|
| Kingdom |
Fungi |
| Phylum |
‘mitosporic fungi’ |
| Class |
Hyphomycetes |
It has not been considered important, since the damage
caused is mostly present in older leaves.
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.
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.
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.
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.
There is no information available. The disease is caused
by more than one species of Alternaria which are pathogenic on different
hosts.
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.
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.
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 |
