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His long career was distinguished by the raising of a number of important cultivar s , including Symphoricarpos × doorenbosii , Betula utilis ' Doorenbos ' , and numerous Dahlia s. Perhaps his most famous achievement was the hybrid elm cultivar ' Den Haag ' , indeed it has been postulated that he was the first to think of crossing elms to obtain varieties resistant to Dutch elm disease .
 Doorenbos
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
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 Doorenbos