This can lead to loss of turgor pressure in cells and flaccid tissues, which can lead to wilting in bedding, pot plants and nursery stock.
There are fungal and bacterial pathogens of bedding, pot plants and nursery stock that are commonly referred to as "wilts" because they infect and damage the plant's vascular system, disrupting the flow of water from roots to leaves and causing them to wilt.
The fungal pathogens most usually referred to as wilts are soil-borne species of Verticillium or the closely related Phialophora, or host-specific strains of Fusarium oxysporum. Fusarium wilt can be a particular problem on cyclamen and is an increasing issue for growers of cut flowers, such as stocks and Lisianthus.
The clematis wilt pathogen (Phoma clematidina syn. Asochyta clematidina) behaves differently from Verticillium or Fusarium wilts.
Pelargonium growers need to be aware of the bacterial wilt Ralstonia solanacearum, a notifiable disease because of the threat to potato crops. The main risk to pelargonium in the UK is from importing infected plant material. The ability of the pathogen to lie dormant in latent infections makes it difficult to find in routine inspections.
Correct and accurate identification of the cause of wilt symptoms is crucial, especially because there are so few chemical controls that will deal with true wilts. Clean stock, good hygiene and either soil sterilisation or clean growing media are key control strategies.
The tree, in a garden in Southwell, Nottinghamshire, is the parent of all the commercial and domestic Bramley apple trees in the world. But it is currently ravaged by honey fungus and has been predicted to live only another two years without successful intervention.
How to recognise it
With fungal or bacterial wilts, older or lower leaves, or outer leaves of a rosette plant, will tend to be affected first. Wilting caused by physical root damage or incorrect watering affects upper or younger leaves first. Fungal or bacterial wilts may affect only one side of a plant initially.
Confirm Verticillium or Fusarium wilts by looking for characteristic brown or black discolouration in the conducting tissues of the stems, by cutting through a stem well above ground level. You should be able to trace the discolouration both up and down the stem. Should you find discolouration only at the base of the stem, the wilting is more likely to be associated with root damage than a fungal pathogen.
Fusarium wilt in cyclamen causes brown vascular staining in the corms, but this can also be caused by the Botrytis fungus.
Verticillium and Fusarium wilt can persist for many years in soils or in growing media and plant debris in the form of resistant mycelium or survival spores.
Root wounds caused by pests, such as nematodes, may provide a way in but the pathogens will also attack roots damaged during transplanting, young roots and root systems of plants already weakened by stress or other diseases.
The wilting symptoms are caused by damage to the water-conducting vessels. In addition to physical blockage caused by the fungal mycelium, cell collapse and exuded gums and other materials, toxins produced by the pathogen are believed to play a role.
Plants commonly susceptible to Fusarium include aster, begonia, cyclamen, daphne and dianthus. Those susceptible to Verticillium include acer, antirrhinum, aster, berberis, begonia, chrysanthemum, catalpa, Cotinus, dahlia, geranium, Fraxinus, impatiens, Paeonia, pelargonium, petunia, prunus, ribes, sorbus and Tilia.
Fungi can grow and reproduce on the dead plant material of a range of species, do not need ready-made entry points such as wounds and do not produce extensive hyphal growth in plant tissue.
Obvious wilting of leaves - leaf stems that bend downwards followed by yellowing and shrivelling of leaves.
In clematis wilt, young leaves may droop suddenly and the upper surfaces of leaf stalks may blacken. Affected leaves may die quickly. Patches on stems at or near ground level. Clematis not coming into growth in spring may be an early symptom.
Treatment: biological control
Streptomyces species are naturally occurring soil bacteria that are the main ingredient of commercial preparations used in some countries to control fungal pathogens, including Fusarium wilt. Some are available as biological root inoculants that can help to improve resistance to environmental stress, promote nutrient uptake and stronger growth.
Trichoderma spp. can enhance plants' natural protection against a range of fungal pathogens, including Fusarium. Products include Trianum P/Trianum G or T34 Biocontrol*. Products based on Bacillus subtilis include Serenade ASO or Revive.
For field-grown crops, there is a range of specially developed mustards called the Caliente series from Tozer Seeds. The large amount of organic matter produced during growth is chopped and incorporated into moist soils, which release isothiocyanates. These compounds may give some control over a range of soil-borne diseases.
Treatment: cultural control
- Remove and destroy or securely quarantine any plants showing wilt symptoms, or propagation material that may have originated from them.
- Quarantine or monitor any bought-in stock. Always obtain bought-in material from a reputable source.
- Use a good horticultural disinfectant between crops and disinfect capillary mats at least once a year. Disinfect tools or equipment between plants or batches.
- In pot plants, Fusarium and Verticillium wilts may spread faster on ebb-and-flood benches than on capillary matting. Clematis wilt is more likely to be spread by overhead irrigation than by drip.
- Growing media containing bark, wood fibre or crushed shells may help to suppress wilt.
- Do not pot too deeply and avoid root damage during potting or transplanting.
- Ensure correct irrigation and nutrition for strong growth.
- Control root pests, which may provide entry wounds for wilt fungi, and weeds, which may harbour the pathogens.
Treatment: chemical control
Active ingredient Chloropicrin
IRAC code 8B
Formulation Custo-Fume* (Custodian), K&S Chlorofume* (K&S Fumigation)
Action(s) Needs to be applied with specialised injection equipment with polythene sheeting (150 gauge) being laid down over soil as the treatment proceeds.
Active ingredient Dazomet
HRAC code Z
Formulation Basamid (various)
Action(s) Granular soil fumigant for pre-planting use outdoors or under protection.
Active ingredient Metam-sodium
HRAC code Z
Formulations Various including Discovery (United Phosphorus)
Action(s) Partial sterilant for glasshouses, nurseries and outdoor soils.
Active ingredient Prochloraz
FRAC code 3
Formulation Scotts Octave (Everris)
Action(s) Broad-spectrum protectant and eradicant fungicide with known activity against Fusarium.
Active ingredient Thiophanate-methyl
FRAC code 1
Formulation Cercobin WG* (Certis)
Action(s) Systemic fungicide with protectant and curative activity. Restricted use to crops under permanent protection.
- See more at www.horticultureweek.co.uk/plant-health
Fully updated by Dove Associates
Use plant protection products safely. Always read the label and product information before use.
* Extension of Authorisation (EoA) required for use in protected/outdoor ornamental plant production.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.