Spatiotemporal Description of Epidemics Caused by Phoma ligulicola in Tasmanian Pyrethrum Fields

ABSTRACT Spatial and temporal patterns of foliar disease caused by Phoma ligulicola were quantified in naturally occurring epidemics in Tasmanian pyrethrum fields. Disease assessments (defoliation incidence, defoliation severity, incidence of stems with ray blight, and incidence of flowers with ray blight) were performed four times each year in 2002 and 2003. Spatial analyses based on distribution fitting, runs analysis, and spatial analysis by distance indices (SADIE) demonstrated aggregation in fields approaching their first harvest for all assessment times between September and December. In second-year harvest fields, however, the incidence of stems with ray blight was random for the first and last samplings, but aggregated between these times. Spatiotemporal analyses were conducted between the same disease intensity measures at subsequent assessment times with the association function of SADIE. In first-year harvest fields, the presence of steep spatial gradients was suggested, most likely from dispersal of conidia from foci within the field. The importance of exogenous inoculum sources, such as wind-dispersed ascospores, was suggested by the absence of significant association between defoliation intensity (incidence and severity) and incidence of stems with ray blight in second-year harvest fields. The logistic model provided the best temporal fit to the increase in defoliation severity in each of six first-year harvest fields in 2003. The logistic model also provided the best fit for the incidence of stems with ray blight and the incidence of flowers with ray blight in four of six and three of six fields, respectively, whereas the Gompertz model provided the best fit in the remaining fields. Fungicides applied prior to mid-October (early spring) significantly reduced the area under disease progress curve (P < 0.001) for defoliation severity, the incidence of stems with ray blight, and the incidence of flowers with ray blight for epidemics at all field locations. This study provides information concerning the epidemiology of foliar disease and ray blight epidemics in pyrethrum and offers insight on how to best manage these diseases.     

Development of potato late blight epidemics: disease foci, disease gradients, and infection sources

ABSTRACT Natural potato late blight epidemics were studied to assess the relative impact of various inoculum sources of Phytophthora infestans in Southern Flevoland (the Netherlands) from 1994 through 1996. Disease surveys were combined with characterization of isolates for mating type and DNA fingerprint pattern using probe RG57. Seventy-four percent of the commercial potato fields with early foci were clearly associated with nearby infested refuse piles. Characterization of isolates from refuse piles and fields confirmed the association. Infected seed tubers, volunteer plants, and infested allotment gardens appeared to be of minor importance for late blight development in potato fields. Several foci in refuse piles, potato fields, and allotment gardens contained more than one genotype. Due to favorable weather in August 1994, infested organic potato fields became major inoculum sources, resulting in the spread of P. infestans to adjacent conventional potato fields. Analyses of disease gradients, both at the field and regional levels, confirmed the role of the organic fields as mid-season infection sources. The mean slope of field gradients downwind of refuse piles (point sources) was significantly steeper (100-fold difference) than the mean slope of field gradients downwind of organic fields (area sources). The genotypic composition of the P. infestans populations along the gradient and of the source populations in the organic potato crops did not differ significantly. Analysis of the region gradient revealed genotype-specific disease gradients. Control measures are recommended.     

Spatiotemporal relationships between disease development and airborne inoculum in unmanaged and managed Botrytis leaf blight epidemics

Comparatively little quantitative information is available on both the spatial and temporal relationships that develop between airborne inoculum and disease intensity during the course of aerially spread epidemics. Botrytis leaf blight and Botrytis squamosa airborne inoculum were analyzed over space and time during 2 years (2002 and 2004) in a nonprotected experimental field, using a 6 x 8 lattice of quadrats of 10 x 10 m each. A similar experiment was conducted in 2004 and 2006 in a commercial field managed for Botrytis leaf blight using a 5 x 5 lattice of quadrats of 25 x 25 m each. Each quadrat was monitored weekly for lesion density (LD) and aerial conidium concentration (ACC). The adjustment of the Taylor's power law showed that heterogeneity in both LD and ACC generally increased with increasing mean. Unmanaged epidemics were characterized in either year, with aggregation indices derived from SADIE (Spatial Analysis by Distance Indices). For LD, the aggregation indices suggested a random pattern of disease early in the season, followed by an aggregated pattern in the second part of the epidemic. The index of aggregation for ACC in 2002 was significantly greater than 1 at only one date, while it was significantly greater than 1 at most sampling dates in 2004. In both years and for both variables, positive trends in partial autocorrelation were observed mainly for a spatial lag of 1. In 2002, the overall pattern of partial autocorrelations over sampling dates was similar for LD and ACC with no significant partial autocorrelation during the first part of the epidemic, followed by a period with significant positive autocorrelation, and again no autocorrelation on the last three sampling dates. In 2004, there was no significant positive autocorrelation for LD at most sampling dates while for ACC, there was a fluctuation between significant and non-significant positive correlation over sampling dates. There was a significant spatial correlation between ACC at given date (t(i)) and LD 1 week later (t(i + 1)) on most sampling dates in both 2002 and 2004 for the unmanaged and managed sites. It was concluded that LD and ACC were not aggregated in the early stage of epidemics, when both disease intensity and airborne conidia concentration were low. This was supported by the analysis of LD and ACC from a commercial field, where managed levels of disease were low, and where no aggregation of both variables was detected. It was further concluded that a reliable monitoring of airborne inoculum for management of Botrytis leaf blight is achievable in managed fields using few spore samplers per field.     

Spatial and Temporal Analyses of Bacterial Blight of Onion Caused by Xanthomonas axonopodis pv. allii

ABSTRACT Bacterial blight of onion is a severe disease, which emerged over the past decade in several onion-producing areas. This disease currently is observed in both the Old and New Worlds. Although the causative agent, Xanthomonas axonopodis pv. allii, is potentially seedborne, seed transmission and significance of seedborne initial inoculum for the development of bacterial blight of onion previously has not been assessed. This article describes experimental work designed to evaluate the biological importance of seedborne X. axonopodis pv. allii as an initial inoculum source, and examine the temporal and spatial dynamics of the disease. Over 3 years, outbreaks of bacterial blight of onion always were induced in experimental plots sown with naturally contaminated seed lots, with a contamination rate determined as 0.04%. Analyses of disease patterns indicated a likely seedborne origin for the inoculum associated with the early stages of epidemics. Spatial analyses performed with several statistical methods indicated aggregated patterns of disease incidence data. Primary foci enlarged over time, and a few distinct secondary foci sometimes were established after occurrence of wind-driven rains (with gusts up to 15 m s(-1)). Distances between primary and secondary foci ranged from less than 1 m (satellite foci) to 25 m. It remains possible that longdistance dispersal of inoculum was at least partly involved in the later stages of epidemics.     

Temporal and spatial dynamics of watermelon gummy stem blight epidemics

The effect of the distance of initial inoculum on the intensity of watermelon gummy stem blight, caused by Didymella bryoniae, was studied in a naturally-infected rainfed commercial field. The shorter the distance from the focus, the sooner was disease onset and the earlier maximum disease levels were achieved. Maximum disease incidences were reached earlier than maximum severities, but eventually destructive levels were observed for both disease incidence and severity. Disease progressed at similar rates, irrespective of the radial distance from the focus. A detailed study of the disease temporal progress was conducted in inoculated rainfed experimental fields with commercial genotypes Crimson Sweet (susceptible, S) and Riviera (moderately resistant, R). The Gompertz model best described the disease progress curves, and estimated apparent infection rates were 0.049 and 0.020 respectively for S and R genotypes. In addition, spatial pattern studies were conducted during the dry season in overhead irrigated experimental plots, inoculated with point-source foci. Disease intensity gradients were better explained by the Exponential model than by the Power Law model. Gummy stem blight distribution was classified as aggregated by the Ordinary Runs procedure. Two different spatial autocorrelation methods (2DCorr and LCOR) revealed strong short distance spatial dependencies. Long distance positive correlations between quadrats were observed along with periods of higher progress rates. The dynamic patterns of the epidemics of gummy stem blight in watermelon described here are consistent with epidemics of polycyclic diseases with splash-dispersed spores.     

Response of black rot in cabbage to spatial distribution of inoculum

Disease progress of black rot in cabbage crop was studied over three years in field plots to compare the effects of uni-focal and multi-focal inoculum applied in equal amounts per plot. Disease progress (plant incidence and leaf incidence) was plotted over time, three dimensional maps were made, and disease aggregation was studied by means of geostatistics, black-black counts and Moran's I statistic. Black rot progress was primarily due to focus expansion. Secondary foci may appear at short distances from the initial focus but they usually merge with the expanding initial focus. Anisotropy occurred occasionally but was of minor importance. Disease proceeds faster in plots with multi-focal inoculation than in those with uni-focal inoculation. Probably, serious epidemics in Dutch cabbage fields originate from large numbers of foci.     

Influence of canopy structure on sheath blight epidemics in rice

Sheath blight (caused by Rhizoctonia solani) is one of the most important constraints in achieving high grain yield in intensive rice production systems. Canopy structure can influence the development of sheath blight epidemics. The objective of this study was to determine the effect of canopy structure parameters such as shoot number, leaf area index, biomass production, contact frequency, light transmittance and plant height on the development of sheath blight epidemics in commercial fields. Field experiments were conducted in both early and late seasons of 2009 and 2010 in Wuxue, Hubei province, China. The effects of nitrogen (N) rate and hill density on structure and production parameters and sheath blight severity were investigated. Sheath blight severity was recorded as a sheath blight index or relative lesion height on inoculated and uninoculated plants in each crop. Lesion length was measured on inoculated plants in 2010. The results showed that the sheath blight index increased with an increase of N rate and hill density in uninoculated plots in each trial. Stepwise multiple regression analysis demonstrated that contact frequency was consistently related to sheath blight. Lesion length on inoculated plants was not correlated with canopy structure. These results indicate that canopy structure influences sheath blight epidemics. A ‘healthy’ canopy resulting from appropriate crop management practices can be used to suppress sheath blight epidemics in rice.     

Development of natural late blight epidemics in pure and mixed plots of potato cultivars with different levels of partial resistance

Late blight, caused by Phytophthora infestans , is the most severe disease of potato worldwide. Controlling late blight epidemics is difficult, and resistance of host cultivars is either not effective enough, or too easily overcome by the pathogen to be used alone. In field trials conducted for 3 years under natural epidemics, late blight severity was significantly lower in a susceptible cultivar growing in rows alternating with partially resistant cultivars (mixtures) than in unmixed plots of the susceptible cultivar alone. Partially resistant cultivars behaved similarly in unmixed and mixed plots. Mixtures of cultivars reduced disease progress rates and sometimes delayed disease onset over unmixed plots, but did so significantly only for the slowest epidemic. This suggests that reduction of area under the disease progress curve (AUDPC) in mixtures resulted from the cumulative action of minor effects. Disease distribution was focal in all plots at all dates, as shown by Morisita's index values significantly exceeding 1. Significant yield increases for the susceptible cultivar, and occasionally for the partially resistant ones, were observed in mixed-cultivar plots compared with single-cultivar plots. These results show that cultivar mixtures can significantly reduce natural, polycyclic epidemics in broadleaved plants attacked by pathogens causing rapidly expanding lesions.     

Assessment of the impact of mid-season late blight infection on disease development and yield of potato variety Russet Norkotah in Maine

Development of late blight of potatoes caused by Phytophthora infestans (US 8 fungal genotype, A2 mating type) was monitored in two Russet Norkotah commercial fields at Fort Fairfield in 1996 and Duncan Farm in 1997. Experimental plots representing various disease treatments (low, moderate, high and random late blight severity levels) were established in two fields in 1996. In 1997, only low and high disease treatments were established. The application of fungicides for late blight control was conducted in both years. Late blight incidence and severity were assessed in each plot of each treatment. Components of late blight disease development, tuber blight incidence and tuber yields were determined from each plot. Progress of late blight disease was rapid in 1996 but not in 1997. During the 1996 cropping season, mean disease incidence and severity in the random disease treatment plots were 84 and 21% respectively within 10 days of disease detection. In 1997, low levels of late blight severity were detected in the field plots. Average numbers of late blight leaf and stem lesions on infected plant and fungal sporangia on the diseased leaf were not significantly affected by disease treatment. Late blight foliar severity significantly affected potato tuber yields. Lowest tuber yield was obtained in plots with high disease levels and highest yields were recorded in plots with low late blight severity in 1996. Late blight severity was significantly correlated with tuber yield but not with per cent tuber blight.     

Qualification of a Plant Disease Simulation Model: Performance of the LATEBLIGHT Model Across a Broad Range of Environments

ABSTRACT The concept of model qualification, i.e., discovering the domain over which a validated model may be properly used, was illustrated with LATEBLIGHT, a mathematical model that simulates the effect of weather, host growth and resistance, and fungicide use on asexual development and growth of Phytophthora infestans on potato foliage. Late blight epidemics from Ecuador, Mexico, Israel, and the United States involving 13 potato cultivars (32 epidemics in total) were compared with model predictions using graphical and statistical tests. Fungicides were not applied in any of the epidemics. For the simulations, a host resistance level was assigned to each cultivar based on general categories reported by local investigators. For eight cultivars, the model predictions fit the observed data. For four cultivars, the model predictions overestimated disease, likely due to inaccurate estimates of host resistance. Model predictions were inconsistent for one cultivar and for one location. It was concluded that the domain of applicability of LATEBLIGHT can be extended from the range of conditions in Peru for which it has been previously validated to those observed in this study. A sensitivity analysis showed that, within the range of values observed empirically, LATEBLIGHT is more sensitive to changes in variables related to initial inoculum and to weather than to changes in variables relating to host resistance.     

Temporal Dynamics of Phytophthora Blight on Bell Pepper in Relation to the Mechanisms of Dispersal of Primary Inoculum of Phytophthora capsici in Soil

ABSTRACT The effect of components of primary inoculum dispersal in soil on the temporal dynamics of Phytophthora blight epidemics in bell pepper was evaluated in field and growth-chamber experiments. Phytophthora capsici may potentially be dispersed by one of several mechanisms in the soil, including inoculum movement to roots, root growth to inoculum, and root-to-root spread. Individual components of primary inoculum dispersal were manipulated in field plots by introducing (i) sporangia and mycelia directly in soil so that all three mechanisms of dispersal were possible, (ii) a plant with sporulating lesions on the soil surface in a plastic polyvinyl chloride (PVC) tube so inoculum movement to roots was possible, (iii) a wax-encased peat pot containing sporangia and mycelia in soil so root growth to inoculum was possible, (iv) a wax-encased peat pot containing infected roots in soil so root-to-root spread was possible, (v) noninfested V8 vermiculite media into soil directly as a control, or (vi) wax-encased noninfested soil as a control. In 1995 and 1996, final incidence of disease was highest in plots where sporangia and mycelia were buried directly in soil and all mechanisms of dispersal were operative (60 and 32%) and where infected plants were placed in PVC tubes on the soil surface and inoculum movement to roots occurred with rainfall (89 and 23%). Disease onset was delayed in 1995 and 1996, and final incidence was lower in plants in plots where wax-encased sporangia (6 and 22%) or wax-encased infected roots (22%) were buried in soil and root growth to inoculum or root-to-root spread occurred. Incidence of root infections was higher over time in plots where inoculum moved to roots or all mechanisms of dispersal were possible. In growth-chamber studies, ultimately all plants became diseased regardless of the dispersal mechanism of primary inoculum, but disease onset was delayed when plant roots had to grow through a wax layer to inoculum or infected roots in tension funnels that contained small volumes of soil. Our data from both field and growth-chamber studies demonstrate that the mechanism of dispersal of the primary inoculum in soil can have large effects on the temporal dynamics of disease.     

Spatial analysis of epidemics of <Emphasis Type="Italic">Grapevine leafroll associated virus-3</Emphasis>

Several vineyards in Rías Baixas and one in the Ribeira Sacra (Spain) were monitored and the spatial pattern of leafroll-diseased grapevine was analysed at several dates. Unidimensional aggregation analysis (ordinary runs), bidimensional analysis, and disease gradients analysis were used as methods of study of spatial aspects of epidemics of GLRaV. At very low insect populations the mealybug Planococcus citri transmitted the GLRaV-3 from infected plants to healthy ones planted between them in an experimental plot at Beluso; an initial focus of leafroll-infected grapevines was detected 2 years after planting at the area where the vectors had been located infesting the old plants. Within 8 years some new foci appeared and coalesced, and the incidence reached >80%. In three commercial plots where no vectors were observed, the spatial analysis of the diseased plants showed three different situations. In Meaño, the study of the evolution of the spatial patterns of diseased plants between 1992 and 2005 suggested slow vectorial field transmission of GLRaV-3. In Goián the analysis for only 2 years suggested random distribution; therefore the viruses were arriving with the planting material, but the runs analysis of some lines suggested incipient spread of GLRaV-3. In Portomarín the incidence of both GLRaV-2 and 3 was low and their distribution was random, without any evidence of field spread. These examples of the study of the spatial analysis of leafroll-infected plants may be helpful to determine whether or not spread of the viruses is occurring, and the best control measures to take.     

Measurement of healthy and diseased haulm area for assessing late blight epidemics in potatoes

The advantages of assessing disease severity by visual grading and by measuring healthy and diseased plant area were compared in four mild to moderate late blight epidemics during spring and in two severe epidemics during autumn in Israel. Disease progress curves obtained through visual grading showed a continuous increase, but the area of lesions tended to fluctuate during the cropping season and often reached a maximum in plots in which the total amount of foliage was also largest. The healthy haulm area differed with disease intensity and undefined seasonal and cultural influences. The decrease in healthy haulm area was not related to expansion of the blighted area only, but also to the breaking of plants at stem lesions. This phenomenon was especially evident in the warm spring season and was not determined by visual grading of disease severity. Each kind of assessment revealed different features of the epidemics and suited different applications. Visual grading enabled the easiest comparison of epidemic patterns. Lesion areas reflected patterns of inoculum potential, while healthy haulm areas reflected the integrated influences of factors affecting the crop and disease and thus provided useful data for simulating epidemics and for estimating yield losses.     

Host diversity can reduce potato late blight severity for focal and general patterns of primary inoculum

ABSTRACT The use of host diversity as a tool for management of potato late blight has not been viewed as promising in the past. But the increasing importance of late blight internationally has brought new consideration to all potential management tools. We studied the effect of host diversity on epidemics of potato late blight in Oregon, where there was little outside inoculum. The experimental system consisted of susceptible potato cv. Red LaSoda and a highly resistant breeding selection, inoculated with local isolates of US-8 Phytophthora infestans. Potatoes were grown in single-genotype plots and also in a mixture of 10 susceptible and 26 resistant potato plants. Half of the plots received inoculation evenly throughout the plot (general inoculation) and half received an equal quantity of inoculum in only one corner of the plot (focal inoculation). The area under the disease progress curve (AUDPC) was greater in single genotype stands of susceptible cv. Red LaSoda inoculated throughout the plot than with stands inoculated in one focus. The host-diversity effect on foliar late blight was significant in both years of the investigation; the AUDPC was reduced by an average of 37% in 1997 and 36% in 1998, compared with the mean disease level for the potato genotypes grown separately. Though the evidence for influence of inoculum pattern on host-diversity effects was weak (P = 0.15), in both years there was a trend toward greater host-diversity effects for general inoculation. Statistical significance of host-diversity effects on tuber yield and blight were found only in one of the two years. In that year, tuber yield from both the resistant and susceptible cultivar was increased in mixtures compared with single genotype stands and tuber blight was decreased in mixtures for susceptible cv. Red LaSoda.     

Detection of Pseudomonas syringae pv. aptata in Irrigation Water Retention Basins by Immunofluorescence Colony-staining

Bacterial blight of cantaloupe (Cucumis melo) caused by Pseudomonas syringae pv. aptata was first observed in south-western France and has since spread to all cantaloupe-growing areas of this country. Use of pesticides registered for this disease has proved ineffective and no commercial cultivars of cantaloupe are resistant to this blight. To develop control strategies for this disease, the principal sources of inoculum were investigated. Among the different sources of inoculum studied, we report the isolation of P. syringae pv. aptata from irrigation water retention basins in south-western France using the immunofluorescence colony-staining (IFC) method. In this study, the pathogen was detected at a low concentration (12 and 70cful^–1) in two different retention basins. These results suggest that P. syringae pv. aptata can survive in water used to irrigate cantaloupe crops and could be a source of inoculum for epidemics of bacterial blight. To develop control strategies for this bacterial disease, the importance of water retention basins as sources of inoculum for bacterial blight of cantaloupe needs to be evaluated relative to other potential sources such as seeds, plants from nurseries and plant debris in the soil.     

Late-blight epidemics on potato in Finland, 1933–2002; increased and earlier occurrence of epidemics associated with climate change and lack of rotation

Changes in the incidence and onset of potato late-blight epidemics in Finland were investigated and compared with possible changes in climate, presence of soil-borne inoculum, and aggressiveness of Phytophthora infestans populations. Datasets were constructed from leaf blight assessments in cultivar trials or fungicide tests carried out at eight experimental sites during the periods 1933–1962 and 1983–2002. Additional data were obtained from late-blight monitoring projects carried out from 1991 to 2002. From 1998 to 2002, the risk of blight outbreak was 17-fold higher compared with the periods 1933–62 and 1983–1997. Simultaneously, the outbreaks of the epidemics began 2–4 weeks earlier. The changes observed were associated with a climate more conducive to blight in the late 1990s. Lack of rotation also advanced blight epidemics by an average of 9 days in 1998–2002, but it did not have this effect in 1992–1997, suggesting that soil borne inoculum may not have been a significant threat to potato until the late 1990s. The aggressiveness of the P. infestans isolates seemed to have only minor effect on the onset of the epidemics after 1991, as the apparent infection rate remained unchanged despite weather conditions more favourable to late blight in the late 1990s. As a consequence of the more frequent and earlier epidemics, the sales of fungicides used against late blight in Finland increased 4-fold from the 1980s to 2002.     

The explicit dependence of quadrat variance on the ratio of clump size to quadrat size

ABSTRACT In the past decade, it has become common practice to pool mapped binary epidemic data into quadrats. The resultant "quadrat counts" can then be analyzed by fitting them to a probability distribution (i.e., betabinomial). Often a binary form of Taylor's power law is used to relate the quadrat variance to the quadrat mean. The fact that there is an intrinsic dependence of such analyses on quadrat size and shape is well known. However, a clear-cut exposition of the direct connection between the spatial properties of the two-dimensional pattern of infected plants in terms of the geometry of the quadrat and the results of quadrat-based analyses is lacking. This problem was examined both empirically and analytically. The empirical approach is based on a set of stochastically generated "mock epidemics" using a Neyman-Scott cluster process. The resultant spatial point-patterns of infected plants have a fixed number of disease foci characterized by a known length scale (monodisperse) and saturated to a known disease level. When quadrat samples of these epidemics are fit to a beta-binomial distribution, the resulting measures of aggregation are totally independent of disease incidence and most strongly dependent on the ratio of the length scale of the quadrat to the length scale of spatial aggregation and to a lesser degree on disease saturation within individual foci. For the analytical approach, the mathematical form for the variation in the sum of random variates is coupled to the geometry of a quadrat through an assumed exponential autocorrelation function. The net result is an explicit equation expressing the intraquadrat correlation, quadrat variance, and the index of dispersion in terms of the ratio of the quadrat length scale to the correlative length scale.     

Phytophthora infestans in a subtropical region: survival on tomato debris, temporal dynamics of airborne sporangia and alternative hosts

Little is known about inoculum dynamics of late blight caused by Phytophthora infestans in tropical/subtropical areas, particularly in Brazil. The objectives of the present study were to assess (i) the survival of the pathogen on stems, leaflets and tomato fruits, either buried or not in soil; (ii) the pathogenicity of P . infestans to mostly solanaceous plant species commonly found in Brazil that could act as inoculum reservoir; and (iii) the temporal dynamics of airborne sporangia. Phytophthora infestans survived in tomato plant parts for less than 36 days under greenhouse and field conditions. In greenhouse tests, pathogen structures were detected earlier on crop debris kept in dry than in wet soil conditions. Isolates of two clonal lineages of P. infestans , US-1 from tomato, and BR-1 from potato, were inoculated on 43 plant species. In addition to potato and tomato, Petunia  ×  hybrida and Nicotiana benthamiana were susceptible to the pathogen. Airborne inoculum was monitored with Rotorod and Burkard spore traps as well as with tomato and potato trap plants. Sporangia were sampled in most weeks throughout 2004 and in the first two weeks of 2005. Under tropical/subtropical conditions, airborne inoculum is abundant and is more important to late blight epidemics than inoculum from crop debris or alternative hosts.     

Epidemics of ray blight on pyrethrum are linked to seed contamination and overwintering inoculum of Phoma ligulicola var. inoxydabilis

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.