Articles
PATHOGENICITY OF A STRAIN OF VENTURIA INAEQUALIS RACE 6 ON APPLE CLONES (MALUS SPP).
Article number
484_75
Pages
443 – 448
Language
Abstract
About 40 apple cultivars possessing the gene Vf derived from Malus floribunda 821 are resistant to the five races of Venturia inaequalis previously described.
Race 6, recently identified, is able to overcome the resistance of some of these cultivars.
Thirty-five apple clones were tested for their susceptibility to one strain of race 1 and to one strain of race 6. These clones represented a large sample of the resistance sources used by apple breeders throughout the world (selected clones and their ancestors). Race 6 induced symptoms on nearly all the Vf selections but also on ‘Nova Easygro’ (reportedly Vr), on a selection carrying Va, and on ‘Jonsib’. On the other hand, race 6 did not attack susceptible cultivars such as ‘Granny Smith’, three selections from different polygenic sources of resistance, Malus baccata jackii (Vbj), R 127.40.7A (Vr) and PI 172623 (Va). The pedigrees of three hybrids selected for their Vf resistance were analysed.
All the parents resistant to race 1 involved in these pedigrees were susceptible to race 6, except the ancestor M. floribunda 821, and one of its F2 selected seedlings (F2 26829-2-2). The other F2 (F2 26830-2) was susceptible.
These results confirmed the hypothesis of a complex genetic background for resistance to Venturia inaequalis in wild species, which has been partly lost during the breeding process.
The variability of the resistance genes introgressed in recent selections could be lower than believed, and reduced to the Vf gene in most cases.
Apple breeders should focus increasingly on finding polygenic sources of resistance in order to obtain durable scab resistance.
Race 6, recently identified, is able to overcome the resistance of some of these cultivars.
Thirty-five apple clones were tested for their susceptibility to one strain of race 1 and to one strain of race 6. These clones represented a large sample of the resistance sources used by apple breeders throughout the world (selected clones and their ancestors). Race 6 induced symptoms on nearly all the Vf selections but also on ‘Nova Easygro’ (reportedly Vr), on a selection carrying Va, and on ‘Jonsib’. On the other hand, race 6 did not attack susceptible cultivars such as ‘Granny Smith’, three selections from different polygenic sources of resistance, Malus baccata jackii (Vbj), R 127.40.7A (Vr) and PI 172623 (Va). The pedigrees of three hybrids selected for their Vf resistance were analysed.
All the parents resistant to race 1 involved in these pedigrees were susceptible to race 6, except the ancestor M. floribunda 821, and one of its F2 selected seedlings (F2 26829-2-2). The other F2 (F2 26830-2) was susceptible.
These results confirmed the hypothesis of a complex genetic background for resistance to Venturia inaequalis in wild species, which has been partly lost during the breeding process.
The variability of the resistance genes introgressed in recent selections could be lower than believed, and reduced to the Vf gene in most cases.
Apple breeders should focus increasingly on finding polygenic sources of resistance in order to obtain durable scab resistance.
Authors
L. Parisi, Y. Lespinasse
Keywords
apple scab, Malus baccata, M. floribunda, M. pumila
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