A paper on the above appeared in The Plantsman, December 1993, Vol 15, p.148-160, and the summary below is by permission of the Editor of The Plantsman.
Clematis wilt is caused by a vulnerability to a number of fungi of which Ascochyta clematidina (Phoma clematidina Thüm.) and Coniothyrium clematidis-rectae have been identified. The involvement of Ascochyta clematidina was first reported by Gloyer in 1915 and has been verified by independent workers in the United Kingdom, The Netherlands and New Zealand. Coniothyrium is the most lethal fungus in The Netherlands. This may be true elsewhere due to the diversification of trade in clematis.
It is probable that the loss of resistance developed from the very active hybridisation programme in the period 1860 to 1880. The author is of the opinion that the clematis carrying the vulnerability may be C. lanuginosa and C. fortunei.
The fungus attacks young succulent plants, especially just before blooming. Woody stems are more resistant. The lesion girdles the plant at or between nodes killing the stem above it. The lesion may be below or above soil level and it may or may not kill a plant. The stems collapse within days of the onset of symptoms. Stem rot and leaf spot are linked. Leaf spot is more likely to occur on thin-leaved varieties and more on the lower side of a leaf than the upper.
An important precipitating factor is damage to the stem. Other precipitating factors are high humidity and a temperature between 20oC and 25oC.
The species are not completely immune but have better defence mechanisms. In hybrid clematis, varieties vary in their resistance to wilt.
Wilted stems should be severed below the site of attack to prevent spread of infection to the roots.
The best method of control is by modern fungicides. Systemic fungicides should be put on to wet soil and watered in. The application should start early in the growing season and be repeated every month. It should be noted that benomyl and carbendazim are most effective at 0.2% strength of solution. The fungicide used should be changed at least annually.
Plant hygiene is very important in the garden and in the nursery. Dead material should be removed immediately and burnt. Sterilisation of the medium for growing cuttings helps cuttings to become established. Spraying cuttings with fungicides is worthwhile. As infected plants can produce good cuttings, this fact might lure the grower to complacency.
But further research on every aspect of clematis wilt is still needed. Some of the avenues to be explored are:
1. The use of fungicides to control wilt is promising, but further trials would identify more effective fungicides. Optimum concentrations of fungicides and the best methods of application need to be established.
2. Two of the fungi causing wilt have been studied. Other suspected fungi need similar study.
3. A study of plant breeding, provided that sufficient data is available, may be able to identify the clematis that introduced vulnerability at the time of hybridising work in the last century. This clematis and its progeny could be eliminated from hybridising programmes or genetic engineering remove the vulnerability (see 6 below).
4. It should be possible to establish whether an alkaline medium reduces the incidence of wilt.
5. The system of propagating and growing clematis plants should be examined with a view to introducing proven hygiene procedures.
6. Biotechnology offers techniques for overcoming plant vulnerability to fungi - by producing new plants with disease resistance, by identifying the genes responsible for the vulnerability and removing them or by introducing a gene responsible for the production of defensive enzymes.
7. To explore ways of increasing immunity such as: by pre-immunising with less damaging fungi or by spraying with salicylic acid and similar synthetic compounds to create an 'induced resistance'.
8. In the New Zealand trials there is a suggestion that the species clematis have strong defence mechanisms against wilt. It might be possible to identify fungi or agents involved in the mechanisms that might be antagonistic to the wilt fungi. Work on this area has been reported from Russia.
9. The optimum use of fertilizers, especially in the first two years, should be established.
10. Varieties vary in their resistance to wilt. To establish vulnerability of each variety would be a guide to the grower, of value in hybridising and useful information for the plant geneticist.
11. The optimum period for alternating fungicides should be established.
12. The value of sterilising soil and media for cuttings should be estimated.