Agriculture Reference
In-Depth Information
species. On tuber surfaces it can develop numerous, tiny
black microsclerotia, visible at 10× magnification. The
infected periderm usually darkens during storage. Lesions
can appear silvery and distinguished from silver scurf by
tending to be irregular in shape and without a distinctive
margin (Plate 16.9). Disease development is favoured by
warm, wet conditions in field or store but it does not spread
in store. Control measures are generally cultural with long
rotations and avoidance of severely infected seed. Early
harvesting, dry curing and cool storage inhibit blemish
development.
surface often immediately surrounded by a sunken ring
(Plate 16.12). Although they can be superficial, some
cultivars are penetrated, sometimes leading to secondary
rotting. The disease is best avoided by planting clean seed
early and using resistant cultivars. Aphicides have limited
affect as the virus is nonpersistent.
Invertebrates
Aphids
Aphids are mainly a problem for seed storage because they
are virus vectors, however where aphid numbers build
up on a stored crop they can spoil the appearance of a crop
and promote weight loss. Sprouting tissue is particularly
at risk. The bulb and potato aphid (
Rhopalosiphoninus
latysiphon
), in particular, can build up in store because it is
a pest of the underground potato plant. Its close association
with potatoes means that it is capable of transmitting
Potato Leaf Roll Virus and Potato Virus Y causing
significant virus spread.
Silver scurf
Helminthosporium solani
causes silver scurf, a pervasive
blemish disease likely to be present wherever potatoes are
grown. Lesions are generally round with dark margins, and
often coalesce into larger silvery patches (Plate 16.10).
The lesion damages periderm leading to moisture loss.
Spores (conidia) can give the lesion a sooty appearance,
especially in warm, wet conditions. It is possible for the
disease to spread in store as spores are released. Diagnostic
features are short, black thread-like conidiophores, seen at
10× magnification. Infection is mainly perpetuated on seed
tubers. There are no resistant varieties so clean, or fungi-
cide, treated seed is the first control option. Harvesting
early is prudent because the longer mature tubers remain in
the ground, the more likely the progeny are to become
infected. Dry curing and cool storage will inhibit disease
development. Avoid condensation.
Potato rot nematode
Potato rot nematode (
Ditylenchus destructor
) has a
widespread world distribution but is a relatively minor pest
because it is intolerant of desiccation. Subsequently, it is a
pest of cool, moist soils. The nematode enters underground
tubers, through lenticels, multiplies rapidly and continues
to develop in store. Affected tubers have sunken areas with
cracked, wrinkled, detaching skin revealing discoloured
flesh. The use of clean seed, long rotations (with weed host
control) and nematicides are effective.
Skin spot
Polyscytalum pustulans
causes skin spot, an unsightly
blemish that begins as dark grey spots and can develop into
pimples with dark sunken rings. They can be spaced indi-
vidually or clumped, especially around eyes or scuffed skin
(Plate 16.11). Blemishes can penetrate the tuber flesh,
especially under cold storage, causing peeling losses. The
blemish is not visible on tubers until after six weeks of
storage at the earliest. Some thin skinned cultivars are
particularly susceptible. The disease is exacerbated by wet
harvests and early cold storage. Curing in warm, dry
conditions soon after lifting is beneficial. Disease-free or
fungicide treated seed reduce disease incidence.
Potato tuber moth
The potato tuber moth,
Phthorimaea operculella
(Zeller),
is a serious pest of stored potatoes in warmer countries.
Larvae can tunnel through tubers in storage, leading to
secondary rotting. Damaged tubers are unsaleable for
either fresh or processing. To limit the problem only crops
with low infestations should be stored and, as the larvae are
fairly intolerant of cooler conditions, rapid transit into
store is beneficial.
Wireworms
Wireworms are the larval stage of “click” beetles in the
family Elateridae. Several species e.g.
Agriotes
spp. and
Limonius
spp. can damage in stored potato tubers. Larvae,
about 2 mm long after hatching and growing to 40 mm or
more at maturity are slender, light coloured with a hard
cuticle and have three pairs of small legs near the head.
The larvae tunnel into tubers, producing holes about 3 mm
in diameter, tunnels can extend throughout the tuber.
Virus infection
Potato tuber necrotic ringspot disase (PTNRD), caused by
the aphid borne PVY
NTN
virus is a disease that develops
during storage, particularly in Europe and North America.
Necrosis, tending to be associated with warm, late growing
conditions, is seen as distinctive raised rings on the tuber
