Quantifying Loss Caused by Ray Blight Disease in Tasmanian Pyrethrum Fields

Abstract

The efficacy of newly implemented fungicide recommendations on reducing the intensity of ray blight disease caused by Phoma ligulicola to achieve site-specific attainable yield potentials in Tasmanian pyrethrum fields was quantified over two seasons in 46 and 51 fields during the 2003 and 2004 growing seasons, respectively. Disease intensity and yield in two plots (10 × 24 m), one following the commercial fungicide protocol recommendations and the second receiving no fungicide, were assessed in each pyrethrum field. The commercial fungicide protocol consisted of one application of azoxystrobin at 150 g a.i./ha, followed by two applications of a tank mixture of difenoconazole at 125 g a.i./ha and chlorothalonil at 1,008 liters a.i./ha at 14- to 21-day intervals. This program resulted in significant decreases in defoliation severity and the incidence of stems and flowers with ray blight, and increases in the height of stems and number of flowers produced per stem in October and November. In plots receiving the commercial fungicide protocol, the dry weight of flowers was increased by 76 and 68% in 2003 and 2004, respectively. Moreover, pyrethrin yield increased by 81 and 78% when the commercial fungicide protocol was used compared with the nontreated plots. Tobit regression was used to examine the relationships and thresholds among disease intensity measures (defoliation severity, stem severity, and incidence of flowers with ray blight) assessed just prior to harvest. This regression utilized a left-censored regression model to define subminimal thresholds, as none of the disease intensity measures could be less than 0. Defoliation severity had a threshold of 35.3% before stem severity linearly increased and a threshold of 38.2% before the incidence of flowers with ray blight linearly increased. Finally, the threshold for stem severity was 13.7% before the incidence of flowers with ray blight linearly increased. These thresholds can be used to assist growers in making disease management decisions with the objective of minimizing loss of flowers by maintaining defoliation severity below the critical point at which the incidence of flowers with ray blight begins to linearly increase.

Diseases of Pyrethrum in Tasmania: Challenges and Prospects for Management

“…Tomato spotted wilt virus (TSWV) is also prevalent, although infected plants generally are asymptomatic (76). Ray blight is the most prevalent disease in early spring and is the primary target of spring fungicide programs (60,62,68,71,75). Sclerotinia and Botrytis flower blights can be observed when the crops are flowering from early November until harvest in December, and are managed using a fungicide program during summer flowering.…”

Site-Specific Risk Factors for Ray Blight in Tasmanian Pyrethrum Fields

“…Results of this study identified weather and other site-specific factors that impact the risk of ray blight defoliation exceeding 40%-a level of defoliation associated with a linear increase in the loss of flowers due to ray blight (31). For the September model, the product of the number of days with rainfall ≥0.1 mm and average maximum temperature during the last 14 days was a significant predictor of ray blight severity in September.…”

“…The methodology used to assess disease intensity in fields sampled in 2003 and 2004 has been reported (31,32), and this protocol was also followed in 2005. In the current study, disease intensity assessments were made during 12 to 16 20 flowering stems were systematically selected from each of three linear transects covering the entire width of each nontreated plot (60 stems per plot in total).…”

“…This value was assigned based on practical considerations that a pyrethrum grower would be more likely to identify and classify fields using a 40% defoliation severity than the 38.2%. Fields with defoliation severity below 40% were assigned a value of zero and those greater than or equal to the 40% defoliation severity were assigned a value of one to generate a binary response variable for use in subsequent statistical analyses (31).…”

“…inoxydablis, can reduce plant growth and pyrethrin yield in Tasmania (31,34,36,40). Ray blight was first observed causing minor losses on pyrethrum flowers in Tasmania in 1995 (41).…”