Thielaviopsis basicola (Black root rot of tobacco and ornamentals)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Hypocreomycetidae
Order	Microascales
Family	Ceratocystidaceae
Genus	Berkeleyomyces

Scientific Name

Berkeleyomyces basicola

Scientific Name Synonyms

Thielaviopsis basicola

Chalara elegans

Common Name

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

Scientific Name Synonym

Chalara elegans Nag Raj & W.B. Kendr.

Pathogen

Thielaviopsis basicola is a soilborne fungus in the phylum Ascomycota. T. basicola reproduces asexually by producing two types of conidia, endoconidia and aleuriospores. Endoconidia (also called phialospores) (7.5 to 19 × 3 to 5 µm) are single-celled hyaline spores with slightly rounded ends that are produced within elongate terminal phialides (Figures 1-3). If undisturbed, the conidia stick together end-to-end, forming long unbranched chains. Aleuriospores are darkly pigmented, cylindrical spores that contain 2 to 8 cells and measure 24 to 55 µm in length, with the terminal cell having a rounded apex (Figure 4). At maturity and before germinating, the cells fragment along the transverse septa, leaving two to eight short-cylindrical one-celled spores (6.5 to 14 × 9 to 13 µm) usually referred to as chlamydospores (Figure 5). Reproduction in this species is entirely asexual, but molecular evidence indicates that T. basicola fits within the teleomorph genus Ceratocystis.

Hosts, Signs, and Symptoms

Thielaviopsis basicola infects a wide range of hosts, causing root diseases on over 200 plant species. While primarily a root rot pathogen, symptoms may also include stem rot and damping off on some hosts. Black root rot is particularly severe on Japanese holly, pansy and other Viola species, and on tobacco. Other ornamentals commonly affected by black root rot are Madagascar periwinkle, calibrachoa, petunia, cyclamen, poinsettia, florist’s cineraria, and geranium. Many herbaceous perennials also are susceptible to black root rot, including Asarum, Heuchera, Tiarella, and Phlox subulata.

The primary sign observed on diseased tissue is the dark brown aleuriospores that appear black in mass. On susceptible hosts, the abundant sporulation can enhance the dark or black appearance of the lesion and gives the disease its common name, black root rot. When observed under magnification, both aleuriospores and endoconidia are usually present.

Above-ground symptoms of black root rot are typical of many other root rots or root problems and include chlorosis, defoliation, stunting, and wilting (Figures 6 and 7). Infected root tissue first develops dark brown to black elongated lesions (Figure -). The disease can quickly progress until much of the root system may take on a black discoloration (Figures 8 and 9). The cortical tissue collapses and the epidermis and cortical tissue may slough off, but decay is not as generalized as with Pythium or Phytophthora root rots. Co-infection with Pythium is common. Symptoms may include necrosis of the stem below the soil surface on some plant species. On poinsettia, longitudinal cracking of the stem may begin below the soil surface and extend up the stem.

Ecology

T. basicola is a hemibiotrophic plant pathogen, with an initial biotrophic phase as it invades and colonizes living cells followed by a necrotrophic phase with the death of the plant cells. Chlamydospores of T. basicola can survive in soil for several years and serve as the primary source of inoculum. The fungus is spread when healthy roots come in contact with infected roots or infested soil and when aleuriospores are dispersed by splashing water. In the greenhouse, previously infected pieces of root that remain attached to pots, flats, and plug trays are an important source of inoculum for subsequent crops. Additionally, fungus gnats and shore flies can spread T. basicola, when feeding on root tips and other plant parts. Active black root rot infection sites are a preferred food source for these insects. Soil environmental conditions, especially cool soil temperatures (13 to 18°C) and pH values above 5.5, are critical for the development of black root rot. Temperatures that are unfavorable for the growth of the host may result in maximum disease severity. Movement of the pathogen from field to field is attributed to the movement of infested soil on equipment.

Geographic Distribution

M. J. Berkeley and C. E. Broome described T. basicola on peas and on Nemophila sp. in England in 1850. Thielaviopsis basicola has been reported from most regions of the world, most commonly in areas with cool and moist climates. The fungus is found in all major production areas for very susceptible crops.

Management

Cultural

In greenhouses and nurseries, incoming plants of susceptible species must be inspected to be sure that they are free from black root rot. This may be difficult in early stages of infection. Care must be taken that pots, flats, and potting mixes do not come in contact with native soil.

Environmental conditions are important for disease management and prevention. Soil temperatures should be favorable for the growth of the plant and soil pH should be kept low to limit disease development. In areas where pansies are used for winter color in the landscape, they should be transplanted only after the weather has cooled in the fall. Black root rot is generally more severe at soil pH values >5.6 and suppressed under more acidic conditions (pH < 5.2). In large nurseries or greenhouses, it may be possible to treat beds or bulk soil with aerated steam. All areas must be brought to a temperature of 71 to 82°C for 30 minutes. Steam treatment for used pots and flats is also recommended. For plastic items, the center of the stack should be held at 66 to 71°C for one hour. Chemical disinfectants can also be used.

Soil solarization with clear polyethylene tarps as well as soil flooding has been shown to be effective in reducing or eliminating T. basicola soil populations in field soils.

Crop rotation with grain crops is recommended for management of black root rot affecting tobacco. Leguminous cover crops and cotton are susceptible to T. basicola and should be avoided.

In tobacco, resistance is well characterized with complete single dominant gene resistance from Nicotiana debneyi and a number of sources of partial resistance in N. tabacum.

The incorporation of a biological control product based on Trichoderma or Pseudomonas into potting media could be beneficial for managing black root rot. Pseudomonas fluorescens strain CHA0, isolated from a suppressive soil, has been shown to be very effective in managing black root rot on tobacco. Other strains of P. fluorescens also have been reported to control black root rot.

Chemical

Fungicide treatments for management of black root rot are protective in nature. Infected plants and those adjacent to them in pots and flats should be discarded. In the ornamentals industry, fungicide drenches with either thiophanate-methyl, fludioxonil, triflumizole, or thiophanate-methyl and etridiazole should be used preventatively or at the first indication of disease.

Sterol-inhibiting fungicides like flusilazole and triadimenol also have been shown to be effective in disease control for field crops. Fungicides with different modes of action can be applied in alternation, in tank mixes, or in pre-mixed products to deter the development and build-up of resistant strains. The use of a non-selective fungicide as the final spray of the season is strongly encouraged.

In greenhouses, benches need to be first cleaned and then sanitized between crops using a disinfectant based on one of a number of active ingredients, including sodium hypochlorite and copper oxychloride. Effective disinfestation of T. basicola from greenhouse surfaces is difficult to achieve.

Diagnostic procedures

The fungus is easily identified by its two distinct spore types. After washing roots, examine them carefully under the dissecting microscope for typical blackened sections and streaks. Often the shape of individual aleuriospores can be distinguished. If there is any doubt, suspect roots can be examined further at 100x. Caution: roots and crowns should always be examined before rinsing, which may remove evidence of other pathogens such as Cylindrocladium/Calonectria. If diagnosis is unclear, incubate affected tissue within a surface-sterilized carrot bait following this carrot bait protocol. If T. basicola is present, chlamydospores of the fungus will be visible, under magnification, in the carrot tissue.

T. basicola forms a gray-brown to black colony on most nutrient media, but is likely to be overrun by other soil fungi. Mycelial growth is greatest at 20 to 25°C and pH levels of 4.0 to 6.5. One of the most widely used selective media for T. basicola is the medium TB-CEN developed by Specht and Griffin. Diseased roots can be immersed in this medium for isolation, but it functions best when used for detection in soil assays (D. Shew, personal communication). Plates should not be discarded as negative until after at least 10 days of incubation.

Acknowledgments

• 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)