Which genes of apple scab resistance keep control against pathogen V. inaequalis in future? R. Vávra 1, T. Litschmann 2, V.

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1 162 Short Contributions Which genes of apple scab resistance keep control against pathogen V. inaequalis in future? R. Vávra 1, T. Litschmann 2, V. Falta 3 Abstract Attacks of apple scab on the collection of genotypes carrying particular scab resistance genes (Bus et al., 2009) in locality Holovousy (the eastern Czechia) from 2011 to 2013 was the objective of this study. Evaluations were done after primary infections on leaves on genotypes planted in spring 2010 into randomized blocks with five repetitions. Severity was assessed according to the scale 1 9 (Lateur and Populer, 1994). The genotype H(1) Golden Delicious was infected by pathogen V. inaequalis with high intensity in the grade from 6.8 to 9.0. Attacks on the next scab sensitive genotype H(0) Gala were also high in the grade from 4.8 to 9.0. Only genotypes H(5) 9-AR2T196, H(7) M. floribunda 821, H(11) Hansen s baccata #2 and H(15) GMAL 2473 remained healthy without any damage. Remaining genotypes were infected. Genotypes H(4) TSR33T239, H(14) Dülmener Rosenapfel, H(9) J34, H(2) TSR34T15 and H(13) Durello di Forli were infected in very low rate in maximum grade 1.6; 1.7; 2.0; 2.4 and 2.8 respectively. Infections on genotype H(3) Q71 were recorded in maximum grade 5.8. Severe symptoms of the disease were recorded on the genotype H(8) B 45 in maximum grade 6.2. Genotype H(6) Priscilla were infected in the last year of evaluation in grade 2.3 while H(7) M. floribunda 821 was not attacked indicating occurrence of race no. 6 of pathogen V. inaequalis in this location. Spreading of apple scab races overcoming genes Rvi6 that is the most involved genes in scab resistant cultivars reduce their growing. Cultivars carrying scab resistance genes that are not yet attacked by scab and cultivars with polygenic resistance are promise for breeders and growers. Keywords: Venturia inaequalis, apple scab, host genotype, scab races, apple scab resistance Introduction Cultivation of apple scab resistant cultivars was considered in past time to be a solution for control of the pathogen Venturia inaequalis (Cooke) Wint. Plant breeding programs in numerous countries have attempted to develop resistant varieties to eliminate the most important disease in apple orchards caused by pathogen V. inaequalis. But occurrence of apple scab races overcoming genes of resistance complicated this assumption. The occurrence of apple scab on cultivars and genotypes carrying different resistance genes are one of actual problems of apple growing. Several V. inaequalis races were defined to be able infect different cultivars and species of apples used by plant breeders as sources of resistance. Resistance to apple scab originating from Malus floribunda clone 821 was the most widely form of resistance used in apple breeding programs. Two races virulent on the genotypes carrying most-used type of resistance given by Vf (Rvi6) gene were founded up recent time. The race 6 described at first time in Germany in 1993 (Parisi, 1993) can infect some progenies of M. floribunda 821. The race 7 described at first time in England in 1994 (Roberts, 1994) can infect all progenies of M. floribunda 821. According 1 Research and Breeding Institute of Pomology Holovousy Ltd., Holovousy 1, Hořice, Czech Republic, vavra.vsuo@seznam.cz 2 AMET group, Žižkovská Velké Bílovice, Czech Republic 3 Crop Research Institute, Drnovská 507/73, Praha 6 - Ruzyně

2 Short Contributions 163 the new nomenclature of resistant genes was the Vf gene named as Rvi6 (Bus et al., 2009). The first appearance of apple scab on resistant Rvi6 cultivars was recorded in four locations in the Czech Republic in the year 2006 (Blažek and Vávra, 2006). From this time up to the year 2013 lot of locations with occurrence of apple scab on resistant cultivars carrying Rvi6 gene were recorded. The spreading of scab on genotypes carrying different scab genes of resistance gives evidence of high selection pressure elicited by host genotypes. This results in spreading virulent populations of pathogen V. inaequalis able to overcome genotypes carrying genes of resistance with annually higher intensity. Material and Methods Symptoms of the pathogen V. inaequalis were evaluated on the collection of genotypes carrying different genes of resistance. Each genotype contains single gene responsible for scab resistance. The overview of the evaluated genotypes and genes of resistance is given in the table 1. Genotypes were planted in the spring 2010 on rootstock M.9 in five randomized blocks. The genotype A723-6 was planted in the year 2011, the genotype Malus baccata jackii was not evaluated due to inaccessible material. The cultivar Gala was used as the genotype without the gene of resistance. Symptoms were recorded after primary infections in middle of June on leaves using scale 1 9 (0: none evaluated, 1: 0 %, 2: 1-5 %, 3: inter-grade, 4: ±25 %, 5: inter-grade, 6: ± 50 %, 7: 75 %, 8: > 90 %, 9: over 90 % leaves infected by scab), according Lateur and Populer, Table 1: Evaluated genotypes carrying different genes of resistance Genes of resisteance Host genotyp former name new name H(0) Gala 0 0 H(1) Golden Delicious Vg Rvi1 H(2) TSR34T15 Vh2 Rvi2 H(3) Q71 (Geneva x Braeburn) Vh3 Rvi3 H(4) TSR33T239 Vh4 Rvi4 H(5) 9-AR2T196 Vm Rvi5 H(6) Priscilla Vf Rvi6 H(7) F1 M. floribunda 821 Vf, Vfh Rvi6, Rvi7 H(8) B45 (Pacific Beauty x M. sieversii GMAL4302-X8) Vh8 Rvi8 H(9) J34 (Gala x Dolgo) Vdg Rvi9 H(10) A (Wocester x PI172623) Va Rvi10 H(11) Hansen s baccata #2 Vb Rvi11 H(12) Malus baccata jackii Vbj Rvi12 H(13) Durello di Forlì Vd Rvi13 H(14) Dülmener Rosenapfel Vdr1 Rvi14 H(15) GMAL 2473 Vr2 Rvi15

3 164 Short Contributions Results Attacks of scab on the collection of genotypes carrying particular scab resistance genes are given in the table 2. The numbers of infections in the period from April till June and their intensity according data obtained from the meteo station (Meteo Uni, AMET) in locality Holovousy are demonstrated in the figure 1. The year 2010 was characteristic by high number of infection of light intensity. The weather conditions in this year were favourable for development of scab. In the year 2011 was recorded the lowest number of scab infection. Weather conditions in years 2012 and 2013 were favourable for the development of apple scab in locality Holovousy. In the year 2013 the number of strong infections was higher than middle and low infections resulting in very high occurrence of scab on host genotypes. The highest attacks were recorded on scab sensitive genotypes Gala and Golden Delicious with attacks evaluated in grades 4.8 and 6.8 in the first years of evaluations and in the grade 9 evaluated in the year Only genotypes 9-AR2T196, M. floribunda 821, Hansen s baccata #2 and GMAL 2473 were evaluated without any attacks. On the genotypes Q71 and B45 were recorded attacks during the all period of evaluation from the year 2010 till All remaining genotypes were attacked by the pathogen V. inaequalis in different grades only in particular years (figure 2). Table 2: Scab attacks on genotypes in years (grade 1 9) Year of evaluation Host genotyp Mean H(0) Gala H(1) Golden Delicious H(2) TSR34T H(3) Q H(4) TSR33T H(5) 9-AR2T H(6) Priscilla H(7) M. floribunda H(8) B H(9) J H(10) A H(11) Hansen s baccata # H(12) Malus baccata jackii H(13) Durello di Forlì H(14) Dülmener Rosen H(15) GMAL Mean

4 Short Contributions low middle high Figure 1: Number of scab infections and their intensity Figure 2: Scab attacks on genotypes in years (grade 1 9) Discussion From results is evident that the pathogen V. inaequalis is infecting host genotypes with annually higher intensity. Scab sensitivee cultivars Gala and Golden Delicious suffered by scab in the year 2010 with high scab infection pressure lower intensity than in the year 2013 when those cultivars were evaluated in the highest grade and even defoliation was observed. Also on genotypes Q71, B45 and A723-6 weree observed higher apple scab attacks in last year than in the first year of observation. This is proof that the highh selection pressure from the host plants exists. The development of highly virulent populations of V. inaequalis pathogen continues in the orchards. Those populations are able to infect host plants with annually higher intensity. Spreading of apple scab races overcoming genes Rvi6 that is the most involved gene in scab resistant cultivars can reduce their growing. Cultivars carrying scab resistance genes that are not yet attacked by scab and cultivars with polygenic resistance are promise for breeders and growers. The question which genes will work against apple scab in future is discussed. Breeders have to involve genotypes with polygenic resistance into apple scab resistant cultivars. Breeding with monogenic resistant genotypes seems to be insufficient even with accumulations of those genes in one cultivar due to high infection pressure from the host side.

5 166 Short Contributions Acknowledgements This work is financed by the Technology Agency of the Czech Republic in the framework of the project TA References: Bus V. G. M., Rikkerink E. H. A., Aldwinckle H. S., Caffier V., Durel C. E., Gardiner S., Gessler C., Groenwold R., Laurens F., Le Cam B., Luby J., Mac Hardy W., Meulenbroek B., Kellerhals M., Parisi L., Patocchi A., Plummer K., Schouten H. J., Tartarini S., van de Weg W. E A proposal for the nomenclature of Venturia inaequalis races. Acta Horticulturae 814: Blažek, J., Vávra, R Scab attacks on apple cultivars with genes of resistance in the Czech Republic. Hort. Science. 12: Lateur M., Populer C Screening fruit tree genetic resources in Belgium for disease resistance and other desirable characters. Euphytica 77: Parisi L., Lespinasse Y., Guillaumes J., Kruger J A new race of Venturia inaequalis virulent to apples with resistance due to the Vf gene. Phytopathology, 83: Roberts, A. L., Crute, I. R Apple scab resistance from Malus floribunda 821 (Vf) is rendered ineffective by isolates of Venturia inaequalis from Malus floribunda. Norw. J. Agric. Sci. 17: