Cause of Dutch Elm Disease
A fungal disease, affecting Britain’s trees, Dutch Elm Disease, so named by a Dutch scientist, is caused by two European bark beetles—the the Scolytus scolytus and the Scolytus multistriatus—that feed upon the bark of the elms, transmitting fungal spores to the trees. When the fungus gets into an elm it blocks its water transport system causing the tree eventually to wilt and die.
Dutch elm disease became first apparent in the Netherlands in 1910, identified in 1920 by Bea Schwarz, a Dutch phytopathologist, and arrived in Britain in 1927. Three known species of ascomycete microfungi—Ophiostoma ulmi, Ophiostoma himal-ulmi, and Ophiostoma novo-ulmi—cause the disease.
Britain’s First Wave of Dutch Elm Disease
The first wave of Dutch elm disease, hereafter referred to as DED, that swept over Britain beginning in 1927 was fairly mild, killing only a few trees, and causing others to have yellowing branches that died. By 1940 this episode of DED had run its course.
Britain’s Second Wave of DED
In approximately 1967, a load of Rock Elm U. thomasii logs from North America brought a virulent and highly contagious strain of DED to Britain, causing the death of 25 million British elms. The disease made a deadly onslaught on the country as it moved northward, reaching Edinburgh in the late 70s and the Highlands of Scotland by 2006.
Forestry studies made in 1982 suggested that O. novi-ulmi would not die out as had the original epidemic, but would return cyclically to attack subsequent generations of elms, as soon as they were large enough for the beetles to forage on them. That is what is occurring in southern Britain at the present time. In many place as many as 50 to 90 percent of the elms are dying.
DED in Cornwall and East Anglia
Most of the mature Cornish elm and East Anglian smooth-leaved elms have been destroyed by DED. However is some areas of East Anglia some smooth-leaved trees have survived. Perhaps there survival can be attributed to the pendulous twigs mature trees have making them repugnant to the beetles.
DED in Scotland and North-west England
The epidemic has proceeded slower in it attacks on the wych elm, U. glabra. The 1970s epidemic is still in progress here. The beetles do not like the bark of these trees very much. Since the wych elm does not produce suckers, it does not get disease from people digging up its roots. Furthermore, the fungus Phomopsis that also attacks a dying wych elm serves to control the beetle populations in the area. The northern climate probably helps to keep the beetle population down. The disease is still active in Scotland in Glasgow, the warmer area of Nairn, and on the East coast North of Aberdeen
Diagnosis of DED
Currently DED is being caused by Ophiostoma novo-ulmi. Wilting or yellowing of leaves is noticeable in early summer. In Fall they turn brown and drop from the tree. The branches that are affected die from their tips and often become crooked on the ends. The bark of affected branches may have dark brown or purple longitudinal marks. If you cut through an affected branch you will discover a ring of dark staining.
In the 1970s, Lignasan BLP, carbendazim phosphate, was used to treat DED. It has to be injected into the base of infected trees, but it was not very effective. It is still available today with the name “Elm Fungicide.” This had to be injected into the base of the tree using specialized equipment, and was never especially effective. Although the disease cannot be eliminated when a tree is infected, Arbotect, thiabendazole hypophosphite, needs to be injected into the tree every two or three years to control the disease. Some people have tried Alamo, propiconazole, used mostly for treating OakWilt, but it is not as effective as Arbotect.
In the late 1980s, the University of Amsterdam invented a biological vaccine titled Dutch Trig® that is non-chemical and non-toxic, composed of spores of a fungus strain of Verticillium albo-atrum to be injected in Spring into the tree. It is supposed to produce an immune response in the tree that will protect the tree from DED for one season. It will not cure infected trees, but will prolong their life from 5 to 10 years at best.
Care of Elm Trees
If you have your elms pruned, be certain that the company doing the pruning cleans their blades with a 10 per cent solution of sodium hypochlorite before touching your tree. The bleach will kill any DED spores that might be on the blades. Moreover, if you have an infected tree its roots can carry the disease to healthy trees.
An attempt has been made to grow trees that are resistant to DED. A ten year evaluation begun in 2005, the National Elm Trial, is being made of 19 cultivars in the United States to determine if they are resistant. The Dutch have raised over 1000 cultivars and evaluated them for their disease resistance. They found several successful cultivars: “Columella,” “Nanguen,” “Lutèce,” “Wanoux” and Vada. The Dutch released “Columella;” the French Institut National de la Recherche Agronomique bought patents for “Lutèce” and Vada, putting them through twenty years of field trial before releasing them commercially in 2002 and 2006.