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.     

Forecasting the spread of aerially transmitted crop diseases with a binary classifier for inoculum survival

The risk of between‐field spread of disease is typically omitted from crop disease warning systems, as it is difficult to know the number and location of inoculum sources and thus predict the abundance of inoculum arriving at healthy crops. This study explores the utility of a simple approach to predicting risk of between‐field spread, based on the estimated probability that inoculum will survive the transportation process. Using potato late blight as a case study, the effect of solar radiation on the viability of detached Phytophthora infestans sporangia was assessed. A model to estimate the probability of spore survival was derived using a binomial generalized linear mixed model (GLMM), and receiver operating characteristic (ROC) curve analysis and cross‐validation were used to evaluate the global performance of the model as a binary classifier for discriminating between viable and nonviable sporangia. The model yielded an area under the ROC curve of 0.92 (95% CI = 0.90–0.93), signifying an excellent classification algorithm. Inspection of the curve provided a number of suitable decision threshold (or cut‐off) probabilities for discriminating between viable and nonviable sporangia. The classifier was tested as a forecasting system for potato late blight outbreaks using 10 years of outbreak data from across Great Britain. There was a marked differentiation among the cut‐offs, but the best prediction outcome was an accuracy of 89% with an alert frequency of 1 in 7 days. This model can be easily modified or the methodology replicated for other pathosystems characterized by airborne inoculum.     

Cumulative and residual effects of different potato cropping system management strategies on soilborne diseases and soil microbial communities over time

Soilborne potato diseases and soil microbial community characteristics were evaluated over 8 years in different potato cropping systems designed to address specific management goals of soil conservation, soil improvement and disease suppression. Results were compared to a standard rotation and non‐rotation control in field trials in Maine. Standard rotation consisted of barley underseeded with red clover, followed by potato (2‐year). Soil‐conserving system (SC) featured an additional year of forage grass and reduced tillage (3‐year, barley/timothy–timothy–potato). Soil‐improving system (SI) added yearly compost amendments to SC, and the disease‐suppressive system (DS) featured crops with known disease‐suppressive capability (3‐year, mustard/rapeseed–sudangrass/rye–potato). Systems were established in 2004, evaluated with and without irrigation, and actively managed until 2010, with potato also planted in 2011 and 2012 to examine residual effects. All rotations reduced soilborne diseases black scurf and common scab, and increased yield after one rotation cycle (3 years), but diseases increased overall after two rotation cycles. DS maintained lower soilborne disease levels than all other rotations, as well as high yields, throughout the study. Cropping system effects became more pronounced after multiple cycles. SI system and irrigation both resulted in higher yields, but also higher levels of soilborne disease. Cropping system and irrigation effects were significant even after systems were no longer maintained. Soil microbial community data showed significant changes associated with cropping system, and differences increased over time. Cropping system strategy had significant and lasting effects on soil microbiology and soilborne diseases, and can be used to effectively enhance potato production.     

Diversität, Pflanzenernährung und Prognose: Ein integriertes Konzept zum Management der Kraut-und Knollenfäule in der Ökologischen Landwirtschaft

In organic potato production adequate plant nutrition and plant protection have to be achieved by long term and preventive management strategies. Late blight, caused by Phytophthora infestans Mont. De Bary is one of the most important diseases in organic potatoes and besides varietal resistance its management in Germany relies mainly on the limited use of protective copper based fungicides. The currently allowed amounts of copper of 3?kg?ha^?1 and year are more than necessary for plant nutrition and thus are in contrast to the aim of sustainability of organic farming. The results of the presented studies show that it is usually possible to achieve good disease control with only 1–1.5?kg?Cu?ha^?1 even under high disease pressure with the help of the forecasting model BioPhyto-PRE which was developed for organic farming. To increase the efficiency of less effective fungicides such as plant extracts in organic farming, disease pressure has to be reduced as much as possible by management approaches. Four on-farm experiments showed that strip intercropping of potatoes does reduce disease pressure in 12–16?m wide strips with neighbour spring wheat or carrots. A cost-benefit analysis using the prices of the year 2004 showed that growing potatoes without treatment in most cases was the most lucrative option with slightly increased benefits by strip cropping the potatoes. Multiple stepwise regressions show that in organic farming the yield loss relationships between late blight and potatoes can only be determined when N-uptake is integrated into the model. Overall, only very limited yield benefits are achieved by the application of copper fungicides, in most cases due to the limited nutrient supply in organic systems. It is concluded that an integrated forecasting model for late blight management in organic farming will have to include data on growth stage dependent plant nutrient uptake and variety specific properties to allow for efficient and minimised fungicide applications.     

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.     

Importance of the proportion of grassy weeds within legume crops in the perpetuation of Gaeumannomyces graminis var. tritici

Various grass species susceptible to infection by Gaeumannomyces graminis var. tritici were mixed-sown into a legume crop in order to assess their influence on density of inoculum and take-all disease in a subsequent crop of wheat.
In a pot experiment take-all inoculum increased ( P < 0.001) in all treatments containing a proportion (from 20 to 100%, in increments of 20%) of grass in subterranean clover. In a plot trial, most severe take-all occurred in the 20% legume/80% grass stands and least in the 100% legume and 80% legume/20% grass stands. Total grain weight was highest ( P <0.1) after the 100% legume stands. There was no difference in severity of take-all after pure stands of medic, subterranean clover and lupin, but there was more severe take-all after the grass-infested medic stands than after those of subterranean clover ( P < 0.1) or lupin ( P < 0.05). No significant differences ( P > 0.1) in yield occurred in wheat following any of the legumes or mixed stands.     

The importance of the infected seed tuber and soil inoculum in transmitting Potato mop‐top virus to potato plants

Potato mop‐top virus (PMTV), the cause of spraing in potato tubers, is transmitted by Spongospora subterranea, the cause of powdery scab, and by planting infected seed tubers. This study was undertaken to determine the relative importance of these sources of infection in seed potato production in Scotland. The transmission of PMTV from tested seed tubers to daughter plants was examined over 2 years and six cultivars. The development of foliar symptoms varied with year and cultivar. Infection of daughter tubers derived from PMTV‐infected seed tubers was more prevalent on plants affected by foliar symptoms than those without symptoms. The rate of transmission of PMTV from infected seed tubers to daughter tubers ranged from 18 to 54%. Transmission was affected by cultivar and by origin of seed tubers used for a cultivar, but not by a cultivar's sensitivity to PMTV infection. The incidence of PMTV in daughter tubers of cv. Cara grown from seed potatoes from one source (common origin) by more than 25 seed producers was examined over two successive generations. The incidence of PMTV in daughter tubers was not correlated with that in the seed tubers but appeared to be strongly associated with soil inoculum. The incidence of PMTV was correlated with powdery scab in those crops in which both were present. There was some evidence from soil tests conducted in 2006 using a tomato bait plant and real‐time RT‐PCR that planting PMTV‐infected seed potatoes could increase the risk of introducing the virus into land not infested by PMTV.     

Real‐time PCR detection of Erwinia amylovora on blossoms correlates with subsequent fire blight incidence

Fire blight is the most devastating bacterial disease of rosaceous plants. Forecasting fire blight infections is important to allow for countermeasures that reduce economic damage in pome fruit production. Current computerized forecasting models are solely based on physical factors such as temperature and moisture, but not on the actual presence of the pathogen Erwinia amylovora. Although the inoculum concentration is considered to be crucial for infection and disease outbreak, most current approaches used for identification of fire blight inoculum including morphological, biochemical, serological, and DNA‐based methods are nonquantitative. Based on a real‐time PCR approach previously published, an improved protocol to be used directly on whole bacteria in the field is described. The method allows for early detection and quantification of the pathogen prior to the occurrence of first symptoms. There is a clear correlation between bacterial abundance and subsequent disease development. However, in some cases, no disease symptoms could be observed despite a pathogen load of up to 3·4 × 10⁶ cells per blossom. Integration of the amount of pathogen detected into refined prediction algorithms may allow for the improvement of applied forecasting models, finally permitting a better abatement of fire blight.     

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.     

Effect of two-component cultivar mixtures on development of wheat yellow rust disease in the field and greenhouse in the Nepal Himalayas

Wheat yellow rust (WYR), caused by Puccinia striiformis f. sp. tritici (PST), is a major disease of wheat, and deployment of a single cultivar often leads to disease epidemics. Effect of inoculum level, foliar fungicide spray, and wheat cultivar mixtures were evaluated on disease development in the field and greenhouse in Nepal. Treatments were arranged in a split–split plot design with three replications in both experiments. Two inoculum levels of PST (low and high) were main plot factors; nontreated control and foliar spray of fungicides (Mancozeb and Bayleton) were subplot factors; and two-component cultivar mixtures, composed of different ratios of a susceptible (S) and a resistant (R) cultivars (90:10, 80:20, and 50:50, 100:0, and 0:100) were sub–subplot factors. WYR severity was assessed at different time intervals, and disease development was calculated as area under the disease progress curve (AUDPC). Inoculum level did not cause significant differences in AUDPC in the field but did in the greenhouse. Foliar spray of fungicides reduced the AUDPC in the greenhouse and field. In both experiments, AUDPC values were low in cultivar mixtures compared with a pure stand of a susceptible cultivar. As the proportion of resistant cultivar increased compared with the susceptible cultivar in the S:R mixture component, disease severity decreased with a consequent increase in grain yield. The greater yield obtained with cultivar mixtures compared with only the susceptible cultivar, independent of inoculum level and fungicide spray in the field, revealed a promising strategy to manage WYR in Nepal.     

Severity of phytophthora root rot and pre‐emergence damping‐off in subterranean clover influenced by moisture,temperature, nutrition,soil type,cultivar and their interactions

Studies were carried out in controlled environment rooms reflecting field situations. In the presence of the devastating soilborne pathogen Phytophthora clandestina, subterranean clover (Trifolium subterraneum) seedling emergence was significantly affected by moisture, soil type, temperature and cultivar. The level of rotting of tap and lateral roots was significantly affected by nutrition, soil type, temperature and cultivar. There were significant interactions involving temperature, moisture, soil type and cultivar; cultivar resistance, high moisture, high or medium temperature, high nutrition and sand soil all contributed towards less pre‐emergence damping‐off and tap and lateral root disease and to greater clover productivity. Host resistance of subterranean clover cultivars was critical for reducing disease severity and increasing productivity, even when favourable environmental conditions for severe disease occurred. In the presence of P. clandestina, the most resistant cultivar, Seaton Park, performed best under a high temperature, high nutrition and high moisture combination, but showed lower productivity under conditions of low nutrition or lower temperature, even when moisture level was high. In contrast, less resistant cultivars Riverina and Meteora had less disease and greater productivity under low moisture conditions. Findings reflect field observations that pre‐emergence damping‐off and root disease from P. clandestina in subterranean clover is particularly severe under colder conditions and in nutritionally impoverished sandy soils, and demonstrate how variations in soil type, nutrition, moisture, temperature and cultivar have profound effects on the expression and severity of phytophthora pre‐emergence damping‐off and root disease and the productivity of subterranean clover forages.     

Fusarium graminearum as a dry rot pathogen of potato in the USA: prevalence,comparison of host isolate aggressiveness and factors affecting aetiology

A 2004–2005 survey of potatoes from stores in the north‐central potato‐producing region of the USA showed that the predominant causes of dry rot were Fusarium graminearum and Fusarium sambucinum. Isolates of F. graminearum originally isolated from potato tubers with dry rot (n = 15), wheat kernels with scab (n = 15), and sugarbeet tap roots with decay (n = 5) were tested for aggressiveness to potato tubers. There were no significant differences in aggressiveness among isolates of F. graminearum, regardless of original host, as measured by their ability to cause dry rot. These findings may have implications for survival of F. graminearum inoculum since potatoes, wheat and sugarbeets are frequently used in crop rotation in the region. Fusarium graminearum required larger wounds for infection of potato tubers than F. sambucinum. Plug‐removal injury, simulating a stolon‐removal injury, resulted in equal incidence of dry rot caused by the two Fusarium species, whereas abrasion and bruising injury were sufficient for infection and dry rot development by F. sambucinum, but not F. graminearum. A change in harvest practices from vine‐killing prior to harvest to mechanical vine‐killing on the day of harvest may be a factor affecting the onset of dry rot caused by F. graminearum, since this process often causes large wounds at the stem end of the tubers when the stolon is forcibly removed.     

Relationship between Spongospora subterranea f. sp. subterranea soil inoculum level,host resistance and powdery scab on potato tubers in the field

The relationship between initial soil inoculum level of Spongospora subterranea f. sp. subterranea (Sss) and the incidence and severity of powdery scab on potato tubers at harvest was investigated. In all experiments soil inoculum level of Sss (sporeballs/g soil) was measured using a quantitative real‐time PCR assay. Of 113 commercial potato fields across the UK, soil inoculum was detected in 75%, ranging from 0 to 148 Sss sporeballs/g soil. When arbitrary soil inoculum threshold values of 0, <10 and >10 sporeballs/g soil were set, it was observed that the number of progeny crops developing powdery scab increased with the level of inoculum quantified in the field soil preplanting. In four field trials carried out to investigate the link between the amount of inoculum added to the soil and disease development, disease incidence and severity on progeny tubers was found to be significantly (P < 0·01) greater in plots with increasing levels of inoculum incorporated. There was a cultivar effect in all years, with disease incidence and severity scores being significantly greater in cvs Agria and Estima than in Nicola (P < 0·01).     

Activation of defence responses to Phytophthora infestans in potato by BABA

Late blight caused by Phytophthora infestans is one of the most devastating diseases of the potato crop. Resistance breeding and current fungicides are unable to control the rapidly evolving P. infestans and new control strategies are urgently needed. This study examined mechanisms of dl ‐β‐aminobutyric acid (BABA)‐induced resistance (IR) in the potato–P. infestans system. Leaves from two cultivars that differ in their degree of resistance, Bintje and Ovatio, were analysed after foliar treatment with BABA. Rapid activation of various defence responses and a significant reduction in P. infestans growth were observed in leaves treated with BABA. In the more resistant cultivar, Ovatio, the activation was both faster and stronger than in Bintje. Microscopic analysis of leaves treated with BABA revealed induction of small hypersensitive response (HR)‐like lesions surrounded by callose, as well as production of hydrogen peroxide (H₂O₂). Molecular and chemical analyses revealed soluble phenols such as arbutin and chlorogenic acid and activation of PR‐1. These results show a direct activation of defence responses in potato, rather than priming as reported for other plant species. They also show that the efficiency of BABA‐IR differs between cultivars, which highlights the importance of taking all aspects into consideration when establishing new methods for disease management.     

近年来中国马铃薯晚疫病的时空演变特征及防控情况分析

为系统全面地了解马铃薯晚疫病在我国的发生演变特征及防控情况,基于2008—2017年国家统计局官方数据、《中国农业统计资料》《全国植保专业统计资料》及全国马铃薯品种分布等数据,对全国及各马铃薯产区晚疫病的发生及防治情况进行综合量化分析。结果表明：马铃薯晚疫病在我国连年发生,年平均发生面积约197.47万hm²,占马铃薯总种植面积的40.66%;晚疫病造成马铃薯年平均产量损失占所有病害所致产量损失的63.54%,远高于其他病害,损失巨大;在我国马铃薯4大产区中,马铃薯晚疫病在北方一作区的年平均发生面积最大,约73.44万hm²,而在西南混作区造成的年平均产量损失最高,约14.29万t;晚疫病发生与马铃薯种植区域的重心在空间上呈现相似的迁移趋势;以费乌瑞它为代表的高感晚疫病品种在全国大面积种植,但近年来部分抗病品种的种植面积正在逐年增加;近年来我国对马铃薯晚疫病的防治面积大幅度提升,但仍太过依赖化学防治,应加大绿色防控技术的推广和应用力度。     

Evidence for presence of the founder Ia mtDNA haplotype of Phytophthora infestans in 19th century potato tubers from the Rothamsted archives

Late blight remained a significant disease for potato growers in Europe long after the famine of the 1840s. Of the four mitochondrial haplotypes of Phytophthora infestans, only the Ia mitochondrial DNA (mtDNA) haplotype has been identified previously in infected potato leaves from famine‐era herbarium specimens collected in England, Ireland and Europe in the 19th century. Long‐term soil fertility experiments were conducted on potato between 1876 and 1901 in Rothamsted to investigate effects of combinations of organic manures and mineral fertilizers on disease and yield. This report identifies for the first time the same Ia mtDNA haplotype of P. infestans in three diseased tubers from 1877 from the long‐term Rothamsted trials, thus providing the earliest evidence of the presence of the founder Ia mtDNA haplotype of P. infestans in potato tubers in England. Soil amendments had a significant impact on disease and yield. A real‐time PCR assay was used to detect and quantify P. infestans in tubers. The level of pathogen DNA was greatest in tubers from highest yielding plots that received combinations of inorganic nitrogenous and mineral fertilizers and least in tubers from plots with organic farmyard manures or non‐nitrogenous mineral fertilizers. The Ia mtDNA haplotype was also confirmed from diseased potato leaves during the same time period. Thus, the founder Ia mtDNA haplotype survived in potato tubers after 1846 and was present over 30 years later in the UK.     

Spatiotemporal variation in airborne sporangia of Phytophthora infestans: characterization and initiatives towards improving potato late blight risk estimation

This study investigated the value of using real‐time monitoring of Phytophthora infestans airborne inoculum as a complement to decision support systems (DSS). The experiment was conducted during the 2010, 2011 and 2012 potato production seasons in two locations in New Brunswick, Canada. Airborne sporangia concentrations (ASC) of P. infestans were monitored using 16 rotating‐arm spore samplers placed 3 m above the ground. The first cases of late blight (2010 and 2011) were detected 6–7 days after the first ASC peak, and all samplers captured their first sporangia within the same week (at 3‐ and 9‐day periods). The cumulative ASC curve and the risk curves from two DSS (PLANT‐Plus and Pameseb Late Blight) had the same shape but different magnitudes. In both locations, the negative binomial distribution fitted the data better than the Poisson distribution, which is indicative of heterogeneity, and based on Taylor's power law, the heterogeneity increased with increasing ASC. Therefore, the present results suggest that spore‐sampling network devices may be a suitable approach for early detection of incoming inoculum and, when combined with DSS, represent a potential aid for targeting the optimal time to apply a disease‐control product. In this context, cumulative ASC can be a counterweight to the DSS risk estimate: a high risk combined with significant ASC will trigger fungicide spraying. Moreover, spore sampling can be used to assess the efficiency of management strategies by means of examining the area under the inoculum progress curve.     

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.     

Studies of crop loss in potato blight caused by Phytophthora infestans

Relationships between healthy haulm area (HHA) and yield, and between losses of HHA and yield in the potato late blight system, were evaluated in several seasons and fields variously affected by the disease in Israel. The results showed that because of the different yield potential of potatoes cultivated in different conditions, evaluation of crop losses cannot be separated from evaluation of yield potentials of the crop as influenced by environmental and cultural factors.
In one calculation the yield and the HHA in the healthiest plot in each field were equated to 100% and losses in other plots were derived from comparisons with this plot. In such a calculation the relationships of yield loss to HHA loss in all fields and seasons fitted one regression equation, and a high correlation was obtained between both parameters. However, the actual haulm areas and yield weights differed widely from one field to another, even in the healthiest plots. Also, a similar percentage of destroyed haulms sometimes corresponded to different areas of the remaining haulms and to different yields. So no correlation was found between HHA and yields over all fields and seasons. Such a correlation was found only for several plots, variously affected by late blight, in the same field and season and in different fields and seasons yields responded in a different way to increases in HHA.     

Exploiting host resistance to reduce the use of fungicides to control potato late blight

Field trials in 1996, 1997 and 1998 with six potato cultivars differing in levels of foliar and tuber race-nonspecific resistance to late blight were treated with 100, 50 and 33% of the recommended dose of the fungicide fluazinam at application intervals of 7, 14 and 21 days. Using a mixed inoculum of six or seven indigenous isolates of Phytophthora infestans small potato plots were inoculated via infector plants. A foliar blight model for the relationship between the effects of resistance, fungicide application and disease pressure was developed using multiple regression analysis. Cultivars with a high level of quantitative resistance offered the greatest potential for fungicide reduction. The model showed that the effect of resistance on integrated control increased exponentially with increasing cultivar resistance. Reducing fungicide input by lowering the dose resulted in less foliar disease than extending application intervals. The higher the disease pressure, the greater the risk associated with reducing fungicide input by extension of application intervals. The field resistance of cultivars to tuber blight mainly determined the frequency of tuber infection. Exploiting high foliar resistance to reduce fungicide input carried a high risk when cultivar resistance to tuber blight was low. When field resistance to tuber blight was high, a medium level of resistance in the foliage could be exploited to reduce fungicide dose to c . 50%, provided application was at the right time. At a high level of field resistance to both foliar and tuber blight, application intervals could be extended.