Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Order	Botryosphaeriales
Family	Botryosphaeriaceae
Genus	Macrophomina

Scientific Name

Macrophomina phaseolina

Scientific Name Synonyms

Macrophomina phaseoli

Sclerotium bataticola

Botryodiplodia phaseoli

Tiarosporella phaseolina

Rhizoctonia bataticola

Macrophoma phaseolina

Common Name

charcoal rot

Author: Michelle S. Oliveira, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Macrophomina phaseolina is a widespread, non-specific pathogen that can infect more than 500 hosts in about 100 families, including crops and weeds (3, 4). Economic crop hosts include cotton, groundnut, jute, maize, millet, potato, sesame, soybean and other beans, sunflower, sweet potato, tomato, and tobacco (3). The fungus is a highly variable pathogen, differing in microsclerotial sizes and the presence or absence of pycnidia (4).

Microsclerotia, 50-70 µm diameter (60-200 µm diameter when produced in laboratory), are black, smooth and round to oblong shape, uniformly reticulate, formed from hyphal agregates. Pycnidia, when present, are immersed in the host tissue and erumpent when mature. They are dark to grayish, globose, and membranous, with 100-200 µm in diameter and a truncate ostiole. Pycnidiospores are hyaline and single celled, ellipsoidal or ovoid, with a length-width ratio of 3:1, measuring 14-30 x 5-10 µm. Hyphal branches are formed in a right angle and may form a cross-wall at the constriction point, which can be confused with Rhizoctonia (1, 2, 3, 4).

Symptoms and Signs

Symptoms appear in hot dry weather. On seeds, the fungus can be asymptomatic, remaining as microsclerotia on the fissures or under the coat tissue (4). Symptomatic seedlings show brownish to black discoloration at the soil limit and above. The seedlings eventually die (4). If older plants are infected, leaves lose vitality, turn yellow, wilt, and die (1, 2, 4). In infected cucumbers, the host shows a reaction to the pathogen infection exuding sap out of the affected area. The sap becomes dark brown when it dries (1). Water absorption is blocked by the formation of numerous microsclerotia in xylem going up on the stems. This results a powdered charcoal appearance and causes a reddish brown or grayish black color to it (1, 3, 4). Disease progresses and complete death of the plants varies in a time frame of 7-21 days (1).

Ecology and Spread

Microsclerotia and mycelia of M. phaseolina overwinter on the soil or in plant debris and serve as a primary inoculum. The fungus can survive up to 2-3 years as a mycelium, in asymptomatic seeds, or as microsclerotia in the soil or plant debris (3, 4). Under dry conditions, Macrophomina microsclerotia can survive up to 15 years in the soil (5). The fungus cause disease symptoms in hot and dry weather, the ideal conditions where most of its hosts are cultivated. M. phaseolina competes well with other soil pathogens when soil nutrient levels are low and soil temperatures are high, above 30 degrees Celsius (2, 3). During these conditions, the fungus is attracted by crop roots exudates. The microsclerotia germinate and hyphae will penetrate the roots directly or through wounds. Colonization of the host is systemic and the plants start showing the first symptoms of wilting. Pycnidia, when formed, are enclosed at the plant tissue and will release conidia (i.e., the secondary inoculum) when mature. Plant debris is a niche for the formation of new microsclerotia that will overwinter again in dry soils. Seeds, produced in the same field by healthy plants, can become contaminated with the microsclerotia as a latent infection and disperse the pathogen to a new production field (5). Transportation of the soil on transplants or machinery can also spread the disease.

Geographic Distribution

Charcoal rot has a worldwide distribution and can be found in all the areas where one of its major hosts is cultivated (e.g., beans, beets, brassicas, corn, cotton, cucurbits, peanuts, potato, sorghum, etc.).

Management

Rotate susceptible crops with comparatively poor hosts.
Fertilize the crop to promote vigorous growth.
Irrigate to keep soil moisture high.
Fumigate or flood the soil before planting to reduce primary inoculum.
Use fungicides to treat seeds and transplants before planting.
Consult your local extension specialist for legal and efficacious fungicide products available in your state. Remember, the label is the law and the product applicator is responsible for reading and following all chemical labeling.

Diagnostic Procedures

When the first symptoms of wilting appear, take the whole plant from the field (with roots) and cut it open to look for the following diagnostic features:

reddish brown discoloration of the vascular tissue; and

presence of microesclerotia, 50-70 µm diameter, on the stem and root surfaces.

This fungus will grow well on common culture media such as PDA, but its isolation is selective in RB media. The fungal colonies will grow with a greyish black color like round small charcoal chunks.

The pathogen can be isolated and quantified from the soil where the crop was cultivated through real-time PCR using TaqMan and SYBR Green assay techniques (5).

Resources and References

Compendium of Cucurbit Diseases. 1996. APS Press, St. Paul-MN.
Compendium of Peanut Diseases. 1997. APS Press, St. Paul-MN.
Compendium of Sorghum Diseases. 2000. APS Press, St. Paul-MN.
Compendium of Soybean Diseases. 1999. APS Press, St. Paul-MN.
Kaur, S., Dhillon, G.S., Brar, S.K., Vallad, G.E., Chand, R., Chauhan, V.B. 2012. Emerging phytopathogen Macrophomina phaseolina: biology, economic importance and current diagnostic trends. Critical Reviews in Microbiology, 38(2): 136–151.

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)