Location: Forage Seed and Cereal Research UnitTitle: Carpogenic germination of sclerotia of Sclerotinia minor and ascosporic infection of pyrethrum flowers
|O'MALLEY, THOMAS - Tasmanian Institute Of Agricultural Research|
|HAY, FRANK - Tasmanian Institute Of Agricultural Research|
|SCOTT, JASON - Tasmanian Institute Of Agricultural Research|
|Gent, David - Dave|
|SHIVAS, ROGER - Queensland Government|
|PETHYBRIDGE, SARAH - Cornell University - New York|
Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2015
Publication Date: 4/30/2015
Citation: O'Malley, T.B., Hay, F.S., Scott, J.B., Gent, D.H., Shivas, R.G., Pethybridge, S.J. 2015. Carpogenic germination of sclerotia of Sclerotinia minor and ascosporic infection of pyrethrum flowers. Canadian Journal of Plant Pathology. 37:179-187.
Interpretive Summary: The fungus Scleronitia minor is reported to conduct a portion of life cycle, called carpogenic germination, only rarely. During 2007 to 2009, the fungus was found in pyrethrum flowers in Tasmania, Australia, suggesting carpogenic germination. Laboratory studies confirmed that the morphological characteristics and DNA of the organism indeed matched that of S. minor. Eight of 10 S. minor isolates obtained from flowers also germinated carpogenically in the laboratory, and were able to cause flower infection. To the best of our knowledge, this is the first report of S. minor causing flower disease of pyrethrum. It is also one of only a few examples of carpogenic germination of S. minor. This finding has implications for the management of the Sclerotinia disease complex affecting pyrethrum and, more broadly, potential carpogenic germination of the fungus in other diseases.
Technical Abstract: Evidence for carpogenic germination of sclerotia and infection of flowers by ascospores of Sclerotinia minor is rare. During 2007 to 2009, isolates with morphological characteristics consistent with S. minor were obtained from surface-sterilized pyrethrum flowers collected from fields in Tasmania, Australia. The isolation frequency of S. minor from flowers, in 2007, 2008, and 2009, was 15.8%, 5%, and 1.4%, respectively, in each of three years. The prevalence of Sclerotinia minor isolated from flowers varied between 10.3% and 60%. Sclerotia with apothecia, consistent in size with the formal description of S. minor, were collected from the soil in one field during early flowering. Colonies produced from individual ascospores from this isolate were identified as S. minor. Eight of 10 S. minor isolates obtained from flowers also germinated carpogenically in the laboratory following conditioning. Phylogenetic analysis based on the internal transcribed spacer region grouped the pyrethrum isolates with S. minor, and distinct from published sequences of other Sclerotinia spp. Species-specific primers developed to differentiate the four major Sclerotinia spp. (S. sclerotiorum, S. minor, S. homoeocarpa, and S. trifoliorum) were also used to confirm the isolate identity. Pathogenicity of S. minor to pyrethrum flowers was confirmed in the greenhouse using ascosporic inoculum. Symptoms of infection were withered, necrotic and/or abscised ray florets, and light-brown and pitted disk florets. To the best of our knowledge, this is the first report of S. minor causing flower disease of pyrethrum. It is also one of only a few examples of carpogenic germination of S. minor. This finding has implications for the management of the Sclerotinia disease complex affecting pyrethrum in Australia.