Epidemiology of Verticillium wilt of olive trees in Jordan

This study was conducted during 1995-1996 growing season, to determine the factors affecting the outbreak and spread of Verticillium wilt of olive in two locations: in Al-Baqu'a, representing the upland region, and in Al-Dhulial, repesenting the desert plains in Jordan. The cultivar used was improved Nabali. The main casual agent of olive wilt in Jordan -Verticillium dahliae. This species is characterized by forming microsclerotia and mycelium as resting structures fo the pathogen. The primary wilting symptoms appeared in both locations in mid February 1995. Disease incidence and severity gradually increased until mid June where the temperature was suitable for disease development. No further disease developement occurred at high temperatures during the summer months, with temperature range between 28 - 36 deg C. Disease incidence and severity in Al-Dhulial area was higher than that in Al-Baqu'a due lo increase of irrigation (two times weekly) in comparison with the uplands (Al-Baqu'a) which were supplementally irrigated once every 10 days (during the summer season). However, the young trees in Al-Baqu'a seemed to be more sensitive to symptoms expression. Nurseries played a major role in distributing infested olive seedlings to new areas as proved by isolation of the fungus from these nurseries. Nine nurseries were verified having the wilting pathogen among thirty nurseries surveyed. Fallen leaves were found to harbor the fungus and Verticillium dahliae was isolated from these leaves indicating its importance as a source of inoculum and the build up of pathogen population in the soil. Verticillium dahliae isolated from olive branches had the ability to infect wide range of weeds, vegetables and legumes, under greenhouse conditions, these hosts could be considered as alternative hosts to the fungus. Isolation of fungus from olive trees was very successful during spring and winter of 1995 growing season, while low percent of fungal isolation during summer to early fall were noticed, as indicated by fungal isolation from branches. Survey results showed that Verticillium wilt of olive was present in all locations examined wherever olive trees were grown under irrigation, while no wilting signs were noticed in the examined areas under rainfed conditions. The disease severity was greatest in newly established orchards compared to old once. The results of fungal growth under controlled laboratory temperatures showed that Verticillium fungus developed well at 15 deg C and 25 deg C, but was the most favorable at 20 deg C, while fungal growth was greatly restricted at 10 deg C and 30 deg C. Tested olive cultivars showed different reactions to infection by Verticillium dahliae. K18 cultivar was the only tolerant one and showed no wilting symptoms. Rasie, Nabali and Grossa slightly responded to infection, while Turkey was the most sensitive cultivar. The fungus was easily reisolated from all examined cultivars three months after initiation of the experiment