DOCTORAL THESIS (PhD)

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1 DOCTORAL THESIS (PhD) VESZPRÉM UNIVERSITY GEORGIKON AGRICULTURAL FACULTY CROP PRODUCTION AND HORTICULTURAL SCIENCES DOCTORAL SCHOOL Consultant: Dr. habil. GÉZA FISCHL university professor TESTING FOR RESISTANCE OF POTATO VARIETIES TO LATE BLIGHT AGENT (PHYTOPHTHORA INFESTANS [MONT.] DE BARY) AND THE EFFECT OF CERTAIN ENVIRONMENTAL FACTORS ON DISEASE RESISTANCE Prepared by: Dr. LÁSZLÓ GERGELY Keszthely 2004

2 1. Background and research objectives The pathogen of potato late blight is a microorganism of special biology, traditionally classified as a fungus, Phytophthora infestans (Mont.) de Bary. On a worldwide scale it is considered to be the most important potato disease because the yield loss under favourable (cool and rainy) weather conditions may even reach %, due to complete defoliation and subsequent tuber rot. The two global migrations of the notorious pathogen from its assumed site of origin, Mexico had serious social and economic consequences. The first caused the Irish famine and mass emigration in the middle of the 19 th century ( ), while the second one has completely transformed the genetic constitution of local populations throughout the world since the 1980s. One of the major changes was the spreading of P. infestans races with complex virulence, altering the strategy in breeding for resistance. Due to the vulnerability of race-specific (vertical) resistance, horizontal types of resistance (e.g. field resistance) have gained priority in current breeding programmes. In Hungary, according to the former practice of variety testing introduced by Kálmán Hinfner ( ), resistance tests of potato late blight were based on natural foliar and tuber infections occurring in the nationwide field variety trials. However, the continuity of resistance tests could not have been ensured in the last few decades, because of the increasing frequency of hot and dry growing seasons, as well as the more efficient chemical control carried out in field trials. The changed circumstances have called for the development and/or adaptation of provocation testing methods providing a reliable and continuous assessment of race non-specific (horizontal) resistance of potato genotypes to late blight agent, in both the foliage and tubers. I have been dealing with a significant field of the integrated late blight control, varietal resistance (genetic control) since 1988, when I started resistance tests with potatoes at the Phytopathology Department of the National Institute for Agricultural Quality Control in Budapest. The methodological difficulties of resistance tests essential for the state registration of potato varieties (the restricted usability of natural infections, the different genetic background of foliar and tuber resistance, the uncertainties involved in the methodology of inoculation and disease assessment) were a great challenge to me. It was a motivation for me to apply for the doctoral programme of PATE (Pannon University of Agriculture) entitled The effect of environmental conditions on the resistance to fungal diseases within the Theoretical bases of integrated pest management methods in I wish to contribute with my work to the development of the integrated pest management of potatoes. 1

3 I set the following objectives for my studies: 1. Development and/or adaptation of provocation testing methods suitable for measuring race non-specific (horizontal) resistance of potato varieties and variety candidates to late blight agent. 2. Comparative analysis of testing methods for assessing foliar resistance. 3. Comparative analysis of testing methods for assessing tuber resistance. 4. Studying the correlation between the foliar and tuber resistance. 5. Studying the durability of horizontal resistance to potato late blight. 6. Studying the effect of certain environmental factors on disease resistance. 7. Characterization of the horizontal resistance of registered potato varieties to late blight, in both the foliage and tubers. 2

4 2. Materials and methods 2.1. Materials and sites of resistance tests The materials of all tests were variety candidates studied for state registration by the National Institute for Agricultural Quality Control (OMMI) and/or registered potato varieties. The inoculum used for the resistance tests consisted of a mixture of Hungarian isolates of Phytophthora infestans and contained at least 7 virulence factors in all cases, duly representing the complex race character of domestic populations. The isolates were obtained from the following persons: H-K/91, 2001/04: Terézia Dula (Plant Protection and Soil Conservation Service /PPSCS/ of County Heves, Eger), H-3.1/93, US-8: József Bakonyi (Plant Protection Research Institute, Budapest) and 2001/21/3: István Wolf (Veszprém University, Potato Research Centre, Keszthely). The isolates were maintained on the leaves of tomato plants (PPSCS of County Heves) and multiplied on the foliage of susceptible potato varieties (OMMI, Monorierdő). The foliar resistance to potato late blight was assessed at the Postcontrol Variety Testing Station of OMMI in Monorierdő (under walk-in plastic tunnel) and at the Phytopathology Station of OMMI in Röjtökmuzsaj (field trials) in provocation tests adjusted to this purpose. The incidence of tuber late blight was tested in field variety trials under natural infection (Phytopathology Station of OMMI, Röjtökmuzsaj) and by using laboratory methods (whole-tuber inoculation, treatment of tuber disks with elicitor) Testing methods for evaluating foliar resistance to late blight Provocation tests under walk-in plastic tunnel My provocation method based on whole-plant inoculation has been used for resistance tests of potato variety candidates since Sixty budcuttings were cut out from first generation tubers of each genotype tested. After breaking their dormancy with chemicals (soaking in a solution of 0.1 % gibberellic acid (GA 3 ) + 1% thiourea for 10 minutes, followed by washing with tap water), they were planted in peat beds under plastic tunnel, in a two-replicated micro-plot trial with randomized complete block design (30 plants/plot). A 1:1 mixture of two Hungarian isolates (H-K/91, H-3.1/93) of P. infestans was used as inoculum with complex race character (virulence types: and (8).10.11, respectively) that could overcome at least 7 dominant resistance genes (R-genes) in potatoes. 3

5 Plants at leaf stage were inoculated by spraying a suspension of 3 x 10 4 sporangia/ml on the foliage, in late afternoon (30 ml/plot). The number of sporangia per ml was determined by using a Bürker chamber. Disease severity (infected leaf area %) was assessed 14 to 16 days after inoculation on a 1-10 grade scale of increasing susceptibility, estimating the diseased area of the whole leaf surface for each genotype (Gergely 1999). Genotypes were classified into 5 susceptibility categories according to late blight severity on their foliage. Classification was based on the deviation of the disease severity of a particular variety from the current trial mean. Therefore, regarding the trial mean as 100%, a certain genotype is resistant (1) if its disease severity does not exceed 25% of the mean, moderately resistant (2) if its disease severity is 25-75% of the mean, intermediate (3), if its disease severity is % of the mean, susceptible (4), if its disease severity is % of the mean and highly susceptible (5), if its disease severity exceeds 175 % of the mean. In the cases when the trial mean is over 50%, the percentage values of the actual disease severity correspond to the following ratings of resistance/susceptibility: (1) 0-20% = resistant, (2) 21-40% = moderately resistant, (3) 41-60% = intermediate, (4) 61-80% = susceptible, (5) % = highly susceptible (Hinfner and Homonnay 1966). In the last 5 years ( ) the horizontal resistance of 60 potato variety candidates and registered varieties was assessed in the foliage. The durability of horizontal resistance to potato late blight was determined by comparing foliar infection rates of 18 potato genotypes originating from distant years (1990, 1995, 1997 and 2001). Comparability was allowed by the fact that the susceptible (Cleopatra) and intermediate (Désirée) standard cultivars were involved over the whole period of the provocation tests Field provocation tests The ADPC (Area under the Disease Progress Curve) method described by Shaner and Finney (1977) was used for quantifying the field resistance in the foliage. As a result, the progress rate of the disease was determined for each genotype by calculating the integral, the size of the area under the epidemic curve. The field resistance of 36 registered potato varieties was assessed over a period of 3 years ( ) in fungicide-free micro-plot trials (10 plants/plot) with two replications. The blocks of the genotypes tested were surrounded by the virus-resistant cultivar Százszorszép (spreader=inner source of infection), which was inoculated with a 1:1:1 mixture of 3 domestic isolates of P. infestans (H-K/91, 2001/04, 2001/21/3). Isolate H-K/91 is of 4

6 complex race character (see point ), while the virulence type of the other two isolates is not known. After irrigation, plants were inoculated with a suspension of 2-3 x 10 4 sporangia/ml 1-2 weeks after crop cover, in the evening hours. The spontaneous spreading of the disease was promoted with sprinkler irrigation if necessary. Three check cultivars were involved in the tests: White Lady (1=resistant), Désirée (3=intermediate), and Impala (5=highly susceptible). The severity of foliar late blight was determined at 5-8-day intervals from the onset of first symptoms until the end of the vegetation period and expressed in % of the infected leaf area. The level of field resistance expressed by the ADPC values was calculated with the following formula (Shaner and Finney 1977): n ADPC= [(X i+1 + X i )/2] [t i+1 - t i] i = 1 where x i = infected leaf area (%) at the i th observation, t i = time (days) at the i th observation and n = total number of observations. Genotypes were classified into susceptibility categories based on their relative ADPC values (RADPC) comparing to the most susceptible cultivar as follows (Jenkins and Jones 2003): 1 = (resistant) 2 = (moderately resistant) 3 = (intermediate) 4 = (susceptible) 5 = (highly susceptible) Testing methods for evaluating tuber resistance to late blight Resistance tests in field variety trials under natural infection The incidence of blighted tubers of 27 registered potato varieties was tested in the years (1996, 1998 and 1999) when rainfall at the end of the growing season induced an assessable tuber infection. Disease severity was symptomatically assessed in samples of 2x100 tubers per variety following a 2-month storage after harvesting (% of infected tubers). When observing the typical external symptoms (lead-grey, slightly sunken, hard to touch spots on the tuber skin), the tubers were cut through the stem-end in order to check the presence of rust-coloured dry rot infiltrating in the tissues 5

7 below the skin. In case of uncertain diagnosis, tuber halves were placed in a moist chamber and incubated at 18 C for hours. Then the sporulation of Phytophthora infestans was examined under stereomicroscope. Genotypes were classified into susceptibility categories as described under Laboratory testing of horizontal tuber resistance The horizontal resistance of the tubers of 35 registered potato varieties was tested to late blight for two years ( ). Sato s wholetuber inoculation technique (1995) was applied, completed with a method for evaluating disease severity based on digital image processing. Four mature tubers of medium size from each genotype were stored at 3-5 C for 4-5 months. Before testing, the tubers were conditioned at room temperature for 24 hours. Then their surface was disinfected with 1:10 dilution hypochlorite, followed by rinsing with tap water. After this, the tubers were pretreated in beakers, soaking them in tap water for 2 hours (simulating moistening in the ridges). Before inoculation, tap water was removed and replaced with a sporangium suspension (30-50 sporangia/ml), and the tubers were kept there at 18 C for 4 hours. The inoculum consisted of a 1:1:1 mixture of 3 Hungarian Phytophthora infestans isolates (H-K/91, 2001/04, 2001/21/3). Inoculation was followed with a thorough rinsing with tap water, then the tubers were placed on plastic nets put on moist filter paper in covered plastic boxes and incubated at 18 C for 7 days in darkness. In order to evaluate disease severity, the tubers were cut through the stem-end, the other part of the tuber was removed and digital photos were taken from the 4 tuber halves kept from each variety. The size of the diseased tuber tissues (showing brown discolouration) was measured in the colour pictures, using a rasterbased image processing software (Photoshop 6.0), and calculating the ratio (%) of the total area (pixel) of the significant sample parts and the total area (pixel) of the tuber samples (Berke 1994). Genotypes were classified into susceptibility categories as described under Elicitor assay with potato tuber disks The sensitivity of the tubers of 29 registered potato varieties was evaluated to cell-free homogenate of P. infestans mycelia (as elicitor) in 2001, applying the method described by Érsek et al. (1989). The tubers, stored at 3-5 C for 6 months and conditioned at room temperature for 2 days before treatment, were disinfected on the surface with 1:10 dilution hypochlorite. Then 18 mm-diameter tissue cylinders were cut out from them using a cork borer, and sliced to 2 mm-thick disks. The disks, after rinsing them with distilled water several times, were placed on wet filter paper (8 from each genotype) in covered plastic boxes and desiccated at room temperature until the treatment. During the treatment, 30 µl elicitor preparation 6

8 was dropped on each of 6 disks and the same volume of distilled water was dropped to 2 disks as controls. Then the drops were dispersed using a flatheaded glass rod. After a short desiccation, the boxes were incubated at 18 C for 72 hours in darkness. After incubation, black-and-white photos were taken of the treated and untreated tuber disks of each potato genotype. The degree of the hypersensitive tissue reaction (necrosis) was measured with digital image processing, by determining the discoloured area of potato disks compared to the control (Berke 1994). The elicitor preparation was made from one-week-old mycelium of the US-8 race of Phytophthora infestans grown on a selective pea-agar medium. The previously frozen 1.1 g air-dry mass of mycelia was homogenized with 20 ml distilled water in a blender, and treated with ultrasound in an MSE device at maximum capacity for 10 minutes. Finally, the supernatant of the material centrifuged at 5000 g for 25 minutes was sterilized for 20 minutes Correlation studies Correlations were calculated between the following variables using linear regression analysis: 1. the foliar resistance of potato varieties to late blight determined with two methods (testing in plastic tunnel and in the field) (n = 25), 2. the severity of tuber late blight under natural infection and artificial inoculation (n = 27), 3. the horizontal late blight resistance of tubers and their sensitivity to elicitor (n = 27), 4. the horizontal resistance to late blight in the foliage and tubers (n = 34), 5. the growing season of varieties and horizontal late blight resistance in their tubers (n = 31) and 6. the amount of whole and late-season (August and September) precipitation and the incidence of potato tuber blight under natural infection (n = 10) Meteorological observations In the resistance tests carried out under walk-in plastic tunnel (Monorierdő) the air temperature and relative humidity from the inoculation to the appearance of the first symptoms were measured with a thermohygrograph placed in the crop. In the field provocation trials (Röjtökmuzsaj) the meteorological data were locally recorded by an automatic measuring station. 7

9 3. Results 3.1. Foliar resistance of potato varieties to late blight Based on the results of provocation tests on resistance carried out in plastic tunnels during the past 5 years ( ), we concluded considerable differences among the genotypes in the severity of foliar late blight. The disease severity ranged between 0-3% (minimum) and 85-95% (maximum), expressed in percentage of the infected leaf area (F%). Only 5 (8%) of all the genotypes tested (n=60) showed sufficient level of resistance (resistant and moderately resistant types: F%=0-20 and 21-40, respectively). Further 10 varieties (17%) behaved as intermediate types (F%=41-60), and 45 varieties (75%) proved to be susceptible or highly susceptible (F%=61-80 and , respectively). The highest level of resistance was observed on the plants of the early variety candidates Derby and Vitara, as well as of the midseason registered cultivars White Lady, Raja and Solara. In contrast, the genotypes with foliage of the highest susceptibility were the early Berber, Adora, Annabelle, Arosa and Amorosa, as well as the midseason Lady Rosetta, Redstar, Aladin, Margarita and Dura. The susceptible host-parasite interaction was characterized by large, rapidly expanding lesions with abundant sporulation. In the resistant interactions, hypersensitive reaction (HR) was observed on certain genotypes, while slowly increasing lesions and/or poorer sporulation were recorded on the others. The rather big differences found in the growth of lesions and their extension on the foliage were not expressed in the intensity of sporulation. An intermediate sporulation was recorded only for the 5 varieties showing the highest level of resistance, all the other varieties were characterized by intensive sporangium production. During the 5 years of the study, the vulnerability of the race-specific (vertical) resistance became evident for certain genotypes (Derby, Cupido, Innovator, Cicero, Raja and White Lady), as since 2000 sporulating lesions of variable size were observed on the foliage of varieties formerly responding with a hypersensitive reaction to infection. Among them cultivar White Lady was affected to the least extent (F%=3.0), while cultivars Cupido, Innovator and Cicero showed symptoms characteristic of the intermediate types (F%= ). The averages of the susceptibility ratings of registered varieties and variety candidates were equally 3.9, indicating that the majority of available and prospective potato cultivars have susceptible foliage to late blight. Studying the duration of resistance to potato late blight it was concluded that while the foliage of the early standard cultivar Cleopatra proved to be consistently susceptible, certain mid-late cultivars (e.g. Désirée, Kondor, Agria) maintained their intermediate horizontal resistance 8

10 over the whole period of the study (12 years). In the first year (1990), an extremely high level of horizontal resistance (F%= ) was observed on the foliage of three Hungarian variety candidates (Kastia, Galant, Luna). Unfortunately, they were excluded from the further studies, because several undesired traits prevented their registration. However, certain moderately resistant cultivars (e.g. Solara, Signal) appeared later, that have been registered since then and included in Hungary s National List of Varieties. The provocation test under plastic tunnel, developed by us has proved to be reliable and well-reproducible for assessing horizontal resistance to potato late blight in the foliage. This method is suitable for the differentiation of the main types of behaviour to the disease (resistant, intermediate and susceptible) and the determination of their proportion in a particular variety choice. Its great advantage is that it can also provide information in the years unfavourable for late blight development (e.g. 2003), allowing continuous data supply for the quality control and registration of varieties. As by using this method, it is possible to assess the resistance of numerous potato genotypes within a relatively short time (45-50 days) at a low cost, the screening procedure described under has been an integral part of OMMI s variety testing methodology since In the provocation tests ( ) assessing field resistance to potato late blight in the foliage, it was concluded that 2000 was the most favourable and 2001 was the least conducive for disease development, based on the ADPC means of the trials. The highest field resistance was observed on the plants of the Hungarian cultivar White Lady, the German Panda and the Dutch Raja. In contrast, with their consistently high ADPC values, the most susceptible cultivars were Adora, Lilla, Lady Rosetta, Impala and Agata. The first symptoms of potato late blight appeared days later on White Lady, the cultivar showing the highest level of field resistance, compared to the susceptible cultivars. The extension of the incubation period was the most characteristic of the genotypes carrying even R-gene(s) and formerly responding with HR in the tests under plastic tunnel. The moderate growth of the lesions proved to be the most important and general component of the horizontal resistance. In the field trials there was no opportunity to reveal differences in the degree of sporulation that could be attributed to the weather conditions unfavourable for the formation of sporangia (high daily mean temperature, shortage of rainfall and low relative humidity). Because of the significant seasonal effects, the determination of the field resistance of potato varieties needs to be based on the mean ADPC s of at least 3 consecutive years. This resistance of quantitative type can be reliably measured using the ADPC method of frequently assessing foliar infection and calculating the area under the disease progress curve. Resistance ratings of the genotypes, based on their relative ADPC values 9

11 (RADPC), comparing them to the most susceptible one of the current varieties also proved to be correct. The results of the provocation tests carried out for determining foliar resistance to late blight in plastic tunnel and in the field corresponded well to each other: a close correlation was confirmed between the resistance ratings of the genotypes according to the two methods (r=0.80, P=5%). The ratings of the registered varieties involved in both trials (n=25) were the same or there was only a one-degree difference between the phytopathological qualification of the same variety Tuber resistance of potato varieties to late blight There were only three years in the last decade (1996, 1998 and 1999) when the incidence of potato tuber blight could be evaluated in small-plot variety trials. Weather conditions in these years were especially favourable for the spontaneous occurrence of the disease, based on the trial means of disease severity (17.1, 8.1 and 7.5%, respectively) that was related to the amount of whole and late-season (August and September) precipitation. At the trial site (Phytopathology Station of OMMI, Röjtökmuzsaj), the major disease affecting tubers was Phytophthora dry rot in all the three years. The other fungal and bacterial rots (Fusarium spp., Alternaria spp., Erwinia spp.) occurred only sporadically on the tuber samples. Of the registered potato varieties (n=29), the lowest incidence of tuber blight (% of blighted tubers = F%) was recorded for the cultivars White Lady, Solara, Rosara, Raja, Panda and Santé that did not reach 5% in the average of three years. In contrast, the highest susceptibility (F% > 20.0) was observed on the tuber samples of cultivars Lady Rosetta, Góliát, Romina and Karlena. The average level of tuber resistance (3.0) exceeded that of the foliage by almost one rating (3.9) based on the trial mean of resistance ratings of the genotypes tested. The race non-specific (horizontal) resistance of 35 registered potato varieties was evaluated to late blight using whole-tuber inoculation in the laboratory in 2002 and None of the varieties was qualified as resistant (1) in the inoculation tests well simulating the process of natural tuber infection. Eleven varieties proved to be moderately resistant (2), further 15 ones showed intermediate behaviour (3) and 9 varieties were susceptible to different extent (4 or 5). A close positive correlation (r=0.86, P=5%) was confirmed between the severity of tuber blight under natural infection and artificial inoculation on the genotypes tested. On the basis of it, the horizontal resistance of tubers can be reliably tested applying the whole-tuber inoculation, especially in hot and dry years when no assessable natural infection occurs in field variety trials. The inoculation technique described under is 10

12 well-reproducible and, completed with a disease evaluation based on digital image processing, allows a more accurate, quantitative determination of the diseased area of tuber tissues. There was no correlation between the growing season of cultivars and the horizontal tuber blight resistance (r= 0,01, P=5%). In the treatment of tuber disks with elicitor the most intensive hypersensitive reaction was measured on the cultivars, such as Lady Rosetta, Amorosa and Romina that proved to be the most susceptible in the whole-tuber tests, indicating that they did not have horizontal type of resistance to late blight. At the same time high horizontal resistance was observed in both the tubers and foliage of some cultivars (such as White Lady, Panda) being the least sensitive to the elicitor treatment. However, the negative correlation between the elicitor sensitivity of tubers and their horizontal resistance to late blight was expressed only on some of the extremely behaved (resistant or highly susceptible) genotypes. The revealed moderate negative correlation (r= 0.56, P=5%) does not allow to use the elicitor method in resistance tests The relationship between foliar and tuber resistance to late blight In our earlier resistance tests (1998 and 1999) a low correlation was found between the resistances expressed on the foliage and tubers (r= , P=5%). In the recent years, however a moderate correlation (r=0.65, P=5%) was confirmed between the late blight resistance of the different plant parts that could be attributed to the improvement of methodology having taken place since The effect of environmental factors on the resistance to potato late blight In the provocation tests carried out in plastic tunnel, the optimal environmental conditions for disease development (air temperature, relative humidity, free water on the leaves) and/or shorter photoperiod led to an increase in susceptibility of certain varieties. In the field provocation trials ( ), using the annual ADPC rankings, it was confirmed that the field resistance of certain cultivars (such as White Lady, Panda and Raja) was consistently expressed, independently of the year, even under changing environmental conditions. In contrast, a few cultivars were rather sensitive to the seasonal effects (e.g. Red Scarlett, Ukama and Cicero). Confirming the decisive role of meteorological factors in the development of potato late blight, the extremely high daily mean temperature and the low 11

13 relative humidity in summer of 2003, led to the complete failure of resistance test in spite of the double inoculation and sprinkler irrigation. Of the environmental factors, the amount of whole and late-season (August and September) precipitation (>400 mm and >160 mm, respectively) had the greatest impact on the natural late blight infection of potato tubers Characterization of the resistance of potato varieties to late blight We were the first in Hungary to publish not only estimated but quantified data on the horizontal resistance of registered potato varieties to late blight expressed in both plant parts. Of the genotypes included in the National List of Varieties, the Hungarian cultivar White Lady, the German Panda and the Dutch Raja were outstanding for their high level of horizontal resistance in both the foliage and tubers. In contrast, on the list of registered varieties 11 genotypes were found to be susceptible or highly susceptible expressing in both plant parts. Of these cultivars Lady Rosetta, Romina and Aladin proved to be the most susceptible. 12

14 4. New research results 1. I worked out a reliable and well-reproducible method for measuring the race non-specific (horizontal) resistance of potato genotypes to late blight in the foliage. This screening technique, based on whole-plant inoculation under plastic tunnel, has been an integral part of OMMI s methodology since In field provocation trials ( ), using a method based on frequent evaluations of foliage infection and calculation of ADPC (Area under the Disease Progress Curve) values, I demonstrated that the field resistance of potato cultivars to late blight can be reliably characterized by the mean ADPC s of at least three consecutive years. 3. I concluded that, by applying an evaluation method based on digital image processing, the horizontal tuber resistance of potato varieties to late blight can be more exactly and reliably measured than by traditional visual evaluations. 4. I confirmed a moderate negative correlation (r= 0.56, P=5%) between the sensitivity of tubers to elicitor treatment and their horizontal resistance to late blight. Therefore, the elicitor method is not suitable for quantifying tuber blight resistance. 5. In Hungary I published the first quantified and not only estimated data on the complex horizontal resistance of registered potato cultivars to late blight expressed in both plant parts (foliage and tubers). 6. On the basis of annual ADPC rankings originating from field provocation trials ( ), I demonstrated that the field resistance of certain potato cultivars (e.g. White Lady, Panda and Raja) to late blight was consistently expressed, independently of the year, even under changing environmental conditions. In contrast, several cultivars (e.g. Red Scarlett, Ukama and Cicero) showed inconsistent behaviour in the years of the study. 13

15 5. Recommendations Based on a comparative analysis of the methods for assessing foliar resistance to late blight, I consider the provocation test under plastic tunnel the most suitable method for variety registration due to its reliability and reproducibility. However, the field provocation tests (ADPC method) are recommended for quantifying the field resistance of registered varieties, because even their performance in commercial growing may be predicted by using them. The whole-tuber inoculation method and the evaluation of disease severity based on digital image processing are recommended for determining the horizontal tuber resistance to late blight. The method allows us to obtain reliable data even in hot and dry growing seasons when no assessable natural infection occurs in field variety trials. As, even with standardizing the methods, only a moderate correlation can be confirmed between the late blight resistance of the foliage and tubers, I believe it is justified to include both plant parts in the resistance tests. Considering that horizontal resistance does not provide complete foliar protection (partial resistance), the moderate but lasting sporulation of the pathogen increases the risk of tuber infection. Therefore, breeding for resistance needs to aim at reaching at least intermediate level of resistance in both the foliage and tubers. Registered potato cultivars possessing not only a high level of blight resistance but multiple disease resistance (e.g. White Lady, Santé) can be recommended for integrated and especially organic cultivation. These cultivars may considerably contribute to the improvement of the health status of potatoes and to the reduction of pesticide pollution of the environment. 14

16 6. Publications in the field of the dissertation Scientific publications 1. Gergely, L. und Förster, H. (1995): Prüfung der Laubresistenz der Kartoffelsorten gegen Phytophthora-Befall im Inokulierungsversuch. Bericht 46. Arbeitstagung der Arbeitsgemeinschaft der Saatzuchtleiter im Rahmen der Vereinigung österr. Pflanzenzüchter, BAL Gumpenstein, Gergely L. (1999): [Foliar and tuber resistance of potato varieties to late blight (Phytophthora infestans (Mont.) de Bary).] Burgonyafajták lombés gumóellenállósága a burgonyavésszel (Phytophthora infestans [Mont.] de Bary) szemben. Növényvédelem, 35: Gergely L. (2000): [Evaluation of foliage and tubers for susceptibility to late blight in potato variety candidates.] Burgonya-fajtajelöltek fitoftórafertőzöttségének értékelése lombozaton és gumón. Növényvédelem, 36: Gergely L. (2001): [Differences in the susceptibility of the foliage of potato varieties and variety candidates against late blight.] Burgonyafajták és -fajtajelöltek lombozatának eltérő fogékonysága a burgonyavésszel szemben. Növényvédelem, 37: Gergely, L., Lönhárd, M. and Proksza, P. (2003): Durability of dual resistance of potato varieties to late blight (Phytophthora infestans [Mont.] de Bary) and common scab (Streptomyces scabies [Thaxt.] Waksman et Henrici). Acta Phytopathologica et Entomologica Hungarica, 38: Gergely, L. (2004): Field resistance of registered potato cultivars to late blight agent, Phytophthora infestans. Georgikon for Agriculture, 7: Publications in popular science 1. Gergely L. (1991): [Genetic control against potato late blight.] Genetikai védelem a burgonyavésszel szemben. Agrofórum, 2 (2-3):

17 2. Proksza P. and Gergely L. (1991): [The results of potato variety trials carried out in 1990.] Az évi burgonya fajtakísérletek eredményei. Burgonyatermesztés, 17 (2): Gergely L. (1993): [Possibilities of genetic control against the major potato diseases.] A genetikai védelem lehetőségei a burgonya legfontosabb betegségeivel szemben. In: Integrált termesztés a szántóföldi kultúrákban 9., Budapest, Proceedings, Gergely L. and Proksza P. (1993). [Potato variety choice with special attention to disease resistance.] A burgonyafajta-választék különös tekintettel a betegség-ellenállóságra. Agrofórum, 4 (3): Proksza P. and Gergely L. (1993): [The results of potato variety trials carried out in 1992.] Az évi burgonyakísérletek eredményei. Burgonyatermeszés, 19 (1-3): Proksza P. and Gergely L. (1994): [Information on the results of comparative small-plot trials with potato varieties carried out in 1993.] Tájékoztató a kisparcellás burgonya fajtaösszehasonlító kísérletek évi eredményeiről. Burgonyatermesztés, 20 (2): Proksza P. and Gergely L. (1996): [Current issues of the registration of potato varieties and their description.] A burgonyafajta-minősítés időszerű kérdései és a fajták ismertetése. Agrofórum, 7 (2): Gergely L. (1999): [Resistance studies with potato varieties at the National Institute for Agricultural Quality Control.] Burgonyafajták rezisztenciavizsgálata az Országos Mezőgazdasági Minősítő Intézetben. Mag (kutatás, termesztés, kereskedelem), 13 (2): Gergely L. (2000): [Testing for resistance of potato varieties at the National Institute for Agricultural Quality Control.] Burgonyafajták betegség-ellenállóságának vizsgálata az Országos Mezőgazdasági Minősítő Intézetben. Burgonyatermesztés, 1 (2): Gergely L. ( ): [Phytopathological data of small-plot comparative trials with early and mid-season potato varieties.] Korai és középérésű burgonyafajták kisparcellás fajtaösszehasonlító kísérleteinek kórtani adatai. In: Békési P. Gergely L. et al.: Az OMMI Növénykórtani Osztályának évi munkái, OMMI, Budapest. 16

18 11. Gergely L. ( ): [Phytopathological traits and characterization of registered potato varieties.] Államilag elismert burgonyafajták kórtani tulajdonságai és jellemzése In: Czirák et al.: Burgonya leíró fajtajegyzékek ( ), OMMI, Budapest. 12. Lönhárd M., Proksza P. and Gergely L. (2003): [Durability and efficacy of resistance to late blight, common scab, potato wart and potato cyst nematode of potatoes (Solanum tuberosum L.)] A burgonya (Solanum tuberosum L.) ellenálló képességének állandósága és hatékonysága a burgonyavésszel, a varasodással, a burgonyarákkal és a gyökérrontó fonálféreggel szemben. Burgonyatermesztés, 4 (3): Lectures given in the field of the dissertation 1. Gergely L. (1994): [Evaluation of foliar resistance of potato varieties to late blight (Phytophthora infestans [Mont.] de Bary).] Burgonyafajták lombrezisztenciájának értékelése fitoftóra- (Phytophthora infestans [Mont.] de Bary) fertőzéssel szemben. In: Növényvédelmi Fórum 94, Keszthely, Proceedings, Gergely L. (1996): [Assessment of foliar resistance of potato varieties to late blight (Phytophthora infestans [Mont.] de Bary).] Burgonyafajták lombrezisztenciájának vizsgálata fitoftóra- (Phytophthora infestans [Mont.] de Bary) fertőzéssel szemben. In: 42. Növényvédelmi Tudományos Napok, Budapest, Proceedings, Gergely L. (1996): [Genetic control against the major potato diseases.] A burgonya legfontosabb betegségeivel szembeni genetikai védelem. In: I. Tiszántúli Növényvédelmi Fórum, 1996, Debrecen, Proceedings, Gergely L. (1997): Foliar and tuber resistance of potato varieties to late blight (Phytophthora infestans [Mont.] de Bary).] Burgonyafajták lomb- és gumórezisztenciája fitoftóra- (Phytophthora infestans) fertőzéssel szemben. In: VII. Keszthelyi Növényvédelmi Fórum, 1997, Keszthely, Proceedings, Gergely L. (1997): [Possibilities for controlling late and early blight of potatoes.] A burgonya fitoftórás és alternáriás betegségeivel szembeni védekezés lehetőségei. Meeting organized by Summit-Agro Hungária Ltd, Budapest,

19 6. Gergely L. (1998): [The role of disease resistance in the integrated production of potatoes.] A betegség-ellenállóság szerepe a burgonya integrált termesztésében. (Potato Sector Forum) Burgonya Ágazati Fórum, PATE, Keszthely, Gergely L. (1998): [Assessment of foliar resistance of potato varieties and variety candidates to late blight (Phytophthora infestans [Mont.] de Bary)] Burgonyafajták és -jelöltek lombellenállóságának vizsgálata fitoftóra- (Phytophthora infestans) fertőzéssel szemben. In: 44. Növényvédelmi Tudományos Napok, Budapest, Proceedings, Gergely L. (1998): [The role of resistant potato varieties in the sustainable agricultural production.] A rezisztens burgonyafajták szerepe a fenntartható mezőgazdasági termelésben. In: XL. Georgikon Napok, Keszthely, A versenyképes magyar mezőgazdaság az évezred küszöbén, Proceedings II: Gergely L. (1998): [Resistance status in the host-parasite interactions of certain vegetable crops.] Rezisztenciaviszonyok egyes zöldségnövények gazda-parazita kapcsolataiban. Biológiai alapok a növénytermesztésben, országos konferencia (National Conference), Rezisztencia a növény- és környezetvédelem szolgálatában, Budapest, Gergely L. (1999): [Resistance studies in potato variety trials.] Rezisztenciavizsgálatok a burgonya fajtakísérletekben. (Potato Sector Forum) Burgonya Ágazati Fórum, PATE, Keszthely, Gergely L. (1999): [Assessment of foliar and tuber resistance of potato variety candidates to late blight (Phytophthora infestans [Mont.] de Bary)] Burgonya-fajtajelöltek lomb- és gumórezisztenciájának vizsgálata fitoftóra- (Phytophthora infestans) fertőzéssel szemben. In: 45. Növényvédelmi Tudományos Napok, Budapest, Proceedings, Gergely L. (2000): [Assessment of foliar and tuber resistance of potato variety candidates to late blight (Phytophthora infestans [Mont.] de Bary)] Burgonya-fajtajelöltek lomb- és gumórezisztenciájának vizsgálata fitoftóra- (Phytophthora infestans [Mont.] de Bary) fertőzéssel szemben. In: 46. Növényvédelmi Tudományos Napok, Budapest, Proceedings,

20 13. Gergely L. (2000): Foliar resistance of potato varieties to late blight. [Burgonyafajták lombozatának ellenállósága a burgonyavésszel szemben.] Meeting of EAPR-Eucarpia Section, Warsaw, Breeding research for resistance to pathogens and for quality traits, 3. (abstr.) 14. Gergely L. and Hertelendy P. (2000): [Results of resistance studies with potato variety candidates.] Burgonya-fajtajelöltek rezisztenciavizsgálatának eredményei, In: II. Nemzetközi Növényvédelmi Konferencia, Debrecen, Proceedings, Gergely L. (2001): [Foliar resistance of potato variety candidates to late blight.] Burgonya-fajtajelöltek lombozatának ellenállósága a burgonyavésszel szemben. In: 47. Növényvédelmi Tudományos Napok, Budapest, Proceedings, Gergely L. (2002): [Vulnerability of vertical resistance in controlling potato late blight.] A vertikális rezisztencia sebezhetősége a burgonyavész elleni védekezésben. In: 48. Növényvédelmi Tudományos Napok, Budapest, Proceedings, Gergely L. (2003): [Field resistance of registered potato varieties to late blight.] Államilag elismert burgonyafajták szántóföldi rezisztenciája a burgonyavésszel szemben. In: XIII. Keszthelyi Növényvédelmi Fórum 2003, Proceedings, 17. Other publications in Hungarian language 1. Fischl G. and Gergely L. (1990): [Gangrene of potato tubers in relation to production site and varieties.] A burgonya fómás gumókorhadása a termőhely és a fajták függvényében. Növénytermelés, 5: Lönhárd M., Proksza P. and Gergely L. (2002): [Durability and efficacy of the resistance of potatoes to viruses in the light of nationwide variety trials and Hungarian breeding experiences]. A burgonya vírusokkal szembeni ellenálló képességének állandósága és hatékonysága az országos fajtavizsgálatok és a hazai nemesítési tapasztalatok tükrében. Növénytermelés, 3: