Black rot

Black rot  is economically important.  It results in heavy losses in seedbeds, in the field and more importantly, in storage.  The transplants eventually die in seedbeds; reduces yield in the field  and renders roots in storage unmarketable because of the unsightly black lesions, rotting and  bitter taste.  

Geographical distribution

Africa (Congo Democratic Republic, Cote d’Ivoire, Ghana, Seychelles, South Africa, Uganda); Americas- Central (Costa Rica, El Salvador, Guatemala,  Nicaragua, Panama,  Suriname, North (Mexico, USA;  South (Brazil, Colombia, Ecuador, Guyana,  Peru, Venezuela); Asia (Brunei, Cambodia, China, Indonesia, Japan, Korea, Malaysia, Myanmar, Philippines, Thailand); Australia; Caribbean (Cuba, Grenada, Dominican Republic, Haiti, Jamaica, Puerto Rico, Saint Lucia, Trinidad and Tobago);  Europe (France, Italy,  Poland, United Kingdom); and  Pacific Islands (American Samoa, Fiji, Papua New Guinea, Samoa, Solomon Island).

The disease is called black rot because of its symptoms  on storage roots. However, the disease can be serious on young sprouts and adult plants. Infected sprouts develop black sunken necrotic lesions or cankers on the stem under the soil.

In the field plants are stunted and chlorotic due to the cankers present in the underground stem. Chlorotic leaves drop.

On storage roots the first symptoms are brown, slightly sunken spots not more than 0.5 cm in diameter. These spots continue to enlarge in storage, transit and marketing, enlarging up to 5 cm in size. The lesions become firm, dry and black and  can coalesce to cover the entire root. A particular feature of this disease is the presence of fungal structures (long necked perithecia) protruding from the affected tissue. The lesions are generally shallow, however in some cases a deeper rotting develops due to the presence of secondary organisms, generally bacteria.

The tissues in the rotten spots and their surroundings have a  bitter taste but fruity odour.

The perithecia are superficial or partly to completely immersed, brown to black, globose, 140-220 µm in diameter, unornamented, with  long black neck, pale brown to subhyaline toward the tip, tapering slightly, up to 900 mm long and  with erect or moderately divergent, hyaline ostiolar hyphae. The ascospores  are elliptical with a gelatinous sheath forming a brim, giving a hat-shaped appearance, hyaline, non-septate, smooth  and measure 4.5-8 x 2.5-5.5 µm. The conidiophores  are slender, arising laterally from the hyphae, septate, phialidic, hyaline to very pale brown, up to 160 µm, usually tapering towards the tip and producing a succession of conidia through the open end. The conidia  are cylindrical, truncate at the ends, hyaline, smooth walled, 11-25 (15) x 4-5.5 µm. The chlamydospores are terminal in chains, obovate to oval, thick walled, brown, of 9-18 x 6-13 µm. A thick walled endoconidial form is present.

The fungus is parasitic on a wide range of hosts, but there is physiological specialization, and presence of different strains. The sweetpotato strain is an important wound parasite that affects mainly fleshy roots in storage, transit, and marketing, although it also causes damage to plants in seedbeds and in the field. Any sort of wounding increases the possibility of infection, but the fungus can also penetrate through lenticels.

Sweetpotato roots can carry the infection from the field, and when infected roots are used for propagation, infected shoots are produced. In storage, roots develop black rot when temperatures are favourable. It has been found that disease development is most rapid at 25^oC, but is reduced at higher temperatures.

The fungus is also soil-transmitted by chlamydospores that survive in dry soil and plant debris after harvest. Once in storage, insects such as sweetpotato w

eevil and rodents can spread the infection.

The bitter taste comes from the phytoalexin, ipomeamarone, produced by the storage roots in response to C. fimbriata infection while the fruity odour is from the volatile compound, amyl acetate produced after infection.

Host Range

The sweetpotato strain of C. fimbriata only infects sweetpotato but many plants, especially trees, are hosts to other strains. 

Use of  healthy material  for transplanting.

Curing storage roots immediately after harvest.

Fumigation of crates/containers and storeroom.

Control of insects and rodents.

Crop rotation for five years.

Host-plant resistance

Use of less susceptible cultivars.

Recommended fungicides for fleshy root treatment are Benomyl and Thibendazole used for dipping planting materials and spraying the roots before storage.

Cheo, P.C. 1953. Varietal differences in susceptibility of sweet potato to black rot fungus. Phytopathology 43:78-81.

Clark, C.A. and J. Moyer. 1988. Compendium of Sweet Potato Diseases. APS Press. 74 p.

Holliday, P. 1980. Fungus Diseases of Tropical Crops. Dover Publications, Inc. New York 607 p.

Morgan-Jones, G. 1967. Ceratocystis fimbriata. C.M.I. Descriptions of Pathogenic Fungi and Bacteria. No. 141.

Contributed by: Teresa Ames

References

Dark, sunken lesions on the base of sprouts grown from infected storage roots (W. Martin, APS).

Black sunken lesion on storage root of a purple-skinned cultivar (W. Martin, APS).

Storage roots of a brownish orange-skinned cultivar with black rot lesions (W. Martin, APS).

The internal and external appearance of a black rot lesion (C. Clark, APS).