206 research outputs found
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Minimizing Crop Damage Through Understanding Relationships Between Pyrethrum Phenology and Ray Blight Disease Severity
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
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Estimation of Pyrethrum Flower Number Using Digital Imagery
Flower number is the primary determinant of yield in pyrethrum (Tanacetum cineariifolium). Traditional estimates of flower numbers use physical harvesting of flowers, which is time consuming, destructive, and complicated. The precision of flower number estimates may be highly influenced by spatial heterogeneity of plant density and vigor. Here, we examined the potential for digital image analysis to enable rapid, nondestructive assessment of flower number. This technique involved removal of pixels with color profiles not typical of the disc florets of pyrethrum. Particle counting was then performed using defined size and shape parameters to estimate flower numbers. Estimates of flower number based on image analyses were correlated with physical harvests of flowers, with estimates representing about an average of 32% of total flower numbers present within a sampling unit. This relationship was consistent across all observed flower densities. Covariate analysis indicated that occurrences of crop lodging and over mature flower canopies had significant, detrimental effects on system predictions. Pyrethrum flowers were spatially aggregated within fields with the degree of aggregation greatest at the lowest flower densities. Based on modeled flower distributions, eight quadrats (0.49-m² sampling unit) were sufficient to achieve a cv of 0.1 in a 600-m² plot area in all but the lowest flower densities. The utility of this approach for biomass assessment in pyrethrum and other Compositae is discussed.This is the publisher’s final pdf. The published article is copyrighted by the American Society for Horticultural Science and can be found at: http://horttech.ashspublications.org/Keywords: Tanacetum cinerariifolium, spatial variability, remote sensing, image analysis, yield estimation, sample siz
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Spatiotemporal Characterization of Sclerotinia Crown Rot Epidemics in Pyrethrum
Sclerotinia crown rot, caused by Sclerotinia minor and S. sclerotiorum,
is a disease of pyrethrum in Australia that may cause substantial decline
in plant density. The spatiotemporal characteristics of the disease
were quantified in 14 fields during three growing seasons. Fitting the
binary power law to disease incidence provided slope (b = 1.063) and
intercept (ln(A[subscript p]) = 0.669) estimates significantly (P ≤ 0.0001) greater
than 1 and 0, respectively, indicating spatial aggregation at the sampling
unit scale that was dependent upon disease incidence. Covariate
analyses indicated that application of fungicides did not significantly
influence these estimates. Spatial autocorrelation and spatial analysis
by distance indices indicated that spatial aggregation above the sampling
unit scale was limited to 20 and 17% of transects analyzed, respectively.
The range of significant aggregation was limited primarily
to neighboring sampling units only. Simple temporal disease models failed to adequately describe disease progress, due to a decline in
disease incidence in spring. The relationships between disease incidence
at the scales of individual plants within quadrats and quadrats
within a field was modeled using four predictors of sample size. The
choice of the specific incidence–incidence relationship influenced
the classification of disease incidence as greater than or less than 2%
of plants, a provisional commercial threshold for fungicide application.
Together, these studies indicated that epidemics of Sclerotinia
crown rot were dominated by small-scale aggregation of disease.
Larger scale patterns of diseased plants, when present, were associated
with severe disease outbreaks. The spatial and temporal analyses
were suggestive of disease epidemics being associated with localized
primary inoculum and other factors that favor disease development at
a small scale
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Changes in Distribution and Frequency of Fungi Associated With a Foliar Disease Complex of Pyrethrum in Australia
In Australia, pyrethrum (Tanacetum cinerariifolium) is affected by a foliar disease complex that can substantially reduce green leaf area and yield. Historically, the most important foliar disease of pyrethrum in Australia has been ray blight, caused by Stagonosporopsis tanaceti, and other fungi generally of minor importance. Temporal fluctuations in the frequency of fungi associated with foliar disease were quantified in each of 83 fields in northern Tasmania, Australia, during 2012 and 2013. Sampling was conducted throughout winter (April to July), spring (August to September), and summer (November) representing different phenological stages. Microsphaeropsis tanaceti, the cause of tan spot, was the pathogen most prevalent and isolated at the highest frequency, irrespective of sampling period. The next most common species was S. tanaceti, whose isolation frequency was low in winter and increased in spring and summer. Known pathogens of pyrethrum, Alternaria tenuissima, Colletotrichum tanaceti, and Stemphylium botryosum were recovered sporadically and at low frequency. Two species of potential importance, Paraphoma chrysanthemicola and Itersonilia perplexans, were also found at low frequency. This finding suggests a substantial shift in the dominant pathogen associated with foliar disease, from S. tanaceti to M. tanaceti, and coincides with an increase in defoliation severity in winter, and control failures of the spring fungicide program. Factors associated with this finding were also investigated. Sensitivity of M. tanaceti and S. tanaceti populations to the fungicides boscalid and cyprodinil collected prior to and following disease control failures in the field were tested under in vitro conditions. A high proportion (60%) of the M. tanaceti isolates obtained from fields in which no response to the spring fungicide program was found were insensitive to 50 µg a.i./ml boscalid. This represented a 4.2-fold increase in the frequency of this phenotype within the M. tanaceti population over 2 years. No shifts in sensitivities to cyprodinil of M. tanaceti and S. tanaceti, or S. tanaceti to boscalid, were observed. Considering the increase in defoliation severity over winter, the benefits of applying fungicides in autumn, in addition to the commercial standard (spring only), were quantified in 14 individual field trials conducted in 2011 and 2012. Mixed-model analysis suggested fungicide application in autumn may improve pyrethrum growth during late winter and early spring, although effects on defoliation and yield were minimal. The increasing prevalence and isolation frequency of M. tanaceti and boscalid resistance within the population is of concern and highlights the urgent need for adoption of nonchemical methods for disease management in Australian pyrethrum fields.This is the publisher’s final pdf. The published article is copyrighted by American Phytopathological Society and can be found at: http://apsjournals.apsnet.org/loi/pdi
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Carpogenic germination of sclerotia of Sclerotinia minor and ascosporic infection of pyrethrum flowers
Carpogenic germination of sclerotia and infection of flowers by ascospores of Sclerotinia minor is rare and seldom documented in most hosts. During 2007–2009, S. minor isolates were obtained from surface-sterilized pyrethrum flowers collected from fields in Australia. The isolation frequency of S. minor from flowers in 2007, 2008 and 2009 was 15.8%, 5% and 1.4%, respectively. During these years, the prevalence of S. minor in flowers amongst pyrethrum fields varied between 10.3% and 60%. Sclerotia with apothecia, consistent in size with S. minor, were collected in one field. Colonies from individual ascospores from this isolate were identified as S. minor. A subsample of 10 S. minor isolates was selected for further studies. Phylogenetic analysis based on the internal transcribed spacer region grouped these isolates with S. minor, and distinct from published sequences of other Sclerotinia spp. Species-specific primers developed previously to differentiate the four major Sclerotinia spp. (S. sclerotiorum, S. minor, S. homoeocarpa and S. trifoliorum) were used to confirm identity. Of the 10 S. minor isolates, eight were able to carpogenically germinate in vitro. Pathogenicity of S. minor to flowers was confirmed in the greenhouse using ascospores. This study is one of the few instances documenting the ability of S. minor to infect floral tissues and the first documentation of S. minor causing flower disease of pyrethrum. These findings serve as a scaffold for further investigations into the mechanisms of flower infection by S. minor and have implications for the management of the Sclerotinia disease complex affecting pyrethrum in Australia.This is the publisher’s final pdf. The article is copyrighted by Canadian Phytopathological Society (Société Canadienne de Phytopathologie) and published by Taylor & Francis. It can be found at: http://www.tandfonline.com/toc/tcjp20/currentKeywords: pyrethrum, Sclerotinia flower blight, Sclerotinia minor, Sclerotinia sclerotiorum, carpogenic germinationKeywords: pyrethrum, Sclerotinia flower blight, Sclerotinia minor, Sclerotinia sclerotiorum, carpogenic germinatio
The Ascomycete Verticillium longisporum Is a Hybrid and a Plant Pathogen with an Expanded Host Range
Hybridization plays a central role in plant evolution, but its overall importance in fungi is unknown. New plant pathogens are thought to arise by hybridization between formerly separated fungal species. Evolution of hybrid plant pathogens from non-pathogenic ancestors in the fungal-like protist Phytophthora has been demonstrated, but in fungi, the most important group of plant pathogens, there are few well-characterized examples of hybrids. We focused our attention on the hybrid and plant pathogen Verticillium longisporum, the causal agent of the Verticillium wilt disease in crucifer crops. In order to address questions related to the evolutionary origin of V. longisporum, we used phylogenetic analyses of seven nuclear loci and a dataset of 203 isolates of V. longisporum, V. dahliae and related species. We confirmed that V. longisporum was diploid, and originated three different times, involving four different lineages and three different parental species. All hybrids shared a common parent, species A1, that hybridized respectively with species D1, V. dahliae lineage D2 and V. dahliae lineage D3, to give rise to three different lineages of V. longisporum. Species A1 and species D1 constituted as yet unknown taxa. Verticillium longisporum likely originated recently, as each V. longisporum lineage was genetically homogenous, and comprised species A1 alleles that were identical across lineages
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