In early spring 2014 ResistantElms continued working to develop new elm hybrids, aiming to build on the experience gained during the creation of the Arcadia progeny in 2013.
The principal goals remain (a) to continue crossing the best clones of proven resistance in order to broaden the resources available to those who want to plant elm; and (b) to try to select for morphological characteristics closest to those of the strains of elm which have traditionally grown in this country. A subsidiary aim is to create trees which are unencumbered by patents and licences. While nobody can sensibly object to paying a few pence extra per tree to a licensee, it is a different matter when highly promising new clones are given to exclusive licensees who immediately price them out of reach.
Numerous factors render successful elm hybridisation singularly difficult to achieve. The parent trees must be producing good flowers and pollen, and must be doing so – or must be induced to do so – at exactly the same time. This is hard to achieve unless the plants (particularly those which are to be used as females) are containerised so that they can be moved between different temperature regimes to accelerate or delay their development. In addition, certain elms are female-sterile, and can be used only as male parents. Some begin to flower only late in their development, so that the flower is simply not yet available or is unreachably high in the canopy (that problem was overcome in the extreme case of the 140 foot Chichester elms at Queens’ College Grove in Cambridge by using twigs brought down by gales and squirrels). Genetic barriers to crossing range from those which make it difficult to some instances in which it is known to be impossible. While cross-fertilisation can create hybrid vigour it can also have the opposite result, producing stunted seedlings which fail to develop or die outright. Most frequently, and for reasons which are likely to remain obscure, the attempt just doesn’t work. The ripe seedcases prove to be empty; like those of Virgil’s Stygian elm.
FL 462 x Patriot
The tall, vigorous and attractive FL 462 began to develop flowers on saplings no more than three years old even before the turn of the year. The only pollen to hand when the flowers opened was that of Patriot, and the overlap was only just enough for two or three days of pollination attempts. Nevertheless, a high proportion of the seed developed, and the germination rate was very high. The seedlings proved to be rather uniform, prompting concern that they might be nothing more than FL 462 selfed, ie. minus the Patriot element. However, FL 462 flowers without the addition of Patriot pollen seemed to produce no fertile seed at all. Yet in the hybridisation game not even that is particularly good evidence of success, because the donor pollen can act as a “pollen mentor” or catalyst to self fertilisation without contributing its own genes.
At present the problem is how to get these seedlings safely through their first season, because their development outstrips their stability.
FL 493 hybridisation
Various crosses were attempted using the promising FL 493 as the female parent (there was insufficient pollen to use it as a male). All failed. FL 493 produced plenty of flower buds very early in the season, but their development slowed. Most fell off without opening. It took high heat to induce any successful flowering, and as the flowers matured they were overtaken by leaf buds which grew through them and destroyed most of them. The very few seeds which made it through to ripening were all infertile. This may be a consequence of the plants’ very young age.
Morfeo x Sapporo Autumn Gold
This was an obvious cross to undertake in view of the proven resistance of both parents and their plentiful flowering. Synchronicity was again a problem, with only the dregs of Sapporo pollen left by the time Morfeo flowered. Nevertheless six fertile seeds went on to germinate. The seedlings show fairly clear genetic variation, and some are promising. The likelihood of good disease resistance seems high, given the performance of both parents.
Back-crossing is a working label for the attempt to build back some of the genes of native elms into resistant hybrids. The theory is that one obtains a progeny which at one end of the range may include an individual combining good disease resistance with at least some of the native elm’s desirable morphology. Since the native trees in use are mature specimens, there is generally no lack of pollen.
A number of crosses were attempted in 2013 and 2014. All but one failed completely, presumably because of some fundamental incompatibility between the two parent plants. This is odd, since there is nothing which on the face of it fundamentally differentiates elms traditionally growing in this country from those developed by the professional breeding programmes. One possibility is that the more complex a tree’s ancestry, the less likely it is to prove fertile
One back-cross worked, after a fashion: Morfeo x U. minor from Tonge Mill, Kent. Two seedlings germinated. Both made very little growth. One died of mild sun-stroke. The other remained diminutive for weeks, and seemed likely to expire from inherent lack of vigour. It then began very slowly to develop, but seems a natural bonsai despite the fact that its male parent reaches 100 feet. Of course when a progeny consists of a single tree, the chances of that individual inheriting the “correct” mix of its parents’ desirable characteristics are vanishingly small.
Nevertheless, this experience has at least one positive outcome. It provides a persuasive answer to fears that the planting of modern hybrids could lead to the genetic “pollution” of native elms. Given the level of care which produced just two seedlings, and those of doubtful viability, it seems virtually inconceivable that such hybridisations could make progress in the field. In the case of wych elm, however, it may be that greater caution is advisable. The final one of this year’s experiments indicates why.
Columella open pollinated
A handful of seed from the two specimens of the highly resistant clone Columella growing in Clerkenwell, London, included a fair proportion which were fertile. The seed had been open-pollinated, and the female parent trees were growing a stone’s throw of young wych elms which flowered at around the same time. The germination rate was high.
After about two weeks of growth, several of the seedlings abruptly died. In some cases this was the result of stem rot, though it was unclear what the trigger may have been. In others there was no obvious cause.
As the remaining seedlings develop, they show marked genetic variation but fall broadly into three groups. Those in the first closely resemble each other, show characteristics of Columella, and may be Columella selfed (ie. Columella is both the female and the male parent). In the second group are diverse and currently healthy seedlings which may perhaps be Columella x glabra (wych elm). The third group consists of fairly diverse and generally pale seedlings which are declining. This may be phenomenon of “hybrid lethality”, which in certain crosses such as U. laevis x U. villosa prevents seedlings surviving beyond their third month.
The photograph below shows (from left to right) sample specimens of (a) FL 462 x Patriot; (b) Morfeo x Sapporo Autumn Gold; (c) Columella open-pollinated; and (d) the sole surviving Morfeo x Tonge Mill U. minor.