Management

Detection

Molecular analysis is the most reliable way of detection of pathogens even if it is time-consuming and costly (Stenlid et al. 2011).

There is an urgent need for developing more effective molecular tools for the identification of DED agent hybrids (Konrad et al. 2002) with the help of high-throughput or hopefully portable molecular detection (Luchi et al. 2020) at an early stage of the disease development to protect the plants in the stage of prevention.

Preventive measures and suppression measures

• Elms resistant to Dutch elm disease

Breeding for resistance began in the Netherlands in 1928 and continued until about 1992 (Heybroek 1993, 2000, Scheffer et al. 2008). Early emphasis was placed primarily on selecting for resistance within native species, especially Ulmus glabra and U. carpinifolia and their various hybrids. During the second disease pandemic in Europe that peaked during the 1970s (Brasier 2000) decimated many surviving native populations and some of the early “resistant” cultivars (e.g., ‘Commelin’) (Scheffer et al. 2008). This led to more extensive use of Asian elm germplasm, particularly the Himalayan elm, U. wallichiana, as a source of resistance genes. More recent elm breeding efforts in Spain and Italy emphasize the native European species Ulmus glabra and U. carpinifolia (=U. minor) but rely on the Siberian elm (U. pumila) as the source of disease resistance genes (Santini et al. 2003, Solla et al. 2000, Scheffer et al. 2008). Siberian elm seems better adapted to the warmer, drier parts of the Mediterranean region than to the cooler, moister climates of Great Britain and the Netherlands where other diseases take their toll on U. pumila. Similar breeding experiences took place in the United States beginning in the 1930s with early efforts focused on identifying resistant individuals in North American species, especially U. americana (Smalley and Guries 1993, Scheffer et al. 2008). Subsequent programs shifted to exploiting either selections within a resistant species, such as various cultivars of U. parvifolia developed by the U.S. Department of Agriculture (Townsend 2000) or released by private nurseries, or interspecific hybrids.

In 2005, the National Elm Trial (USA) began a 10-year evaluation of 19 cultivars in plantings across the United States. The trees in the trial were exclusively American developments; no European cultivars were included. Based on the trial's final ratings, the preferred cultivars of the American elm (Ulmus americana) are ‘New Harmony’ and ‘Princeton’. The preferred cultivars of Asian elms are the Morton Arboretum introductions and ‘New Horizon’ (National… 2018, Griffin et al. 2017).

New trials in different European countries have been started for now.

Respond & Control

• Biological control

The University of Amsterdam developed a biological vaccine by the late 1980s. DutchTrig (Scheffer et al. 2008) is nontoxic, consisting of a suspension in distilled water of spores of a strain of the fungus Verticillium albo-atrum that has lost much of its pathogenic capabilities, injected in the elm in spring. injections with Verticillium WCS850 restricted new infections by the Dutch elm disease pathogen to less than 1% annually. However, infections through root grafts are not controlled and the treatment must be repeated annually (Scheffer et al. 2008). Preventive treatment is usually justified only when a tree has unusual symbolic value or occupies a particularly important place in the landscape. Pheromone traps can catch millions of beetles, but they have never proven effective in controlling Dutch elm disease (Peacock 1981).

• Sanitation and beetle population control

Eradication of Dutch elm disease has often been attempted, but without success. All dying and dead elm wood should be promptly destroyed; at a minimum, the bark has to be pared from the wood. Otherwise, new beetles will emerge to sustain the epidemic. Eradication program is more effective while initiated immediately upon discovery of the disease (Scheffer et al. 2008).

More success stories exist, such as the Integrated Elm Program of the City of Hamburg, Germany, which reports losses due to Dutch elm disease of less than 1%, and similar programs in a number of cities in North America (Scheffer et al. 2008).

• Fungicide treatment

Control of the pathogen by fungicide treatment has focused mainly on benzimidazoles (benomyl, carbendazim, and thiabendazole) and sterol biosynthesis inhibitors, like Arbotect 20-S® and Alamo®. Both products are often effective as a single treatment when properly applied by exposed root flare injection at the highest dosage rates allowed (Stennes 2000). Arbotect 20-S appears effective in protecting elm trees for up to 3 years (Scheffer et al. 2008).