Use of Survival Analysis to Assess Management Options for Ray Blight in Australian Pyrethrum Fields

The effects of fungicide, cultivar and plant density on the time-to-death of pyrethrum flowers affected by ray blight (caused by Phoma ligulicola var. inoxydablis) in Australia were analysed using nonparametric Kaplan–Meier (KM) estimates and accelerated failure time (AFT) models with aWeibull probability distribution. Analyses using KMestimates and AFT models yielded similar results. Themedian survival time (T) for all flowers in the fungicide trial was estimated at 53 days [95% confidence interval (CI) = 43–53] in 2000 and 60 days (CI = 51–60) in 2001. In both years, all fungicides tested except copper oxychloride significantly (P £ 0Æ0495) increased the duration of flower survival compared with nontreated plots. Significant variation (P < 0Æ0001) was noted between years and among four cultivars in terms of flower survival, with T values for different cultivars ranging from 41 to 81 days, and averaging 69 days (CI = 60–69) in 2005 and 64 days (CI = 56–64) in 2006 for all cultivars. Planting at a quarter the density currently recommended increased flower survival by 41Æ8% (v2 = 29Æ19; P < 0Æ0001), but did not increase yield. Linear regression indicated that defoliation severity accounted for at least 94% of variation in median survival time. Improvedmanagementmay be achieved via an integrated strategy incorporating these factors.