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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.

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 | 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.

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.

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