Phoma Clematidina

''Calophoma clematidina'' is a fungal plant pathogen and the most common cause of the disease clematis wilt affecting large-flowered varieties of ''Clematis''. Symptoms of infection include leaf spotting, wilting of leaves, stems or the whole plant and internal blackening of the stem, often at soil level.van de Graaf P. (1999). ''Biology and Control of Phoma clematidina, causal Agent of Clematis Wilt''. PhD thesis, University of Derby, Derby, UK. Infected plants growing in containers may also develop root rot. 3, 3·7·67.

Taxonomy

The asexual stage (anamorph) of the fungus was first described by the German botanist and mycologist Felix von Thümen in 1880 as ''Ascochyta clematidina''. Based on new scientific insights into the differences in spore formation between species, it was reclassified as ''Phoma clematidina'' by the Dutch mycologist Gerhard Boerema in 1978. Boerema G.H. and Dorenbosch M.M.J. (1979). "Mycologisch taxonomisch onderzoek". ''Verslagen en Mededelingen Plantenziektenkundige Dienst Wageningen'' 153 (Jaarboek 1978): 17-21. More recently, multi-locus phylogenetic analyses led to the fungus being reclassified again as ''Calophoma clematidina''.

Genetic sequencing has suggested that ''Calophoma clematidina'' is heterothallic which means that two compatible strains (mating types) of the fungus would need to come together under the right environmental conditions to produce a sexual stage (teleomorph). Both mating types of ''Calophoma clematidina'' are known to occur in Europe, and yet no sexual stage (which is most likely to be a ''Didymella'' species) has ever been described.

Molecular phylogenetic analyses have revealed that some fungal isolates recovered from wild ''Clematis'' species, previously identified as ''Calophoma/Phoma clematidina'', are in fact two closely related species of ''Phoma'' and ''Ascochyta'' with sexual stages in ''Didymella''. ''Didymella clematidi'' has been successfully used as a biological control agent of ''Clematis vitalba'', which is seen as an invasive plant in New Zealand. . Unlike ''Calophoma clematidina'', the two closely related ''Didymella'' species (and their anamorphs) have not been found on large-flowered ''Clematis'' varieties.

The previous misidentification of these species means that some literature referring to ''Calophoma clematidina'' or ''Phoma clematidina'', particularly that on the biological control of '' C. vitalba'', is actually describing work on ''Didymella clematidis'' and its ''Ascochyta'' anamorph.

The situation is further complicated by the existence of another ''Calophoma'' species known to infect ''Clematis'': ''Calophoma clematidis-rectae'' (syn. ''Coniothyrium clematidis-rectae''). This species is closely related to ''C. clematidina'' and has in the past been implicated in cases of clematis wilt in the Netherlands. Blok I. (1965). "Verwelkingsziekte in clematis". ''Jaarboek Proefstation voor de Boomkwekerij te Boskoop 1964'': 82-87.

Biology

The biology of ''Calophoma clematidina'' is now reasonably well understood thanks to research into clematis wilt at the University of Canterbury in New Zealand Smith G.R. (1987). ''Leaf Spot and Wilt of Clematis caused by Phoma clematidina (Thum.) Boerema''. PhD thesis, University of Canterbury, Christchurch, New Zealand and the University of Derby and ADAS in the UK.

A full overview of its life cycle was first produced in 1999. Within a nursery or garden, the fungus is mainly spread through splash dispersal of its asexual spores (conidia) which are formed in bulbous fruiting bodies called pycnidia. Pycnidia can be formed on any infected part of the plant showing symptoms, including leaves, stems and roots. The pycnidia exude the spores in a sticky mass which is splash dispersed to other plants nearby during rain or irrigation. Contact, such as with pruning tools, may also spread the spores and, in addition, it is suspected that certain insect species may act as contact vectors. Movement of infected plant material forms an important method of spread to previously unaffected nurseries or gardens.

Unlike many other plant pathogens, ''Calophoma clematidina'' is not a biotroph which means that it can infect plants but does not need them to survive. The fungus can live saprophytically on dead plant material or organic matter as well. In addition, it forms thick-walled resting spores (chlamydospores) which increase its survival in plant debris and soil during unfavourable conditions.

Disease development

Many publications report ''Calophoma clematidina'' to be a wound pathogen. However, although wounding may aid infection, scientific trials have shown that, in susceptible ''Clematis'' varieties, the fungus can cause extensive leaf spotting and wilt without any previous damage to plant tissues.

When spores of ''Calophoma clematidina'' land on a leaf of a susceptible plant under the right environmental conditions (moisture, temperature ), they will germinate within 6 hours and infect leaf tissues directly or via leaf hairs (but never via stomata). Initial symptoms are small brown lesions which then rapidly spread, eventually killing the whole leaf. Unless the plant abcises the leaf in reaction to the infection, the fungus will spread via the leaf stalk to the node of the stem.

Infection may also occur directly into the stem. This is particularly common at or below soil level where the right amounts of moisture are more likely to persist. Once in the stem, ''Calophoma clematidina'' destroys the internal tissues causing a distinctive, localised black discolouration. Such stem infections can be hugely destructive as they block the plant's vascular system and will lead to the wilting and death of all plant mass above the site of entry, no matter how substantial.RHS Clematis wilt
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Relatively recently, it was discovered that ''Calophoma clematidina'' can also infect the roots of ''Clematis'' and be a cause of black root rot. In practice, this has been found to be a particular issue in containerised plants.

Host susceptibility

Many gardening publications express a view on the susceptibility of different ''Clematis'' varieties and species to clematis wilt based on observations in practice, but very few comment on their susceptibility to disease caused by ''Calophoma clematidina'' in particular compared with other causes of wilt.

Scientific trials have shown that large-flowered varieties are especially susceptible to disease caused by ''Calophoma clematidina'' in line with their vulnerability to wilt observed in practice. It is believed that hybrids which have '' Clematis lanuginosa'' in their ancestry are most susceptible to wilt. Such hybrids include many cultivars in the early flowering, large-flowered group (such as ''C.'' 'Nelly Moser' and ''C.'' 'Elsa Späth') and, to a lesser extent, cultivars in the late flowering, large-flowered group (such as ''C.'' 'Jackmanii' and ''C.'' 'Perle d'Azur').Howells J. (1994)
"Vulnerability to clematis wilt in large flowered clematis". ''The Clematis'', p. 51.
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''Clematis viticella'' and related varieties are also susceptible to infection with ''Calophoma clematidina'', yet are reported to wilt less often than large-flowered varieties, possibly because they are very vigorous and outgrow infections quite rapidly. Most other cultivated ''Clematis'' species and their varieties are largely resistant to both infection with ''Calophoma clematidina'' and wilt in general.

''Calophoma clematidina'' has sporadically been reported from species of ''Clematis'' growing in the wild, including '' Clematis orientalis'' in Russia and '' Clematis pubescens'' in Australia. Reported isolations from wild '' C. vitalba'' in Europe have proven to be an unnamed ''Didymella'' species rather than ''Calophoma clematidina''.

As biological control

This fungus has been reported as having been successfully used as a biological control agent of ''Clematis vitalba'', which is seen as an invasive plant in New Zealand. However, further genetic studies of the isolates used in those trials have revealed that the fungus released as biocontrol agent had been misidentified and was not ''Calophoma clematidina'', which thus far has never been found on ''Clematis vitalba''.

Management

Benzimidazole fungicides are used. Benzimidazole resistance has occurred.

References

{{Taxonbar, from=Q7187110

Fungi described in 1880
Fungal plant pathogens and diseases
Ornamental plant pathogens and diseases