IDENTIFY, PREVENT, TREAT ONION WHITE ROT
Of all the pests and diseases which affect onions White Rot is definitely the most serious and it is also a common problem especially on allotments. The fact that the disease can remain in the soil for up to 16 years also contributes to the devastating effect it has.
Combine that with the fact that there are currently (2018 / 2019) no guaranteed cures for this disease and you will understand it is one of the gardeners top enemies as far as onions, garlic and leek are concerned
HOW TO RECOGNISE ALLIUM WHITE ROT
One of the most annoying features of white rot is that by the time you realise your onions have it they will have become severely infected. The signs become apparent above ground level in July to August for spring sown / planted onions and in May time for Japanese onions. By that time you may well be expecting onion foliage to begin to yellow as maturity approaches but it may, in truth, be the first sign of White Rot.Dig an infected onion up and you will see it looks rotten and feels soft to the touch. White and powdery mould will have formed on the base of the onion with black spots in advanced cases. There will be no signs of grubs or pupae (unless as a secondary infection) because this is a fungal disease (Sclerotium cepivorum) and not caused by a pest.
LIFE CYCLE OF ALLIUM WHITE ROT
Understanding the life cycle of this pest is key to deciding how you wish to deal with it. It begins in a most unusual way for a garden pest. Tiny organisms (called sclerotia) are in the soil (see below for how they get there) and they wait until they sense that onions and members of the onion family, such as garlic and leeks, are growing in the ground nearby. They do this by detecting the odours given off by onion roots.
When onions are detected the organisms germinate and produce even smaller fungi which latch onto the roots. As the fungi mature they produce more sclerotia within a few days and the cycle goes on and on throughout the season. When late autumn arrives they become dormant and are ready to reproduce next time they detect onions. Nasty organisms aren’t they! They appear to benefit nothing in the plant world other than themselves.
If the life cycle of this disease has a weakness it appears to be the fact that they germinate only once. Having germinated, if they do not find a plant to infect they will die. See the section below about treating white rot for more details for exploiting this part of the life cycle.
HOW DOES WHITE ROT GET INTO MY GARDEN / ALLOTMENT?
When ground is infected with white rot it can remain in the soil for many, many years and is almost certain to re-infect any new onion plants. Almost all allotments are infected to some degree or other although many gardens have no infection at all. Where the fungus does exist, it is found in the top parts of ground. The most common method of transferring it from an infected area to an uninfected area is on your boots, gardening implements and from infected plants. Cleanliness is therefore key if you want to avoid introducing white rot into your garden.
TREATMENT OF ALLIUM WHITE ROT
Currently there are no chemical treatments available to the amateur gardener to kill white rot fungi. Even in the commercial agricultural world the available treatments are very limited and do not guarantee eradication of this disease.
PREVENTING ALLIUM WHITE ROT
Cleanliness is the first key rule primarily to avoid bringing any soil into your garden / allotment which may be infected. Boots, spades, forks any plants grown in soil are the culprits as far as importing this disease into your soil.
Always buy plants, seeds and sets from reputable suppliers and beware anything bought at boot sales and offered to you by neighbours.
The above paragraphs are well-researched and proven but they do not tell the whole story about preventing allium white rot. Field scale trials in 2007 indicated that adding specially prepared onion waste into ground infected by white rot was very effective at clearing the ground of infection. The waste onion materials were composted so that they reached a minimum temperature of 50°C for three days. The compost was turned frequently to ensure all parts reached this temperature and killed all bugs and seeds. It was applied to the ground in spring and a crop, other than onions, was grown in the land. Next year, onions were planted and they grew very well with little signs of white rot.
The thinking behind the field trials was that the application of onion compost to the ground stimulated the white rot fungus to believe onions were growing. The fungus then germinated (they can only germinate once) and died because no onions were present in the ground.
Do what you will with that information but two other factors must be taken into account before the amateur gardener attempts to mimic the field trial at home. First, the composted onion material must be added to the ground within three months of the composting process to ensure that the chemicals which cause the fungi to germinate are at a high level. Finally, the fungi are temperature sensitive and will only germinate when the soil temperature is between 10°C and 20°C. Below that temperature range the fungi become dormant, above that range and they simply will not germinate.
For those wanting to investigate the control of white rot further, the following article investigates how the addition of garlic powder has the potential to reduce white rot. This article also investigates the effect of flooding on the fungi.
Other common pests and diseases which affect onions include: