Botrytis blues

According to numerous reports I've received, up and down the country it has been a really bad year for allium diseases, especially during the late ripening process in the ground or in storage. The main culprit is a fungus called botrytis, although there are several types of botrytis that act in slightly different ways. You may know them commonly as grey mould or neck rot. Botrytis leaf blight, or leaf blast (below) is a less common fungus, and certainly not one I've witnessed personally.

The important things to know are that the spores overwinter on infected garden detritus and leaf tissue. It can also store on dirt piles in storage sheds and there are reports it can survive on seeds for 3 years and then transmit to the emerging seedlings. As well as spreading through the soil via the mycellium, spores can also be spread on the wind so as you can see there is a wide scope for being affected at some point or other, even in greenhouses.

The best time (or worst depending on your viewpoint) for infection is during cool, moist, humid conditions which for us in the UK is during mid to late summer when we're thinking of getting our onions up. With the cool summer we've just had you can see why botrytis infections have gone through the roof this year. The botrytis will enter the onion through the tiniest of wounds in the skin or leaves, or via the succulent necks after the tops have been removed.

There are a number of things we can do to minimise losses apparently. First of all consider immersing your seed in a suitable fungicide prior to sowing. For this purpose I have acquired some Rovral fungicide and will be doing this to my seed next season for the first time. If growing in the greenhouse, make sure everything is scrupulously clean and with this in mind I will be giving everything a good spraying and drenching with Jet 5. I'm reliably informed that another, cheaper product called Red Label will also do a good job. Failing that, try a solution of cheap bleach, and add a dash of bleach to your water butts to kill any pathogens lurking in the water.

During the season make sure that you remove all weeds from the greenhouse borders and edges, or from the outside beds. Any onion debris infected or otherwise should be cleared and disposed off, although not on the compost heap. Do not apply nitrogen in the mid to late season of the onion's growth, as this will delay ripening and increases the likelihood of storage losses as the tissue will be too soft and therefore susceptible. Do not water 2 to 3 weeks before lifting and apply a late spraying of fungicide (Rovral) just before lifting when it's not raining or rain is forecast. When actually lifting take extra special care not to damage the onion's tissue, and consider letting the foliage die back naturally rather than cutting as is advocated by most of us in the showing game these days. I've always cut the tops off which I think should be fine if you're ripening off in the house but if you're leaving them in a garage, shed or other outhouse then it may be advantageous to leave the foliage on to dry out and die back naturally. Cool, dry conditions with good airflow are best for storing onions. Ivor Mace has his in a spare bedroom I believe.



It has been a disastrous year for storing shallots for much the same reason. I know of one grower who binned several hundred shallots that all went rotten with botrytis, and I myself have also had to discard about 60% of my crop. After a few 'giveaways' I only have 15 bulbs left for replanting. Dave Thornton reckons he suffers very few losses and the precaution he takes is to scrape the base plate of the bulb so that it is moist. This is then dipped in a Rovral solution to the recommended manufacturer's rate and he is convinced this is why he gets very few rotten bulbs, so I will be doing that to my remaining shallots this weekend hopefully. Another problem with my staged shallots this year was a marked purple tinge to the ripened skins, which I have also suffered from time to time in the past. DT informs me that this is a fungus called 'psytocyanin' and is the result of fluctuating storage temperatures between night and daytime. As I store mine outside on a wire rack I can see that this would certainly have been the case this season, the night-time temperatures this year in June/July resembling much cooler climes. I shall be taking steps to store them in the greenhouse next season to see if I can improve this, as you will certainly be marked down on condition because of it.

Although leeks are also alliums of course, they do not seem to be affected by botrytis as far as I can tell.