Should we start planting Ash with natural Chalara resistance even if they aren’t of UK provenance, e.g. Danish ‘Tree 35′?
AD: We should be very clear that tree 35 is not ‘resistant’. It tolerates the fungus better than most but it still gets infected. We do not know what is likely to happen with such trees over 20-40 years. The plan it to see if there are different genetic determinants in different trees that tolerate the fungus. If there are, it may be possible to cross them with each other and combine the characters to increase tolerance.
DM: It isn’t certain whether ‘Tree 35′ is going to be tolerant against the UK population of ash dieback. Tree 35 has shown to have great tolerance, but it isn’t clear how it will be in 20 – 30 years and we want to be able to create long term resistance. That said, there are great lessons to be learned from the genetic makeup of this tree and understanding how it has reached this tolerance is going to be of great benefit. In the end we would like to achieve a UK population of resistant trees, with UK-specific diversity, as our tree population is genetically different from the Danish population.
JW: Before we can go ahead with widespread planting of ash trees such as Tree 35, we have to be sure about the extent of its resistance. However, just because trees/seeds are not of UK provenance doesn’t mean we should exclude them. The releases from a number of programmes breeding for resistance to Dutch elm disease have made use of a wide range of elm species from Asia to produce resistant elms. Also, many of the broadleaf trees planted in Britain, including oaks raised after the Napoleonic wars, have depended on seed from other European countries.