Eleftherios C. Tjamos

Selection and screening of endorhizosphere bacteria from solarized soils as biocontrol agents against Verticillium dahliae of solanaceous hosts

Verticillium dahliae antagonistic endorhizosphere bacteria were selected from root tips of tomato plants grown in solarized soils. Fifty-three out of the 435 selected bacterial isolates were found to be antagonistic against V. dahliae and several other soilborne pathogens in dual cultures. Significant biocontrol activity against V. dahliae in glasshouse trials was demonstrated in three of 18 evaluated antagonistic isolates, provisionally identified as Bacillus sp. Although fluorescent pseudomonads were also isolated from root tips of tomato plants, none of the tested isolates exercised any significant antagonistic activity against V. dahliae in dual cultures. So these isolates were not tested in glasshouse trials in this study. Finally, two of the most effective bacterial isolates, designated as K-165 and 5-127, were shown to be rhizosphere colonizers, very efficient in inhibiting mycelial growth of V. dahliae in dual cultures and successfully controlling Verticillium wilt of solanaceous hosts. In glasshouse experiments, root dipping or soil drenching of eggplants with bacterial suspension of 107cfu ml−1 resulted in reduced disease severity expressed as percentage of diseased leaves (40–70%) compared to the untreated controls under high V. dahliae inoculum level (40 microsclerotia g−1 soil). In heavily Verticillium infested potato fields, experiments with potato seeds dusted with a bacterial talc formulation (108cfu g−1 formulation), showed a significant reduction in symptom development expressed as percentage of diseased potato plants and a 25% increase in yield over the untreated controls. As for their effectiveness in increasing plant height, both bacterial isolates K-165 and 5-127 produced indolebutyric, indolepyruvic and indole propionic acids. Both antagonists are considered as plant growth promoting rhizobacteria bacteria since significantly increased the height of treated plants compared with the untreated controls. Chitinolytic activity test showed that both isolates were able to produce chitinase. Testing rhizospheric and endophytic activity of the antagonists it was shown that although the bacteria are rhizosphere inhabitants they also preferentially colonize the endorhizosphere of tomatoes and eggplants. Fatty acid analysis showed that isolate K-165 could belong to Paenibacillus alvei while 5-127 to Bacillus amiloliquefaciens.

Symptom development, pathogen isolation and Real-Time QPCR quantification as factors for evaluating the resistance of olive cultivars to Verticillium pathotypes

Verticillium wilt is the most serious olive disease in the Mediterranean countries and worldwide. The most effective control strategy is the use of resistant cultivars. However, limited information is available about the level and source of resistance in most of the olive cultivars and there are no published data using microsclerotia, the resting structures of Verticillium dahliae, as the infective inoculum. In the present study, we correlated symptomatology and the presence of the fungus along with the DNA relative amount (molecules μl−1) of a defoliating (D) and a non-defoliating (ND) V. dahliae strain in the susceptible cv. Amfissis and the tolerant cvs Kalamon and Koroneiki, as quantified by the Real-Time QPCR technology. The viability of the pathogen in the plant tissues was confirmed by isolating the fungus on PDA plates, while symptom assessment proved the correlation between the DNA relative amount of V. dahliae in plant tissues and cultivar susceptibility. It was further demonstrated that the D and ND strains were present at a significantly higher level in cv. Amfissis than in cvs Kalamon and Koroneiki. It was finally observed that the relative amount of the pathogen in roots was lower than in stems and shoots and declined in plant tissues over time. These data constitute a valuable contribution in evaluating resistance of olive cultivars or olive root-stocks to V. dahliae pathotypes.

Phenolic Responses of Resistant and Susceptible Olive Cultivars Induced by Defoliating and Nondefoliating verticillium dahliae Pathotypes

Verticillium wilt is the most serious olive disease worldwide. The olive-infecting Verticillium dahliae pathotypes have been classified as defoliating (D) and nondefoliating (ND), and the disease is mainly controlled in olive orchards by using resistant or tolerant cultivars. Limited information is available about the nature of resistance in most of the olive cultivars. In the present study, the phenolic responses of the susceptible to V. dahliae olive cv. Amfissis and the resistant cv. Koroneiki upon D and ND V. dahliae infection were monitored in relation to the fungal DNA levels in the vascular tissues with the purpose to explore the defense mechanisms of olive trees against V. dahliae. Quantitative polymerase chain reaction revealed that the decrease in symptom severity shown in Koroneiki trees was associated with significant reduction in the growth of both V. dahliae pathotypes in the vascular tissues compared with Amfissis. In Koroneiki trees, the levels of o-diphenols and verbascoside were positively associated with the DNA levels of the D and ND pathotypes. In addition, a positive association was observed between the levels of verbascoside and the fungal DNA level in Amfissis trees, whereas a negative association was revealed between the fungal DNA level and the total phenols and oleuropein content in both cultivars. The levels of verbascoside were clearly higher in Koroneiki trees compared with Amfissis trees, indicating for the first time in the literature the involvement of verbascoside in the defense mechanism of olive trees against V. dahliae.

Implementation of soil solarization in Greece: conclusions and suggestions

Abstract The use of soil solarization in Greece is described, indicating methods of reducing the period of solarization by using impermeable plastic sheets and combining it with the addition of antagonistic bacteria.

Distribution and establishment of the biocontrol fungus Talaromyces flavus in soil and on roots of solanaceous crops

Abstract Ascospores of Talaromyces flavus were applied to eggplant, tomato and potato as either a drench or in spherical granules (alginate prill). In subsequent samplings, the percentage colonization of roots was significantly greater than that of root tips. However, when populations of T. flavus were expressed as colony forming units g −1 fresh weight, recovery of T. flavus was significantly greater in root tips. Populations were lower in rhizosphere soil than from roots and root tips and populations in non-rhizosphere soil were the lowest of any location. Vigorous washing of the roots removed most of the T. flavus , indicating that T. flavus was superficially associated with the roots. Populations of T. flavus were greater when it was applied as a drench rather than in prill.

First Report of Alternaria alternata as the Causal Agent of Alternaria Bud and Blossom Blight of Olives

A severe blight of buds and flowers in olive trees (Olea europaea L.) was observed in the Aitoloakarnania region, West-Central Greece during the 2015-2017 growth seasons. Symptoms appeared in buds and expandable flowers of >100 trees cv. Kalamon in 30 olive groves (50-60% disease incidence). Necrotic buds and flowers appeared scattered in the tree reaching up to 70% disease severity in some trees. Microscopic observations of dead buds and flower tissues showed abundant Alternaria-like conidia. 50 dead buds and 50 dead flowers were collected from 50 olive trees cv. Kalamon in December 2015, surface-sterilized in 0.5% NaClO for 10 min, rinsed with sterile ddH2O, immersed in 70% EtOH for 3 min, rinsed again with sterile ddH2O, dried in sterile filter paper and finally placed in potato dextrose agar and in Rose-Bengal media. Plates were incubated at 25οC with a 12h photoperiod for 5 days. Dark green fungal colonies with septate hyphae and conidia were consistently isolated from all diseased buds and flowers. ...