Clubroot of Cruciferous Crops
Note Number: AG0565
Ian Porter, Knoxfield
Updated: July 1996
Onions will store for up to eight to 10 months provided they are treated correctly, during and after harvest, and prevented from sprouting. The growing conditions will also influence the quality of the onions in storage. Generally, onions grown slowly in cool temperate climates will store for longer periods than onions grown under irrigation in hot climates. Extended periods of wet conditions in the four to six weeks prior to harvest will promote the storage rots caused by Aspergillus and Penicillium spp., especially in hot areas.
Other factors which will increase diseases in storage include: High crop densities in the field;
- Prolonged periods of wet conditions during crop maturity;
- Damage due to pre-harvest diseases and pests;
- Insufficient curing (field or artificial) at 27oC with a relative humidity of 80 per cent;
- Leaving mature crops in the field too long;
- Rough handling during harvesting and grading;
- High temperature and humidity during storage.
Table 1 Major storage diseases of onions.
|Storage disease||Causal organism||Symptoms|
|Blue green mould||Penicillium spp.||Dusty green masses under surface scales.|
|Black mould||Aspergillus spp.||Sooty black masses under surface scales.|
|Neck rot||Botrytis spp||Dusty grey mould and rot in neck of bulb.|
|Fusarium rot||Fusarium spp.||White fluffy growth and soft rot at base or neck of onion.|
|Bacterial rots||Pseudomonas and|
Strong smelling watery rot of the bulb.|
Storage rots are caused by pathogenic fungi and bacteria which live on organic matter and onion debris in the soil. These organisms are common in soil and will attack several different host plants. They become a major problem wherever onions are grown in a short rotation.
Soil containing spores from the fungus or bacterium get washed or blown into the neck or base of the crop during the growing phase of the onion and cause disease when conditions are suitable during storage.
Green-dusty spore masses develop along the mid ribs and under the surface scales when the weather is moist before harvest. In storage the disease develops rapidly, especially in humid conditions when a sparse green, felt-like growth occurs in the neck, and on both surfaces of the outer scales.
This occurs in a similar way to blue-green mould and frequently the two fungi occur together. Sooty-black spore masses usually develop underneath the surface scales and tend to be much more prominent than blue-green mould.
Hot weather in the field and during storage favours development and rapid spread of this disease. Temperatures over 30oC, with a relative high humidity, provide the ideal environment for black mould.
The fungus causing the disease is carried on the seed. It remains inactive during the growing stages of the crop and generally there are no visible symptoms of neck rot until onions have been stored for eight to 10 weeks. A dusty-grey mass of spores develop on infected onions, the neck softens and occasionally black structures called sclerotia, up to 1.5 cm diameter, sometimes form beneath the scales. A soft-brown rot develops and moves into the bulb.
Neck rot control
Seed treatment with registered fungicides is essential to control neck rot in storage. Check with the seed company that seed has been treated, particularly seed obtained from cooler regions. Forced air drying at temperatures of 30oC will reduce disease but, on the other hand, may encourage the growth of black mould. Note: Neck rot and infection by Botrytis on other parts of the plant can occasionally occur in the fields when conditions are humid and wet.
Generally this disease causes minor losses, but rotten onions in a bag can cause other onions to deteriorate. In warm climates of 28-32oC, the rot normally starts in the field at the base of the onion and progresses into the bulb causing a soft watery rot. This is often associated with bacterial decay. See Agriculture Note: Fusarium rot of onions.
Bacterial rots (slippery skin, sour skin)
High field temperatures, greater than 30oC, favour bacterial decay - this means these rots are more of a problem in warm climates. Bulbs may look sound but when cut open, the main scales are brown and water soaked. When pressure is applied, the inner core may be forced from the onion. Infected bulbs have a pungent smell and rot very rapidly.
General control for storage diseases
There are no fungicides registered for post harvest treatment of onions.
- Use good crop hygiene which includes proper crop rotations, sound growing practices and disposal of crop residues.
- Turn onions frequently during field curing.
- Avoid windrowing during periods of wet weather
- Handle bulbs carefully at harvest to minimise damage.
- Keep bulbs continually dry after harvest.
- Maintain good airflow and ventilation, low temperatures less than 20oC, and a low relative humidity during storage of less than 80 percent.
- Forced air curing at 27oC and a relative humidity of less than 80 percent before storage will help minimise storage rots developing. See Agriculture Notes Onion curing and storage 1: 2: and 3.
Correct diagnosis is essential for effective pest and disease control. A commercial diagnostic service is available at the Institute for Horticultural Development (IHD). For further information, phone Crop Health Services on(03) 9210-9222 or fax (03) 9800 3521.
For further information on registered chemicals, phone Chemical Information Service on (03) 9651 7317.
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