Sanitation of olive plants infected by Verticillium dahliae using heat treatments

Abstract

Verticillium wilt, caused by the fungus Verticillium dahliae, is currently the most important disease affecting olive in the Mediterranean basin. There are no effective treatments for controlling this disease. The use of infected nursery stocks has largely contributed to the spread of the pathogen, and therefore the development of treatments to preventively sanitize the propagation stock is critical in the nursery industry. This study describes novel techniques to achieve this aim. The effects of several temperature-exposure time combinations were evaluated: (i) the survival of pathogen on culture medium (PDA); (ii) the pathogen viability on infected shoots and plants; (iii) the vegetative growth of plants of several cultivars; and (iv) the rooting ability of cuttings. The colonies of the pathogen growing in PDA were killed after 8 h and 60 min of exposure at 40 and 47°C, respectively. Temperatures ≥42°C for at least 2 h were lethal for the pathogen infecting the shoots. Likewise, moist hot air treatments at 42-44°C for 6-12 h eradicated the pathogen, without compromising the viability of the plants. Five olive cultivars were also evaluated and classified according to their thermotolerance as follows: sensitive (Chiquitita), moderately sensitive (Koroneiki, Frantoio and Picual) and heat tolerant (Arbequina). However, the optimized sanitation methods were applicable to all of the cultivars. Finally, heat treatments were applied to unrooted cuttings, which severely affected their rooting ability. Thus, this study developed a hot air treatment to produce V. dahliae-free olive nursery plants.