BREEDING ROOTSTOCKS FOR TREE FRUIT CROPS

Identification of problems and prioritising breeding objectives based on those problems are essential first steps in a rootstock improvement program. For all tree fruits, incorporating resistances to critical d iseases and pests will facilitate fruit production in a social environ ment demanding reduction in pesticide usage. Diseases caused by variou s Phytophthora species are important and can be catastrophic for all m ajor tree fruit crops; breeding for resistance to Phytophthora has gen erally been successful. Very large initial seedling populations are re quired to permit suitably rigorous early screening; the breeding team should anticipate odds of 1:10(4) to 1:10(6) that any given seedling w ill be commercially successful. In preliminary orchard tests, positive selection for dwarfing and precocity induction can be made by the fif th year. More rigorous second tests should be made with a number of co mmercial varieties in several locations. Although almost all rootstock improvement programs now rely on conventional breeding methods, throu gh application of genetic engineering, the Malling 26 apple (Malus dom estica) rootstock has been successfully transformed from being highly susceptible to fire blight to being moderately resistant.