Selecting Verticillium Wilt-resistant Olive Genotypes by Classical Breeding

Verticillium wilt is the most destructive disease in olive (Olea europaea) production in the Mediterranean basin as well as in countries where olive is also being planted nowadays. The disease is caused by the fungus Verticillium dahliae. As other soilborne pathogens, control of the disease is very difficult to achieve and the use of resistant cultivars or rootstocks is one of the most efficient measures. However, nowadays the number of cultivars identified as resistant to the disease is very low. Due to these reasons, obtaining olive material resistant to Verticillium wilt is nowadays a major objective in the olive breeding program of the University of Cordoba, Spain. A screening procedure was developed, using the root dip inoculation technique with a highly virulent isolate of the fungus. This protocol has allowed evaluating the resistance to V. dahliae in several thousands of olive seedlings in controlled conditions. About 10 percent of the evaluated genotypes were highly resistant to the disease. In a second phase, the selected resistant genotypes are grown under forced conditions and after 6 months are planted in fields highly infested by the pathogen, where their field resistance to the disease, together with other agronomical traits are tested during 3-4 years. Then, the desired genotypes are replicated for further field trials. The genotypes evaluated following this procedures have been obtained from crosses and open pollination of olive cultivars, wild olives and other Olea species. According to the results, cultivar ‘Frantoio’ and a few wild olive accessions usually yielded more than 40% of resistant genotypes and therefore are the best sources to breed for Verticillium wilt resistance. However, a low proportion of resistant genotypes was also obtained in many crosses, even in those obtained from susceptible parents. Results about inheritance of resistance to Verticillium wilt in olive are also presented.