Click on any
image for a more
detailed view

Disease Cycle and Epidemiology

Click here for more information about the disease cycle and epidemiology.

Rhizoctonia species are strong saprophytes. They are able to survive for extended periods of time in the absence of living host plants by feeding on decaying organic matter. When conditions are not favorable for growth, these fungi persist as mycelium or as sclerotia in the thatch and soil. When a host plant is present and environmental conditions are favorable, Rhizoctonia species begin to colonize the surface of the potential host plant with long, unbranched hyphae called runner hyphae (Figure 27). Depending on the isolate and host species, the runner hyphae may branch and penetrate through stomata or give rise to structures called infection cushions, which are aggregates of convoluted hyphae (Figure 28). Infection cushions of R. solani form most frequently along the edges of the vascular bundles (Figure 29) or over stomata. The infection cushions are involved in enzymatic degradation and physical penetration of the leaf surface, providing entrance into the plant for the pathogen. When sufficient moisture is present, Rhizoctonia species spread from plant to plant through the growth of hyphae among plant leaves (Figure 30).

Figure 27	Figure 28
Figure 29	Figure 30

Rhizoctonia species are necrotrophic pathogens, meaning that they kill host cells before colonizing them. This is accomplished through the secretion of enzymes and toxins in advance of fungal growth.

Epidemiology

Brown Patch
Brown patch may occur when minimum air and soil temperatures exceed 18ºC (64.4ºF) and 15ºC (59ºF), respectively, but severe disease outbreaks occur only at higher temperatures (>25ºC or 77ºF). Periods of high relative humidity (≥95%) for at least 10 consecutive hours are required for disease development. Irrigation and/or rainfall events exceeding 2.5 cm (1 in.) are also conducive to brown patch.

Characteristics of the site and turf stand may also favor disease development. In general, factors that impede evaporation or drainage (poor air movement and soil drainage, excessive shade) are conducive to brown patch. Excessive nitrogen fertilization creates dense, lush turf that is highly susceptible to R. solani. Frequent use of organic fertilizers has also been associated with increased brown patch severity.

Large Patch
Large patch occurs in the fall and spring, when warm-season turfgrasses are entering or exiting their period of winter dormancy. Infection of the lower leaf sheaths by R. solani occurs whenever the temperature of the thatch layer is between 10ºC (50ºF) and 21ºC (69.8ºF), and continuous moisture is available for at least 48 hours. Warm-season grasses are not growing vigorously during the spring and fall, and are highly susceptible to pathogen attack. Patches may also develop in the summer during periods of cool weather, especially in wet or shaded sites.

Recent evidence indicates that the majority of large patch development occurs in the fall. Symptoms are observed in the spring because plants infected the previous fall have not yet recovered. As conditions become optimal for the growth of warm-season turfgrasses in late spring/early summer, recovery occurs due to the spreading growth habit of these grasses.

Excessive moisture levels in the soil, thatch, and lower turf canopy encourage large patch development. Factors such as poor drainage, shade, restricted air movement, or excessive irrigation increase the severity of this disease.

Leaf and Sheath Spot
Turf diseases caused by R. zeae and R. oryzae are more prevalent than R. solani at high temperatures (30ºC or 86ºF). Other factors that encourage brown patch development (i.e. moisture, excessive nitrogen) also favor the development of leaf and sheath spot.

Yellow Patch
Yellow patch development occurs at temperatures below 20ºC (68ºF). Yellow patch occurs most frequently in temperate climates during the transition from fall into winter and winter into spring. Factors that encourage brown patch development (i.e. moisture, excessive nitrogen) also favor the development of yellow patch.

RETURN TO TOP