Isolation, identification and molecular characterization of physiological races of apple powdery mildew (Podosphaera leucotricha)

Abstract

The development of apple varieties displaying durable resistance against powdery mildew is one of the major aims in apple breeding programmes worldwide. For a reliable judgment of the resistance of different Malus genotypes, an extended knowledge about the virulence of the pathogen is necessary. To prove the existence of physiological races of Podosphaera leucotricha , 31 monoconidial isolates of the obligate biotrophic fungus representing five locations within Europe have been established and maintained over a period of 3 -4 years. The isolates were maintained on in vitro shoots of the highly susceptible apple cv. Gibb's Golden Gage. An AFLP-based DNA fingerprinting protocol was developed and, using 54 stably reproducible AFLP markers, a dendrogram revealed genetic variability among different isolates of P. leucotricha . Although the molecular characterization of the isolates showed an overall low level of genetic variability, the high phenotypic diversity among European isolates suggest that sexual reproduction may also be involved in the disease cycle of the pathogen in Europe. Phytopathological tests using detached leaves of a collection of 36 Malus genotypes allowed the differentiation of five selected isolates by their virulence patterns. A high level of diversity in terms of virulence was obtained in P. leucotricha. From the present study, based on apple breeding germplasm, cultivars and Malus species, it can be concluded that physiological races of P. leucotricha do indeed exist in Europe.

Development of apple powdery mildew on sources of resistance to Podosphaera leucotricha, exposed to an inoculum virulent against the major resistance gene Pl‐2

“…It is, however, not known what might occur under higher selection pressure. The existence of virulent isolates has previously been demonstrated for MIS (Lespinasse 1983), and suspected for ÔWhite AngelÕ and M. hupehensis (Lespinasse 1989, Urbanietz andDunemann 2005). As a consequence, a strategy based on pyramiding of major resistance genes (Evans 1997, Kellerhals et al 2004) should be treated with caution.…”

Mapping of the apple powdery mildew resistance gene Pl1 and its genetic association with an NBS‐LRR candidate resistance gene

“…Podosphaera leucotricha is an obligate parasite and does not grow on artificial media. The existence of races has been demonstrated (Caffier and Laurens 2005, Urbanietz and Dunemann 2005), but in contrast to apple scab ( Venturia inaequalis ), race specific pathogenicity tests are not easily to apply to larger genetic mapping experiments. Evaluation of powdery mildew resistance by using a natural inoculum is strongly influenced by environmental conditions and further factors.…”

“…This may be attributed to the presence of different races of the mildew fungus, the higher inoculum dose in the greenhouse, the physiological status of the apple trees or the changing climatic conditions being more favourable in the greenhouse. Regarding the race spectrum, it is probable from the molecular studies on the pathogen (Urbanietz and Dunemann 2005) that in the greenhouse experiment only a single or a few genetically similar races were present, whereas outside in the field a more complex race spectrum occurred. To consider this situation, the greenhouse and field scoring data were separated and separate linkage maps were constructed which finally proved to be comparable regarding the location of the Pl1 locus.…”

Paecilomyces farinosus destroys powdery mildew colonies in detached leaf cultures but not on whole plants

“…For various research purposes, many laboratories maintain isolates of different powdery mildew fungi on detached leaves or leaf segments of cereals (Brown and Wolfe, 1990;Wyand and Brown, 2003), cucurbits (Bardin et al, 1997;Nicot et al, 2002;Shishkoff and McGrath, 2002;Romero et al, 2003), tomato (Kiss et al, 2001), tobacco (Szentiva´nyi and Kiss, 2003), apple (Scheewe and Ketzel, 1994;Urbanietz and Dunemann, 2005), and other crops. In general, the leaves of the host plant are surface-sterilized, placed in small plates or tubes on artificial media, for example on benzimidazole agar or on mannitol sucrose agar (MSA), then the leaves are infected with conidia of a given powdery mildew isolate under near sterile conditions and kept for 2-6 weeks at 15-20°C under artificial illumination.…”