There is little published
information on the damage caused by these Fusarium diseases.
Surface rot, caused by Fusarium oxysporum, infects fleshy roots
usually in storage, through wounds made during harvest. However, it can be
present at harvest on roots damaged by splitting or insect or nematode feeding. Circular, light to dark brown,
dry and firm lesions are found on the storage roots. The infection may start at
the end attached to the mother root or at wounds and may slowly extend to the
cortical tissue but do not usually get beyond the vascular ring. The
surface of the root shrinks and the flesh underneath dries out. A white mycelium
outgrowth is sometimes observed after which the root becomes hard and mummified.
Fusarium root rot, caused by Fusarium solani, is initially hard to differentiate from surface rot.
However, as Fusarium root rot develops, it penetrates the vascular ring
and the lesions acquire a target-like pattern with concentric light and dark
brown rings. In advanced stages lens-shaped cavities develop inside the
lesions which often contain white mould fibres (hyphae). Surface rot, on the
other hand, is restricted to the cortex, and the lesion surface has usually a
uniform brown colour.
F. oxysporum, the pathogen causing
surface rot is morphologically undistinguishable from F. oxysporum
f.sp batatas, the form causing
Fusarium wilt though they differ in the way they cause disease on
sweetpotato. Like F. oxysporum, F. solani, the root rot-causing
Fusarium, produces
chlamydospores and
conidia with the latter having smaller
macroconidia than the former.
These fungi are soil-borne and can persist in the soil for many years.
Infection is usually through wounds obtained during and after harvesting. Both
diseases develop during storage but do not spread to other roots unless new
wounds occur.
F. oxysporum only infects the mother roots and does not transmit
infection to the sprouts. F. solani, on the other hand, can spread from
the mother roots to the sprouts. Disease dissemination in the field or plant
beds occurs when infected cuttings from sprouts are used as planting material.
In this case, plants may die at an early stage.
Surface rot may become more common under the following conditions: a)
harvesting is done when soil is wet and where mechanical damage is more likely
to occur; (b) harvesting when soil is excessively dry and when storage root skin
is easy to peel off and get wounded; and (c) when harvested roots are exposed to
extreme low or high temperatures for a long period of time.
The primary host is sweetpotato but these fungi also attack several
Ipomoea species, and a number of Convolvulaceae. While cross inoculation
studies have not been reported, it is likely that these fungi cause cortical
rots in other plants.
The symptoms described above are good indications of the diseases. With a
cross section of the storage root, a uniform brown discolouration of the cortex
will indicate the presence of F. oxysporum while discolouration
with light and dark brown target-like pattern showing pits with white mould
inside indicate the presence of F. solani.
In the laboratory, inspection of thin sections of affected tissue under a
compound microscope will show the presence of macroconidia, microconidia or
chlamydospores.
Use of healthy planting material. Obtain cuttings from sprouts 5 cm above
the soil line. Shoots pulled from the mother roots should not be used.
Harvest roots when soil is neither too wet nor too dry to avoid wounding
during harvest.
Proper curing and handling of storage roots after harvest
to avoid surface rot during storage.
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.
Clark, C.A. and J.W. Moyer. 1988. Compendium of sweetpotato diseases. 1988.
APS Press. 74 p.
Contributed
by: Vilma Amante and Jane O'Sullivan
