Phomopsis Canker of 'Robusta' Poplar

Abstract

'Robusta' poplar (Populus X Euroamericana) is used extensively in the north central part of the United States as a fast-growing shade tree. This hybrid is asexually reproduced by stem cuttings and is commonly grown in tree nurseries. A stem canker suspected to be caused by Phomopsis macrospora T. Kobayashi and Chiba, has caused considerable losses in nursery grown trees in Minnesota. This fungus causes a similar canker disease in cottonwood (P. deltoides) in Mississippi, (1, 2) and Paulownia (Paulownia tomentosa) in Japan (3). In July 1974 a random sample of 103, 2-yearold nursery-grown 'Robusta' poplar trees were examined for cankers. Sixteen percent of the trees were not cankered and the remaining 84% had one or more basal (below 30cm) or aerial (above 30cm) stem cankers above ground (Fig. 1, 2). The majority of the diseased trees (44%) had basal bankers and only 13% had aerial cankers. Both types of cankers were found on 27% of the trees, and as many as 4 cankers were found on 7% of the trees. No trees had been killed by the fungus, but 10% were broken at the canker and would be rogued from the nursery. Phomopsis macrospora was isolated from 100% of the basal and aerial stem cankers from 15 trees selected at random. Two species of Fusarium and one species of Verticillium were occasionally isolated from basal cankers. The identification of P. macrospora was confirmed by Dr. Colin Booth, Commonwealth Mycological Institute, Kew, England. Three 2-year-old 'Robusta' poplar trees were inoculated with P. macrospora, Fusarium sp. (isolates 1 and 2), Verticillium sp., or sterile grain. The fungi were grown on sterile grain (oats) for about 30 days. Drill and knife wounds were made about 5 mm deep into xylem tissues at 15 and 45 Fig. 1, 2. Main stem cankers on 'Robusta poplar' caused by Phomopsis macrospora.