Improving Fungicide-Based Management of Ray Blight Disease in Tasmanian Pyrethrum Fields.

Ray blight disease, caused by Phoma ligulicola var. inoxydablis, is a serious threat to the Tasmanian pyrethrum industry. The management of this disease relies upon the strategic application of fungicides in early spring. A range of fungicides were assessed for their efficacy in controlling ray blight disease in Tasmanian pyrethrum fields, and the primary objective of this study was to increase fungicide options available to growers in different resistance groups. Fungicides were assessed under in vitro conditions, within five replicated-plot field trials over three seasons (2004 to 2006) and in single-plot trials over eight fields in 2005. In each of the field trials, regular assessments of disease intensity (defoliation severity and the incidence of stems with ray blight), stem height, and the number of flowers produced on each stem were made using stems as the primary sampling unit. Canopy reflectance at 830 nm and the Difference Vegetative Index, measured using a handheld multispectral radiometer, also were used to compare fungicide effects on green leaf area. The effect of fungicides on the dry weight of flowers, pyrethrin content within the flowers, flower maturity, and pyrethrin yield were determined. Under in vitro conditions, boscalid reduced both conidial germination and mycelial growth at concentrations of at least 0.16 μg/ml. In field trials 1 and 2 (in 2004), the premixed formulation of pyraclostrobin + boscalid (Pristine) increased pyrethrin yield by an average of 79% compared with nontreated plots over the two locations. Furthermore, in single-plot trials, pyraclostrobin + boscalid increased pyrethrin yield by 134 and 60% compared with the industry-recommended protocol (single application of azoxystrobin at 150 g a.i./ha [Amistar WG] and two additional applications of a tank mixture of difenoconazole at 125 g a.i./ha [Score] and chlorothalonil at 1,008 liters a.i./ha [Bravo 720] at 14- to 21-day intervals) and nontreated plots, respectively. In field trials 3 (in 2005) and 4 and 5 (in 2006), similar yield benefits also were produced by applying pyraclostrobin (Cabrio SC) or boscalid (Filan) alone or in combination with chlorothalonil (Bravo 720) at 1.4 liters of product per hectare, regardless of the rates of pyraclostrobin (250 and 125 g a.i./ha) and boscalid (500 and 250 g a.i./ha) used. These data were used to recommend the incorporation of boscalid to improve the fungicide-based management of ray blight disease. This decreases the number of applications of both strobilurin and triazole fungicides which have been used extensively for the management of ray blight and other diseases in Tasmanian pyrethrum fields and are prone to fungicide resistance development.