Click on any
image for a more
detailed view

Pathogen Biology

Rhizoctonia
Rhizoctonia belongs to a group of fungi called the "Mycelia Sterilia." These fungi do not produce asexual spores, but grow by producing thin, vegetative strands called hyphae (Figure 20). In recent years, the sexual stages, or teleomorphs, of many Rhizoctonia species have been characterized and classified in the basidiomycete genera Thanatephorus, Waitea, and Ceratobasidium. However, these sexual stages are rarely seen in nature, so the classification in the Mycelia Sterilia has been maintained.

Figure 20

Because Rhizoctonia species often do not produce spores, these fungi are identified by characteristics of their hyphae. Rhizoctonia hyphae produce branches at right and acute angles to the main hypha, the branch hypha is slightly constricted at the branch origin, and there is often a septum near the branch origin (Figure 20). Rhizoctonia species also produce specialized hyphae composed of compact cells called monilioid cells (Figure 21). The monilioid cells fuse together to produce hard structures called sclerotia (Figure 22), which are resistant to environmental extremes, allowing the fungus to survive adverse conditions.

Figure 21

Figure 22

Rhizoctonia species can be divided into two groups based on the number of nuclei present in cells of the hyphae. This characteristic can be assessed by staining hyphae with a fluorescent dye that binds specifically to the DNA in the nucleus. Binucleate Rhizoctonia species have two nuclei in each cell (Figure 23). Rhizoctonia cerealis is the only binucleate species that causes turfgrass disease. Multinucleate species have three or more nuclei in each cell (Figure 24). Rhizoctonia solani, R. zeae, and R. oryzae are multinucleate species.

Figure 23

Figure 24

Species of Rhizoctonia are further divided into groups based on the ability of hyphae to fuse with one another, a phenomenon known as anastomosis. Anastomosis is the primary means of genetic recombination in Rhizoctonia, therefore, isolates within an anastomosis group are closely related to one another. As a result, isolates within anastomosis groups (AGs) tend to have similar host ranges. In some cases, anastomosis groups may be further divided into intraspecific groups based on morphological or physiological characteristics.

Rhizoctonia solani
Rhizoctonia solani forms colonies on potato dextrose agar (PDA) that range in color from buff to black (Figure 25). Sclerotia are produced on the surface of cultures after 4 to 6 weeks, and are irregularly shaped, light tan to black, and usually >1 mm (0.04 in.) in width (Figure 26). The optimum temperature for growth of R. solani in pure culture ranges from 18ºC (64.4ºF) to 28ºC (82.4ºF). This species is divided into 11 AGs. Isolates in AGs 1 and 2 are commonly associated with turfgrass disease.

Figure 25

Figure 26

Rhizoctonia zeae and Rhizoctonia oryzae
Rhizoctonia zeae and R. oryzae produce colonies on PDA that are white, buff, or salmon colored (Figure 25). These species can be differentiated from one another based on sclerotial morphology. The sclerotia of R. oryzae are salmon colored, 1 mm (0.04 in.) to 3 mm (0.12 in.) in diameter, and form on the agar surface or submerged in the agar. The sclerotia of R. zeae are initially white, but eventually turn to orange then red or dark brown, are uniformly spherical, and form submerged in the agar medium. Each species grows optimally in pure culture at 32ºC (89.6ºF), and contains only one anastomosis group (R. zeae - WAG-Z; R. oryzae - WAG-O). Both of these species are less sensitive to dicarboximide fungicides than R. solani. In addition, Rhizoctonia zeae is less sensitive to benzimidazole fungicides than R. solani. These differences have important implications in the management of diseases caused by Rhizoctonia species, and increase the importance of proper disease diagnosis.

Rhizoctonia cerealis
Colonies of R. cerealis on PDA are white to buff colored (Figure 25), growing optimally at 23ºC (73.4ºF). Sclerotia are generally smaller than those produced by other Rhizoctonia species (0.3-1.2 mm, 0.01-0.5 in. diameter), are irregular in shape, and are initially white to yellow, eventually turning brown in color (Figure 22). This species contains a single anastomosis group, designated AG-D.

An additional Rhizoctonia species, designated Rhizoctonia AG-Q, has been associated with a disease similar to yellow patch in Japan. This species also produces white to buff colonies on PDA, but produces no sclerotia and grows more slowly than R. cerealis in culture.

RETURN TO TOP