Wolfgang Jarausch

Characteristics of the Spread of Apple Proliferation by Its Vector Cacopsylla picta

The distribution and natural phytoplasma infection of Cacopsylla picta were investigated during a long-term field survey between 2002 and 2009 in commercial and abandoned apple proliferation-infected orchards throughout Germany, northern Switzerland, and eastern France. Comparable population dynamics were described for the different sites whereas considerable variations in the absolute population densities were observed among the years. Individual polymerase chain reaction (PCR) testing revealed, for each year, a rather stable natural infection rate with ?Candidatus Phytoplasma mali? of ?10% for overwintered adults of C. picta. Both genders were equally highly infected although more females were caught. The overall male/female ratio was 1:1.5. No direct correlation was found between the infection status of the orchard and the infection rate of overwintered C. picta. No influence of agricultural practices was seen. However, a relationship between the incidence of the disease and the vector population density became evident on a regional scale. Successful transmission of ?Ca. P. mali? occurred each year with overwintered individuals as well as with new adults. The transmission efficiency varied among the years within 8 to 45% for overwintered adults and 2 to 20% for individuals of the new generation. The load of single C. picta with ?Ca. P. mali? was determined by quantitative real-time PCR. High phytoplasma titers were measured in overwintered adults already at their first appearance in the orchards after remigration from their overwintering hosts. Thus, the data indicate the transmission of the disease on a regional scale by remigrant adults of C. picta and at a local scale within the same season by emigrant adults which developed on infected plants.

A permanent rearing system for Cacopsylla pruni, the vector of ‘Candidatus Phytoplasma prunorum’

Cacopsylla pruni (Scopoli) (Hemiptera: Psyllidae) has gained great importance in the past decade because it is the only described vector of ‘Candidatus Phytoplasma prunorum’, the causal agent of European stone fruit yellows (ESFY) disease in Europe (Carraro et al., 1998; Jarausch et al., 2001, 2008). ESFY disease causes important losses in Prunus spp., such as apricot, Japanese plum, and peach, in the Mediterranean Basin and Central Europe (Marcone et al., 2010). Cacopsylla pruni is a European and Central Asian species (Lauterer, 1999) that is strictly oligophagous on Prunus spp., completes one generation per year, and overwinters as an adult on shelter plants, usually conifers (Ossiannilsson, 1992; Hodkinson, 2009). Very recently, C. pruni was shown to be a complex of two genetic groups that have yet to be distinguished morphologically (Peccoud et al., 2013). Nevertheless, the biology of both genetic groups is similar. At the end of winter or early spring, C. pruni adults migrate from the overwintering plants back to their host plants to mate, reproduce, and die (Jarausch et al., 2007; Jarausch & Jarausch, 2010). Eggs hatch and develop until the beginning of July when new-generation adults disperse to shelter plants for oversummering and overwintering (Labonne & Lichou, 2004; Cermak & Lauterer, 2008). Several efforts have been undertaken to follow the whole life cycle of this univoltine psyllid species under natural conditions. Using branch cages, Th ebaud et al. (2009) could show that C. pruni was not able to overwinter on Prunus but on conifers in higher altitudes (between 550 and 1 260 m above sea level). Almost no overwintering C. pruni were found on conifers in low altitudes next to the areas where the Prunus host plants grow. Our aim was to establish a permanent breeding colony under controlled conditions in the laboratory, which is useful for breeding experiments, studies of particular hibernation features, and detailed analysis of characteristics involved in pathogen transmission. Only recently, a permanent culture of Cacopsylla picta (Foerster), the main vector of ‘Ca. Phytoplasma mali’, was successfully established in our laboratory mimicking the whole life cycle of this univoltine psyllid species under experimental conditions (Jarausch & Jarausch, 2014). The same approach was adopted for rearing C. pruni despite some species-specific particularities. Hence, we achieved for the first time a permanent culture of C. pruni under controlled conditions over a period of more than 2 years. These experimental data furthermore prove that C. pruni is univoltine and that conifers only act as shelter plants for overwintering and no reproduction occurs on these plants.

Inheritance of apple proliferation resistance by parental lines of apomictic Malus sieboldii as donor of resistance in rootstock breeding

To study inheritance of Malus sieboldii-derived apple proliferation resistance, 14 cross combinations were performed with the tetraploid apomictic M. sieboldii and first and second generation parental lines as donor of resistance and Malus x domestica scion cultivars and apple rootstocks as donor of pomological traits. In the progeny examined mainly three classes were present consisting of mother-like plants with the allele composition of the maternal apomict (ML), hybrids based on fertilization of an unreduced egg cell (hybrid I), and fully recombinant plants (hybrid II). Two-year screening of inoculated plants in the nursery revealed that progeny classes ML and H I responded similarly to infection and that about half of the progeny showed satisfactory resistance. No appropriate resistance was identified in progeny class H II. This might be due to the fact that in fully recombinant offspring M. sieboldii haplotypes have been reduced from 4n to 1-2n or were entirely lost. Following nursery-growing, promising trees were evaluated for six more years in the orchard. Nearly all of them showed satisfactory resistance but were mostly less productive and more vigorous than trees on clonal standard rootstock M9. However, mainly among the offspring of progeny 4608 × M9, resistant genotypes were identified showing pomological properties similar to M9.