|SCOTT, JASON - Tasmanian Institute Of Agricultural Research|
|Gent, David - Dave|
|PETHYBRIDGE, SARAH - Plant And Food Research|
|GROOM, T - Botanical Resources Australia Pty Ltd|
|HAY, FRANK - Tasmanian Institute Of Agricultural Research|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2013
Publication Date: 1/31/2014
Citation: Scott, J.B., Gent, D.H., Pethybridge, S.J., Groom, T., Hay, F. 2014. Crop damage from Sclerotinia crown rot and risk factors in pyrethrum. Plant Disease. 98:103-111.
Interpretive Summary: Sclerotinia crown rot is a constraint to production of pyrethrum, which is the source of the natural pyrethrin insecticides. Studies were conducted to determine the effectiveness of current management practices, how severely the disease impacts yields, and risk factors for disease development. We found that commercial management practices did indeed reduce disease occurrence, but were associated with only a modest improvement in flower yield. Disease development in spring, when pyrethrum flowers are developing, was linked to density of the pyrethrum canopy and how much disease was present in the preceding autumn. In turn, these factors were associated with planting date and rain events after planting. From this study, it appears that fungicide applications can be made more strategically and are best targeted to fields at greatest risk of the disease.
Technical Abstract: Sclerotinia crown rot, caused by Sclerotinia sclerotiorum and S. minor, is a prevalent disease in pyrethrum fields in Australia. Management involves the application of fungicides during the rosette stage of plant development during autumn to early spring in fields approaching first-harvest, although estimates of crop damage and the efficacy of these tactics are poorly understood. Plots were established in 86 pyrethrum fields in Tasmania, Australia during 2010 to 2012 to quantify crop damage from Sclerotinia crown rot, efficacy of management practices, and to identify risk factors for disease outbreaks. On average, commercial management practices for Sclerotinia crown rot reduced disease incidence 43 to 67%, compared to nontreated plots. There was a weak, but significant, relationship between relative increase in flower yield when fungicides were applied and the incidence of crown rot (R2 = 0.09; P = 0.006), although the mean number of flowers produced was similar regardless of fungicide applications. Flower yield was positively associated with canopy density in spring (S = 0.39; P = 0.001). Moreover, canopy density in spring was linked by both direct and indirect effects to canopy density during autumn and winter, which in turn were associated with planting date and previous rain events. Assessment of canopy density and disease incidence in autumn allowed correct prediction of disease incidence exceeding a provisional disease incidence threshold of 2% in spring in 72% of fields. In a subset of 22 fields where Sclerotinia crown rot and canopy development was monitored over two years, canopy density in the autumn following the first harvest had a negative relationship with Sclerotinia crown rot incidence the preceding year (R2 = 0.23; P = 0.006). On average however, current commercial management efforts provided only small increases in flower yield in the current season. Fungicide applications and other disease management practices appear best targeted to fields at highest risk of the disease, specifically early planted fields with well-developed plant canopies, and Sclerotinia crown rot present during early autumn.