How to recognise it
The symptoms of the different types of wilts (see ‘other useful information’ below for the different types) are all similar. The symptoms take various forms:
- The lower leaf stalks bending downwards (called epinasty).
- General wilting, which may initially recover at night.
- Leaves yellow and then shrivel (where the leaves grow as a rosette the outer layers are affected first).
- A bluish striping on one side of raspberry canes.
- Sudden and simultaneous wilting of leaves in hot weather.
While environmental conditions (such as under- or over-watering and root damage) can cause similar symptoms, in these cases the younger leaves are usually the first affected. With wilt the older leaves are also affected and, often, the symptoms appear initially on only one side of the plant.
The easiest way to identify whether the problem is wilt is to look inside the stem for discolouration. If you cut off a stem well above ground level you will see a black or brown discolouration of the xylem vessels which conduct water around the plant. If a discolouration is only found at the base of the plant then this is more likely to be due to root damage or disease than due to wilt.
Why it’s a problem
It is believed that wilt causes toxins to be introduced to the water conduction system (xylem) within the plant, which can lead to blockages of the transport vessels (either cause directly by the fungal infection or due to cells being killed and then blocking the tubes). This can prevent the plant from receiving water supplies being brought in by the roots.
Some wilt fungi can survive in the soil for several years, readily infecting new plants. Wilts are often associated with the presence of eelworms; it appears that the wilt fungus uses the eelworm entry points to infect the root, although plants with undamaged roots can also suffer from wilt.
Where you are likely to find it
Wilts affect a wide range of plants. It is most commonly seen on acers, antirrhinums, asters, beans (French and runner), begonias, barberries (Berberis), brassicas, catalpas, cherries, chestnuts, chrysanthemums, Cotinus, cyclamen, cucumbers, dahlias, daphnes, delphiniums, Dianthus (including carnations and pinks), elms, fuchsias, godetias, heleniums, Koelreuteria, limes, lupin, paeonies, peas, poppies, potatoes, privet, quince, raspberries, rhododendrons, rhubarb, robinias, roses, strawberries, sumach, sweet peas, tomatoes and wallflowers.
It can affect plants grown in glasshouses as well as outdoor crops or ornamentals.
How to prevent it
Some wilt-resistant raspberries, strawberries and tomatoes are available to gardeners, and some susceptible plants are grafted onto wilt-resistant rootstock.
When planting in glasshouse beds do not plant too early or into cold soil. Tomatoes grown repeatedly in glasshouse beds can be particularly vulnerable and alternative growing media, such as containers, growbags or ring culture would be better.
How to get rid of it
If you have an outbreak in a glasshouse raising the temperature can help plants to recover, although the effectiveness of this varies by the type of wilt you have (and it’s impossible to diagnose the specific fungus causing wilt without scientific investigations). Increasing the shading and watering by spraying the leaves (rather than watering at the base of the plant) may also help. Packing the bases of the stems with damp soil-less potting compost can also help with mild infections by encouraging new, healthy roots to emerge further up the stem.
If you have had a wilt infection in a glasshouse, once the plants have been dealt with, the glasshouse and all equipment within it should be thoroughly disinfected.
Any tools you have used to deal with plants which may have wilt should also be disinfected and gloves/hands washed thoroughly.
Outdoors any affected plants should be removed and destroyed (not put on the compost heap). As much soil as possible should be removed from the affected area and you should avoid planting any of the plants which are susceptible to wilt in the area for as long as possible (several years is best). Crops such as carrots, parsnips and parsley can be grown, as these seem to be resistant to wilt.
Is it good for anything?!
No
Other useful information
Wilts are caused by various different fungi, most of which are part of the Deuteromycete class of fungi (“imperfect fungi”). This includes, for example, the following wilts:
- Verticillium wilt is the principle genus of wilts and includes Verticillium dahliae and Verticillium alboatrum.
- Fusarium wilt, including Fusarium oxysporum, which mainly affects China asters (Callistephus), carnations and peas.
- Diplodina passerinii.
Some wilts may also be caused by bacteria, which is mainly seen with carnations and wallflowers.
