Fiann Ó Nualláin offers advice on preventing and treating a fungal infection that disrupts healthy growth.
CLUBROOT is a fungal infection of Plasmodiophora brassicae — and in that scientific name we spy “brassicae”, a large clue to what it infects.
So apart from cabbage, kale, cauliflower, broccoli, Brussels sprouts and Asian greens, it can also disrupt the healthy growth of kohl rabi, swedes, turnips and radishes.
Its common name gives the clue to the type of damage it does. The spores exist in the soil and can enter the plant via its root hairs.
The infection causes the roots of affected plants to swell asymmetrically — to “club” up.
The swelling clubroot becomes more and more ineffective at water and nutrient absorption, leading to stunted growth and long-term cultivation complications.
Malformed roots make plants more prone to wilt and sustaining root rot and ultimately to premature death. The upshot is that crop yields can be reduced by more than 50%.
The big issue with clubroot is that it can linger in the soil for many years and so limit brassica-growing potential onsite for many years to come.
The traditional advice is to swap to an eight-year crop rotation system.
That’s some length of quarantine and not everyone has the space to be adding in more raised beds for an extended system. You can, however, swap to growing your brassicas in containers with spore-free compost.
The site where the spores first manifested will still need to be quarantined for at least seven years or utilised to grow edibles not affected by those spores.
It sounds scary but really its just a big nuisance rather than a total nightmare. We can adapt and survive — thanks to good old gardener’s resilience.
While the pathogen presents as malformed roots, there are ways to spot before the lifting at harvest.
Constantly wilting plants and stunted growth is a big clue but that’s also a sign of poor watering and nutritional deficiencies.
Lift and check, by all means, if this is occurring through the growing year but do note that clubroot-affected plants can display a purple tinge to their foliage.
So while the convention is to wait it out and avoid planting brassicas in previously affected sites for seven-eight years, in truth Plasmodiophora brassicae can survive 20 years in soil without a host crop. So in a way it can be a case of keeping your fingers crossed that it has starved off in time.
You might want to be a bit more proactive and utilise soil drenches or phytoremediation to cull the spore levels.
Before we get to that, I should note that many weeds are in the brassica family and they will keep it ticking along if allowed to fill the gaps in your next crop or in spore affected empty beds — so be ruthless with bittercress, shepherd’s purse and any wild mustards.
It can also find a host in your ornamental borders — in the likes of wallflowers, stock, candytuft, sweet alyssum and honesty — so using the infected bed as a cut-flower patch for a few years’ interim might not do the trick.
In terms of proactivity current conventional fungicides have little effect on this one but enriched soil seems to diminish the disease.
Soil drenches of nettle and comfrey will keep soil at a good health while you wait for levels to drop.
Raising your soil’s pH is another option as most spores find replication more difficult in more alkaline conditions — so some liming will help.
You can go heavy and lime it more than once a year if you will be leaving it fallow for some years but take it easy if you plan to keep that area productive with other crops.
With phytoremediation the best crop to grow is winter rye (Secale cereale) it is not affected by the spores and it seems to diminish the viability of clubroot spores over a growing season.
It can be sown now to over winter or be sown in spring. It can be utilised as green manure and tilled in as early as three-four weeks after germination.
You could figure into your crop rotation plan to have secale in that bed every year as spring or autumn crop and still utilise the place over summer for other non-host crops
The other proactive thing is to not just starve it out but cook it off.
You will have to wait for end of spring to get that ball rolling but come summer you can harness the suns radiant energy and solarise the soil. In spring weed and water then cover.
The idea is to tarp over the affected area with some clear plastic sheets and magnify the suns rays into the area and raise the soil temperature to levels that will kill the fungal spores present.
In the process the higher heat concentration will also kill off many other disease-causing organisms including nematodes, and even cook a broad array of weed seeds.
You will need to leave the plastic on and the soil heating for four to six weeks to be fully effective.
It may not kill off all the club root spores in a single season but certainly it will but a dent in their aspirations.
A mix of summer solarisation, winter rye and non-host plants or fallow practice at other times will speed up the elimination process.
Be cautious when composting and amending soil that you are not moving spores about and be vigilant with hygiene of tools and boots when moving from work in an infected area to other sections of the garden.
Prevention being better than cure; crop rotation is key and you can also select disease resistant brassica varieties to grow.