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Kingdom |
Fungi |
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Phylum |
Ascomycota |
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Order |
Hypocreales |
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Family |
Hypocreaceae |
There
are no exact figures published on the damage caused by Fusarium wilt. However,
planting susceptible varieties were noted to produce more than 50% losses. The
availability of resistant varieties can definitely relegate this important
disease into a minor problem.
Argentina, Brazil, China, Hawaii, India,
Indonesia, Japan, Malawi, New Zealand, Peru, Puerto Rico, Taiwan, USA, Uruguay.
F. oxysporum f.sp batatas produces a white aerial mycelium
and purple pigment characteristic of the species. Erect,
hyaline conidiophores are formed
successively producing conidia which
accumulate into groups. It produces microconidia, macroconidia and
chlamydospores forming bud cells in liquid medium. Microconidia are generally
one celled, very seldom two celled, hyaline measuring 2 - 3.5 x 5 - 12µm.
Macroconidia are mostly 3- septate but some have 4-5 septa They are boat shaped
to oblong and 3 - 4 x 25 - 45µm in size. Chlamydospores
are formed mainly in the inner cells of macroconidia; they are thick walled,
with dense protoplasm, spherical with 7 - 10µm in diameter. Bud-cells are round
or oblong in shape, 1 - 2 x 1 - 1.5µm.
Discolouration of the vascular tissues of the
stem is an early and diagnostic symptom. Frequently, this is one-sided
with only one portion of the vascular ring discoloured.
The most obvious symptom of Fusarium
wilt in the field is an interveinal
yellowing of leaves followed by wilting. The older leaves dry out and can drop
from the plant or stay hanging from the stems. The disease may develop at any stage of development.
Yellowing of leaves is sometimes one sided; this occurs when only a part of the vascular
system is invaded by the fungus.
Other symptoms include stunting of the vines, cracking or splitting of stem,
or premature flowering.
The lower stem may appear purplish and the cortex can rupture and expose brown
to black affected tissue. The surface of the vine killed by Fusarium wilt has a pinkish extramatrical
growth w/ numerous macroconidia and microconidia.
Veinal blackening may extend to at least the proximal
end of the storage roots which may appear normal unless cut vertically. It may
also cause fibrous root necrosis.
Fusarium solani and F.
moniliforme have also been isolated from sweetpotato storage roots infected
with F.oxysporum f.sp. batatas.
Fusarium wilt is a disease mostly present
in subtropical regions of the world. The optimum temperature for infection is
around 30°C, but the disease can develop at lower temperature and across a wide
range of soil moisture- from 28 to 75%. However, the highest damage occurs in
fields where the moisture is low.
The fungus is soil- borne and can persist
in the soil for many years. Infection is usually through vascular wounds such as
those obtained when procuring cuttings for planting or when leaves are detached
from the stems. The fungus, however, cannot penetrate the callus that has grown
over the wounds.
The disease can affect vines at any stage
of development, but when infected transplants (from mother roots) are used for
propagation, plants die at an early stage.
Once the soil has been infested, the
infection persists in plant refuse, because the fungus produces resistant
structures - the chlamydospores - that can survive in the soil for several
years. When sweetpotatoes are harvested mechanically, the possibilities of
infection are higher, because of wounding.
Chlamydospores germinate when enough
moisture is present in the soil. They produce a germ tube that enters the root
through natural openings, or those produced by nematodes and natural wounds due
to root growth. Germination of chlamydospores can be impaired in certain soils,
due to the presence of antagonistic microorganisms, alkalinity, and deficient
moisture.
Disease transmission is through infected
plant material and through contaminated soil. The disease can be initiated in a
field when infected cuttings are used as planting material. Irrigation water,
human movement and use of implements previously used on an infected crop may
also cause the spread of the disease. Sometimes the disease is just found in
patches across the field.
The primary host is sweetpotato but the
fungus also attacks several Ipomoea species, and a number of
other Convolvulaceae. Experimentally, F.oxysporum f.sp. nicotianae, the
cause of Fusarium wilt in tobacco was found to cause wilt in sweetpotato and
conversely, F. oxysporum f. sp. batatas caused wilt in susceptible
tobacco.
An accurate and easy way to detect the disease in the field is to obtain a transverse section of the stem near the soil
surface. Brown to purple discolouration of the vascular system will confirm the
presence of F.oxysporum f.sp. batatas as the disease develops in
the vessels of stems and roots. The same discolouration will be observed in a
cross section of the proximal end of a storage root attached to an infected
plant.
In the laboratory, inspection of thin sections of affected tissue under a compound microscope will show the presence
of macroconidia, microconidia or chlamydospores.
Host-plant resistance
The development of resistant varieties is the most important control measure,
and has turned the disease from a major threat to a minor problem in many areas. There are many resistant varieties from breeding programs now available in
several countries such as USA, China and Japan. Early examples are Goldrush and
Tinian, the latter being widely used in breeding backcrosses that produce
moderately resistant F1 material with field-immune progeny. Many additional
cultivars with high levels of resistance have been developed since.
In Argentina the varieties Tucumana lisa and Brasilera blanca were also found
resistant to the disease.
Cultural control
Use of certified healthy planting
material. If cuttings would be obtained from sprouts, cut 5 cm above the soil
line. Shoots pulled from the mother roots should not be used. Storage roots used
for propagation, should come from healthy plants.
Solarization
under plastic kills the mycelium and conidia up to 20 cm deep.
Crop rotation with non-host plants.
Proper handling of storage roots
after harvest to avoid surface rot during storage.
Chemical control
Fungicides such as
carbendazim, benomyl, and thiabendazole are generally used as dip
treatments for propagation material.
Fusarium
oxysporum f. sp. batatas. IMI Descriptions of Fungi and Bacteria No. 1112. 2
p.
Clark, C.A. 1988. Principal
bacterial and fungal diseases of sweetpotato and their control. Report of the
First Sweetpotato Planning Conference: Exploration, Maintenance, and Utilization
of Sweetpotato Genetic Resources, Lima Peru, 23-27 February 1987. International
Potato Center. pp 275-289>.
Holliday, P. 1970. Fusarium
oxysporum f.sp. batatas. CMI Descriptions of Pathogenic Fungi and
Bacteria No. 212. 1 p.
Jatala, P., Boluarte T., and
Guevara E. 1993. Interaction of Meloidogyne incognita and Fusarium
oxysporum on resistant and susceptible sweetpotato clones. Abstracts 25th.
Annual meeting. Organization of Tropical American Nematologists. Cochabamba,
Bolivia 4-9 April l993.
Nielsen L.W. 1977. Control
of sweetpotato Fusarium wilt with benomyl and thiabendazole. Plant Disease
Reporter 61: 1-4.
Contributed by: Teresa
Ames |
