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.     

Phytophthora infestans's 10-year truce with Holland: a long-term analysis of potato late-blight epidemics in the Netherlands

A sequence of 47 potato late-blight ( Phytophthora infestans ) epidemics in the Netherlands, from 1950 to 1996, was analysed using agronomic and meteorological variables. The intensity of annual epidemics was characterized by an index of disease intensity (DI, 0 = absence of late blight; 4 = severe epidemic). Three periods were identified, with average DIs of 2·9, 0 and 2, respectively. Period I (1950–68) had relatively regular epidemics; period II (1969–78) was virtually blight free; and period III (1979–96) showed large variations in disease intensity. Disease-enhancing factors were number of days with precipitation, and number of hours with temperatures between 10 and 27°C and relative humidity >90% during the growing season. Limiting factors were number of hours with temperatures >27°C, and amount of global radiation. Linear discriminant analysis of DI using the blight status of the previous year and meteorological variables correctly classified up to 40 years out of 47 (87·0%), with five out of the six incorrectly classified years falling in period III. Blight status of the previous year and number of days with precipitation were important discriminating variables.     

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.     

Prediction studies supported by computer on potato late blight in central Anatolia in Turkey*

Prediction of potato late blight epidemics, caused by Phytophthora infestans (Mont) de Bary, was studied in three different villages of Bolu Province having large potato growing areas with the Winstel and Ullrich Schrodter models in the years 2002–05. During these years, a late blight outbreak was observed only in 2005 with the disease being less apparent in the other years. The Ullrich Schrodter model was found to poorly predict potato late blight epidemics in 2005. The Winstel model gave first warnings too early but correctly predicted late infections. Both the A1 and A2 mating types were found in Central Anatolia for the first time, in Bolu province.     

Multi‐scale modelling of infection pressure from Phytophthora infestans*

Management of potato late blight could benefit from prediction of the risk posed to potato fields from external inoculum sources of Phytophthora infestans. Influx of inoculum depends on a complex interplay of population biological, atmospheric and spore survival processes, and is difficult to predict. This research aims at building tools for such prediction. BLIGHTSPACE is a spatio‐temporal model (parameterized for potato late blight) that has been developed and utilized to study the progress of epidemics in individual fields and networks of fields. A quasi‐Gaussian plume model was developed to provide long‐range transport of spores within BLIGHTSPACE. Numerical results compared favorably with experimental data. A further submodel for the survival of spores during long‐range transportation has been added. Integration of these three submodels will create an experimental arena for comparing control options for potato late blight.     

Resistance to acylalanines in Phytophthora infestans in The Netherlands1

In The Netherlands in 1980 a severe late-blight epidemic involving metalaxyl-resistant strains of Phytophthora infestans did considerable damage to the potato crop. As a consequence metalaxyl or metalaxyl-containing products were withdrawn from the Dutch fungicide market for the control of potato late blight. In 1981 the majority of the P. infestans isolates obtained from various parts of the country were sensitive to metalaxyl. In three areas where metalaxyl mixtures were used on a relatively large scale metalaxyl-resistant strains dominated the population. Incidental use of metalaxyl mixtures in 1982 and 1983 showed adequate late-blight control and, when in 1984 an early and in some areas severe epidemic developed, a metalaxyl/mancozeb mixture was reintroduced for curative application in combination with conventional fungicides. Although disease control was adequate, resistant strains were present in a small number of fields. Whether resistant strains will increase in frequency and threaten the usefulness of mixtures of acylalanines will heavily depend on how well potato farmers adopt strategies involving a limited use of mixtures of acylalanines in a spray schedule with conventional late-blight fungicides.     

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.     

Efficacy of the R2 resistance gene as a component for the durable management of potato late blight in France

Many race-specific resistance genes to potato late blight are overcome in France, but the disease appears later on genotypes carrying the R2 gene. This study examined whether R2 could contribute to durable late-blight control in France, and analysed the conditions of its performance. Plants grown from tubers of different physiological ages showed no difference in R2 expression in field and climate-chamber experiments, demonstrating that the delay in epidemic onset provided by R2 was not the result of gene inactivation in old plants. Among isolates collected at one site, those virulent on R2 were classified into three AFLP profiles. AFLP-VII comprised exclusively isolates virulent to R2 , whereas AFLP-IV and AFLP-V included both virulent and avirulent isolates. No significant aggressiveness differences were observed between virulent and avirulent isolates from AFLP-V; however, isolates from AFLP-VII were significantly less aggressive than virulent isolates from AFLP-V. These results indicate that: (i) the delayed onset of epidemics on R2 cultivars is the result of the breakdown of R2 by virulent isolates; (ii) aggressiveness of isolates virulent to R2 depends primarily on the genetic background of the pathogen where the mutation to virulence occurs; and (iii) this mutation does not lead per se to lower pathogenic fitness. It is suggested that R2 is unlikely to make a lasting contribution to late-blight control in France, and that diversification strategies such as cultivar mixtures might not considerably increase its durability.     

Influence of host diversity on development of epidemics: an evaluation and elaboration of mixture theory

ABSTRACT A spatiotemporal/integro-difference equation model was developed and utilized to study the progress of epidemics in spatially heterogeneous mixtures of susceptible and resistant host plants. The effects of different scales and patterns of host genotypes on the development of focal and general epidemics were investigated using potato late blight as a case study. Two different radial Laplace kernels and a two-dimensional Gaussian kernel were used for modeling the dispersal of spores. An analytical expression for the apparent infection rate, r, in general epidemics was tested by comparison with dynamic simulations. A genotype connectivity parameter, q, was introduced into the formula for r. This parameter quantifies the probability of pathogen inoculum produced on a certain host genotype unit reaching the same or another unit of the same genotype. The analytical expression for the apparent infection rate provided accurate predictions of realized r in the simulations of general epidemics. The relationship between r and the radial velocity of focus expansion, c, in focal epidemics, was linear in accordance with theory for homogeneous genotype mixtures. The findings suggest that genotype mixtures that are effective in reducing general epidemics of Phytophthora infestans will likewise curtail focal epidemics and vice versa.     

Nine years of practical experience with phenylamide resistance in Phytophthora infestans in the Netherlands

Resistance to phenylamide fungicides in thePhytophthora infestans population in the Netherlands decreased from a level of 77% of the samples containing resistant strains in 1980 to 0% in 1986 with decreasing use of metalaxyl, the only phenylamide fungicide registered for use against late blight in this country. Use of metalaxyl decreased because the fungicide alone and combinations of metalaxyl with preventive late blight fungicides were not commercially available from 1981 through the first part of the potato growing season of 1984. When metalaxyl was made available again in August 1984 in a mixture with mancozeb and from 1985 in a mixture with almost full rates of maneb and fentinacetate its use remained limited because growers were advised to apply the combination only two times per season in critical situations only to control the severe late blight epidemics of 1984 and 1985. Moreover application on seed potato crops was not allowed. In 1986 the combination was hardly used because the mild epidemic was easily controlled by preventive compounds only.In 1987 and 1988 phenylamide resistance increased to 16 and 42%, respectively, inherent to a general use of the combination in both years in most potato growing regions of up to two applications to combat a very severe epidemic.In lots of seed potatoes harvested in 1988 the percentage of blighted tubers yielding resistant isolates significantly differed from 13% for regions where seed potatoes are predominantly grown for table and processing crops to 41% for a region where predominantly starch potatoes are grown, both as seed and production crops. The more intensive use of metalaxyl combinations in the latter area through the years as compared to the other areas is assumed to be responsible for the higher incidence of resistance.In most years application of the combination induced a significant shift in the population to resistance as is indicated by the higher frequency at which resistant isolates were present in leaf samples taken from treated fields or tuber samples from the harvested crop.The strategy adopted by Dutch potato growers to combat phenylamide resistance in late blight, involving limited use of metalaxyl mixtures to a maximum of two times in critical situations only, has kept up to now resistance in the population at a level at which metalaxyl still significantly contributes to the efficacy of the combination in controlling the disease. Never-theless the observed increase in resistance in the population of 1988 after two consecutive late blight years indicates that this may change if severe epidemics occur each year.     

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.     

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.     

Modeling Infection of Strawberry Flowers by Botrytis cinerea Using Field Data

ABSTRACT The incidence of strawberry flower infection by Botrytis cinerea was monitored in unsprayed field plots in three successive years together with meteorological data and numbers of conidia in the air. There were large differences in conidia numbers and weather conditions in the 3 years. Three sets of models were derived to relate inoculum and weather conditions to the incidence of flower infection; by inoculum only, by weather variables only, and by both inoculum and weather variables. All the models fitted the observed incidence satisfactorily. High inoculum led to more infection. Models using weather variables only gave more accurate predictions than models using inoculum only. Models using both weather variables and inoculum gave the best predictions, but the improvement over the models based on weather variables only was small. The relationship between incidence of flower infection and inoculum and weather variables was generally consistent between years. Of the weather variables examined, daytime vapor pressure deficit and nighttime temperature had the greatest effect in determining daily incidence of flower infection. Infection was favored by low day vapor pressure deficit and high night temperature. The accuracy and consistency of the weather-based models suggest they could be explored to assist in management of gray mold.     

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.     

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.     

The effect of phenylamide fungicides on the control of potato late-blight (Phytophthora infestans) in England and Wales from 1978 to 1992

During the period 1978–1992, phenylamide fungicides in co-formulation with the dithiocarbamate fungicide mancozeb were tested for the control of potato late-blight in 51 separate field experiments in England and Wales. Whilst there was a general trend over all the experiments which indicated that foliage blight was less severe where the phenylamide + dithiocarbamate mixture had been used, the benefit was more marked in some than in others. Despite the detection of phenylamide resistance, at some sites the mixture gave better control of foliage blight than the dithiocarbamate alone. At one site where continuous data were available, the additional benefit of the phenylamide compound for control of foliage infection was lost after 1986 coinciding with a rise in phenylamide resistance from 31.7% in 1986 to 81% in 1987. Where foliar blight epidemics occurred, yield responses to fungicide programmes compared with unsprayed controls ranged from 0 to +118.5% with a mean response of +30.2%. This is equivalent to 30.8 and 12.92 t/ha respectively. Standardized yield differences were calculated to allow inter-trial and inter-year comparisons and showed no benefit from the phenylamide fungicide applied at 14-day intervals in 33 out of 38 experiments where foliar blight epidemics occurred. At one site, Cusum analysis of standardized yield differences following treatment with the phenylamide + dithiocarbamate mixture and the dithiocarbamate alone showed a mean benefit from the phenylamide mixture of 2.23 t/ha during the period 1978 to 1986. From 1987 to 1992, the yield benefit dropped to a mean level of 0.68 t/ha a decrease of 69.5%. In experiments where blight was not recorded, fungicide treatments had no deleterious effect on yields. Over all the trials, there was no effect of fungicide treatment on the incidence of tuber blight at harvest.     

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.     

Aerial photography used for spatial pattern analysis of late blight infection in irrigated potato circles

ABSTRACT Spatial and temporal dynamics of late blight were investigated from color, infrared aerial photographs of five commercial potato fields in the Columbia Basin during epidemics in 1993, 1995, and 1998. Aerial photographs were taken one to four times at 6- to 21-day intervals. Photographs were scanned and pixels, representing approximately 1 m(2) in the field, were used in the analysis. Late blight-infected plants were aggregated as indicated by runs analysis. Significant z-tests were computed for four directions during each sampling date in each of the five fields. Absolute z-values for runs analysis increased, indicating increasing aggregation in the four directions, as disease incidence increased in the early and midphases of the epidemics in each field. Variograms indicated the existence of autocorrelation among infected plants in four directions; the range of influence increased as disease incidence increased except at the highest levels of disease. Late blight was observed to spread in fields as foci. Late blight foci enlarged in size, produced distinct daughter foci, and coalesced. A field where initial inoculum likely originated from infected seed tubers exhibited less initial aggregation than the other fields, perhaps due to a different source of primary inoculum. Aerial photography coupled with spatial analyses of late blight-infected plants was an effective technique to quantitatively assess disease patterns in relatively large fields and was useful in quantifying an intensification of aggregation during the epidemic process on a large scale.     

Computer aids for plant protection in Latvia*

The main agricultural crops where decision support systems (DSS) can be used via the Internet in Latvia are winter and spring cereals and potato. Two PC‐based models, forming part of a Danish DSS, were tested under the agroecological conditions of Latvia in 1999/2002: PC‐Plant Protection to control diseases in cereals and several modifications of the NegFry model for prediction of potato late blight. The results of 4 years of trials suggest that models that satisfy the needs of one pest may not fit another. The main reasons for failure to adapt PC models are differences in cultivar susceptibility, differences in pathogenicity, simultaneous action of other organisms and spatial placement of crops (forests, rivers and fallow land). For example, it is well known that, with the recent global migration of more aggressive strains and populations of Phytophthora infestans, late blight epidemics have become less predictable and, at the same time, less controllable in potato‐growing areas. For cereals, there is a different spectrum of prevailing pathogens, causing different levels of damage, requiring incorporation into models of thresholds corresponding to local conditions. Data from weekly monitoring of local fields, warnings about the local situation and meteorological information via the Internet are the most important computer‐aided elements for experts in plant protection.     

Stability of partial resistance in potato cultivars exposed to aggressive strains of Phytophthora infestans

Potato cultivars were evaluated for their resistance responses to aggressive strains of Phytophthora infestans in field and laboratory experiments. Analysis of variance revealed differential cultivar-by-isolate interactions for both foliar and tuber blight resistance. Differential responses occur as revealed by specific susceptibilities of cultivars to certain pathogen genotypes and changing rank order. In general, severity of late blight epidemics as observed in the haulms did not correlate well with foliar blight resistance ratings as presented in the National List of Recommended Potato Varieties. No significant correlation was found between tuber blight incidence under field conditions and the tuber blight rating in the National List. Also, there was no relation between the field and laboratory tuber blight resistance assessments. A significant association was demonstrated between late blight infection in the foliage and tuber blight incidence under field conditions. The presence of differential interaction, independent of R-gene-based resistance, indicates some adaptation of P. infestans to partial resistance and consequently adverse effects on the stability and durability of partial resistance to potato late blight.