Taxonomy

Economic importance

Leaf spot and stem blight of sweetpotato caused by Alternaria bataticola is a foliage and stem disease that is very important in Eastern and Central Africa and Brazil. Although no literature has been found on the losses caused by this disease, it has been known  to cause important damage due to plant death.

Geographical distribution

Brazil, Burundi, Ethiopia, Japan, Kenya, Papua New Guinea, Rwanda, and probably in some other African countries.

Symptoms

The disease is first observed as small, brown to black oval lesions with a typical bulls’eye appearance of concentric rings, on leaves, stems and petioles. Blackened veins are observed on leaf undersurface. As the disease progresses, the lesions become necrotic usually surrounded by a wide yellow halo; soon after the whole leaf blade turns chlorotic and drops. Bases and middle sections are more affected than the vine terminals. The ground under affected vines is often carpeted with blackened leaf debris.

On petioles and stems, lesions are initially grey, but as the lesions enlarge, they become black and sunken. Ultimately, the petioles and stems are girdled and the plant dies.

The mycelium of A. bataticola is fuscous brown to almost hyaline, septate, and branched. Colonies on agar are gray- green. Conidiophores are single or in bundles, unbranched, erect or slightly curved, 2 - 7 septate, pale brown to fuscous-brown, 47 - 80 x 7mm. Conidia are solitary , elongate, obclavate muriform, transversely 5 -12 septate, longitudinally 0 - 8 septate, pale to fuscous-brown, smooth walled, conidium body. The conidial beak is long, filiform, colourless to pale brown, septate, often branched once and occasionally up to three times and measures 16 -128 x 3 - 6mm.

Biology and ecology

The fungus survives in soil and plant debris. The air-borne conidia are spread through infected planting material by wind, splashing rain, and water.

High relative humidity or free water is necessary for infection and sporulation, conditions common in tropical regions due to continuous rain. The disease and lesions increase with altitude as observed in Africa. During dry conditions the lesions get a silvery tone.

In other species of Alternaria  the presence of different toxins have been reported such as alternaric acid, which might be the reason why leaves drop.

The disease, as well as the causal agent, have been described only few years ago, in Africa (Ethiopia and Burundi) and South America (Brazil).

Host range

Sweetpotato is the only known host.

Diagnosis

This disease can be differentiated from other leaf spots and blights because of its severity. There are no other foliage diseases, with the exception of leaf and stem scab (Elsinoe batatas) , which can be so destructive.

There are other Alternaria species that also cause leaf blight but  they only attack the basal mature leaves.

Management

The use of clean planting materials.

It has been suggested that red-skinned  are more resistant than white-skinned varieties.

Although no records on the use of fungicides have been found, dyrene and mancozeb are specific fungicides to control Alternaria spp. in other crops and can probably be effective against A. bataticola .

References

van Bruggen. A.H.C. 1984. Sweet potato stem blight caused by Alternaria sp.: A new disease in Ethiopia. Neth. J. Plant Pathol. 90: 155-164.

David, J.C. 1991. Alternaria bataticola. IMI Descriptions of Fungi and Bacteria No.1071.

Lopes, C.A. 1994. Leaf spot and stem blight of sweet potato caused by Alternaria bataticola: A new record to South America. Plant Disease. 78: 1107-1109.

Contributed by: Teresa Ames

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Diagnosis

Management

References

Alternaria leaf spot

Alternaria blight on leaves, petioles and stems of sweetpotato (C. Lopes, Embrapa, Brazil).

Black sunken lesions on stems (C. Lopes , Embrapa, Brazil).