Java black rot

Economic importance

Java black rot is one of the most significant postharvest diseases of sweetpotato. It can damage a huge quantity of storage roots depending on the length of time in which they are in storage. The disease can also be severe in seedbeds when infected roots are used as a source of planting material. Although there is no economic estimate of damage, it is evident that under certain environmental and storage conditions significant yield and postharvest  losses are possible.

Geographical distribution

The disease is present worldwide.

Lasiodiplodia theobromae is a fungus that grows well in a culture medium (potato-dextrose-agar) in the laboratory, forming fluffy gray colonies initially which become black with age. The mycelium is grey to black. A few days after infection the mycelium forms black stromatic structures containing pycnidia. The pycnidia are round or elongated, ostiolated, generally aggregated, usually setose, up to 5 mm in size. The conidiophores are simple, rarely branched, hyaline, cylindrical, forming a mat in the inner surface of pycnidia. The conidia which arise from the tip of conidiophores are hyaline, unicellular, and granulated when young, becoming cinnamon to fawn, bicellular with longitudinal striae when mature and measure 20-30 x 10-15 µm. They are somewhat subovoid to ellipsoid-oblong in shape.

Symptoms

The most obvious symptoms are those found in fleshy roots few days after harvest. Brown to reddish brown, round sunken lesions with solid black centre, surrounded by soft, pinkish ring of decaying tissues are observed. Soon after, the lesions become hard, sunken and completely blackened due to the presence of mature mycelium and stromatic tissue. When infection starts in one or both ends of the fleshy root, the entire root dries out and mummifies. During the drying process, black dome-shaped structures bearing pycnidia emerge through root periderm and an abundance of black powdery spores is shed.

In early stages of infection the symptoms of Java black rot can be confused with those caused by Ceratocystis fimbriata (black rot) and Macrophomina phaseolina (charcoal rot).  However, lesions caused black rot are usually confined to the outer layers, in distinct circular lesions.  Charcoal rot initially spreads through the outer cambial layer without forming superficial lesions.  Java black rot spreads usually from one end of the root, through all layers of tissue. In some cases, the rot is limited to one end of the root and does not spread further.

Biology and ecology

Java black rot occurs in seedbeds, attacking underground parts of the plant through soil borne conidia when infected sprouts have been used as propagation material.

Fleshy roots are usually infected in the field through wounds made during harvest by inoculum present in the soil or from the infected mother plant. Secondary infections occur in  storage when insects  carrying the spores infest the storage roots.

L. theobromae requires warm temperatures (20-30^oC). Humidity is not crucial, however, when it is too high the disease does not develop.

When infected fleshy roots are stored after harvest, the fungus starts developing and after one week or two. A black pimple-like growth is observed on the surface of the roots. These are pycnidia, containing thousands of conidia, which are the propagation structures of the fungus.

Due to its ability to live on decaying matter L. theobromae survives in the soil on plant refuse for several years.

Cacao (die-back), citrus (stem end rot of the fruit), banana (finger rot), avocado (stem end rot), mango (stem end rot), onion (collar rot), apple (stem rot), cacao (pod rot), yam, cassava, groundnut and melon.

Use of  healthy roots or cuttings as propagation material.

Use of transplants cut above the soil line.

Avoid wounding the roots during harvest and handling.

Curing roots before storage. Storage roots should be cured at 30-34^oC and 90% relative humidity for 4-7 days depending on the cultivar. After curing, maintain roots at 15-16^oC during storage.

Use of clean and disinfected storage containers. 

Crop rotation for 3-4 years, if possible.

Differences in susceptibility have been noted among lines and cultivars from studies done in different countries.

Dipping propagation material (fleshy roots, sprouts) in Benomyl or Captan. To prevent rotting, the same fungicides are used as a spray for roots before bedding or storing.

Ames, T., N.E.J.M. Smit, A.R. Braun, J.N. O’Sullivan and L.G. Skoglund. 1996. Sweetpotato: Major pests, diseases, and nutritional disorders. International Potato Center (CIP). Lima, Peru. 152 p.

Clark, C.A. and J.W. Moyer. 1988. Compendium of sweetpotato diseases. 1988. APS Press. 74 p.

Holliday, P. 1980. Fungus diseases of tropical crops. Cambridge University Press. 607 p.

Palomar, M.K., A.D. Solis and H.S. Bandala. 1980. Sweetpotato storage root rot disease in the Philippines. Ann. Trop. Res. 2:111-121.

Punithalingam, E. 1976. Botryodiplodia theobromae. CMI Descriptions of Pathogenic Fungi and Bacteria No. 519.

Vasquez , E.A. and C.E. Sajise. 1989. Pests of sweetpotato: Insects, mites and diseases. PRIS and PRCRTC, Visayas State College of Agriculture, Baybay, Leyte, Philippines. 66 p.

Contributed by: Teresa Ames

Taxonomy

Economic importance

Geographical distribution

Morphology

Symptoms

Biology and ecology

Host range

Management

References

Internal and external views  of storage roots with Java black rot (J. Lo, APS).

Black sunken lesion on storage root (T. Ames).

Lesion restricted to one end of a storage root (C. A. Clark).

Black stromatic masses erupt through the root surface, and shed black powdery spores (C. Clark, APS).