Article submitted by Diane

I have white rot, Sclerotium cepivorum, on my allotment, a soil borne fungal disease which attacks all members of the alium family. Leeks seem to survive it best, shallots and garlic the worst.  On average over the last 9 years I have lost half of the onion crops to it.  First the leaves yellow and on inspection the roots are rotted away and a white mould is forming on the basal plate.

There is no cure, and it can persist in soil for decades, but I have been looking into ways of combating it and improving the survival rate at least.

Last year I gave up planting in rows in a conventional manner and spot planted onions among other crop, and in odd corners - the theory being that if they were infected they could not pass it on to neighbouring onions. The result - I lost about 30% , which is a marked improvement.

This goes against established wisdom of crop rotation, but if the disease is already all over the plot then you aren't going to cause it by planting the same thing in successive years, and wide random spacing means some onions will dodge it.

In addition to this practice I also plan further experiments based on ideas which are being researched in more formal academic and industrial forms. The following quote comes from one of several articles that a quick google search on "white rot" and "research" came up with.

http://westernfarmpress.com/garlic-and-onion-industry-targets-white-rot-cure

"On biostimulants, Mike Davis, plant pathologist at the University of California, Davis, has been researching “DADS,” or diallyl disulfide, a compound that when applied in the spring or fall mimics the odor of garlic plants and causes premature germination of white rot sclerotia before the next crop is planted.

The compound, combined with field sanitation, destroys 95 percent of the sclerotia, but the remainder is sufficient to set off an infestation that would prevent the field from being profitable, Davis said.

“But I'm a fan of DADS,” he added. “With that 95 percent we can really slow down the spread of white rot. So we need to combine it with another strategy.”

Related studies with garlic juice as a biostimulant through irrigation water have been done by Fred Crowe, emeritus plant pathologist with Oregon State University at Madras, an area where garlic seed crops for California are grown."

So in a 'home-made' version of this research I am 'inoculating' one area of soil  with chopped onion waste and dilute garlic juice in early spring and in another planting a sacrifice crop of spring onions before putting onion sets and plants in.  The spores of the white rot fungus react to chemicals specific to aliums, and come to life when the soil is at about 15 degrees C. In the case of the bed with onion waste they will find themselves without living hosts, and die (I hope). Where they attach themselves to the spring onions they will have short lives as I pull these out and burn them.  There will no doubt still be some of the sclerotia left but on a domestic scale some losses are acceptable.

I will then plant red and white onions in these beds with wide spacings, I can use the space by inter-planting with rows of carrots and with parsnips between  individual onions. (I can get the rows of carrots out without disturbing the onions shallow roots, the parsnips will still be growing when I carefully lift the onions. The scent of the onions may help mask the roots from carrot fly.)

Lovely theories, I will report back in autumn on the onion's survival rate, it would be great if others did the same and we could compare notes.