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VEGETABLE  DISEASES  (allium white rot)

ALLIUM  WHITE  ROT  affecting, garlic, elephant garlic, onions, shallots , leeks.

This is one of the most insidious diseases for alliums as it is carried in the soil, and very persistent for up to 20 years. It tends to build up in the soil where alliums have been cropped for several years and is frequently present on Allotments.

The presence of white rot is usually spotted as the alliums are harvested. It is seen as a white fungus growth on the bulbs and roots. The roots and the bulb are frequently rotting. When seen, the bulb, roots and the surrounding soil should be sterilised or binned. Never add to the compost heap, as this will only spread the disease.

The white rot infected allium bulb produces thousands of “sclerotia”, which look like very small black seeds. These remain in the soil for a few years until the “sclerotia” sense the presence of allium roots   close by. The “sclerotia” then germinate and attach themselves to the allium roots and infect them.

The disease can be brought into your plot by the addition of contaminated soil or by planting sets or plants that are carrying the disease. You should only buy sets that are specially sold for planting and growing and that have been inspected and certified as being disease free. Therefore do not plant sets from the Supermarket that have been sold as food. While safe to eat, they may be carrying disease that you cannot see. Where leeks are bought as seedlings, try and check with the seller that they have been grown in disease free soil.

As onions and shallots can now be grown from seed, doing this will prevent white rot being transferred to your soil along with the sets or plants.

There is no chemical cure for white rot available to the general public. Strict crop rotation can help to delay the build up of the disease, but as the dormant “sclerotia” can remain viable for up to 20 years, it is not the answer to the problem.

Proposed Organic control of “white rot”.

This method of organic control of white rot was suggested by Professor Fred Crowe, Oregon State University, after ongoing tests of the treatment.

When white rot has infected an allium bulb, it forms thousands of little black seed like, “sclerotia”, which fall off and remain in the soil, unless the infected bulb and the adjacent soil have been removed and sterilised. When the “sclerotia” infected ground is dug over, the “sclerotia” will be distributed throughout the digging depth. These “sclerotia” will become dormant over the Winter, but in the Spring will germinate as soon as the ground is warm enough and they sense the presence of allium roots close by.

The control method is to trick the dormant “sclerotia” into thinking that there is an allium growing beside it, by watering on a ground-up garlic bulb solution of 1 part garlic solution  to 1000 parts water, say one ground-up garlic bulb to a watering can of water. This should be watered onto 1 sq m of damp soil. It is best to be watered onto the ground when rain is expected, so as to take the garlic water deep into the soil to contact the maximum number of “sclerotia”.

After carrying out the first treatment using the ground-up garlic water, turn over the soil with a fork. Then apply the ground-up garlic water again. This will enable the soil to be treated to a fork depth, hopefully killing most of the white rot sclerotia.

Strip the garlic cloves of their papery wrapping and cut off the basal plates before you grind the cloves up, to cut down the danger of introducing any disease. You should be able to taste and smell the garlic in the solution. It should be applied when the ground temperature is between 10c and 20 c with an optimum temperature of 15c.

It should be watered onto the ground that you intend to use for alliums, during the preceding year while the ground is moist and warm.

It is also possible to use garlic powder which you can find in equestrian stores or on-line, as it is used for the treatment of horses. This should be applied at a rate of 250 lb to the acre. Roughly 125 Kg per 4000 sq m, or roughly 30 gm per sq m. This has the advantage of having been sterilised and unable to pass on any allium infections but is not so effective as using fresh garlic bulbs.

Once the sclerotia have been tricked into germination by the garlic liquid or powder, there are no alliums for them to feed on and so they die, thus breaking the reproductive cycle of white rot.

Interestingly, it seems that UK farmers are already successfully using a similar method of applying heat-treated onion waste to trick the dormant “sclerotia” to wake up, and subsequently die in the absence of the host allium crop. See http://www.farmersguardian.com/fighting-allium-white-rot-and-the-uk%E2%80%99s-waste-problems-with-composted-onions/23430.article

Results of garlic water treatment.

Having just harvested alliums from treated ground using the above method, I can confirm that it seems to be remarkably effective in controlling and reducing white rot. If you have problems with white rot at the moment, you have nothing to lose and everything to gain from giving this method a try.