Marta Mari

Biological control of Monilinia laxa and Rhizopus stolonifer in postharvest of stone fruit by Pantoea agglomerans EPS125 and putative mechanisms of antagonism.

Treatment of stone fruits (apricot, peach and nectarine) with Pantoea agglomerans strain EPS125 decreased the incidence and diameter of lesions of brown rot caused by Monilinia laxa and soft rot caused by Rhizopus stolonifer. Root control was achieved on fruits either wounded and subsequently inoculated with the pathogens or non-wounded and naturally infected from orchards. The efficacy of biocontrol was dependent on the concentration of the biocontrol agent and pathogen. At medium to low pathogen dose, optimal EPS125 concentrations were above 10(7) CFU ml(-1). The median effective dose (ED(50)) of EPS125 was 4.5x10(4) in M. laxa and 2.2x10(5) CFU ml(-1) in R. stolonifer. However, EPS125 was more effective in M. laxa than in R. stolonifer as indicated by the ratio between ED(50) of the biocontrol agent and pathogen (K(z)/K(x)) which was 166 and 1263, respectively. Interactions between the strain EPS125 and the fruit surface, and M. laxa and R. stolonifer, were studied to determine the mechanisms of protection from postharvest rots. The strain EPS125 colonizes, grows and survives on stone fruit wounds. Significant inhibition of conidial germination and hyphal growth of R. stolonifer and M. laxa was achieved when the fungal and EPS125 cells were cocultivated on peel leachate or nectarine juice. However, no effect was observed when the antagonist and the pathogen cells were physically separated by a membrane filter which permits nutrient and metabolite interchange. Therefore, a direct interaction between the strain and the pathogen cells is necessary for antagonism, without a significant contribution of the production of antibiotic substances or nutrient competition. Preemptive exclusion by wound colonization and direct interaction with the pathogen is proposed as the mechanism of biocontrol.

Fungicidal Activity of Plant Volatile Compounds for Controlling Monilinia laxa in Stone Fruit

Nine plant-volatile compounds were tested for their activity against Monilinia laxa, the cause of brown rot in stone fruit. In vitro trials on conidial germination and mycelial growth showed a consistent fungicidal activity of trans-2-hexenal, carvacrol, and citral, whereas trans-cinnamaldehyde, hexanal, (-)-carvone, eugenol, 2-nonanone, and p-anisaldehyde exhibited a progressively lower inhibition. The best inhibitor of conidial germination was trans-2-hexenal (effective dose for 50 and 90% inhibition [ED50 and ED95] = 7.53 and 9.4 μl/liter, respectively; minimal inhibitory concentration [MIC] = 12.3 μl/liter], whereas carvacrol was the best inhibitor of mycelial growth (ED50 and ED95 = 2 and 3.4 μl/liter, respectively; MIC = 6.1 μl/liter). The three most active compounds in in vitro studies also were tested in vivo as postharvest biofumigants. The best control of brown rot was with trans-2-hexenal (efficacy ranging from 46.2 to 80.3%, depending on cultivar), whereas citral and carvacrol resulted in a lower efficacy of 40 and 32.9%, respectively. Fumigation with trans-2-hexenal at concentrations that stopped decay did not cause any visible disorders to plum, whereas it was phytotoxic to apricot, peach, and nectarine and produced off-odors or off-flavors in all species of stone fruit tested.

Postharvest biological control of grey mould (Botrytis cinerea Pers.: Fr.) on fresh-market tomatoes with Bacillus amyloliquefaciens

Abstract Antagonistic bacteria were evaluated for their effectiveness in postharvest control of grey mould on fresh-market tomatoes. Grey mould was reduced in fresh-market tomatoes treated with antagonists and artificially inoculated with Botrytis cinerea , and stored at 20 °C for at least 7 days. One strain, 5PVB ( Bacillus amyloliquefaciens ) was particularly effective. This strain apparently did not produce extracellular antibiotic substances, yet was highly active against the pathogen on both mature-green and red tomatoes. Treatment with 5PVB before storage at 10 °C showed only fungistatic activity against grey mould. Chilling-injured mature-green tomatoes were more susceptible to B. cinerea . On chilled fruits the bacterial antagonist completely controlled the pathogen if the treatment was applied immediately after storage at 2 °C. In fermentation tests on four industrial waste materials, B. amyloliquefaciens grew well and maintained its antagonistic activity when grown in two of them. In pilot experiments with wounded fruits dipped in the bacterial cell and pathogen suspension, the development of decay was effectively inhibited.

Peracetic Acid and Chlorine Dioxide for Postharvest Control of Monilinia laxa in Stone Fruits

The effects of different concentrations of peracetic acid (PAA; 62.5, 125, 250, 500, and 1,000 μg/ml)and chlorine dioxide (ClO2; 12.5, 25, 50, 100, and 200 μg/ml) on germination of Monilinia laxa conidia were tested. Conidia germination was related to the concentration of chemical product used, as well as duration of treatment. Complete inhibition of germination was observed with PAA at 500 μg/ml after 5 min of contact with conidia and with ClO2 at 50 μg/ml after 1 min of contact with conidia. The results of in vitro tests were confirmed by inoculation of fruits with treated conidia. The PAA treatment also was effective 1 h after pathogen inoculation but only on plums, for which a 1,000 μg/ml treatment significantly reduced decay incidence by 50%. In a semi-commercial test, pathogen conidia dipped for 20 min in PAA at 250 μg/ml or ClO2 at 10 μg/ml or for 5 min in PAA at 250 μg/ml were completely inhibited, and no brown rot was observed in inoculated wounded nectarines and plums.

Preliminary studies on the efficiency of endophytes in the biological control of the postharvest pathogens Monilinia laxa and Rhizopus stolonifer in stone fruit

Endophytic bacteria were tested in the postharvest biocontrol of Monilinia laxa and Rhizopus stolonifer in stone fruits. Of a total of 122 strains of these sub-epidermis micro-organisms isolated from various horticultural sources (cucumber, eggplant, pepper, tomato, zucchini, apricot, peach and plum), approximately twenty showed greater than 90% control of the given pathogens. R. stolonifer appeared to be less susceptible than M. laxa to the endophytes. The influence of conidial concentration of M. laxa and of antagonist/pathogen contact time on the effectiveness of biocontrol were also evaluated.

Activity of trans‐2‐hexenal against Penicillium expansum in ‘Conference’ pears

Aims:  To investigate the effects of trans‐2‐hexenal on blue mould disease, patulin content and fruit quality in ‘Conference’ pears.

Biological control of postharvest diseases by microbial antagonists: how many mechanisms of action?

The postharvest phase has been considered an environment for successful application of biological control agents (BCAs). However, the interactions between fungal pathogen, host (fruit), and antagonist are influenced by several parameters such as temperature, oxidative stresses, oxygen composition and water activity that could determine the success of biocontrol. Knowledge of the modes of action of BCAs is essential in order to enhance their viability and increase their potential in disease control. The antagonists display a wide range of modes of action: antibiosis, competition for nutrients and space, parasitism and induction of resistance are considered the main ones. Their efficacy, however, is related to the host and the pathogen; sometimes, these mechanisms could act simultaneously, and it is therefore difficult to establish which is related to a specific antifungal action. The current review presents a brief summary of the research that has led to a better understanding of the mode of action of BCAs with particular emphasis on the most recent literature.

Effects of potassium sorbate on postharvest brown rot of stone fruit.

The effect of potassium sorbate (K-sorb), a low-toxicity chemical, to control Monilinia spp. was investigated. Preliminary in vitro studies found the MIC of K-sorb for conidial germination and mycelial growth was, respectively, 260 and 1,250 mg/ liter. Immersion of naturally infected peach and nectarine fruit in a solution (15 g/liter) of K-sorb for 120 s reduced brown rot by over 80% in four of five trials. Although treated fruits showed a significant reduction in firmness with respect to the control, they did not reach the overripe stage and retained acceptable quality parameters. In an attempt to elucidate the mechanism of action for K-sorb, the inhibition of enzymatic activity by K-sorb was also tested. In a radial diffusion assay, the addition of K-sorb to agarose reduced polygalacturonase (PG) activity across the concentrations considered. The greatest reduction (54.3%, with respect to the control) was obtained at a sorbate concentration of 15 g/liter. PG kinetic activity of Monilinia laxa observed by a spectrophotometric assay peaked after 40 min in all samples tested. PG activity was significantly higher in the control than in the samples with increased K-sorb concentrations. In conclusion, based on these findings, K-sorb can be recommended as a low-toxicity antifungal compound against Monilinia spp. in peaches and nectarines with its mode of action probably depending in part on the inhibition of PG activity in M. laxa.

New Approaches for Postharvest Disease Control in Europe

Alternative methods to fungicide treatments have been studied in order to prevent fruit losses in the postharvest phase. Within these methods the applications of: (a) biological control agents (BCAs), (b) plant bioactive compounds, and (c) physico-chemical methods showed interesting results but still far from a practical application in Europe. Actually, despite the substantial progress obtained with BCAs, any biofungicide has been registered in Europe to control postharvest pathogens, moreover because of their insufficient and inconsistent performance. The use of plant bioactive compounds has shown that the treatment conditions (concentration, form of application, formulation, exposure time, time of treatment, etc.) can deeply influence their efficacy. The different responses found in many studies indicate a cultivar specificity in the product-pathogen-volatile interaction. A barrier to use the plant bioactive compounds may not be efficacy, but rather the off-odours caused in fruits and vegetables and/or the phytotoxicity. Physico-chemical methods include heat, ionising and ultraviolet C irradiation, food additives inducers of resistance. Heat treatments by hot water dips, hot dry air, vapour heat or very short water rinse and brushing appear promising. To overcome the drawbacks that have arisen with the these methods, the integration of the antagonist with other treatments such as low toxic substances (GRAS), heat, etc. has been proposed; this strategy could produce an additive or synergistic effect on disease control and obtain satisfactory levels of disease reduction.

Effectiveness of Trichoderma, Gliocladium and Paecilomyces in postharvest fruit protection☆

Abstract Trichoderma viride, T. harzianum, Gliocladium roseum and Paecilomyces variotii were tested in biocontrol trials against several postharvest pathogens and compared with conventional fungicides. Applied as spray treatments to the fruits, these fungi partially controlled Botrytis cinerea, in strawberry and kiwifruit, Fusarium oxysporum in potato and Alternaria citri in lemon. Trichoderma demonstrated significant control of artificially inoculated B. cinerea in strawberry but proved ineffective against latent infections. Several strains of Trichoderma were found to be pathogenic in kiwifruit and lemon. All three fungal species gave poor mycelial development at 0, 2, 5, and 10°C storage temperatures, resulting in unsuccessful fruit colonization. The overall data indicate some of the difficulties in developing biological control techniques using Trichoderma, Gliocladium and Paecilomyces.