Rhizoctonia solani (Target spot of Tobacco)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Basidiomycota
Subphylum	Agaricomycotina
Class	Agaricomycetes
Order	Cantharellales
Family	Ceratobasidiaceae
Genus	Rhizoctonia

Scientific Name

Rhizoctonia solani

Scientific Name Synonyms

Moniliopsis aderholdii

Corticium solani

Rhizoctonia napaeae

Thanatephorus cucumeris

Common Name

Rhizoctonia damping-off, blight and rot

Author: David Shew and Garrett Ridge, North Carolina State University

Scientific Name Synonym

Thanatephorus cucumeris (A.B. Frank) Donk [Teleomorph]

Pathogen

Rhizoctonia solani is a soil-borne plant pathogenic fungus in the phylum Basidiomycota that exists as thread-like growth on plants or in culture. These thread-like hyphae are first hyaline, then turn buff to dark brown with age. Hyphae are multinucleate and tend to form at right angles at branching points that are usually constricted (Figures 1-2). Cells of R. solani lack clamp connections, but possess a complex dolipore septum with continuous parenthesomes (Figure -). This fungus forms irregularly shaped sclerotia that are light to dark brown in color and measure up to 8 mm in diameter (Figure -). R. solani is not currently known to produce conidia, though it is considered to have an asexual life cycle.

Isolates of R. solani can be separated into different groups based on the ability of hyphae to fuse with one another, a phenomenon known as anastomosis. Isolates within the same anastomosis group (AG) are closely related and tend to have similar host ranges. Members of AG 3 are primarily responsible for target spot on tobacco but R. solani isolates in AG 2-1 have recently been found to also cause target spot.

Target spot is initiated and spread through the aerially dispersed basidiospores of the teliomorph of R. solani, Thanatephorus cucumeris. T. cucumeris forms hymenia on infected plant tissue that are made up of successive layers of club-shaped basidia with four apical sterigmata on which four ellipsoid to oblong, hyaline basidiospores (4 to 5.5 µm × 7 to 10 µm) are borne.

Hosts, Signs, and Symptoms

Rhizoctonia solani infects a wide range of hosts, causing root diseases on over 200 plant species. Isolates of T. cucumeris in AG-3 that cause target spot on tobacco can also cause disease in other solanaceous crops as well as in sugar beet.

In areas infested with R. solani, lower leaves turn brown and the presence of brown web-like mycelium may be observed attached to leaves and stems (Figure -). When favorable conditions occur, cream-colored hymenia are formed on infected leaves or other plant tissue.

Symptoms on leaves develop around 48 hours after infection and begin as small, round, water-soaked spots about 2 to 3 mm in diameter (Figure 3). Lesions expand rapidly during periods of warm temperatures (>24°C) and high humidity. Primary lesions remain distinct even when the lesions continued to expand (Figure 4). Expanded lesions first become light green, almost transparent, with irregular margins and then form large circular spots of concentric rings, taking on a characteristic ‘bullseye’ appearance (Figures 5-6). Tissue in the center of the lesions is thin and may drop out, giving a shot-hole appearance to the leaf (Figure 7). R. solani also causes damping off in tobacco seedlings (Figures 8-10).

Ecology

Rhizoctonia solani relies on both a necrotrophic and hemibiotrophic lifestyle for pathogenicity, as the pathogenic process of R. solani is characterized by the death of plant cells, both before and after penetration and colonization events. The pathogen can survive for many years in soils as sclerotia or as a saprophyte, colonizing soil organic matter. Periods of high relative humidity (>98%), prolonged leaf wetness, and moderate temperature are favorable for disease development. In the greenhouse, mycelia and sclerotia that remain attached to plug trays are an important source of inoculum for subsequent crops. Basidiospores of the teliomorph T. cucumeris serve as the primary and secondary inoculum for this disease. The basidiospores land on leaf tissue and germinate to produce hyphae that infect leaves during periods of high relative humidity. After germination, penetration by appressoria occurs and invading hyphae also grow on the leaf surface and enter stomata to create secondary lesion sites. Once lesions are formed, secondary inoculum is produced by hymenia on the lower and sometimes upper leaf surfaces and is dispersed by wind and rain to surrounding tobacco plants.

Geographic Distribution

R. solani occurs in all parts of the world, and is capable of attacking many different host plants. Target spot frequently is reported on tobacco grown in South America, South Africa, the southern United States, and Canada. The disease first appeared in the U.S. in the 1980s and has remained a significant problem for tobacco production ever since.

Management

Cultural

The most important environmental factor in the development of target spot is moisture. Prolonged leaf wetness and high humidity favors leaf infection, lesion development, and secondary inoculum production. Conditions like these often occur in greenhouses and can be highly conducive to the development of target spot. Ventilation is critical. Plants with stem lesions should not be transplanted into the field.

Sanitation and good growing practices are the best defense against target spot. Plants that are nitrogen-deficient show increased susceptibility to target spot. Severe outbreaks of target spot have occurred in cases where nitrogen has dropped below 50 ppm, a common scenario in outdoor float beds that have received significant rainfall resulting in dilution of fertilizer levels. Maintaining nitrogen within the recommended range of 75 to 125 ppm will help suppress, but not eliminate, this disease. T. cucumeris can survive on used polystyrene trays and has been identified as the principal source of inoculum to new transplants. Steam treatment for used pots and trays is recommended. For plastic items, the center of the stack should be held at to 75 to 80°C for one hour.

Practices that open the plant canopy, increase air circulation, and reduce leaf wetness should reduce the rate of disease development. Early harvesting of lower leaves from infected plants, while reducing the quality of the leaf, greatly reduces subsequent disease development and may save a large percentage of the remaining crop.

All cultivars of flue-cured and burley tobacco are susceptible to target spot. While differences in disease severity may exist among tobacco cultivars, resistance to target spot has yet to be found.

Biological control is another important aspect of R. solani management in tobacco. The efficacy of biological antagonism has been demonstrated in R. solani using Trichoderma sp. Isolates of T. harzianum can reduce the growth of R. solani and enhance disease control in tobacco plants.

Chemical

Fungicides containing Azoxystrobin and Mancozeb have good activity against target spot. These products can be applied preventatively as a foliar spray for target spot.

Diagnostic procedures

The fungus is easily identified by its multinucleate hyphae and right angle branching with a septum often occurring near the branch origin. If diagnosis is unclear, incubate affected tissue in a moist chamber for 2 to 3 days. If R. solani is present, characteristic hyphae will be visible under magnification.

Symptoms of target spot on tobacco are similar to those of brown spot caused by Alternaria alternata. It is simple to distinguish these two pathogens with the aid of a microscope. R. solani will have its characteristic hyphae and basidiospores of T. cucumeris may be present. A. alternata will have brown to light brown obclavate to obpyriform conidia with several vertical and transverse septa.

R. solani forms white to dark brown mycelium when grown on most nutrient media. Mycelia growth is greatest when kept between 20 and 28°C. 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. Diseased roots can be immersed in AWA medium for isolation.

Molecular characterization of R. solani can be conducted using PCR to examine the internal transcribed spacer (ITS) regions with the primers ITS1 and ITS4 to amplify the ribosomal DNA.

Acknowledgements

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)