Minimizing crop damage through understanding relationships between pyrethrum phenology and ray blight disease severity

Authors: PETHYBRIDGE, S - Botanical Resources Australia Pty Ltd; Gent, David - Dave; GROOM, T - Botanical Resources Australia Pty Ltd; HAY, FRANK - Tasmanian Institute Of Agricultural Research

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2013
Publication Date: 11/30/2013
Citation: Pethybridge, S.J., Gent, D.H., Groom, T., Hay, F. 2013. Minimizing crop damage through understanding relationships between pyrethrum phenology and ray blight disease severity. Plant Disease. 97(11):1431-1437.

Interpretive Summary: One of the most damaging foliar disease of pyrethrum is ray blight caused by the fungus Stagonosporopsis tanaceti. The probability of growers incurring economic losses caused by this disease has been substantially reduced by the implementation of a prophylactically-applied spring fungicide program. Research was conducted to related temperature to plant development to aid in timing the first fungicide application of the season. A model was developed that is able to be operated with only daily high and low temperature to estimate development of pyrethrum stems in spring. The timing of the first fungicide application was closely related to its effectiveness in suppressing ray blight, and a threshold point was identified where effectiveness of fungicides was substantially reduced if the first spray was applied after this point. Overall, this research indicated that disease management may be improved by applying the first fungicide substantially earlier in plant development than currently recommended.

Technical Abstract: The most damaging foliar disease of pyrethrum in Australia is ray blight caused by Stagonosporopsis tanaceti. The probability of growers incurring economic losses caused by this disease has been substantially reduced by the implementation of a prophylactically-applied spring fungicide program. This has been traditionally initiated when 50% of the stems have reached between 5 and 10 cm in height. Data collected on the emergence of stems from semi-dormant plants over late winter from 27 fields across northern Tasmania from 2009 to 2011 was used to develop a degree-day model to assist with initiation of the fungicide program. Temporal changes in cumulative proportion of plants with elongated stems were well described by a logistic growth model (R2 = 0.97 across all fields). These models were used to calculate the number of days to when 50% of the sampling units had at least one elongated stem for the calculation of simple degree-days assuming a nominal biofix date of the austral winter solstice. The median date for 50% stem elongation was estimated as 30 August in these data sets. Mean error and root mean square error of degree-day models was minimized when a base of 0°C was selected. Mixed model analysis found prediction errors to be significantly affected by geographic region requiring the use of scalar correction factors for specific production regions. In the Western region, 50% stem emergence was predicted at 590.3 degree-days (mean prediction error = 0.7 days). Moreover, in the Coastal and Inland regions, 50% stem emergence was predicted as 644.6 (mean prediction error = 7.7 days) and 684.7 (mean prediction error = 0.7 days) degree-days, respectively. The effect of fungicide timing for initiation of the spring disease management program and resultant ray blight disease severity (expressed as percent disease control in October) was quantified within 44 fields from 2003 to 2005. This relationship was best explained by a split-line regression with a significant break-point of 513.8 degree-days, which corresponded to 10.7% of sampling units with elongated stems. Overall, this research indicated that disease management may be improved by applying the first fungicide of the program substantially earlier in phenological development of the stems than currently recommended.