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Pethybridge S.J.,New Zealand Institute for Plant and Food Research | Gent D.H.,Oregon State University | Groom T.,Botanical Resources Australia Agricultural Services Pty. Ltd. | Hay F.S.,University of Tasmania
Plant Disease | Year: 2013

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 semidormant plants over late winter from 27 fields across northern Tasmania from 2009 to 2011 were 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 until 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 were 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), compared with 644.6 (mean prediction error = 7.7 days) in the Coastal region and 684.7 (mean prediction error = 0.7 days) degree-days in the Inland region. The importance of fungicide timing for initiation of the spring disease management program in minimizing losses (expressed as percent disease control in October) was also quantified. 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. © 2013 The American Phytopathological Society.


Barimani M.,University of Melbourne | Pethybridge S.J.,Botanical Resources Australia Agricultural Services Pty. Ltd | Vaghefi N.,University of Melbourne | Hay F.S.,University of Tasmania | Taylor P.W.J.,University of Melbourne
Plant Pathology | Year: 2013

A new pathogen of pyrethrum (Tanacetum cinerariifolium) causing anthracnose was described as Colletotrichum tanaceti based on morphological characteristics and a four-gene phylogeny consisting of rDNA-ITS, β-tubulin (TUB2), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and actin (ACT) gene sequences. The fungus produced perithecia in culture, requiring an opposite mating type isolate in a heterothallic manner. The initial infection strategy on pyrethrum leaves involved the formation of appressoria followed by production of multilobed infection vesicles in the epidermal cells. Infection and colonization then proceeded through thinner secondary hyphae, which resulted in the initial production of water-soaked lesions followed by black necrotic lesions. The infection process was suggestive of a hemibiotrophic infection strategy. Moreover, phylogenetic analysis clearly showed that C. destructivum, C. higginsianum and C. panacicola were separate species that also had similar intracellular hemibiotrophic infection strategies as C. tanaceti, which all clustered in the C. destructivum complex. Colletotrichum spp. were detected at 1% incidence in seed of 1 of 19 seed lines, indicating the potential for seed as a source of inoculum into crops. Colletotrichum tanaceti was detected in leaf lesions from 11 of 24 pyrethrum fields surveyed between April and July 2012, at a frequency of 1·3-25·0% of lesions. Anthracnose probably contributes to the complex of foliar diseases reducing green leaf area in pyrethrum fields in Australia. © 2013 British Society for Plant Pathology.


Pethybridge S.J.,Botanical Resources Australia Agricultural Services Pty. Ltd. | Gent D.H.,Oregon State University | Gent D.H.,Purdue University | Hay F.S.,University of Tasmania
Phytopathology | Year: 2011

Ray blight, caused by Phoma ligulicola var. inoxydabilis, is the most damaging disease of pyrethrum (Tanacetum cinerariifolium) in Australia. Data collected from 72 plots in commercial pyrethrum fields since 2001 to 2009 revealed substantial annual variations in isolation frequency of the pathogen during semidormancy of the crop in autumn and winter. Isolation frequency of the pathogen during this time and subsequent outbreaks of ray blight in spring were similar across the eight production regions where sampling was conducted, and isolation frequency of the pathogen was linearly correlated (r = 0.88; P < 0.0001) with subsequent defoliation severity when plants commenced growth in spring. Isolation frequency and defoliation severity also were correlated with the incidence of seed infested with P. ligulicola var. inoxydabilis (r = 0.71 and 0.44, respectively; P < 0.0001 in both correlations). Highly accurate risk algorithms for the occurrence of severe epidemics of ray blight were constructed using logistic regression. A model based solely on isolation frequency of the pathogen over autumn and winter correctly predicted epidemic development in 92% of fields. Another model utilizing the incidence of infested seed and rain-temperature interactions in early autumn (austral March and April) and late winter (austral June and July) had similar predictive ability (92% accuracy). Path analysis modeling was used to disentangle interrelationships among the explanatory variables used in the second logistic regression model. The analysis indicated that seedborne inoculum of P. ligulicola var. inoxydabilis contributes indirectly to ray blight defoliation severity through directly increasing overwintering frequency of the pathogen. Autumn and fall weather variables were modeled to have indirect effects on defoliation severity through increasing overwintering success of the pathogen but also direct effects on defoliation severity. Collectively, the analyses point to several critical stages in the disease cycle that can be targeted to minimize the probability of regional epidemics of ray blight in this perennial pathosystem. © 2011 The American Phytopathological Society.


Pethybridge S.J.,Botanical Resources Australia Agricultural Services Pty Ltd | Ngugi H.K.,Pennsylvania State University | Hay F.S.,University of Tasmania
Plant Pathology | Year: 2010

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 a Weibull probability distribution. Analyses using KM estimates and AFT models yielded similar results. The median 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 ≤ 0·0495) increased the duration of flower survival compared with nontreated plots. Significant variation (P < 0·0001) 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 41·8% (x2 = 29·19; P < 0·0001), but did not increase yield. Linear regression indicated that defoliation severity accounted for at least 94% of variation in median survival time. Improved management may be achieved via an integrated strategy incorporating these factors. © 2010 Botanical Resources Australia.


Scott J.B.,University of Tasmania | Gent D.H.,Oregon State University | Pethybridge S.J.,The New Zealand Institute for Plant and Food Research Ltd | Groom T.,Botanical Resources Australia Agricultural Services Pty. Ltd | Hay F.S.,University of Tasmania
Plant Disease | Year: 2014

Sclerotinia crown rot, caused by Sclerotinia sclerotiorum and S. minor, is a prevalent disease in pyrethrum fields in Australia. Management involves fungicide applications during the rosette stage of plant development from autumn to early spring in fields approaching first harvest. However, estimates of crop damage and the efficacy of these tactics are poorly understood; therefore, plots were established in 86 pyrethrum fields in Tasmania, Australia during 2010 to 2012 to quantify these and to identify risk factors for disease outbreaks. On average, commercial management for Sclerotinia crown rot reduced disease incidence 43 to 67% compared with nontreated plots. There was a weak but significant relationship between relative increase in flower yield when fungicides were applied and the incidence of crown rot (R2 = 0.09, P = 0.006), although the mean number of flowers produced was similar regardless of fungicide applications. Flower yield was positively associated with canopy density in spring (S = 0.39, P = 0.001). Moreover, canopy density in spring was linked by both direct and indirect effects to canopy density during autumn and winter which, in turn, were associated with planting date and previous rain events. Modeling canopy density and disease incidence in autumn correctly categorized disease incidence in spring relative to a threshold of 2% in 72% of fields. In a subset of 22 fields monitored over 2 years, canopy density in the autumn following the first harvest had a negative relationship with Sclerotinia crown rot incidence the preceding year (R2 = 0.23, P = 0.006). On average, however, current commercial management efforts provided only small increases in flower yield in the current season and appear best targeted to fields with well-developed plant canopies and Sclerotinia crown rot present during early autumn. © 2014 The American Phytopathological Society.


Pethybridge S.J.,Botanical Resources Australia Agricultural Services Pty. Ltd | Scott J.B.,University of Tasmania | Hay F.S.,University of Tasmania
Plant Disease | Year: 2012

Ray blight, caused by Phoma ligulicola var. inoxydabilis, causes substantial annual losses in Australian pyrethrum fields. Fifty-nine P. ligulicola var. inoxydabilis isolates were randomly selected from fields in three distinct geographical regions in Tasmania, Australia. Genetic diversity was characterized using random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP). Based on genetic similarities of less than 99%, 56 distinct genotypes (putative clones) were observed. Mean haploid gene diversity of clone-corrected populations ranged between 0.05 and 0.31, and 0.11 and 0.32, for the RAPD and AFLP data sets, respectively. Cluster analysis indicated two distinct groups of isolates supported by all bootstrap replicates. The first cluster contained all but four isolates with representatives from all three populations. The second cluster contained two isolates from the Western and Central populations, respectively, while the remaining isolates were not able to be grouped with any distinct cluster. Analysis of the population structure suggested no evidence for spatial autocorrelation at the smallest distance classes. The presence of linkage disequilibrium was indicated regardless of population scale. Collectively, these findings provided further evidence for the absence or minor role of the teleomorph in the epidemiology of ray blight in Australian pyrethrum fields. © 2012 The American Phytopathological Society.


Gent D.H.,Oregon State University | De Wolf E.,Kansas State University | Pethybridge S.J.,Botanical Resources Australia Agricultural Services Pty Ltd.
Phytopathology | Year: 2011

Rational management of plant diseases, both economically and environmentally, involves assessing risks and the costs associated with both correct and incorrect tactical management decisions to determine when control measures are warranted. Decision support systems can help to inform users of plant disease risk and thus assist in accurately targeting events critical for management. However, in many instances adoption of these systems for use in routine disease management has been perceived as slow. The under-utilization of some decision support systems is likely due to both technical and perception constraints that have not been addressed adequately during development and implementation phases. Growers' perceptions of risk and their aversion to these perceived risks can be reasons for the "slow" uptake of decision support systems and, more broadly, integrated pest management (IPM). Decision theory provides some tools that may assist in quantifying and incorporating subjective and/or measured probabilities of disease occurrence or crop loss into decision support systems. Incorporation of subjective probabilities into IPM recommendations may be one means to reduce grower uncertainty and improve trust of these systems because management recommendations could be explicitly informed by growers' perceptions of risk and economic utility. Ultimately though, we suggest that an appropriate measure of the value and impact of decision support systems is grower education that enables more skillful and informed management decisions independent of consultation of the support tool outputs. © 2011 The American Phytopathological Society.


Pethybridge S.J.,Botanical Resources Australia Agricultural Services Pty. Ltd. | Hay F.S.,University of Tasmania | Gent D.H.,Oregon State University
Plant Disease | Year: 2010

Sclerotinia flower blight, caused by Sclerotinia sclerotiorum, causes substantial losses in Australian pyrethrum fields. The spatiotemporal characteristics of epidemics were characterized in five fields over 3 years. Log likelihood tests indicated that the 7beta;-binomial distribution fit better than the binomial distribution for 92% of the data sets. The index of dispersion, D, was significantly greater than 1 in 97% of the data sets. The estimated parameters of the slope and intercept terms of the binary power law were 1.631 (standard error [SE] = 0.059) and 0.678 (SE = 0.099), indicating a high degree of aggregation at the individual sampling unit scale. In 69% of the data sets, the magnitude of the first-order autocorrelation coefficient 1 r̂ , was significantly greater than 0. In 11 of the 12 epidemics, the monomolecular model provided the best fit, indicative of monocyclic processes. A significant spatial association between apothecia and incidence of Sclerotinia flower blight within the lag of one sampling unit was also quantified. This study suggests that S. sclerotiorum apothecia emergence was closely synchronized with flower development, and epidemics were dominated by localized sources of ascosporic inoculums. This research provides the basis for improved management strategies for Sclerotinia flower blight in pyrethrum. © 2010 The American Phytopathological Society.


Vaghefi N.,University of Melbourne | Pethybridge S.J.,Botanical Resources Australia Agricultural Services Pty. Ltd. | Ford R.,University of Melbourne | Nicolas M.E.,University of Melbourne | And 2 more authors.
Australasian Plant Pathology | Year: 2012

Ray blight disease of pyrethrum (Tanacetum cinerariifolium) is shown to be caused by more than one species of Stagonosporopsis. The Australian pathogen, previously identified as Phoma ligulicola var. inoxydabilis, represents a new species described as Stagonosporopsis tanaceti based on morphological characters and a five-gene phylogeny employing partial sequences of the actin, translation elongation factor 1- alpha, internal transcribed spacers and 5.8S of the nrDNA, 28S large subunit and beta-tubulin 2 gene sequences. Furthermore, the two varieties of Stagonosporopsis ligulicola are elevated to species level as S. chrysanthemi and S. inoxydabilis based on their DNA phylogeny and morphology. © Australasian Plant Pathology Society Inc. 2012.

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