Changes in distribution and frequency of fungi associated with a foliar disease complex of pyrethrum in Australia

Authors: HAY, FRANK - Tasmanian Institute Of Agricultural Research; Gent, David - Dave; PILKINGTON, STACEY - Tasmanian Institute Of Agricultural Research; PEARCE, TAMIEKA - Tasmanian Institute Of Agricultural Research; SCOTT, JASON - Tasmanian Institute Of Agricultural Research; PETHYBRIDGE, SARAH - Plant And Food Research

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2015
Publication Date: 9/1/2015
Citation: Hay, F., Gent, D.H., Pilkington, S., Pearce, T., Scott, J.B., Pethybridge, S.J. 2015. Changes in distribution and frequency of fungi associated with a foliar disease complex of pyrethrum in Australia. Plant Disease. 99:1227-1235.

Interpretive Summary: Pyrethrum is the source of the naturally occurring pyrethrins, which are important natural insecticides used in household pest control products and also in organic agriculture. A new disease complex has emerged in Australia that reduces foliage over winter and deleteriously affect yield. Field and laboratory studies over several years characterized fungi associated with diseased plants and factors that may be related to emergence of the disease complex. A group of plant pathogens were identified, and the most prevalent being Microsphaeropsis tanaceti. The increased prevalence and isolation frequency of this organism coincided with a succession of abnormally wet winters and detection of reduced sensitivity to a commonly used fungicide. Extensive analysis of fungicide trial data found that changes in management approaches may improve pyrethrum growth during late winter and early spring, although latter effects on disease control and yield appear to be variable and situation-dependent. Changes in management paradigms appear necessary.

Technical Abstract: In Australia, pyrethrum is affected by a foliar disease complex which can substantially reduce green leaf area and deleteriously affect yield. Traditionally, the dominant disease in spring has been ray blight, caused by Stagonosporopsis tanaceti, with other foliar diseases more prevalent during autumn and winter. However, since 2009 high disease intensity leading to excessive defoliation over autumn and winter, combined with failures of spring fungicide programs to effectively manage foliar diseases has been observed. Multiphasic field and laboratories experiments identified a complex of fungal pathogens could be associated with symptoms of winter dieback. Microsphaeropsis tanaceti, the cause of tan spot, was most prevalent and had the highest mean isolation frequency from affected plants, irrespective of sampling time. The mean isolation frequency of S. tanaceti was highest in late spring (35%), but was found in less than 11.3% of isolations during autumn and winter. In contrast, Alternaria spp. were recovered most often during autumn and winter (26% isolation frequency), and less than 5% at other times. Other fungal pathogens, including Colletotrichum tanaceti, Itersonilia perplexans, Stemphylium botryosum and Phoma chrysanthemicola were recovered sporadically and in less than 8% of samples in spring. The increased prevalence and isolation frequency of M. tanaceti coincided with a succession of abnormally wet winters and detection of reduced sensitivity to boscalid, which is an integral component of the spring fungicide program. Due to concern over the emergence of tan spot and its role in defoliation over autumn and winter, the efficacy of fungicides applied in early autumn was quantified. Mixed-model analysis suggested fungicide application in autumn may improve pyrethrum growth during late winter and early spring, although latter effects on disease control and yield appear to be variable and situation-dependent. This research indicated that tan spot is a cause for pyrethrum defoliation during autumn and winter and is of increasing importance. Changes in management paradigms may be warranted.