occur among winter wheat cultivars or advanced breeder lines that have widely divergent parentage.

Conclusions and Preliminary Recommendations

Rhizoctonia root rot of cereals appears to be most important in the lower and intermediate precipitation zones in a recrop cereal rotation, particularly in wetter areas and during years with above normal precipitation. Spring barley has been most severely affected, although crop losses have been reported in spring and winter wheat and winter barley. Severe forms of the disease, evident as patches of stunted plants, have not been reported in cereals after non-cereal crops. However, many of the commonly grown alternate crops for this dryland region may also be a host of the pathogen, and mild forms of Rhizoctonia root rot have been documented in winter wheat after peas and lentils.

Clean chemical or mechanical-tillage fallow, which controls volunteer and weeds, appears to be the most effective rotational control. Reduced tillage practices have tended to favor this disease, but the effects of tillage are influenced by crop rotation, herbicide-weed control practices, precipitation area, the pathogen species and population present and many other factors.

Because the disease has only recently been documented in the Northwest, there is still little research information available to help predict where and under what conditions the disease will significantly affect crop yields. It is important to become familiar with the symptoms of Rhizoctonia root rot and closely monitor fields for the disease. Request assistance from county Extension agents, fieldmen and others to help identify suspected problems. Try to keep up to date on new research developments on the disease.

Changing management practices without knowing positively if the disease is present could increase the risk of losses from other production-limiting factors. For example, increasing the number of tillage operations or tillage intensity could reduce yields from loss of soil water by evaporation or runoff, without any assurance of increasing yields through control of the disease. A minor management change that might be considered now would be to allow more time between the application of a preplant herbicide for control of volunteer and weeds, and seeding of the wheat or barley, particularly under no-till and minimum tillage systems.

Through the efforts of Cook, Smiley and cooperating scientists, an initial research effort is underway to help develop a better understanding of the extent, impacts and management of Rhizoctonia root rot in cereals in the Northwest. Additional emphasis and support needs to be placed on research to control Rhizoctonia root rot and associated soilborne diseases in the region.

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 o similar products not mentioned.