Branch dieback was observed throughout the crown of a 25-year-old heartnut, a variety of Japanese walnut, growing in a mixed plantation with black walnut (J. nigra) in central Iowa in the spring of 1997. No cankers were evident. However, hyphal pegs and hyaline, two-celled, fusiform conidia were present on the affected 1996 branch wood. These are characteristic of S. clavigignenti-juglandacearum, the cause of branch and stem cankers on butternut, also called white walnut, (J. cinerea) (1). The fungus was isolated in pure culture from symptomatic twigs. The ability of this isolate to cause canker on butternut was compared with an isolate from butternut in Minnesota by inoculating the stems of potted, 2-year-old butternut seedlings in a greenhouse. Lignified stem portions of the seedlings were wounded by removing a 5-mm2 area of bark with a scalpel, exposing the cambium, and placing a plug cut from the margin of an actively growing culture on potato dextrose agar (PDA) fungus-side-down on the wound. Each of 30 seedlings received both isolates, 15 with the heartnut isolate on top (10 cm from shoot apex), and 15 with it on the bottom (30 cm from shoot apex) position of the stems. Noncolonized PDA control plugs were similarly placed on wounds in the middle position of each stem. Parafilm was used to hold the agar plugs in place and was removed 2 weeks later. After 10 weeks, canker lengths were recorded. Stem segments were surface sterilized, and wood chips from canker margins were placed onto PDA at 20°C. Cankers characteristic of S. clavigignenti-juglandacearum developed on all seedlings at each inoculation site. The means of lengths of cankers for the heartnut and butternut isolates (37.1 and 37.2 mm, respectively) did not differ significantly (P > 0.9). Hyphal pegs and conidia of S. clavigignenti-juglandacearum developed at inoculation sites of stems with both isolates. The fungus was recovered in pure culture from the canker margins. No cankers developed at the control wound sites and all wounds were callused over. This is the first report of S. clavigignenti-juglandacearum on Japanese walnut. This report and an earlier report of this fungus causing dieback on black walnut (2) indicate this fungus may occur on other Juglans spp. than previously reported and could be considered a potential pathogen in walnut plantings.
References: (1) V. M. G. Nair et al. Mycologia 71:641, 1979. (2) M. E. Ostry et al. Plant Dis. 81:830, 1997.
