Susceptibility of potato (Solanum tuberosum L.) foliage and tubers to the US8 genotype ofPhytophthora infestans

Late blight is an important disease of potato(Solanum tuberosum L.) worldwide, and therefore, many potato-breeding programs have prioritized the development of late blight-resistant potato cultivars. Although the emphasis has been to enhance foliar resistance, it is also necessary to evaluate tuber late blight resistance in new breeding lines and new sources of late blight resistance. We report here on the assessment of foliar and tuber resistance and the correlation between these aspects of resistance in a sample of Michigan State University potato breeding lines. Two MSU breeding lines had significantly less infected foliage than the susceptible check cultivars. Tuber susceptibility was significantly different (P < 0.05) only between the most susceptible and the least susceptible breeding lines/cultivars. Foliar and tuber susceptibility to potato late blight were not correlated as low tuber susceptibility was associated both with extremely low (e.g., MSG274-3) and high (e.g., MSE202-3Rus) foliar susceptibility.     

Susceptibility of potato varieties and advanced breeding lines (Solanum tuberosum L.) toPhytophthora infestans (Mont.) de Bary in greenhouse screenings

Late blight (Phytophthora infestons (Mont.) de Bary) has re-emerged as an important pathogen of the cultivated potato (Solanum tuberosum subsp.tubero-sum L.) in North America. The purpose of this study was to evaluate the relative susceptibility of potato germplasm in the greenhouse in order to initiate a breeding program for resistance to the US-8/A2 mating type which is the more aggressive and prevalent strain of late blight. Whole plants of 147 cultivars and breeding lines were evaluated. Percent plant area infection was visually assessed. Seven days after inoculation, infection ranged from 0 to 100% and the overall mean was greater than 50%. Two-thirds of the cultivars and breeding lines tested were very susceptible to the US-8 genotype. The highest resistance was identified in the somatic hybrids betweenS. tuberosum andS. bulbocastanum and their backcross derivatives. Pike and Snowden were less susceptible than the other North American cultivars. Zarevo was most resistant among the European cultivars. Seven of the advanced breeding lines were equivalent to Zarevo in infection levels. The host plant resistance identified among the material tested in this study can be used by breeding programs to develop improved cultivars with resistance to US-8 genotypes of late blight.     

Assessment and analyses for the interpretation of potato late blight response in field studies

Potato seedlings and cultivars were successfully subjected to moderate to heavy late blight pressure in field plots to which a race complex of the pathogen was applied. Multiple assessments through the course of the epidemics revealed differences in overall disease susceptibility or resistance. Differences were also demonstrated in disease response patterns among potato lines with similar overall disease ratings. Use of principal component and cluster analyses established discrete groupings of potato lines. This provides a basis for a more objective interpretation of the results in identifying both overall disease response and pattern (slow or fast) of response of potato cultivars and breeding lines.     

Light use efficiencies of potato cultivars with late blight (Phytophthora infestans)

Summary Potato cultivars of different maturity classes and levels of resistance toPhytophthora infestans were grown under several disease intensities in three field trials. Seasonal courses of ground cover by green foliage and final tuber yields were determined. Light use efficiencies (LUE) were calculated from regression analyses of yield on cumulative light interception. Late blight reduced tuber yields by decreasing cumulative light interception without affecting LUE. No differences in LUE between cultivars or cultivar classes were detected. Therefore, the maintenance of green leaf area is important when breeding potatoes for optimal performance in the presence of late blight. The results support the hypothesis that the correlation between lateness and reported resistance of potato cultivars is due to the vigorous foliage growth of late cultivars.     

Reaction to late blight in response to nitrogen management in Argentine potato cultivars

The objective of this study was to evaluate the reaction to late blight of two potato cultivars in response to nitrogen (N) management under field and in Vitro conditions. In Balcarce trial a reduction in phenols with the addition of excess N was found. Despite the reduction of the concentration of phenols no differences in the relative area under disease progress curve (rAUDPC) were observed between N levels. In Tafí del Valle, the rAUDPC denoted an interaction between cultivar and N level. In Frital, the N0 showed lower rAUDPC than N optimal (Opt) and N excessive (Exc) treatments. In contrast, no differences in rAUDPC between N levels on Pampeana were found. In the detached leaf test, no significant differences between N levels were observed in the threshold of severity of disease and the lesion growth rate. In conclusion, excessive N fertilization did not increase the susceptibility to late blight in this study.     

Foliar reaction toPhytophthora infestans in inoculated potato field trials in Michigan

Late blight, caused byPhytophthora infestans, is the most important disease of potato worldwide and foliar resistance is an important component of managing late blight in the field. The objective of this research was to identify germplasm for use in breeding cultivars with foliar resistance toP. infestans. More than 500 clones were tested from 1997 to 2002 in inoculated (US8 genotype) field experiments conducted at the Michigan State University Muck Soils Research Farm in Bath, Michigan. All of the current commercial cultivars tested were classified as susceptible toP. infestans. The most resistant clones were A90586-11, AWN86514-2, B0718-3, Jacqueline Lee (MSG274-3), MSI152-A, MSJ307-2, MSJ317-1, MSJ453-4Y, MSJ456-2, MSJ456-4, MSJ461-1, MSK101-2, MSK128-1, NY121, LBR8, LBR9, Tollocan, and Torridon. Some of these resistant selections were from crosses with B0718-3, Jacqueline Lee, and Tollocan suggesting that the resistance toP. infestans was transmissible. These resistant clones will provide the opportunity to breed late-blight-resistant cultivars from a diverse pool of cultivated germplasm. Consistent foliar reaction toP. infestans over years suggested that the Michigan State University Muck Soils Research Farm is a valuable location for North American breeders to assess the reaction of potato germplasm to the US8 genotype of late blight.     

马铃薯晚疫病发病程度田间观察记录标准的探讨

目前用于马铃薯晚疫病发病严重程度的描述方法主要分为两类,一是直接计数法,即感病百分数法;二是分级计数法,包括病情指数法及CIP 9级分级法。基于以上描述方法,还可计算出病情发展曲线下的面积(AUDPC),对遗传材料的抗性进行评价。无论何种方法,准确估计和记录晚疫病病斑侵染面积占植株总面积的比例是非常重要的,即使是长期从事晚疫病研究的专业人员有时也会产生较大的偏差。利用标准图谱作为参照,可以有效降低科研人员的判断误差。所以,有必要建立一套标准的彩色图谱,供从事病害调查的技术人员参考,有效地提高对晚疫病发病程度的评估准确性。     

The use of area under the disease progress curve to assess resistance to late blight in potato germplasm

The multiple evaluation of potato cultivais and breeding selections (clones) for disease during the season can be costly and may not be necessary for accurate assessments of disease resistance or susceptiblity. For diseases whose progression can be described by sigmoid curves, an estimate of the area under the disease progress curve from two data points may provide as much information as from repeated assessments. Twentythree clones were planted in a randomized complete block design in Hastings, FL, in 1997 and evaluated for percent diseased foliage 14 times during a 31-day period after late blight was first noticed in the plots. The area under the disease progress curve (AUDPC) was calculated for all 14 assessments. The AUDPC was also calculated for nine sub-sets of the original data set. There was a high correlation between all the methods in the calculation of the AUDPC. The AUDPC calculated based on two dates (representing the beginning of the epidemic and the time until two of the clones were dead from late blight) was as informative as the AUDPC calculated on the entire data set. The AUDPC could be calculated based on any two dates from this time frame as long as one date was shortly after the epidemic started and the other date was as the epidemic was reaching its peak. Much information was lost if AUDPC was calculated based on dates involved only in the early part of the epidemic. A considerable savings in time and effort can be realized by only a few assessments.     

Potato cultivar response to late blight as affected by clonal selection andin vitro culture

Response to foliar late blight for Superior and Sebago was not affected by the two potato seed selection and multiplication methods. Kennebec and Russet Burbank potato plants derived throughin vitro tissue culture techniques had significantly more late blight damage on only 4 of 7 and 2 of 7 observation dates, respectively, than plants derived through a clonal selection system. In practical terms, the disease response differences between the two seed propagation methods were minimal. Similarly, disease response differences among the 2 and 3 years of field multiplication for Russet Burbank and Kennebec, respectively, did not demonstrate any significant disease response trends. In general,in vitro culture plots had slightly higher yields than clonal plots but only Kennebec had a significant yield response. The incidence of late blight storage rot was generally not significantly different but clonally selected Kennebec potatoes had significantly more disease than those produced throughin vitro culture.     

The Pathosystem Solanum tuberosum L.-Phytophthora infestans (Mont.) de Bary in Chapingo,Mexico. Expected,Observed, and Simulated

The 1:1 ratio of the Phytophthora infestans A1 and A2 mating types favors the sexually derived genetic diversity of the late blight pathogen of potatoes, which is widespread in the central highlands of México. This ratio guarantees the successful infection of this pathogen, even in resistant potato hosts. However, wild Solanum species present in the region serve as alternative hosts to the pathogen. Knowledge about the external factors that influence the dynamics of this disease facilitates the assessment and selection for genetically resistant potato cultivars to late blight, in addition to providing the capacity to predict epidemics. This work aimed to assess the expected, observed, and simulated progress of natural P. infestans infection of potatoes during two epiphytotic field seasons (2009 and 2010) at Chapingo, México. Using 8-years of weather datasets, six ideal situations were predicted with four to eight infection cycles of 6 to 12 h each. In comparison to the predictions, the observed effect of the area under the disease progress curve, and its components (AUDPC, RAUDPC, RaRAUDPC), was highly significant, with a low coefficient of variation among the potato cultivars used in the study. In conclusion, we confirm that the LATEBLIGHT-LB2004 model is useful for simulating and predicting late blight epidemics based on the weather conditions of Chapingo, except for the magnitude of the relative humidity threshold variable (RH_threshold), which requires calibration for each cultivar.     

Assessing resistance types and levels to epidemic diseases from the analysis of disease progress curves: Principles and application to potato late blight

Both race-specific (RS) and race-non-specific (RNS) resistances exist in potato against the late blight pathogenPhytophthora infestans. Because these resistance types do not have the same epidemiological effects, their presence, alone or combined, in potato genotypes can be deduced from the analysis of disease progress curves from field experiments, a type of data commonly available to potato breeders, and their comparison with those of standard reference cultivars. The identification of RS resistance is based on the presence of a delay in epidemic onset compared to a susceptible cultivar, whereas the identification of RNS resistance is translated into a reduction of apparent infection rates. These parameters can be easily computed after linearization of the disease progress curves. This paper assesses the reliability of this identification using sets of experimental data, discusses its limitations, and highlights potential applications for breeding and cultivar assessment purposes.     

Effect of cultivar maturity period on the growth and yield of potato plants grown from microtubers and conventional seed tubers

Potato plants of early cultivars grown from microtubers have been reported to have a much lower growth vigor and produce lower yields than microtubers of late cultivars. This study intended to clarify the field performance of plants grown from directly planted microtubers of cultivars with different maturity periods, with a special attention to early cultivars. The experiments were conducted at Hokkaido University, Japan, over four years. Microtubers and conventional seed tubers of the early cultivar Kitaakari, late cultivars Konafubuki and Norin 1, and very late breeding line IWA-1 were planted, and the plant growth and tuber yields were analyzed. The microtuber plants of Kitaakari had a lower initial increase in leaf area index than conventional seed tuber plants, but at the maximum shoot growth had the same leaf area index. This pattern was also observed in the other cultivars. Tuber initiation and tuber bulking occurred on average five days later in microtuber plants than in conventional seed tuber plants of cultivar Kitaakari. At maximum shoot growth, microtuber plants had on average 65% of tuber dry weight of conventional seed tuber plants, with small variation among cultivars. Irrespective of maturity period, microtuber plants showed a higher tuber increase after maximum shoot growth, achieving around 86% of tuber dry weight of conventional seed tuber plants at harvest. From the results of this study we conclude that microtuber plants of early and late cultivars have a similar yield potential relative to conventional seed tuber plants, and microtubers of both early and late cultivars might be used as an alternative seed tuber source for potato production, if necessary.     

Potassium phosphite combined with reduced doses of fungicides provides efficient protection against potato late blight in large-scale field trials

Potato late blight caused by the oomycete Phytophthora infestans is a devastating disease of potato worldwide. Most of the potato cultivars grown in conventional agriculture are susceptible, or at best moderately resistant, and require frequent applications of fungicides to avoid heavy yield losses.In field trials spanning four years, we have investigated the effect of potassium phosphite, an inorganic salt on potato late blight. Potassium phosphite is known to induce defence responses in potato and to also have direct toxic effects on oomycetes, which in turn counteract late blight and tuber blight development. However, the use of this salt is not yet implemented and approved in European potato cultivation. We compared the effect of phosphite alone with fungicides currently used in Swedish potato cultivation. We also investigated the combined use of potassium phosphite and reduced doses of fungicides. Table potato cultivars and starch potato cultivars with different levels of resistance were used.We found that potassium phosphite in combination with reduced doses of fungicides results in the same level of protection as treatments with the recommended full dose of fungicides. These combined treatments reduce the need of traditional fungicides and may also decrease the selection pressure for fungicide resistance development in the pathogen. In relatively resistant starch potato cultivars using phosphite alone gave sufficient protection against late blight. Furthermore, in starch potato a combination of phosphite and fungicides at two-week intervals provided similar protection to weekly applications of fungicide at the recommended dose. Foliar treatment with phosphite also gave protection against tuber blight at similar levels to that of the best-performing fungicide. Our data suggests that potassium phosphite could be used in potato cultivation in temperate regions such as in Sweden, at least in combinations with reduced rates of fungicides. The implementation of the use of phosphite in practical potato crop protection as part of an IPM strategy is discussed. Doses, intervals and combinations could be adjusted to the level of cultivar resistance.     

Relative resistance of potato cultivars to bacterial wilt

Fifty-one potato cultivars were evaluated for reaction to infection byPseudomonas solanacearum Sm. under high disease pressure in a field plot near Tifton, Georgia during the spring of 1978 and 1979. Plots were infested by clipping beds of thickly seeded tomato plants with a rotary mower contaminated with the wilt baterium in 1975, 1976, 1977 and 1978. The plants were incorporated into the soil following disease development. In addition to the use of infested soil, one stem on each potato plant was inoculated in late May by cutting with a knife blade dipped in a bacterial suspension. Most of the cultivars evaluated were highly susceptible to bacterial wilt. Ontario was the only cultivar highly resistant to the disease, with only 1% and 8% dead or dying plants in 1978 and 1979, respectively. Based on tuber infection, Ontario and Snowchip were the most resistant, with 9% and 14% infection, respectively, in 1978 and both 4% in 1979. Saco, Green Mountain and Sebago also showed some resistance.     

Uniformity among virus-free clones of ten potato cultivars

Heat therapy and meristem tip propagation were used to develop two or three virus-free clones of 10 cultivars from the following sources: Epicure from British Columbia and Scotland; Katahdin, Kennebec, Keswick, Red Pontiac and Sebago from British Columbia and New Brunswick; Norchip from Alberta, Quebec and Washington; Norland from Alberta, British Columbia and North Dakota; Red La Soda from British Columbia, Nebraska and North Dakota; and White Rose from British Columbia and North Dakota. Tests prior to treatment showed that all but one of the stocks were infected with potato virus X and/or potato virus S but in only five of the ten cultivars were all clones infected with the same virus or viruses. Field trials with the virus-free clones, which extended over four years, showed that one of the Norchip clones produced fewer tubers per plant than two other clones in three years of the four years, but no consistent differences in yield or specific gravity of tubers were found among the clones of any of the 10 cultivars.     

Phenotypic stability of resistance to late blight in potato clones evaluated at eight sites in the United States

Changes in the fungal pathogenPhytophthora infestans in the United States pose a significant threat to potato production. Sources of resistance to these new genotypes of P.infestans need to be identified for potato breeders to have parental materials for crossing, and the phenotypic stability of late blight resistance in these potato clones needs to be determined. Sixteen potato clones which reportedly have some resistance to late blight were evaluated at eight locations: Florida (FL), Maine (ME), Michigan (MI), Minnesota (MN), North Dakota (ND), New York (NY), Pennsylvania (PA) and Wisconsin (WI) in 1996. Percent infected foliage was recorded at approximately weekly intervals following the onset of the disease at each location. Area under the disease progress curve (AUDPC) was calculated. Clones were ranked for mean AUDPC within location and the nonparametric stability statistics, mean absolute rank differences and variance of the ranks, were analyzed for phenotypic stability. Neither of these statistics was significant, indicating a lack of genotype x environment interaction on the rankings of these clones across locations in 1996. The four clones with lowest AUDPC scores were U.S. clones AWN86514-2, B0692-4, B0718-3 and B0767-2. These clones should be useful parental materials for breeders seeking to incorporate genes for late blight resistance into potatoes.     

Response of tubers of five potato cultivars to co-inoculation with US-1 and US-8 genotypes ofPhytophthora infestans

Summary The severity of late blight tuber rot in five potato cultivars (Green Mountain, Sebago, Dorita, AC Brador and Island Sunshine) resulting from inoculation with US-1/US-8 combinations of Canadian isolates ofPhytophthora infestans was evaluated. In addition, the potential for mixed (A1∶A2) inoculations to result in oospore formation was examined. In studies conducted in 1996 and 1997, cv. Dorita (and Island Sunshine in 1997) developed the least amount of surface or internal necrosis relative to the other cultivars following co-inoculation with US-1 and US-8 isolates ofP. infestans. Differences also were revealed among isolate combinations used for inoculation. The combination of a US-1 isolate from Prince Edward Island with a US-8 isolate from New Brunswick consistently produced the least amount of necrosis in tubers of the various cultivars. There was no evidence of oospore production in any of the tubers examined. AAFC Contribution No. 919     

Quantitative analysis of host resistance,fungicide and weather effects on potato early and late blight using computer simulation models

Models which simulate the development of potato early blight, potato late blight and fungicide dynamics were used to analyze the effects of host resistance, fungicide and weather on individual and combined epidemics of these diseases. Fungicide and host resistance effects were based on (and accurately reflected) products and cultivars available to commercial potato growers in northeastern USA. Moderate resistance to early blight had a 2-3 fold greater effect than did moderate resistance to late blight in suppressing early and late blight, respectively. Analysis of simulation experiments indicated that replacement of an early blight susceptible cultivar with a cultivar moderately resistant to the disease would permit a reduction in the fungicide application frequency from once every week to once every two weeks without loss of disease suppression. Fungicide effect in suppressing late blight was 2-3 fold greater than in suppressing early blight. Variation in seasonal weather (typical of northeastern USA) influenced late blight development more strongly than it affected early blight development, and it affected epidemics of individual diseases more than combined epidemics.     

Inheritance of early blight resistance from open-pollinated 4×-2× potato hybrids

Early blight, caused byAlternaria solani Sorauer, is a serious disease of potatoes that occurs in most potatogrowing regions in the world. There is little resistance to early blight among commercial potato cultivars. However, resistance to early blight in diploid (Solanum phureja-S. stenotomum) potatoes has been identified, and was found to be highly heritable and readily transferred to the tetraploid level via4×-2× crosses. The purposes of this study were to identify good levels of early blight resistance in open-pollinated4×-2× (Solanum tuberosum×S. phureja-S. stenotomum) hybrids selected for horticultural characteristics, to estimate broad-sense heritability for early blight resistance in these hybrids, and to investigate the general and specific combining ability for resistance to early blight from some of these early blight resistant tetraploid selections. Four early blight resistant clones were crossed as female parents with four different male parents in a design II mating scheme to generate 16 families. Approximately 20 randomly chosen offspring per family were visually evaluated for early blight resistance in 1995 and 1996 in a randomized complete block design in Pennsylvania. Broad-sense heritability for early blight resistance among the open-pollinated4×-2× hybrids which were originally selected for horticultural characteristics was 0.91 with a 95% confidence interval of 0.80, 0.94. There were significant differences among female and male parents for area under the disease progress curve, indicating the importance of general combining ability for early blight resistance in this germplasm. The female x male source of variation was not significant, indicating that specific combining ability was not important. The greatest number of resistant progeny were observed in families where both parents were derived from the early blight resistant population; however, at least one highly resistant progeny was produced in all families. These results suggest that the early blight resistance in these clones can be readily incorporated into the commercial tetraploid breeding population.     

Seed transmission of potato spindle tuber virus

Seed transmission of potato spindle tuber virus (PSTV) has special significance for potato breeding programs. Transmission of PSTV has been reported to occur via true seed. Test families in the Frito-Lay breeding program were obtained from self- and cross-pollinations to determine the incidence of infected seedlings from healthy and diseased parents. Diseased seedlings from PSTV-infected Katahdin selfed were scored by visual and tomato index procedures as 100% and 87% infected, respectively. A similar result was obtained among seedlings from the cross of PSTV-infected Sebago x PSTV-infected Katahdin. Symptoms of PSTV infection were not detected by either method among seedlings from healthy Katahdin, selfed. The Frito-Lay, Inc., potato breeding program employs a number of phyto-sanitary procedures to exclude or reduce the possibility of PSTV contamination and spread.