|SCOTT, JASON - Tasmanian Institute Of Agricultural Research|
|Gent, David - Dave|
|PEARCE, TAMIEKA - Tasmanian Institute Of Agricultural Research|
|PETHYBRIDGE, SARAH - Cornell University|
|PILKINGTON, STACEY - Tasmanian Institute Of Agricultural Research|
|HAY, FRANK - Cornell University|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2017
Publication Date: 11/30/2017
Citation: Scott, J.B., Gent, D.H., Pearce, T.L., Pethybridge, S.J., Pilkington, S.J., Hay, F.S. 2017. Mycoflora associated with pyrethrum seed and the integration of seed steam treatment into foliar disease management strategies. Plant Disease. 101(11):1874-1884.
Interpretive Summary: Pyrethrum is the source of pyrethrins, which are naturally-derived compounds that have insecticidal properties. Several foliar diseases can affect pyrethrum, and those of greatest importance can be seedborne. This study documented the occurrence of various fungi from pyrethrum seed lots produced over 15 years, and discovered that six pathogens were present in commercial seed. Heat treatment of seed reduced the level of two of the most important pathogens to near undetectable levels. Various field studies were conducted to quantify how planting heat-treated seed may affect recovery of the pathogens, disease development, and yield. Overall, we found that treating seed generally delayed the development of the most important diseass, but did not completely control them. This study suggests that steam treatment of seed can delay development of the foliar disease complex on pyrethrum, although an extremely low level of remaining infected seed or others ources of inoculum necessitates the use of foliar fungicide applications in spring under the pyrethrum production conditions of Tasmania.
Technical Abstract: A complex of foliar diseases can affect pyrethrum in Australia, but those of greatest importance are ray blight, caused by Stagonosporopsis tanaceti, and tan spot, caused primarily by Didymella tanaceti. Isolation of fungi from pyrethrum seed lots produced over 15 years resulted in recovery of six known pathogens: S. tanaceti, D. tanaceti, Alternaria tenuissima, Colletotrichum tanaceti, Stemphylium botryosum, and Botrytis cinerea. The incidence of S. tanaceti and D. tanaceti isolated from seed varied between 1.3 to 19.5% (mean = 7.7%), and 0 to 25.4% (mean = 5.3%) among years, respectively. Commercial heat treatment of pyrethrum seed via steaming reduced the incidence of D. tanaceti from 10.9 to 0.06% and the incidence of S. tanaceti from 24.6% to nondetectable levels. In a second experiment, both species were reduced to nondetectable levels (<0.20%) from their initial incidences of 22.4 and 2.4%, respectively. In a field study in 2013, colonization of pyrethrum foliage by S. tanaceti was reduced from 21.1 to 14.3% in early winter when heat-treated seed was planted. However, isolation frequency of D. tanaceti was not affected significantly by seed treatment in this year. In a related experiment in 2015, the isolation frequency of D. tanaceti in plots planted from heat-treated seed depend on both prior application of an industry-standard fungicide program and proximity to another pyrethrum field in autumn (May). The fungus was recovered at a similar frequency in fungicide-treated and non-treated plots that were located near other pyrethrum fields (13.8 vs. 16.3%, respectively), whereas recovery of the pathogen was reduced by fungicide applications in geographically remote pyrethrum fields (6.7 vs. 1.4%, respectively). However, these differences in isolation frequency of D. tanaceti in autumn did not obviate the need for later fungicide applications to suppress foliar disease intensity in spring or flower yield in summer. Independent, independent or proximity to other pyrethrum fields.