U. davidiana “Morton” (Accolade)
Accolade is thought to have arisen from a natural crossing of two Japanese elms at the Arnold Arboretum, Massachusetts . It is named for the Morton arboretum, where a mature and much-photographed specimen now grows. In outline it resembles a rather small and delicate American elm, and has therefore achieved a high profile in the US. The leaf is small, dark and very glossy. Dense masses of foliage are formed. Rapid development of the trunk gives good stability from an early age, though the plant is not easy to train into upright growth. Trees grown from open-pollinated Accolade seed at Butterfly Conservation’s Great Fontley trial site have better form and vigour than Accolade itself.
Although Accolade has been widely planted in the US, there are no reliable disease resistance statistics for it. Two trees are known to have contracted elm disease, of which one died. Until proper test results are published, it cannot be strongly endorsed.
Status: introduced to the UK, but subject to patent restrictions and not commercially available. ResistantElms.co.uk has this tree, but is not legally able to propagate it for sale.
Homestead, created by Alden Townsend in Ohio in 1970, is essentially a crossing of two Siberian elms with two European elm hybrids from the earlier years of the Netherlands breeding programme. It makes rapid, erect growth and develops a thick canopy, but is not thought likely to grow particularly large. The leaf is leathery and coarse, with blunt serration and a long stalk. Homestead’s disease resistance is given as 4.5/5 on the Dutch/French scale, but trees are known to have died in inoculation tests. For that reason it is not the ideal choice for a prestigious site. Status: introduced to the UK, free of patent restrictions. Not commercially available but may be obtainable through ResistantElms.co.uk
U. x hollandica “Pioneer”
Pioneer is another Ohio crossing by Alden Townsend in the early 1970s. The tree is an enigma – a straight hybridisation of wych elm and field elm (thus a “Dutch Elm”) which exhibits surprising disease resistance given that parentage. It tends to form a spherical core with a few long branches growing out at various angles to form an open crown. The leaf is understated – rather small, fairly dark green, light gold in autumn, and curiously close to that of Patriot in young specimens.
Pioneer has a landscape or parkland use as far as its form is concerned – broad, open with lower branches attractively downswept when well grown. However, recent inoculation testing in the Netherlands confirms that the tree has only moderate disease resistance, and therefore can only be recommended for a setting in which failure would not matter too much.
Status: introduced to the UK, free of patent restrictions. Not commercially available but may be obtainable through ResistantElms.co.uk.
This is a true U. americana selection which exhibited high resistance to disease in US inoculation trials. It is a large-framed, large-leaved tree for an open site. Unfortunately, an open site is also what it most fears, since it is vulnerable to wind damage during its youth. Side branches must be pruned away before early summer winds split them from the main stem, causing irreparable damage. What’s more, shoots are naturally pendulous under the weight of their plate-like leaves, and while the side-growth will straighten in the course of the season, the leading shoot tends to remain at an angle to vertical.
As Valley Forge nears maturity, it seems to stiffen up and outgrow these cultural problems. Up until that time it is a high maintenance tree, particularly in a windy area, though it is undeniably attractive. As with other US selections, there is a question mark over disease resistance in European conditions.
Status: introduced to the UK, free of patent restrictions. Very limited commercial availability but may be obtainable through ResistantElms.co.uk.
The “Resista” range
The “Resista” elms comprise a selection of four clones made by the Darmstadt nursery Eisele GmbH from amongst a larger number bred in Wisconsin by Eugene Smalley and Ray Guries. New Horizon has two early Netherlands hybrids and Siberian elm as its parentage. Rebona and Regal are crosses of Siberian and Japanese elm. Rebella, a small tree intended as a garden ornamental, is an unusual crossing of American and Chinese elm.
The “Resista” range seems to have been developed to provide an entirely reliable, standardised product to landscapers (chiefly of the urban variety). Specimens are not released until they have a girth of 14-16cm, by which time they will have been transplanted three times. Each plant is micro-chipped as proof of authenticity. It is hard to judge these hybrids’ characters since they take some years to recover from their early standardisation. Regal may be the most interesting, reminiscent of a broad Jersey elm.
Status: patented. May be available in large sizes from some specialist tree nurseries. Regal is not in commerce in this country. Enquiries should be directed to Eisele GmbH.
This, like the contemporary Italian release San Zanobi, is a crossing of the Dutch clone Plantyn with a Siberian elm. Its resistance to elm disease is impressive: only 7.8% defoliation and 3.9% dieback after inoculation. The clone’s vigour is also impressive, but this has led to problems on fertile soils where the tree bulks up before it has the basal development to support itself. On chalk soils, the tree’s growth is naturally slower and consequently better-balanced, but the tendency is for a broad and lopsided crown to develop under most conditions. Without any particularly attractive features of form or foliage, Plinio is stalwart on the right sites, but never a face to launch a thousand ships.
Availability: Status: introduced to the UK, but patented and not commercially available in this country. May be obtainable through ResistantElms.co.uk.
Simpler than many, Fiorente is a straight cross between Siberian elm and European field elm. However, in the words of Alberto Santini of the IPP, the tree has acquired much from its field elm parent. In inoculation trials Fiorente displayed 32.34% defoliation, and 19.89% dieback. This closely resembles the performance of the mid-programme Dutch hybrids “Plantyn” and “Lobel”, and can only be regarded as partial field resistance at best.
Fiorente makes columnar growth in the restricted space available to it on trial sites, but is broader when open-grown. It is unusual among modern elm hybrids in combining strong monopodial growth with only a moderately fastigiate tendency. Its height and girth increments were regarded as exceptional in the earlier years of the Italian programme, but were later exceeded by Siberian x Japanese hybrids.
Stability is excellent, but in view of the low disease resistance this clone should probably go back into the genetic melting plot to pass on some of its desirable characteristics to a new and more resilient seedling.
Status: introduced to the UK, but patented and not commercially available in this country. May be obtainable through ResistantElms.co.uk
This clone’s ancestry is not entirely certain. What is known is that the plant arose from the crossing of FL 148 with U. chenmoui; FL 148 is thought to be a cross between two Siberian elms, but the database of the Italian breeding programme does not establish this beyond doubt. FL 462 is described by the IPP as “resistant”, but in the programme’s usage that tends to indicate only a moderate degree of resistance. The performance of one of the tree’s parents, FL 148, is known with precision from inoculation trials: defoliation of 43.33% and dieback of 17.50%. That is mediocre indeed, but it is likely that U. chenmoui has contributed a higher degree of resistance to FL 462.
FL 462 has a highly attractive form, rare among hybrids – strongly monopodial and slender without being fastigiate. Lower branches are downswept. The tree is highly vigorous. Peeling bark is an oddity casting further doubt on its ancestry. Mixed woodland planting would be tempting; until the clone’s actual resistance is clearer, it cannot be recommended for other sites.
Status: introduced to the UK, free of patent restrictions. Not commercially available, but may be obtainable through ResistantElms.co.uk.
Dutch Elm Committee
U. “Christine Buisman” began life as Clone 24, one of a batch of several hundred seedlings grown in Holland from U. minor seed collected in 1929 in Madrid. The tree was regarded for many years as an U. x hollandica, following Melville, but genetic testing in Belgium subsequently confirmed it to be U. minor. The tree was released in 1937, named in memory of the Dutch phytopathologist Christine Buisman, who had first proved the connection between Graphium ulmi (now Ophiostoma ulmi) and elm disease before her very early death in 1936.
“Christine Buisman” was selected for resistance to the relatively non-aggressive strain of Ophiostoma responsible for the first major European outbreak of elm disease. The view later developed, perhaps as a not unreasonable hunch, that it was not satisfactorily resistant to the highly aggressive O. novo-ulmi which tore through Europe’s elms from the 1960s onwards.
This view is now being re-evaluated. There are indications that Christine Buisman is in fact highly resistant. Tests carried out by Green, Guries and Smalley (1982) in Wisconsin for volume of discoloured wood (VOD) after the inoculation of very young trees in vitro gave figures of 2.2 for the benchmark-resistant Sapporo Autumn Gold, and only 0.6 for Christine Buisman. Some have doubts about the validity of this technique, but the following year’s testing, in which Christine Buisman was not included, gave VOD of 87.2 for English Elm. That figure is very much in line with what would be expected from effective inoculation by the traditional method using a highly aggressive isolate. Meanwhile, Eric Collin reports that Christine Buisman performed better than any other cultivar of U. minor in French testing, and outperformed at least one modern Spanish clone in its ability to recover in the years subsequent to inoculation.
Unfortunately, concerns remain about Christine Buisman’s susceptibility to Nectria cinnabarina (Coral Spot fungus). Although there are reports from some that this is not in fact a problem, at least in mature trees in the Netherlands, there is no smoke without fire. It has been suggested that the cause of the vulnerability is the tendency of wounds to bleed sap for longer than is usual, so that they more readily become infected.
The distinguishing features of Christine Buisman are deeply furrowed bark and a light, open and disordered crown typically formed by three or four major branches departing at upright angles from comparatively low down the trunk. Some low branches leave the trunk at far more obtuse angles, but often change direction abruptly after about a metre’s growth. Branch breakage is frequent, whether because of poor resistance to wind or perhaps girdling by Coral Spot. Leaves are strongly basally asymmetric, broadly oval, mid-green and leathery. Well-grown trees have something of hornbeam or small-leaved lime about them. Trees in less favoured spots seem more like rather compromised birches, recalling Plot Elm but sadly not much of its charm. Maximum height appears to be about 25 metres.
Notwithstanding the renewal of interest in Christine Buisman and the re-examination of its actual disease resistance, the tree seems of limited landscape use. It can develop well when open grown with shelter, but a good many other species would ultimately be more impressive. It may be more suited in aesthetic terms to closer spacing, where repetition might give the eye the key to its rather wayward form which is difficult to assimilate when seen in isolation. Dead wood and breakage in reality would confine it to rustic or woodland settings where those problems would not matter too much.
The gradual rehabilitation of Christine Buisman is welcome, as is any addition to the diversity of resistant elms. However, as matters currently appear, there are better elms available.