Agriculture Reference
In-Depth Information
Blue mould
Various species of Penicillium can cause yellowish blemishes and watery soft
spots on bulbs, followed by a blue-green mould of spore-producing
conidiophores on the infected surfaces. Blue mould is a serious storage disease
of garlic in California. Infected cloves soften and shrivel and can ultimately
become a mass of blue-green spores. These fungi grow commonly on organic
debris in soil and on senescing plant tissue. Invasion of bulbs is usually through
tissue damaged by bruises, wounds, sunscald or freezing.
The Penicillium species that can cause bulb rots have now been classified as a
group of seven species within the Penicillium series Corymbifera , one of which, P.
allii , is the predominant pathogen of garlic and onion (Overy et al. , 2005a). Unlike
most of the ser. Corymbifera, P. allii fails to produce plant cell wall-degrading
enzymes at temperatures below 15°C (Overy et al. , 2005b), and therefore disease
can be prevented by low-temperature storage. Disease control involves careful
bulb handling to avoid damage, and storage at temperatures of 5°C or less at
humidities as low as possible to avoid shrivelling. However, five others among the
seven species in ser. Corymbifera can cause a rot in the base plate of yellow onions,
and they can produce cell wall-degrading enzymes at 5°C; therefore, they have
potential to cause disease in cool storage (Overy et al. , 2005a, b).
Penicillium allii (formerly called P. hirsutum (Overy et al. , 2005b)) can also
weaken or kill recently planted garlic in the field. When infected bulbs are
separated into cloves for planting, these can be infected via wounds created in the
process. To minimize infection, cloves should be planted as soon as possible after
separation. Differences in susceptibility have been found between Argentinean
garlic cvs (Cavagnaro et al. , 2005).
Smudge
Onion smudge, caused by Colletotrichum circinans , is a soil-borne disease that
causes green-black dots on the skin of white onions (see plate 9d) and yellow,
watery cratering of the underlying fleshy tissue. Cultivars with coloured skins are
rarely affected because their skins contain protocatechuic acid, which makes
them resistant to smudge (see 'The Skin of Onion Bulbs' in Chapter 7). Black
resting bodies called stromata remain viable for many years in soil. These can
germinate and produce acervuli, which produce conidia. Conidia germinate to
produce an appressorium via which the pathogen can penetrate the plant cuticle.
Acervuli are circular spots producing spores and are surrounded by
characteristic bristles, which can be seen with a hand lens. The infection is spread
by rain-splash from infected soil. Humid conditions and temperatures of 25°C
enable the pathogen to progress from infection to sporulation within a few days.
Rapid drying after harvesting bulbs is helpful for control, but white-skinned
varieties should be avoided where the disease is prevalent.
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