Onion White Rot, the cure?

Probably a slightly misleading title, but a good way to get attention.

We all know that white rot is a major fungal disease of the onion family, and most of us on NHA have first-hand experience of infected crops.

I’ve been on and off the site since 1985; my most recent spell here started in 2007 on the ominously numbered plot 13. It’s at the bottom east end of the site and the soil is heavy loam with clay in parts. Up till recently the plots in that area suffered from shade cast by very large conifers in the adjoining gardens.

Despite the unpromising site I successfully grew banana shallots on the plot for several years, always at the top with the lightest soil and most sunshine. The crop was good and stored well. After about three years odd bulbs got white rot, and eventually the incidence was so high that I gave up growing any alliums other than leeks, which were unaffected.

About ten years ago I moved up the bank to plot 21, which is very steep and terraced. The soil there is on a cline, changing from very sandy at the top, to medium loam at the bottom. I was assured that there was no white rot on that plot. I quickly discovered that there was white rot and in thin soil it even affected some leeks. Shallots and onions had 95-100% failure rate due to white rot. I was able to harvest the bulbs but this involved skinning and storage in the freezer. Given the cost of banana shallots this was worthwhile. I don’t regard growing onions in white rot ground as worth it given their bulk and low cost.

Around that time, I spoke to Jim Duffy, our resident record keeper and master experimenter. He mentioned the use of garlic powder as a possible ‘cure’ for white rot. Information online suggested a regime of digging in a thorough dose of garlic powder each autumn, followed by a top dressing of compost to the bed. In the spring the bed was limed.

I did this and decided to grow banana shallots from seed the following spring. These were sown 2 to a pot in modules in the greenhouse in early March, grown on and planted out on my plot when the leaves were about six inches high. Growth after planting out was good and by July I had fine bulbs, significantly bigger than Sainsburys stock. Harvest produced 90% unaffected bulbs which stored dry. The balance of affected bulbs was frozen. The next year produced a similar result and I thought I’d cracked it. This was a mistake.

In 2019, a year of decent sunshine, but more rain, I followed the same procedure and every bulb of the 120 that I harvested was affected. I reverted to freezing, which ate freezer space.

This year I decided to have one last throw of the dice. I composted a bed in my twenty-foot polytunnel and direct sowed the seeds, reasoning that control of watering might be key. I didn’t apply garlic powder. I hoed a shallow drill between each row and watered into that once the seedlings were established.

By early July the bulbs were pleasingly large as usual, apart from the ones at the very front of the bed where drainage was sharpest.

I stopped watering for about three weeks and lifted the whole crop with a fork, laying the bulbs to dry on mesh frames in the tunnel. There appears to be NO white rot on any bulb. The accompanying picture shows over 130 bulbs, grown in a bed of about 30 square feet.

I can’t account for the success of this crop in any other way than by control of water. The tunnel has the same soil as outside, and has been added to by liberal application of home-made compost which must have plenty of infection in it.

Maybe worth a try if you have a tunnel, can spare the space and do lots of cooking with expensive French style shallots. The seed retails at about £2 for 200 and the strain was Zebrune.