Tan spot: a new disease of pyrethrum caused by Microsphaeropsis tanaceti sp. nov.

Abstract

The isolation frequency of Microsphaeropsis sp. in spring in association with necrotic lesions on leaves in Tasmanian pyrethrum ( Tanacetum cinerariifolium ) fields has increased substantially since first identification in 2001. Examination of morphological features and sequencing of the internal transcribed spacer region (ITS) resulted in the identification of a new species, herein described as Microsphaeropsis tanaceti sp. nov. The pathogenicity of three M. tanaceti isolates to two pyrethrum cultivars was confirmed by inoculating glasshouse-grown plants in three experiments. No significant differences in the susceptibility of the two cultivars to infection by M. tanaceti were found. Symptoms were tan-coloured spots which coalesced around the margins of the leaves. Therefore, the name 'tan spot' is proposed for this new disease of pyrethrum. The sensitivity of seven M. tanaceti isolates to difenoconazole and azoxystrobin, commonly used fungicides for the management of foliar diseases in spring, was assessed under in vitro conditions. Sensitivity testing for difenoconazole was conducted using a mycelial growth assay on potato dextrose agar, whilst testing for sensitivity to azoxystrobin used a conidial germination assay on water agar. Microsphaeropsis tanaceti was found to be more sensitive to azoxystrobin than difenoconazole, with complete inhibition of conidial germination at concentrations above 0·625 μ g a.i. mL -1. By comparison, concentrations of 50 μ g a.i. difenoconazole mL -1 or greater were required for significant inhibition of mycelial growth. It therefore appears likely that there is currently some control of tan spot as a result of the use of azoxystrobin and to a lesser extent, difenoconazole, for the control of other diseases.

Diseases of Pyrethrum in Tasmania: Challenges and Prospects for Management

“…Diseases caused by S. floridanum have been reported in chrysanthemum (90) and China aster (5). All pyrethrum cultivars currently grown in Tasmania appear to have similar levels of susceptibility to tan spot, winter blight, and pink spot (70,72,73).…”

“…nov.), winter blight (caused by Alternaria tenuissima), and pink spot (caused by Stemphylium botryosum) are commonly found in Tasmanian pyrethrum fields (67,70,72,73) (Fig. 12).…”

“…Symptomless infections with TSWV also are common. Symptoms of tan spot include tancolored spots that coalesce around the margins of leaves (72,73). The prevalence and incidence of tan spot has substantially increased since its first description in Tasmania in 2001 (67).…”

“…The prevalence and incidence of tan spot has substantially increased since its first description in Tasmania in 2001 (67). The occurrence of tan spot varies temporally according to the season and, along with ray blight, contributes to defoliation in spring (72,73). Therefore, the spring fungicide program used by pyrethrum growers in Tasmania targets the management of both diseases to minimize yield losses (68,71).…”

“…Moreover, there is little information that can be directly translated to the highly mechanized pyrethrum production systems of Tasmania, due in part to differences in agronomic practices, cultivars grown, prevalent pathogens, and profit margins. The major diseases affecting production in Tasmania are (in decreasing order of importance): ray blight (68,70,72,73). Tomato spotted wilt virus (TSWV) is also prevalent, although infected plants generally are asymptomatic (76).…”