DED Resistant Elms

The resistant strains or cultivars of American elms now on the market, how assured are we that they are indeed fully resistant? How old are the oldest?

Why is it sometimes said that the current strain of DED is less virulent than it originally was? Is this true? What would cause a disease to become less effective (er, or, should I say, infective)?

Decumbent

Actually, the newer strain is hypervirulent (more virulent). You might be thinking of chestnut blight, a related fungus that can have a hypovirulence induced by a parasitic virus.

I'm not sure if any New World elm species is immune to DED, but several selections have shown various degrees of resistance via innoculation testing. Remember that anything you plant can have its own problems, now or decades from now. Avoid monocultures and geometric planting patterns and you will have a better chance that your landscape will survive even if an individual tree does not.

Guy S.

Hey Decumbent, there is no assurance. Resistant does not mean anything other than resistant. There's nothing full proof out there to the best of my knowledge.

Please read this thread-
http://davesgarden.com/forums/t/511161/

There are many Princeton Elms that are mature that are holding their own quite well. Are these Princetons resistant, I think so. Regarding other cultivars that are allegedly resistant, we won't know for quite a while. Unfortunately, an Elm is not deemed mature until at such time as it attains 60 years of age so we have a way to go on some of the other DED resistant Elms.

Here's my take on this. DED, it came to us as a gift from Asia. I think it was introduced in a shipment of furniture. The causative agent is a fungus called fungus Ophiostoma which is spread by beetles. There are numerous strains out there and some are more lethal than others.

One of the very first strains of this fungus was mild or rather less virulent. That strain arrived in the 1930's The other really nasty strain arrived in the 1940's (novo-ulmi).

There are many factors that affect emergence and re-emergence of diseases such as DED. One thing for sure, it isn't going away anytime soon and strains of it are mutating. Basically, genetic variations are evolving as I type.

I guess my question would be what strain? To the best of my knowledge. the newer strains are considerable more virulent than those of the past. Maybe I'm off base and my wires are crossed but I don't think so. Best to get a hold of Starhill and see what his take is.

Oops, da man himself posted. Looks as if I was typing while Guy was typing.

Very good advice-

Perhaps I was thinking of the chestnut blight. So there's a virus attacking the fungi (or is it bacteria)? How interesting.

Decumbent

Yes, the chestnut blight fungus (Cryphonectria) can get a "virus cold" which slows it down and permits the tree to outgrow it. The virus seems to be more effectve in establishing spontaneously on the European strains of the fungus than on those found in North America, but it's still being considered as one answer to the blight.

Guy S.

If you read carefully on various websites, princeton elm sellers say bad things about liberty elms("18% survival") and liberty elm sellers say bad things about princeton elms("parent tree dead from DED"). While both of these trees may or may not be DED tolerant, I hope these companies are thinking about more than just making money off of them.