Q What is parsnip canker?
A Black or orange patches on the roots are a sign of parsnip canker. The patches usually occur near the shoulder of the roots and the crown of the plants. In severe cases, the roots crack and rot. There are two kinds.
Black canker, which is actually tinged purple. It is considered to be mainly caused by the fungus Itersonilia pastinacae. Other fungi thought to be involved are phoma and Mycocentrospora acerina.
Brown or orange canker is thought to be caused by invasion of the fungus Itersonilia pastinacae. It is not usually damaging, but it gets into damaged roots. It is also responsible for leaf-spot disease.
Caption: Cankers causes parsnips to rot
Q Can I confuse it with anything else?
A The rusty-brown rotting roots caused by carrot fly can be confused with parsnip canker. Look for the tunnels and possibly maggots. Rots caused by sclerotinia don’t usually develop on roots in the ground, but can do on ones in store. The fluffy white mould and associated resting bodies are quite different from canker.
Q Where does parsnip canker come from?
A The fungi survive in the soil and build up to damaging levels if too many parsnip crops are grown in the same place too often. There is some evidence that the disease is carried on the outside of seed, and in seed that has not been cleaned properly. There is no way for a gardener to tell if the seed is contaminated.
Q What makes it worse?
A Predisposing factors include a badly drained soil. A low or acid pH also makes the disease more likely and more severe.
Carrot-fly injury and damage from hoeing can let the disease into the roots. Keeping down carrot fly by covering the roots with fleece or insect-proof mesh in May and June will reduce damage. Hand weeding may be preferable to hoeing, especially if you’ve got cloddy or stony soil.
Early sowings and large roots are worst affected. Sowing in late April, May and even June will help reduce incidence. Spacing more closely than is usual will produce smaller roots. Where canker is a problem, aim for plants about 7.5cm apart.
Q Can it be cured?
A Once the canker has started there is no way to reverse the effect. No amount of watering, liming or fertiliser will have any effect. Once an attack has been detected, use up the roots as fast as possible. Ideally, harvest roots immediately and store in damp sand. Don’t let debris get back on to the vegetable plot. Clear away all crop remains in the spring. This reduces the amount of disease organisms in the soil.
Q Can it be avoided?
A Improving drainage, by putting in drains, double-digging to destroy compacted soil and making raised beds, will reduce the severity. Improving drainage can be difficult on clay soils however. Adding organic matter, such as mushroom compost, well-rotted manure and garden compost, will improve the soil structure, which in turn enhances drainage. Although it is often said that manuring causes fanged or forked roots, there is no evidence this is the case.
If your soil is acidic, add lime to raise the pH to 6.5. If you don’t know the pH, cheap testing kits are available.
Q Can it be prevented?
A Total prevention on soils pre-disposed to canker is unlikely, but the incidence of canker can be greatly reduced by following the guidelines laid out here.
Q What chemicals can be used to control it?
A No chemicals have been found that prevent or cure the disease.
Q Are there any resistant varieties?
A Canker-resistant varieties are available – 'Avonresistor' and 'Cobham Improved Marrow' were the first. Most of the more recent varieties are F1 hybrid varieties and have have some degree of resistance including, 'Albion', 'Archer', 'Gladiator', 'Javelin', 'Palace', 'Panache' and 'Picador'. They should always be used where canker is a problem. They are the most cost-effective control measure although cannot be guaranteed.
Q Are there any varieties I should avoid?
A 'Offenham' is reputed to be especially susceptible, and 'Lancer' seems less resistant than those mentioned above.
Suppliers of insect-proof mesh and fleece
Ferndale Lodge 0844 314 0043 ferndale-lodge.co.uk
Gardening Naturally 0845 680 0296 gardening-naturally.com
Harrod Horticultural 0845 402 5300 harrodhorticultural.com
The Organic Gardening Catalogue 01932 253666 organiccatalogue.com
