Detection of the Diplodia shoot blight and canker pathogens from red and jack pine seeds using cultural methods

Abstract

The shoot blight and canker pathogens Diplodia pinea and D. scrobiculata commonly and abundantly sporulate on seed cones of red pine (Pinus resinosa) and jack pine (P. banksiana) collected from Wisconsin and Minnesota forests. Cultural methods were used to investigate the incidence of these fungi in seed lots obtained from government nurseries in these states. In each of three replicate trials, seeds of each lot were assigned to four treatments before incubation on semi-selective medium: (1) not surface-disinfested; (2) surface-disinfested; (3) surface-disinfested, then inoculated with D. pinea conidia; or (4) not surface-disinfested, then inoculated with D. pinea conidia. For red pine seeds, the mean percentage positive was 2.7% for treatment 1 and 1.3% for treatment 2. Jack pine seeds were less frequently positive than red pine seeds for both treatments 1 and 2. The Diplodia species cultured was identified as D. pinea in almost every case. Diplodia pinea was much less frequently recovered from seeds that were not surface-disinfested and then inoculated (treatment 4), when compared with seeds that were inoculated with D. pinea after surface-disinfestation (treatment 3). Results confirm the potential for dissemination of D. pinea on red pine and jack pine seeds, and caution is warranted before concluding absence of the pathogen based on results using cultural methods with relatively small numbers of seeds. Although the frequency of pathogen-positive seeds was low, the large numbers of seeds planted in nurseries suggest that seeds may be a potentially important route of entry of D. pinea into nursery beds.