Crop Damage from Sclerotinia Crown Rot and Risk Factors in Pyrethrum

Scott, J. B., Gent, D. H., Pethybridge, S. J., Groom, T., and Hay, F. S. 2014. Crop damage from Sclerotinia crown rot and risk factors in pyrethrum. Plant Dis. 98:103-111. Sclerotinia crown rot, caused by Sclerotinia sclerotiorum and S. minor, is a prevalent disease in pyrethrum fields in Australia. Management involves fungicide applications during the rosette stage of plant development from autumn to early spring in fields approaching first harvest. However, estimates of crop damage and the efficacy of these tactics are poorly understood; therefore, plots were established in 86 pyrethrum fields in Tasmania, Australia during 2010 to 2012 to quantify these and to identify risk factors for disease outbreaks. On average, commercial management for Sclerotinia crown rot reduced disease incidence 43 to 67% compared with 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. Modeling canopy density and disease incidence in autumn correctly categorized disease incidence in spring relative to a threshold of 2% in 72% of fields. In a subset of 22 fields monitored over 2 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 and appear best targeted to fields with well-developed plant canopies and Sclerotinia crown rot present during early autumn. Pyrethrum (Tanacetum cinerariifolium (Trevir.) Sch. Bip.) is a perennial herbaceous member of the Asteraceae family grown for the production of the insecticidal pyrethrins, which are contained within oil glands on the surface of flower achenes (4,32). Pyrethrum production in Australia currently supplies approximately 60% of the global market for pyrethrins. The Australian industry is based on an intensive production system characterized by mechanization, monocultures with dense plant populations, selection of high-yielding cultivars with synchronous flower development, and high inputs of fertility and water (18). In this production system, pyrethrum fields can remain productive for three or more years. Elsewhere, pyrethrum typically is grown on a subsistence basis as a low-input crop (13). One of the factors associated with early discontinuation of a commercial crop is reduced stand density and vigor associated with Sclerotinia crown rot, caused by Sclerotinia minor and S. sclerotiorum (18). Sclerotinia spp. are destructive, plurivorous, necrotrophic pathogens of a large range of host species (20), causing significant losses worldwide (2). Sclerotinia spp. can survive for more than 8 years in the soil as melanized sclerotia (1). This survival ability, coupled with the extensive host range of the pathogens and absence of resistance in most hosts, make disease control problematic. Sclerotinia spp. cause two major diseases of pyrethrum (18). First, Sclerotinia crown rot is caused by myceliogenic germination of sclerotia near the soil surface and subsequent infection of the crown leading to wilting and, in severe cases, plant death. The disease is typically observed during the autumn, winter, and early spring months, and has been associated with both S. minor and S. sclerotiorum. Sclerotia of both species may germinate carpogenically to produce airborne ascospores which infect flowers during early summer and cause Sclerotinia flower blight (15). Apothecial emergence is also closely synchronized with flower development in pyrethrum (19). Current management efforts for Sclerotinia crown rot in pyrethrum are centered on regular application of the fungicide procymidone during the autumn and winter, when plants are in the rosette stage. A basic tenet of integrated pest management is management intervention that is informed by the risk of crop damage from a pest (26). Crop damage and yield impacts from Sclerotinia crown rot have not been well characterized in pyrethrum or most other perennial pathosystems, and the economic necessity of disease management efforts require quantification to provide a foundation for both strategic and tactical disease management decisions (21). In this study, we sought to assess the efficacy of current commercial management programs for Sclerotinia crown rot and their association with crop damage. A secondary objective was to clarify risk factors for disease development to identify production situations where management intervention may be justified. Materials and Methods Field sites and data collection. The incidence of Sclerotinia crown rot was assessed in a total of 86 commercial pyrethrum fields in northern Tasmania, Australia during 2010 to 2012. All fields were approaching first harvest and were planted in the preceding spring. Within each field, two plot areas were demarcated for assessment. One did not receive applications of procymidone (Fortress 500; Crop Care Australasia Pty. Ltd.) or other fungicides with known activity against Sclerotinia spp. This was designated Corresponding author: J. B. Scott, E-mail: [email protected] Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of the products or vendors that may also be suitable. Accepted for publication 1 August 2013. http://dx.doi.org/10.1094 / PDIS-06-13-0599-RE © 2014 The American Phytopathological Society