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teresa:
Factsheet
DC16      Onion White Rot


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Onion white rot, Sclerotium cepivorum, is the most serious, widespread, and destructive disease affecting the Allium family. Onions, leeks, garlic, chives, shallots and salad onions can all be affected by this soil-borne fungus. Once in the soil, it is extremely difficult to either control or eradicate.
Typical symptoms
Plants, apparently growing well, suddenly start to die. Older leaves turn yellow and wilt, and examination will reveal that roots have become stunted or rotten. Seedlings keel over, larger plants can easily be pulled out of the ground, and garlic stems pull away easily from the bulb. A few plants in a small patch may be affected at first, then a whole row may show signs as the problem spreads.
As the disease progresses, a white, cottony-looking fungal growth will be seen around the base and up the side of bulbs, with tiny black globules, like poppy seeds, among the fungus. These are the fruiting bodies of white rot, known sclerotia.
Life cycle
Onion white rot is an exceedingly persistent organism. The sclerotia are able to survive for at least 15 years in the soil even without a suitable host plant.
When diseased plants are lifted, sclerotia fall off, then lie dormant in the soil until a member of the Allium family is planted on or reasonably (10cm/4in) near them. The presence of Allium roots stimulates the fruiting bodies to germinate and infect new plants, starting the cycle of disease once again.
Temperature
White rot is temperature dependant. It is active only when the soil temperature is between 10º-20ºC (50º-68ºF). In the UK, this is usually in March/April. At this time overwintered crops, such as garlic, have a well-developed root system, extensive enough to stimulate the sclerotia into germination.
Sources of infection
Infected soil on plants, boots or tools is the usual point of new contamination. There have been occasional cases of infected onion sets but this is very rare. If animals eat infected Allium bulbs, viable sclerotia will be present in their manure, but this is not a common situation.
Prevention and control
•      Hygiene: Quarantine is the best method of avoiding white rot. Don't take chances as infectious soil can be transferred on footwear, tools and seedlings. Discourage other gardeners from walking their mud around your plot. Scrub and rinse thoroughly your own equipment and boots after use in another garden. Don't accept plants from other gardens - these could carry the early stages of the fungus. Buy garlic for planting from a reputable supplier.
•      Cultural control: If you suspect white rot in your garden, use as long a rotation period as possible. Aim for an eight year gap in any suspect area. If you discover a small patch of infection, remove and dispose of affected and adjacent plants, and the surrounding soil. It's worth removing a large quantity of earth to clear out a small area of infection. Avoid growing garlic if you think you might have the disease. It is particularly susceptible and will stimulate sclerotia deep in the soil.

When stimulated by nearby plants, white rot can spread 1-2cm sideways, through the soil. Intertwined roots will also transfer infection so space plants widely. Clumps of multi-sown onions should be 30cm/12in apart. Sow onion seeds, not sets. Onions grown from sets have well developed root systems just as soil temperature triggers disease activity. Seed-sown onions are still very small at this time, with immature roots which are less likely to encourage attack.
•      Harvesting: Badly infected bulbs will not store. They continue to decay, and will contaminate clean crops. Onions whose roots alone have been damaged may store, and although they will be small, they will keep well.
•      Composting: Heat will kill white rot. If you are certain that your compost heap will reach 50ºC for several consecutive days, then it is safe to compost contaminated foliage. Otherwise, burn, or landfill, diseased material.
•      Other things to try: Try growing garlic in large (10cm/4in diameter) holes filled with uncontaminated soil. When infection is triggered in spring, there will be minimal root damage before higher soil temperatures halt the disease. Leeks are worth trying, even on really infected ground. They may survive and crop well due to a combination of soil temperature and root development at a crucial time of growth.

Once a white rot sclerotia has germinated and re-infected a new crop, it dies. If you know you have the disease, plant an Allium crop, wait for signs of infection to appear, then immediately lift and dispose of all the crop and surrounding soil. This might be successful in exhausting and removing most of the infection but would have to be repeated more than once.

Preliminary tests in pots have indicated that garden compost, when added to soil can suppress diseases such as white rot. Some experts consider that the amount of compost needed to affect white rot in a garden situation would cause environmental pollution. Further investigations are needed to confirm these findings.
Not a nice thing to have ???
Teresa

tim:
tell me about it!! - that's why it's all been sanitised this month in the hope that something will come through.

Just cannot go another year buying onions etc. - Tim

ciderself:
I heard today that you could water your patch with Jeyes fluid. Not quite sure what that would do to the soil though - kill all the goody bacteria maybe along with the nasties?

Colin_Bellamy-Wood:
Teresa - just a little thank you for your efforts.   I shall file this in my Gardening Folder.   Don't suffer yet, and don't want to, but if ever I do, or my neighbours do, I'll know exactly what to do, thanks to you.

I have a feeling that Jeyes fluid could work ???

teresa:
Hi Colin and all
Yes Jeyes Fluid could work being a disenfectant used for the garden and animals. You can tell I was brought up on a smallholding JF was always to hand.
Mum used JF watered on her primroses to get rid of the little black slugs that live in the ground from eating the roots of the plants and it worked. Also stopped mice from eating her crocus perhaps that was the smell of it.
Hope it helps others understand what white rot is.
Teresa

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