Verticillium wilt resulting from infection by Verticillium dahliae is one of the most devastating soilborne fungi of the olive tree (Olea europaea L.) worldwide. The pathogen infects a wide variety of plants and can survive in the soil for many years, and chemicals cannot control it. Therefore, sustainable disease management strategies are suggested, with the exploitation of host resistance as the most predominant control measure in practice. In addition, disease risk assessment in commonly used plant genotypes is a prominent issue. In this respect, nine commercially grown Greek olive varieties (‘Amfissis’, ‘Atsiholou’, ‘Chalkidikis’, ‘Koroneiki’, ‘Kothreiki’, ‘Koutsourelia’, ‘Mastoidis’, ‘Megaritiki’, and ‘Tragolia’) and one variety of international interest (‘Picual’) were comparatively evaluated for their resistance to V. dahliae. The roots of young plants were immersed in a concentrated conidial suspension in order to perform an artificial inoculation. We evaluated disease reactions in a 140-day assessment period based on external symptoms (disease severity, disease incidence, and mortality) and calculated the relative areas under disease progress curves (relative AUDPC). The process of qPCR was used to evaluate V. dahliae DNA in vascular tissues and plant growth parameters (height and fresh weight). A cumulative stress response was calculated to consider the overall effect of V. dahliae on olive cultivars. The olive varieties resistance to V. dahliae varied significantly, with ‘Koroneiki’, ‘Tragolia’, and ‘Atsiholou’ being the most resistant. Interestingly, most tested varieties showed a significantly low resistance level, suggesting increased risk for the Greek olive industry due to V. dahliae.
In the present study, combined interactions of the root‐knot nematode Meloidoyne javanica (M.j.) with the soil‐borne fungi Verticillium dahliae (V.d.), Fusarium oxysporum f.sp. radicis‐cucumerinum (F.o.r.c.) or Monosporascus cannonballus (M.c.) against susceptible plant hosts were evaluated. Direct and indirect interactions were tested by applying each pathogen (nematode or fungus) alone or together on the whole plant root system or on each of the two sides of a split‐root set‐up in all possible combinations. Plant–fungi–nematode interactions were estimated by measuring various disease and growth parameters on host plants. A significant increase of verticillium wilt symptoms was observed in eggplant when V.d. and M.j. were applied separately in split‐root plants compared with symptoms in whole root plants inoculated with both pathogens. Root and stem rot and root‐knot symptoms in cucumber were more severe when F.o.r.c. was combined with M.j. in a split‐root set‐up than when plants were only inoculated with a single pathogen on one part of the split‐root set‐up. No significant associations were observed in the case of melon‐M.c.‐M.j. interaction. Gene expression bioassays for cucumber‐F.o.r.c.‐M.j. interaction revealed increased transcriptomic activity for PAL1 gene in plants treated with F.o.r.c at 3 days postinoculation (d.p.i.), whereas high transcriptomic level for DEFENSIN gene was observed primarily in M.j.‐treated plants at 7 d.p.i. The possible interactions between the abovementioned pathosystems are presented and discussed for the first time in literature.
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