clematis wilt -

clematis wilt

Types affected

This is by far the most important disease of the early large-flowering hybrids (pruning group 2) which have predominantly C. lanuginosa or C. patens in their breeding (C. ‘Fortunei’ and C. ‘Standishii’ used by the early breeders). Cultivars such as ‘Vyvyan Pennell’, ‘Henryi’, ‘William Kennett’ and ‘Duchess of Edinburgh’ are particularly susceptible. Late large-flowering cultivars (pruning group 3) which have a weaker background of these species can also be affected but to a lesser degree. C. viticella and the small-flowering species are resistant to wilt.

Symptoms

The first symptoms are typically a wilting of the growing tips as if the plants are lacking water. This is followed by a rapid drying out or a blackening of the leaves and death of one or more shoots. The disease appears suddenly, often just as the plants are about to come into flower. Wilt fungi also infect petioles causing individual leaflets to hang and turn black.

Blackening of the foliage of C. ‘Lasurstern’

From green to crispy within the space of a week: C. ‘Perle d’Azur

Wilting is caused by lesions which block the conducting tissues. The location of the lesions in stems can be traced by slicing affected stems lengthwise in half, starting at the top. In most cases, an internally-blackened area of stem extending about 10cm or so in length will be detected at or just below soil level. Such lesions can, however, be found higher up the stem as well as below ground level. More than one lesion may occur on a single stem.

A lesion located 30cm above soil level on a single stem of C. ‘Perle
d’Azur’

A lesion occurring at soil level on C. ‘Xerxes’

On older plants which have been planted more deeply, similar lesions can be found well below ground level. Shoot and root re-growth often occurs above the affected area but the old lesions serve as an ongoing source of infection. In this example on a 20 year old plant which was going into decline, the roots are also showing disease symptoms (see section on root rots).

An old stem lesion located well below soil level on a 20 year-old ‘Lasurstern’

Conditions for disease development

Moist conditions are needed for infection of the young stems or petioles but symptoms may first appear some time later during dry weather. Blackening of the leaves occurs especially during prolonged periods of rain.

Control methods

First establish that the symptoms are due to the wilt fungus by examining the stems as described above. Remove all affected stems at soil level and destroy them by burning. Remove all plant debris from soil surface around the plant. In spring and early summer ensure that the plants are fertilised and watered well.

Consider applying a fungicide drench around the base of the plant and reapplying at regular intervals to protect new growth. Fungicides which have shown activity include carbendazim and thiophanate methyl (benzimidazoles) but these both have the same mode of action and resistance develops to them very quickly. Other active fungicides include difenoconazole and azoxystrobin which, if used in an alternating spray schedule, could provide an alternative to the benzimidazoles. Follow local regulations concerning the use of fungicides.

Regularly check plants and remove stems showing disease symptoms. Plants are not killed outright by wilt and new shoots will arise from the base. Only in cases where very susceptible cultivars continue to disappoint year after year by wilting just as they are about to flower is there a justification for digging out the whole plant. Do not replant with a large flowered hybrid. Once wilt is present in the garden, it is usually very difficult to eliminate it. Spores of the fungus are spread by rain splash, not by wind, and with good hygiene it may be possible to grow healthy plants in a part of the garden away from the contaminated area.

Wilt is invariably first introduced into the garden on symptom-free nursery plants. Nowadays, some of the leading producers are only too aware that good hygiene is the key to producing disease-free plants and dependence on chemicals is kept to a minimum. This includes starting with healthy plant material, never allowing the plants to come into contact with soil; the use of fresh soil-less, disease-free compost; no reuse of pots; the disinfection of plastic houses and production areas. Their plants are no more expensive than those from other suppliers but you may have to take a little more trouble to find them and to be prepared to pay shipping charges. This is a small price to pay for healthy plants – take the trouble to find these producers. Avoid impulse buying from questionable outlets.