Elm Yellows

In autumn 2013 the UK holder of the licence for U. “Morfeo” noticed unusual colouration in certain plants of that clone. Tests established the presence of the phytoplasma which causes Elm Phloem Necrosis, also known as Elm Yellows (EY). The entire batch of trees was ultimately ordered to be destroyed.

Phloem Necrosis is a disease spread primarily by leaf-hoppers, and affects a wide range of plants. In elm, it attacks the root hairs, and then spreads upwards through the tree’s inner bark. Typical symptoms include distortion, fading and wilting of leaves, formation of witches’ brooms, declining growth, and the death of naturally susceptible trees. There is no effective treatment for the disease.

Until the early 1980s, EY was not thought to be found beyond North America. The American elm, U. americana, is notoriously susceptible to it, and some stands of the tree successfully defended against Dutch Elm Disease were completely destroyed by EY. It has since been found in much of the Mediterranean basin, and is present in France. Some consider that it is likely to be found in Britain as well, although there have been no reports of it excepting the instance at the end of 2013.

European and Asian elms were traditionally thought to be relatively resistant to EY. However, a more complex picture gradually emerged. Siberian elm (U. pumila) and the native wych elm (U. glabra) are still regarded as resistant. In other species, notably the European field elm (U. minor), there is now thought to be significant variation. In Italy, close attention was paid to symptoms of decline in the Chemou elm (U. chenmoui), and presence of EY phtoplasma was detected in some affected plants. U. chenmoui is one of the parents of Morfeo.

Morfeo, in terms of its resistance to elm disease and general characteristics, has been described on this website and elsewhere as one of the most promising new hybrid elms to reach this country. Without a doubt its reputation has now suffered a serious blow. However, the situation should be assessed with an awareness of all the facts.

In Italy, Morfeo has been widely planted and in not a single instance has it shown any signs of EY. Nor have other clones with U. chenmoui parentage proved susceptible. It is beyond dispute that Morfeo is capable of being infected by the disease, as are many if not most elms, but there is only the one occasion on which symptoms were expressed. So the question is, why did that happen?

The propagation technique used by Morfeo’s British licensee involved grafting the clone onto wych elm rootstocks. This is done because it is quicker and easier to build up stock this way than by taking cuttings; wych elm can readily be propagated in quantity from seed and has the advantage (as some see it) that it does not sucker. It was not known to the Italian breeding programme that this technique was to be used; had it been known, it would have been strongly discouraged. Grafting can cause problems which only come to light many years later. In Amsterdam, for instance, it currently is the cause of more loss of elms than elm disease itself.

In this instance, it is possible that the infection travelled through the graft into some of the wych elm rootstocks, presumably because some of those rootstocks did not have the EY resistance normally associated with the species. With the root hairs dying back, it was inevitable that the Morfeo scion would become symptomatic. Any tree, however resistant, would do so. I have put this suggestion to Professor Santini of the Italian breeding programme, who considers that it is likely to be correct.

As for the fact that Morfeo is capable of transmitting EY, further imports from Italy will not do so. It was established in 2004 (Boudon-Padieu et al) that EY phytoplasma are reliably destroyed by soaking cuttings in water at 50 degrees for 30 minutes before setting them out. This method is now used in all propagation carried out by the Italian breeding programme, though it does unfortunately reduce the proportion of cuttings which can be successfully rooted.

In response to the Morfeo incident, the government has made the Plant Health (England) (Amendment) Order 2014, which took effect on 6 May. This requires that any intending importer of elm into this country from another EU state should notify the relevant authorities of the following:

a) The intended date of introduction of the trees
b) The country of export;
c) Their intended first destination after landing
d) The quantity of the trees
e) Their genus and species; and
f) The producer’s identification number

The intention is to monitor the trade in elms to gain more information about its nature and extent, and to raise awareness of the risk of EY. This will of course make it more difficult to source trees such as Lutece and Vada from continental nurseries, but the measure is appropriate and proportionate. More restrictive controls were considered but not adopted, in part because it was recognised that initiatives for the regeneration of the elm would suffer.

ResistantElms.co.uk will of course comply fully with the new legislation, and already adopts the attitude of vigilance recommended by those who framed it.