Four common root diseases of wheat and barley world-wide are, namely: 1) Pythium root rot, 2) Rhizoctonia root rot, 3) Fusarium root and crown rot, and 4) take-all. All are caused by fungi or fungus-like organisms that live in the soil. Different cereal-growing areas of the world will not likely be troubled by all four of these root diseases but will almost certainly have one or two of them in the same fields and even on the same plants. However, except in cases of obvious root disease, as when plants die prematurely with empty heads, as in the case of Fusarium crown rot shown below, these root diseases can go unrecognized or misdiagnosed as lack of soil fertility, drought stress, compact soil, allelopathy, or any number of other causes. (See also Why the Increased Growth and Yield Response of Crops to Soil Fumigation?)
To make the situation more complex, Pythium, Rhizoctonia, and Fusarium root rots each consist of more than one species with different species favored by different cropping systems, soils, and climates but all adapted to roots of wheat and barley. All four of these root diseases, including the root rots caused by multiple species of Pythium, Rhizoctonia and Fusarium, occur on wheat and barley in the U.S Pacific Northwest where they have been studied as extensively or more extensively as a group than anywhere else in the world.
Pythium root rot is both the most common and the most subtle of all four root diseases of wheat and barley. Known to gardeners as the cause of damping off of large-seeded vegetables such as sweet corn and peas, Pythium begins its attack of wheat and barley by infecting the germ (embryo) end of the germinating seed. This leads to stunted and spindly seedlings because of the pathogen using the sugars from the dissolving endosperm for its own nutrition. Most seed treatments going back to early-day chemicals such as Captan prevent this phase of Pythium damage to wheat and barley.
The real damage from Pythium species is due to infection of root tips and the fine rootlets of wheat and barley, and to striping away the root hairs needed by the plant to reach outward into the soil for nutrients. Root tips and the region of root hairs just behind the tip, like embryos, are juvenile tissues, which seem to be the preferred tissues of this pathogen. The photo above shows the growth of a Pythium species (stained red) inside a root hair and growing toward the root cell that gave rise to that hair. The picture immediately below shows the thick walled spores known as oospores of this pathogen, formed in the root tip right behind the root cap. Oospores are then freed into the soil when the root decomposes, to live on and infect another day. Typical wheat field soils in eastern Washington have an average of 150-200 of these oospores per gram of soil in the top 6 inches (15 cm). The third photo below shows the final result of a rootlet from Pythium-infested soil striped of root hairs and discolored owing to the infection along side a rootlet from soil fumigated with Telone C17 to eliminate the Pythium from the soil. Read Abstract here.
The healthy rows on the left received, in the seed furrows at planting, the chemical metalaxyl inhibitory or lethal specifically to Pythium species and its closest relatives, whereas the rows on the left received no metalaxyl. This photo illustrates the uniformity of Pythium root rot within a field, shows that awareness of the disease is only possible with a side-by-side comparison with wheat without the disease.
The damaging effects of Pythium root rot on wheat can also be demonstrated in the laboratory by pasteurizing the soil prior to planting wheat. Pythium species including the oospores are very heat sensitive and are killed in soil by exposure to moist heat at as little as 50 C for 20 minutes. The roots of seedling in the 3- to 4-leaf stage washed free of soil and shown floating in water in the test tubes below show the much larger root system when grown in pasteurized soil than when grown in the same soil not pasteurized. This experiment can be repeated with almost any natural garden or field soil.
Pythium species infect the fine rootlets of many host plants and not just wheat and barley. Accordingly, they are not subject to control by crop rotation, other than periods of a bare fallow when there are no plants. Even then, one year of bare fallow would probably not be long enough to lower the population of this pathogen below a safe threshold. On the other hand, there is evidence that different species and possibly subspecies have different crop preferences, so that rotating crops might also, effectively, “rotate” species, lowering the population of those species adapted to the roots of one host while growing a different host.
Rhizoctonia root rot of wheat and barley is usually caused by one of more of the species Rhizoctonia solani or R. oryzae. Rhizoctonia solani, in turn, consists of several biological species known as anastomosis groups (AGs) and separated on the basis of the ability of their hyphae to “anastomose,” i.e., fuse and exchange nuclei within but not between biological species. Rhizoctonia solani AG 8 is the most important cause of Rhizoctonia root rot of wheat and barley in both the U.S. Pacific Northwest and in the southern Australia wheat belt where it was first described in the 1930s and has been studied extensively.
Root damage caused by R. solani AG8 is readily diagnosed by the characteristic spear-tipping of roots, as shown on barley in the photo below. Plants with root rot caused by AG 8 typically die in patches. The disease in Australia is called bare patch.
Rhizoctonia solani AG 8 has no ability like Pythium to form thick-walled spores, and therefore depends for carry over between host plants on occupancy of root tissues colonized as a pathogen. Even a short period of no plants can greatly lower the potency of this pathogen since root tissues, unlike wheat stems, decomposes relatively rapidly, quickly reducing this food base needed by the pathogen to attack the next crop. This weakness led to the discovery that early and effective elimination of volunteer (greenbridge) plants otherwise allowed to grow in the stubble can greatly lower the damage from this disease—a problem first and incorrectly called “Round-up injury” when direct seeding wheat or barley into cereal stubble. Read Abstract here.
Because of its wide host range, R. solani AG8 also causes root rot of peas, canola, and many other broadleaf crops in addition to cereals. On the other hand, canola is also susceptible to R. solani AG2-1 and it is this biological species that appears to be a more important cause of Rhizoctonia root rot of canola.
Take-all is caused by the fungus Gaeumannomyces graminis var. tritici. In spite of the widespread importance of Pythium and Rhizoctonia root rots, take-all can be considered “king” in that, if it occurs, it will dominate the root disease mixture and make the other two more or less academic. Infections spread over the root system and from plant to plant by pigmented runner hyphae, sending feeder hyphae into the roots as the disease progresses from seedling roots to crown roots and then into the crown. Root infections alone, like Pythium and Rhizoctonia root rots, give rise to nutrient-deficiency symptoms, whereas crown infections cause plants to die and dry up prematurely, producing white heads as shown in the field view below. Growers in Australia call this phase of the disease “ hay-die” because the crop has little further value other than as hay.
The take-all pathogen like Rhizoctonia solani AG 8 has no ability to form survival spores and is therefore depend for its survival between cereal crops on tissues colonized as a pathogen. Unlike R. solani AG 8, it occupies the stems as well as roots, and stems because they decompose more slowly provide a more reliable food base for the pathogen to live in and use as a springboard to infect the next crop. One year out of wheat can greatly reduce the inoculum load of this pathogen in soil, but a two-year break from wheat is better. Take-all on barley tends to be limited to roots, which means that a short break can greatly reduce the inoculum load of this pathogen left after a crop of barley.
Fusarium root and crown rot is caused by F. culmorum in cooler wheat-growing areas of the world, e.g. northern Europe and F. pseudograminearum in warmer wheat-growing areas of the world, e.g, southern Europe. Both species occur in the PNW, with F. culmorum occurring in the more northerly areas of Washington and adjacent northern Idaho and F. pseudograminearum occurring in the more southerly parts of Washington and into northern Oregon. These two pathogens can also occur as a mixture in the same fields.
Fusarium infection of wheat, like take-all, usually starts on the roots and then progresses into the crown and up the main-stem and adjoining tillers (culms), usually extending two or three internodes up the culm, turning them chocolate brown (see photo). Also like take-all, destruction of a functioning crown causes the whole plant to die prematurely, producing the diagnostic white heads.
Both Fusarium species carry over in soil in dead roots and stem tissue colonized as pathogens, with stem tissue providing a safe haven for months and even years because of it slow rate of decay. Both Fusarium species also produced thick-walled spores known as chlamydospores, but those of F. pseudograminearum are relatively short-lived, making this species easier to control by crop rotation. The chlamydospores of F. culmorum can survive dormant for several years in the soil.
Whereas Pythium and Rhizoctonia root rots and take-all are favored by wet cool soil conditions while the crop is young, Fusarium root and crown rot is just the opposite, being favored by warm dry soil conditions and especially conditions that lead to plant water stress.