Materials and Methods

Two hundred fifty F₁ hybrids were obtained from a first cross between ‘Rubira’® and ‘Pamirskij 5’. F₂ families were produced by self-pollination of five F₁ hybrid plants and a pseudo-test cross (TC₁) population was generated by crossing one F₁ hybrid with ‘Big Top’® (clone 5811), a nectarine cultivar susceptible to powdery mildew with green foliage. Selfed progenies (S₁) of the parental cultivars were created at each step of the genetic study. Seeds from all crosses were harvested at full maturity, extracted, and stratified. After radicle emergence, seeds were planted individually in 1-L plastic pots containing a commercial horticultural mix and randomized in a greenhouse. Temperatures were maintained at 23 ± 5 °C and seedlings were watered daily ad libitum with a commercial solution containing micronutrients.

Plants were evaluated for resistance to the green peach aphid (Pascal et al., 2002), scored for red or green leaf color, and pruned before artificial inoculation by S. pannosa once they developed succulent shoots. Initially, the inoculum source derived from the populations endemic to Avignon (France) was maintained on potted susceptible peach seedlings. Inoculations were carried out by blowing or shaking fungal spores from these heavily infected plants onto young test progenies. In addition, heavily infected susceptible plants were grown adjacent to the progenies being tested as a continuing source of inoculum. Plants were evaluated three times over a 4-week period using a well-tried linear rating scale consisting of the following: 0 = no visible symptoms to 4 = white spots completely covering leaves and young stems (Dirlewanger et al., 1996; Foulongne et al., 2003a, 2003b; Pascal et al., 1998). Plants scored 0 and 1 were considered resistant. Segregation ratios were analyzed using standard χ² procedures. Maximum likelihood tables (Allard, 1956) were used to calculate recombination rates for close linkages. The recombination rates were converted to map distances in centiMorgans (cM) using the formula of Kosambi (1944).

Results and Discussion

All S₁ progenies of ‘Rubira’® were uniformly and highly susceptible to infection by powdery mildew in both experiments. Conversely, all S₁ progenies of ‘Pamirskij 5’ as well as all F₁ plants were uniformly and highly resistant (Table 1). These results show that ‘Pamirskij 5’ and ‘Rubira’® are homozygous for resistance and susceptibility to powdery mildew, respectively. The high level of resistance expressed by all F₁ plants indicates that resistance is dominant to susceptibility. Segregation data from the F₂ and TC₁ progenies were tested for goodness-of-fit to expected ratios using χ² analysis. The results of the χ² analysis indicate that the F₂ and TC₁ progenies fit 3:1 and 1:1 ratios, respectively. The data are consistent with powdery mildew resistance in ‘Pamirskij 5’ being controlled by a single homozygous dominant gene that we propose to call Vr2.

Table 1. Segregation ratios for reaction to peach powdery mildew and leaf color of parental selfs, F₁, F₂, and test cross progenies derived from the resistant peach cultivar Pamirskij 5 and the susceptible peach cultivar with red foliage, ‘Rubira’.

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The use of single dominant genes for the development of resistant cultivars is highly controversial because of resistance durability aspects. However, in a perennial plant such as peach, little effort has been devoted to pest improvement and few sources of resistance are available. The use at short term by peach breeders of Vr2 could constitute an essential first step to the development of more stable and long-lasting resistance. Furthermore, this use would be fully justified by the high level of resistance to powdery mildew of ‘Pamirskij 5’, which is clearly more resistant than most current conventional peach cultivars and than resistant cultivars developed by Dabov (Arregui et al., 1999). Nevertheless, to better characterize the resistance genes originating from different genetic backgrounds, it would be necessary to test for allelism the resistance originating from ‘Pamirskij 5’ and ‘Malo Konare’.

Two linkage tests were conducted involving powdery mildew resistance from ‘Pamirskij 5’ and green peach aphid resistance and the red leaf traits from ‘Rubira’®. χ² analysis of segregation data were consistent with independent assortment of powdery mildew resistance and green peach aphid resistance (data not shown). Conversely, distorted segregation ratios were observed in the joint distributions of powdery mildew resistance and the leaf color character (Table 2). In F₂ populations, there was an excess of parental types (susceptible red and resistant green) and a large shortage of recombinant types (resistant red and susceptible green). Recombinants with susceptibility to powdery mildew and green foliage phenotype were not observed in ≈940 F₂ plants. Similarly, no recombinant types were observed in the TC₁ population. From these results we could postulate that the resistant red leaf progeny were heterozygous at both loci (Vr2vr2Grgr) analogous to the F₁ hybrids. These results clearly indicate a very strong linkage in the repulsion phase (χ² = 105.3, P = 0) between the Vr2 resistant allele and the red leaf color allele in ‘Rubira’®, which constitutes a new case of linkage in peach.

Table 2. Evidence for repulsion phase linkage between the genes for resistance to peach powdery mildew and leaf color in peach.

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On the basis of pooled F₂ and TC₁ data (Table 2), the recombination rate between these two genes was calculated as equal to 3.26% and 0%, respectively. The genetic distance converted from the F₂ recombination rate was 0 ± 3.26 cM. Based on TC₁ data, the recombination rate was equal to 0 ± 0.00 cM.

The linkage between Gr and Vr2 could be used for marker-assisted selection in the introduction of the ‘Pamirskij 5’ resistance gene to peach powdery mildew in fruit cultivars. The Gr gene controlling red leaf color has been mapped to linkage group 6 both in a genetic map derived from an interspecific cross with GN22 (Dirlewanger et al., 2004) and another from an intraspecific cross between Japanese peach cultivars Akame and Juseitou (Yamamoto et al., 2005). This would suggest starting the search for linked molecular markers on linkage group 6 and would be consistent with the fact that in P. davidiana, the quantitative trait locus (QTL) with the strongest effect for powdery mildew resistance, was on this linkage group (Foulongne et al., 2003a, 2003b). Vr2 would thus be located on a linkage group distinct from the major quantitative trait locus (QTL) of resistance to peach powdery mildew from P. ferganensis, which mapped on linkage group 7 (Dettori et al., 2001; Verde et al., 2002) and is associated with the Ee foliar gland locus. The diversity of genomic loci implicated in powdery mildew resistance in peach and related species should make possible the pyramiding of different resistant genes to build more durable resistance.