Agriculture Reference
In-Depth Information
in growth may escape detection at harvest and will cause the bulbs to soften and
rot in store unless kept cool; (iii) S. sclerotiorum ; and (iv) bulbs infected with S.
cepivorum (white rot) may continue to rot in store. Among the foliage-infecting
pathogens: (i) Alternaria porri can sometimes cause a watery rot starting in the
bulb neck, which is initially yellow but then becomes purple-red; (ii) B. squamosa
can cause neck rot in white onion bulbs; and (iii) B. cinerea can produce a
disfiguring brown stain on the skins of bulbs, but causes no internal rot.
Other minor diseases of stored bulbs reported include: (i) Diplodia stain, a
grey to black discoloration of the skins of white onions and garlic in Texas
caused by Lasiodiplodia theobromae ; (ii) mushy rot caused by Rhizopus
microsporus , which is geographically widespread but infrequent; and (iii)
yellow mould rot, caused by Aspergillus alliaceus . The latter, like A. niger ,
develops in warm storage (28-32°C) and produces a mass of yellow spores and
black resting bodies (sclerotia). Infection of outer bulb scales by Fusarium
proliferatum has been shown to cause a salmon pink discoloration of the dry
outer scales of white onions in Idaho and Washington State, USA (du Toit et al .,
2003).
Many of these potential pathogens are widespread, growing as saprophytes
and sporulating on decayed matter in soil. Wounds in bulbs or necks are required
for entry by many of them, and damage when onions are topped and harvested is
a vulnerable stage for infection. Rapid drying of wounded necks and tissue can
reduce the infection risk. The temperature optima for development of these
pathogens are indicative of the regions and climates in which they are important
diseases (Maude, 2006). For example, neck rot is a serious problem in temperate
regions but not in warm climates, whereas black mould is commonplace in the
tropics but rare in temperate conditions unless prolonged high temperatures are
used to dry onion bulbs. The climatic conditions favourable to the soil-borne and
foliar infections that can also cause bulb disease are indicated in the sections on
these two classes of pathogen.
Neck rot
Neck rot, as its name implies, causes a soft rot in the neck and upper regions of
infected bulbs. A black mass of sclerotial resting bodies often develops below
the dry outer skin on the decaying tissue. A grey, sporulating mould may also
develop on the surface of the decaying, fleshy scales. The symptoms usually
develop some 2-3 months after the apparently healthy but infected bulbs are
placed in store. Two pathogens, Botrytis aclada (often termed B. allii ) and B.
byssoidea , are exclusively associated with neck rot, the former with sclerotial
and the latter with mycelial neck rot.
Two subgroups of B. aclada have been distinguished, one (type AI) with 16
chromosomes and the other (type AII) with 32, the latter group also having
larger conidia. DNA sequencing has indicated that type AII arose as a hybrid
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