Symptomatology of crown rot of pyrethrum caused by Sclerotinia spp. in Tasmania, Australia

Sclerotinia crown rot (SCR), caused by S. minor and S. sclerotiorum, is an important disease affecting the vegetative tissues of pyrethrum (Tanacetum cinerariifolium) during winter and spring. Symptoms of SCR include bleaching and wilting of leaves leading to necrosis of vegetative tissues. Under severe conditions plant death occurs. This study characterised the progression of SCR symptoms and the relative differences between the two species and forms of inoculums. The relative ability of sclerotia and ascospores to initiate SCR infection was tested under greenhouse conditions. Sclerotia of both species were demonstrated to infect the crown of pyrethrum when myceliogenically germinating sclerotia were in direct contact with the host plant. However, inoculation with ascosporic suspensions of either species failed to initiate SCR symptoms. Under favourable conditions, symptom expression was apparent at 2 to 3 days post inoculation (DPI) with the occurrence of water soaked lesions in leaf tissues. From 20 DPI, infected tissues typically appeared necrotic. Symptoms did not differ between the two species. However, isolates of S. minor were more aggressive with a mean proportion of necrotic leaves at 20 DPI of 0.34, relative to 0.16 (P = 0.01) for S. sclerotiorum. Sclerotial formation by S. minor was also faster, producing sclerotia within 13 DPI, while S. sclerotiorum required 28 DPI. These results support field observations where S. minor is dominant.