A DUTCH ELM DISEASE RESISTANT TRIPLOID ELM

A naturally occurring triploid elm hybrid was found in the American el m, Ulmus americana L., planting on the National Mall in Washington, D. C. Chromosome examinations of mitosis in root tips and meiosis in poll en mother cells showed a chromosome complement of 2n = 3x = 42. The ch romosome alignment at meiotic metaphase I was predominantly 14 bivalen ts and 14 univalents, indicating that one parent was an American elm, which contributed the bivalents through autosyndetic pairing. When DNA underwent restriction digestion with HindIII and probed with a 9.5-ki lobase cloned ribosomal DNA fragment from pea, Pisum sativum L., the t riploid hybrid showed a band not found in American elms. Seed from the open-pollinated parent tree had low viability and seedlings were high ly variable in height, leaf size, and shape. The small leaves of some seedlings suggest that the other parent was a species with leaves smal ler than American elm. The hybrid was found to be resistant to Ophiost oma ulmi (Buis.) Nannf. When twig crotches were inoculated, only 14% d eveloped wilt compared with 63% in the American elm. None of the 22 hy brid trees inoculated developed systemic wilt compared with 8 of the 1 8 American elms inoculated. The hybrid, which is easily propagated by softwood cuttings, has rapid growth, good crown structure, and many ch aracteristics of the American elm.