Alternaria leaf spot

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Causal organism: Alternaria spp.

 

Taxonomy

 

Kingdom Fungi
Phylum ‘mitosporic fungi’
Class Hyphomycetes

 

Economic importance

It has not been considered important, since the damage caused is mostly present in older leaves.

Geographical distribution

The disease is probably present wherever sweetpotato is grown, but no written information has been found on the geographic distribution apart from references from India and Japan.

Symptoms

The presence of brown necrotic lesions on older leaves, with a typical bull’s eye appearance of concentric rings, 1 to 5 mm wide with well defined margins,  is the most evident symptom of the disease. As the disease develops, light brown lesions are formed with concentric rings. Several lesions can fuse and cover a great area of the leaf. When this occurs, the leaf drops. Usually spots are surrounded by a chlorotic halo.

The shape of the lesions on the leaves and the chlorotic halo surrounding the lesion are very similar to lesions produced by Alternaria bataticola., the cause of Alternaria Stem Blight The difference is that A. bataticola is more severe (can kill the plant) and attacks the whole vine (leaves, petioles and stems), at an early stage and through all the crop cycle.

Morphology

Several species of Alternaria can be found in lesions of affected sweetpotato leaves. The most common are A. alternata, A. brassicae and A. solani. There are some differences among these three species but they all have the same shape and colour of conidia. They all produce ellipsoidal or oblong conidia with transverse and longitudinal septae with A. solani and A. porri having a long peak while A. alternata has a short one. In this last species conidia are somewhat short, with fewer septae, and are formed in chains of five or more conidia.

The colour of conidia of the three species varies from olivaceous-brown to dark brown.

Biology and ecology

The fungus remains in plant debris on the soil as mycelium and conidia. Conidia are spread by rain splash, wind and insects.

The conidia germinate and directly enter the leaves through the epidermis of old leaves, killing the tissue in advance, probably by the toxins produced by the mycelia.

Maximum growth of the mycelium in culture is attained at 27°C. Conidia are formed at 20°C when 12-hour light and 12-hour darkness are alternated, producing the same concentric rings present in infected leaves.

There is higher incidence of the disease when dry and rainy periods alternate during cropping.  The disease occurs in all agroecological zones.

Overhead irrigation predisposes plants to the disease.

Some species in this genus produce toxins such as alternaric acid, that may cause the death of cells. Alternaria species  thrive on dead matter.

Several natural enemies have been reported for several Alternaria species, such as: Aureobasidium pululans, Trichoderma koningii and Trichotecium roseum.

Host range

There is no information available. The disease is caused by more than one species of Alternaria which are pathogenic on different hosts.

Diagnosis

In the field,  the characteristic symptoms  on  mature leaves can be useful in diagnosis.  In the laboratory, it is diagnosed  by inducing conidia production and identifying the fungi through a compound microscope.

Management

Cultural control

Destroy and burn crop residues after harvest.

Use clean planting material.

Avoid overhead irrigation.

Host-plant resistance

There is no information about resistance to this disease.

Chemical control

In other crops, such as tomatoes or potato, Alternaria is controlled efficiently with the fungicides chlorotalonil, dyrene and mancozeb but there is no information about using fungicides to control the disease in sweetpotato.

References

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

Ellis, M.B. and Holliday, P. 1970. CMI Descriptions of Pathogenic Fungi and Bacteria No. 248.

Ellis, M.B. 1971. Dematiaceous hyphomycetes. Commonwealth Mycologycal Institute, Kew, Surrey, England. 608 p.

 

 

Contributed by: Teresa Ames

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Diagnosis

Management

References

Alternaria stem blight


Alternaria leaf spot with its characteristic yellow halo around lesion (C. Lopes, Embrapa, Brazil).


Light brown lesions with dark brown concentric rings (G. Lawrence).

Random scatter of dark brown lesions with concentric rings and yellow halo (the purple ring-spots are caused by a virus) (J. O'Sullivan).