Chinese elm (Ulmus parvifolia Jacq.) is native to China, Korea, and Japan and was introduced to the U.S. to replace the American elm, which is highly susceptible to Dutch elm disease. Cultivar Emmer II trademark Allee elm (or Allee elm) is preferred by nurseries for its rich green foliage and beauty of bark. In the summer of 2011, a new disease was observed on Allee elm at a tree farm in FL. Approximately 1% of elms in the same farm and in residential areas in central Florida had similar canker-like symptoms consisting of tan to orange patches of decomposed and loose bark. These symptoms were observed on the main trunks often extending into branches of affected trees. Cankered sections of the trunk were often several feet in length and penetrated the wood to a depth just under the bark into the phloem. To isolate the causal organism, cankers were gathered from 7 trees and tissues from the margin of 1 to 3 cankers per tree were surface sterilized in 1% sodium hypochlorite, plated on PDA, and incubated at 22°C under a 16-h/8-h light/dark cycle for 7 days. Colonies displayed white, fluffy mycelium with sporadic black acervuli containing aggregated conidia. Conidia were 5-celled with two or more apical appendages or hairs; the three central cells were dark brown and the two outer cells were hyaline (1). Based on conidial morphology, these isolates were putatively identified as Pestalotiopsis spp. To identify the species, the rDNA internal transcribed spacer (ITS) region was sequenced for two field isolates (GenBank Accessions JX999998 and JX999999). A BLASTn search in GenBank revealed 100% identity to Pestalotiopsis mangiferae ITS (JX305704.1). To test Koch's postulates, experiments were performed in the field and greenhouse. A mycelial plug of isolate 11-40 was grown on PDA, inoculated on wounded trunk of healthy 18-month-old Allee elms (n = 48) in the field and 7-month-old Allee elms (n = 12) in the greenhouse. Both experiments were set up as a randomized complete block design. The trunk of each tree was wounded with a scalpel to a depth of 5 mm, the wound was inoculated with a 5-mm2 agar plug from a 7-day PDA culture, and the inoculated wound was wrapped with grafting tape. Plants that served as negative controls [n = 20 (field experiment) and n = 12 (greenhouse)] were mock-inoculated with a sterile PDA plug without mycelial growth. After 4 to 6 months, symptoms consisting of loose or fallen off bark developed on all pathogen-inoculated trees but not on control trees. Control wounds healed with no expansion beyond the original 5 mm inoculation zone, whereas Pestalotiopsis-inoculated cankers expanded to 3 to 8 cm in each direction in 6 months. Pestalotiopsis (confirmed by conidial morphology and ITS sequencing) was reisolated from pathogen-inoculated trees but not from control trees. These experiments were repeated with similar results at least three times, each consisting of 15 replications (greenhouse) and 12 replications (field) with additional Pestalotiopsis isolates. Many other Pestaliopsis spp. have been reported on other shade trees through the U.S. and the world. To our knowledge, this is the first report of P. mangiferae on an Ulmus species in the U.S. and the world. Since elm canker mainly affects the trunk, a featured characteristic of the Allee elm, it can potentially result in economic loss to the ornamental industry.
References: (1) Y. Ko et al. Plant Dis. 91:1684, 2007. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.