Occurrence of Emphasis Type="Italic"Venturia ina

Authors: Sylwester Masny

Publish Date: 2016/07/19

Volume: 147, Issue: 2, Pages: 313-323

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Abstract

The aim of this study was to confirm the presence of races in populations of the fungus Venturia inaequalis that are able to overcome specific apple scab resistance genes within the major applegrowing areas of Poland The monitoring was conducted in six orchards located in the north centre and south of Poland The study involved the use of 16 differential genotypes for pathogenicity testing conducted under both greenhouse and orchard conditions In addition the occurrence of apple scab on 10 apple cultivars containing the Rvi6 gene was assessed in four organic orchards in central Poland Apple scab was found on their leaves for the first time in Poland in 2010 The use of differential genotypes containing specific resistance genes suggested that 10 apple scab resistance genes had been overcome by V inaequalis in the orchards monitored in this studyApple scab Venturia inaequalis Cooke Wint is one of the most serious diseases of apple trees in many applegrowing regions It causes damage to the leaves and fruits with negative consequences for fruit quality and yield Currently the most common way to protect apple trees against this disease is the application of fungicides MacHardy 1996 Rossi et al 2007 Holb 2009 In Poland during years which have had particularly favourable conditions for the development of apple scab up to 20 fungicide treatments were found to be necessary in order to avoid losses Meszka and Masny 2006 Masny 2015 It is possible to reduce the use of fungicides in orchards by cultivating less susceptible or resistant apple cultivarsStudies investigating the genetic background of resistance and the breeding of apple trees for resistance were first undertaken in the USA At the beginning of the last century an American fruit grower Charles S Crandall gathered a large collection of species and clones of wild apple trees and began an intensive study on crossbreeding them with cultivated apple cultivars Crandall 1926 He observed that clone 821 of the wild species Malus floribunda was completely resistant to apple scab In the USA in the 1940s two seedlings derived from this clone were used in the PRI breedingforresistance programme as part of a joint project between the Purdue Rutgers and Illinois universities Janick 2006 This work led to the selection of the ‘Prima’ cultivar This cultivar was the first bred by the PRI programme with a satisfactory fruit size and quality Dayton and Mowry 1970 In the second half of the twentieth century other countries had also begun to breed for apple scab resistance Kozlovskaya et al 2000 Sansavini et al 2002However studies have shown that the degree of resistance of some newly bred genotypes has changed over time resulting in the adaptation of V inaequalis and breaking down of existing resistance Williams and Brown 1968 Roberts and Crute 1994 Bénaouf and Parisi 2000 Some of the first studies to investigate physiological races confirmed that isolates of V inaequalis found in different regions of Canada had different virulences towards a particular apple cultivar or group of cultivars Julien and Spangelo 1957 Other studies conducted within the same period in the USA Shay and Williams 1956 Shay et al 1962 demonstrated that many clones derived from different species of Malus collected from farapart regions of the world Australia England Yugoslavia Germany Norway and the USA which were all infected with isolates of V inaequalis but showed different susceptibilities to the disease They allowed for the identification and characterisation of four physiological races of V inaequalis which infected specific apple genotypes and formed conidia According to Shay and Williams 1956 race 1 was commonly observed in the USA and many other countries race 2 was identified in South Dakota and race 3 was found in Nova Scotia Race 4 of V inaequalis was detected at Purdue University in the USA on scabresistant seedlings of R127407 A belonging to M pumila Williams and Kuć 1969 MacHardy 1996 Studies by Williams and Brown 1968 contributed to the detection and characterisation of race 5 of V inaequalis which was capable of infecting apple cultivars derived from Malus micromalus and Malus atrosanguinea which carried the Rvi5 Vm resistance gene Race 6 of V inaequalis was first described by Parisi et al 1993 and is distinguished from the other five races of V inaequalis by its capacity to cause sporulating spots of scab on certain apple hybrids carrying the Rvi6 gene The pathogenicity of this race was investigated by Parisi and Lespinasse 1996 on various apple clones In the 1990s race 7 of V inaequalis was detected in England and described by Roberts and Crute 1994 An isolate from a naturally infected tree of M floribunda also produced symptoms on M floribunda 821 and other cultivars with the Rvi6 gene while other cultivars with the same gene were not infected The discovery of race 8 of V inaequalis was the result of wider research into the interaction between V inaequalis and apple trees and was derived from a population of V inaequalis in New Zealand Bus et al 2005 The first reports the interactions in the pathogenplant system concerned rust and flax and were related to the theory of the complementarity of genes the geneforgene theory proposed by Flor in the 1940s Flor 1971 According to this concept each resistance gene R of the host plant has a counterpart in the genome of the pathogen which determines its avirulence In the case of apple the presence of the Rvi gene which encodes resistance to scab makes it possible to recognise an attack by the fungus V inaequalis due to the presence of the specific avirulence gene AvrRvi making it unable to infect this apple genotype The presence of resistant apple genotypes in a given ecosystem environment exerts selection pressures on the population of V inaequalis and the resulting adaptations eg mutations of the pathogen helps to avoid its recognition when infects an apple tree leading to the emergence of new physiological racesThe definition of a physiological race as proposed by Gessler et al 2006 refers to isolates capable of infecting and producing spores on a specific host After detecting and describing race 6 Parisi et al 1993 added host h6 which carries the Rvi6 Vf gene to the five existing differential hosts This resulted in the identification of the six physiological races of V inaequalis The set of differential hosts has since been extended to include one for each of the scab resistance genes described in the new nomenclature system Bus et al 2009 Over the following years the set of differentiated hosts was modified and broadened by Bus et al 2011 The host h11 represented by M baccata jackii was replaced by the genotype A722–7 M baccata jackii × ‘Starking’ and four hosts were added to the set h14 Dülmener Rosenapfel h15 GMAL2473 h16 MIS op 93051 G07–098 and h17 ‘Antonovka’ APF22 This new system better reflected the emerging complexities associated with the different combinations of genes both in the host and the pathogen Recently the set of differential Malus hosts was complemented by a set of V inaequalis reference isolates characterised by their virulence Caffier et al 2015 Patocchi et al 2009 started the VINQUEST initiative wwwvinquestch to monitor apple scab strains able to overcome a set of apple scab resistance genes which are currently used or could be used in apple breedingThe aim of this study was to assess the virulence diversity of V inaequalis populations in selected orchards and the identification of physiological races in the major fruitgrowing areas of Poland The differential hosts used in our monitoring were based on the set of Malus hosts recommended by the VINQUEST initiative

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