Seasonal fluctuations in fungi associated with pyrethrum foliage in Tasmania

The temporal fluctuations in disease severity (frequency of plants in disease categories) and the frequency of fungal isolations from lesions on foliage of pyrethrum (Tanacetum cineariifolium) were recorded in fields approaching their first harvest throughout 2000 and 2001 in northern Tasmania. In both years, the number of plants with areas of necrotic foliage in the lesser disease categories (< 0.50 of foliage affected) was significantly higher than expected by random chance until August. However, in both years from September onwards this trend was reversed, with the number of plants with areas of necrotic foliage in the higher disease categories (representing > 0.50) being significantly higher than expected by random chance. This corresponded with an increase in the number of consecutive days with rain of at least 0.1 mm and an increase in the frequency of isolation of Phoma ligulicola (a known pathogen) from all lesion types. P. ligulicola was also the most commonly isolated fungus from lesions on stems and buds in 2000 and from necroses of all tissues in 2001. In 2001, necrotic lesions on leaves were differentiated into necrotic spots or marginal necrotic lesions (extending from the margin of the leaf). Alternaria alternata was mostly commonly associated with marginal necroses. A. tenuissima, Stemphylium botryosum and P. ligulicola were the most common fungi associated with necrotic spots on leaves.