Until to-day only one chemical treatment was developed, but it is expensive and must be applied every year. Only very valuable old elms in parks actually are been treated.
The way to a resistant elm variety is long. By breeding a new elm you first have to find crosses with sufficient high resistance, second you want to find elms which are as beautiful as the susceptible elms and third they have to be vital and adapted to the climatic conditions. Trees grow slowly – you can judge a new selected tree not before it has grown for 15 years. More than 100.000 crossings have been tested so far, mostly in the USA.
Together with Smalley and Guries we identified about 50 crosses for further testing in Europe. In Darmstadt we tested these selections for adaptability to our climatic condition and had to test all once more for their resistance because our strains of Ophiostoma ulmi are different for the american. So far we have released about 10 varieties which were then officially tested and registered. Some are currently marketed, some may follow.