Biological Control of Postharvest Diseases of Peaches and Nectarines by Yeasts

Abstract Biocontrol activities of a total of 103 yeast isolates were tested against postharvest diseases of peaches. Seven isolates, with the best efficacy in reducing the number of infected wounds or lesion sizes of Penicillium expansum and Botrytis cinerea, were selected for further large‐scale experiments. In large‐scale experiments, all selected isolates were significantly effective (P ≤ 0.05) in reducing the number of infected wounds and the diameter of lesions caused by P. expansum, B. cinerea, and Monilinia fructicola. DR52 was significantly superior to all the other yeasts in effectiveness against all three pathogens. The efficacy of the other yeast antagonists against B. cinerea and P. expansum was almost equal, while the control of M. fructicola was inferior. DR52 was selected for further storage experiment because its efficacy was higher against the three pathogens and it also showed a different random amplified polymorphic DNA‐polymerase chain reaction pattern compared with other isolates. DR52 was identified by Centraalbureau voor Schimmeelcultures (Baarn, The Netherlands) as Kloeckera apiculata. K. apiculata completely controlled both pathogens after 30 days of storage. Its efficacy declined to an 83.4% reduction in B. cinerea incidence and an 87.5% reduction in P. expansum incidence after 45 days of storage.     

A new improved strategy for the selection of cold-adapted antagonist yeasts to control postharvest pear diseases

Postharvest diseases cause considerable losses of harvested fruits during transportation and storage. Many yeast species have been reported as good antagonists against postharvest pear pathogens. In this work, we used a novel selection strategy that involves the isolation of yeasts from washing fluids, showing biocontrol activity against a regional Penicillium expansum strain (primary screening), originally obtained from fruit wounds after long time storage at ?1/0°C. About 26 isolates representative of the 11 yeast species identified in the 27 selected washing waters were chosen to be evaluated in a secondary screening against a regional Botrytis cinerea strain on pear wounds. Among yeasts tested, 38% showed complete control of P. expansum, but only 15% reduced the decay incidence of B. cinerea to 60–80% at ?1/0°C. These results reveal that some of the yeasts found can be biological alternatives to fungicides in the control of P. expansum and B. cinerea infections. Based on the data obtained, our strategy seems to be much more effective than the previously reported methods in obtaining successful biocontrol agents.     

Postharvest biocontrol of Monilinia laxa,Monilinia fructicola and Monilinia fructigena on stone fruit by two Aureobasidium pullulans strains

The antagonistic effects of yeasts, L1 and L8, isolated from carposphere of ‘Redhaven’ peaches were tested for the first time in the same experiment against three Monilinia species (Monilinia laxa, Monilinia fructicola and Monilinia fructigena) in in vitro and in vivo trials. The two antagonists were selected after preliminary assays for their ability to reduce brown rot in peaches and nectarines, and both were identified by molecular and morphological tools as Aureobasidium pullulans. In in vivo trials, neither the autoclaved cells, nor the sterile culture filtrates of either antagonist showed any significant reduction of rot incidence produced by inocula of the three Monilinia species, while the washed cells of L1 and L8 completely inhibited M. laxa and M. fructicola rots and reduced M. fructigena infections by 70% and 90%, respectively. In other trials, nectarines treated with antagonist cells and inoculated with the pathogens were stored at 0 °C for 21 days, plus 7 days at 20 °C. The low temperature reduced brown rot development, since all fruit were free from disease symptoms on removal from cold storage. However after 7 d at 20 °C, untreated fruit were rotted over 45% depending on the Monilinia species but the antagonists completely inhibited M. laxa and M. fructicola, while M. fructigena infections were reduced by 89.8% and 91.2% by L1 and L8, respectively. For both strains, 10⁸ CFU ml^?1 was the most active concentration, although L1 showed good activity at a concentration of 10⁷ CFU ml^?1. Isolate L8 at the concentration of 10⁷ CFU ml^?1 was ineffective against M. fructicola and M. fructigena, showing no difference between treated fruit and the control, excepting the case of nectarines inoculated with M. laxa, where L8 at the concentration of 10⁷ CFU ml^?1 reduced the brown rot infections with respect to the control. The increase in population density of A. pullulans strains L1 and L8 in the wounds of nectarines stored at 0° or 20 °C was low but sufficient to control brown rot. In conclusion, the present preliminary study identified two antagonistic strains of A. pullulans as active ingredients for the development of biofungicides for postharvest application against three Monilinia species that are responsible for high economic losses in stone fruit crops.     

Effects of ozone treatment on the quality of kiwifruit during postharvest storage affected by Botrytis cinerea and Penicillium expansum

The objective was to reveal the effects of ozone treatment on quality maintenance and resistance to Botrytis cinerea and Penicillium expansum in kiwifruit during postharvest storage. Kiwifruits were treated with 79.44 ppm gaseous ozone for 1 hr once a day for 7 day at 0°C to determine the effects of ozone treatment on the quality and disease incidence caused by B. cinerea and P. expansum in vivo and the growth of B. cinerea and P. expansum in vitro. Ozone treatment significantly reduced the disease incidence of kiwifruit and inhibited the mycelial development and spore germination of B. cinerea and P. expansum. High levels of fruit firmness and titratable acidity were maintained in the ozone‐treated kiwifruit, and the activities of the defence‐related enzymes were remarkably enhanced. Therefore, ozone treatment may be an effective method to maintain the quality of kiwifruit and control its decay during postharvest storage.     

Control of Postharvest Diseases of Sweet Cherry with Ethanol and Hot Water

Complete inhibition of the germination of spores of Penicillium expansum occurred after 10 s exposure to 40% ethanol or more at ambient temperature, while spores of Botrytis cinerea were completely inhibited by 30% ethanol or more. Mortality of the spores of P. expansum and B. cinerea in heated 10% ethanol was higher than in water at the same temperatures. Immersion of naturally inoculated fruit in 20, 30, 40, or 50% ethanol reduced the decay present after storage for 10 days at 20°C similarly and by approximately 60–85%. Immersion of fruit that had been inoculated with the spores of P. expansum and B. cinerea reduced decay by both pathogens after storage for 30 days at 0°C and 5 days at 20°C when 30% or higher concentrations of ethanol were used. The incidence of decay after immersion in water alone for 30 s at 24, 50, 55, or 60°C was 57.7, 44.7, 46.2, and 35.7%, respectively, while 10% ethanol at these temperatures the decay incidence to 52.2, 33.9, 32.8, or 14.7%, respectively. Water treatments at 50, 55, or 60°C alone were not effective against P. expansum, while their efficacies were significantly increased by the addition of 10% ethanol. The most effective treatment was immersion in 10% ethanol at 60°C. Ethanol treatments at 20, 30, 40, or 50% and water treatments at 55 or 60°C significantly reduced natural fungal populations on the surfaces of fruit in all of the experiments. Addition of 10% ethanol to water significantly increased the efficacy of water in reducing the fungal populations at elevated temperatures. None of these treatments caused surface injuries to the fruit or adversely affected stem colour.     

Efficacy of the antagonist Aureobasidium pullulans PL5 against postharvest pathogens of peach,apple and plum and its modes of action

The efficacy of Aureobasidium pullulans PL5 against different postharvest pathogens of fruits (Monilinia laxa on plums and peaches, Botrytis cinerea and Penicillium expansum on apples) were evaluated under storage conditions when applied at 10⁸ cells ml⁻¹ and their interactions were studied in vitro and in vivo to discover the possible modes of action. Under 1.2 °C and 95% relative humidity (RH) for 28 days, the efficacy of PL5 against M. laxa on plums was 45%, reducing disease incidence from 78% to 43%. Under 1 °C and 95% RH for 21 days, the efficacy against M. laxa on peaches was 63%, reducing disease incidence from 79% to 29%. Under 4 °C and 95% RH for 45 days, the efficacy against B. cinerea and P. expansum on apples was 56% and 46%, respectively. In Lilly–Barnett minimal salt medium with the fungal cell walls of pathogens as sole carbon source, PL5 produced β-1,3-glucanase, exo-chitinase and endo-chitinase. Nutrient concentrations had significant effect on pathogen growth reduction by PL5. No attachment was observed in antagonist–pathogen interactions in vitro or in vivo. PL5 completely inhibited pathogen spore germination in PDB at 10⁸ cells ml⁻¹, whereas at 10⁶ cells ml⁻¹ the efficacy was significantly decreased. However, inactivated cells and culture filtrate of PL5 had no effect on pathogen spore germination and germ tube elongation. Our results showed that A. pullulans PL5 could be introduced in commercial formulations to control postharvest pathogens on fruits and its activity was based on secretion of lytic enzymes and competition for nutrients.     

Contribution of Pathogens to Peach Fruit Rot in Northern Greece and their Sensitivity to Iprodione, Carbendazim, Thiophanate-methyl and Tebuconazole Fungicides

This study identified the main pathogens causing fruit rots of mature peaches in northern Greece, the major peach producing area of Greece. The brown rot pathogen Monilinia laxa was responsible for approximately 70% and 78% of rotted peaches in 2005 and 2006 respectively. Serious damage (up to 5%) was also caused with the fungus Phomopsis amygdali. Other pathogens isolated from rotted peaches at a low percentage were Alternaria alternata, Aspergillus niger, Aspergillus flavus, Botrytis cinerea, Sclerotinia sclerotiorum, Fusarium spp., Colletotrichum gloeosporioides, Rhizopus stolonifer and Gilbertella persicaria. Most fungal isolates originated from the rotted peaches were tested for their sensitivity to the fungicides iprodione, carbendazim, thiophanate methyl and tebuconazole at label recommended concentrations. All fungicides inhibited the growth of M. laxa, A. niger, A. flavus, S. sclerotiorum, P. amygdali and B. cinerea on poisoned agar. Apart from iprodione, all other fungicides inhibited the mycelium growth of the pathogen Fusarium sp. The mycelium growth of Fusarium sp. was significantly less with iprodione than control. Only iprodione and tebuconazole were effective against A. alternata and R. stolonifer. Tebuconazole inhibited the mycelium growth of R. stolonifer, while iprodione reduced significantly in comparison to control. The mycelium growth of the fungus C. gloeosporioides was inhibited by tebuconazole and reduced significantly by the fungicides thiophanate methyl, carbendazim and iprodione. Among all the fungi tested, only M. laxa and B. cinerea isolates were found resistant to benzimidazoles [the EC50 (50% effective concentration) value was 100–200 mg/l and 200–300 mg/l for the largest number of thiophanate methyl‐ and carbendazim‐resistant M. laxa isolates respectively, while the biggest number of B. cinerea thiophanate methyl‐ and carbendazim‐resistant isolates showed EC50 value 200–300 mg/l and 300–400 mg/l, respectively]. However, these strains were sensitive to tebuconazole and iprodione. Therefore, these fungicides can be used as an alternative method to control benzimidazole‐resistant Monilinia and Botrytis isolates.     

Effects of yeast antagonist in combination with UV-C treatment on postharvest diseases of pear fruit

The yeast antagonist Candida guilliermondii and ultraviolet-C (UV-C) treatment were investigated for controlling infection following artificial inoculation with Penicillium expansum or Botrytis cinerea, or natural infection in pear fruit stored at 20°C. Applied separately, both C. guilliermondii and UV-C (5 kJ m⁻²) effectively inhibited decay caused by P. expansum or B. cinerea, and natural infection. The combination of C. guilliermondii and UV-C showed better control efficacy. Application of UV-C did not affect the growth of C. guilliermondii in pear fruit wounds, while UV-C induced a significant increase in the activities of chitinase, β-1,3-glucanase, catalase and peroxidase in pear fruit. The mechanism by which UV-C enhanced the biocontrol efficacy of C. guilliermondii may be related to the elicitation of defense responses in pear fruit. The combination of C. guilliermondii and UV-C radiation could be a promising method for the control of P. expansum and B. cinerea in pear fruit.     

The use of a surface coating to ameliorate the rate of spread of post-harvest fungal diseases of top fruit

The impact of TAL Pro-long coatings (a sucrose-ester composition) on the development of some common post-harvest fungal rots of top fruit is reported. Coatings were not observed to be fungistatic but did diminish the incidence of fungal infections on Conference pears held in cold store. At higher temperatures coatings modified the spatial distribution and reduced the rate of spread of lesions within apples and pears. In comparison to experimental controls, TAL Pro-long had a greater effect on those rots caused by Botrytis cinerea Fr., Monilinia fructigena (Aberh. & Ruhl.) Honey and Rhizopus nigricans Lind. than those of Alternaria alternata Fr., M. laxa (Aberh. & Ruhl.) Honey and Penicillium expansum Link em. Thom. The efficacy of the treatments was related to the concentration of the preparations applied, the variety of fruit, the timing of application and the temperature to which produce were subsequently exposed.     

Monilinia species in Hungary: morphology, culture characteristics, and molecular analysis

Monilinia is a well-known pathogen of fruit trees affecting fruit production all over the world. Three species of the Monilinia genus are particularly important with regard to fruit trees and ornamentals, causing serious blossom and twig blight and brown rot in fruits: Monilinia fructicola, Monilinia fructigena, and Monilinia laxa. In this study, Monilinia isolates were compared and identified using classical and molecular methods. Morphological and culture characteristics were determined and pathogenicity testing performed. In addition, internal transcribed spacer regions and a genomic sequence with unknown function were analyzed and compared with sequence data from other Monilinia species in an international database. Four Monilinia/Monilia species were identified: M. fructicola, Monilia polystroma, M. fructigena, and M. laxa. M. fructicola was isolated from imported peach fruits. M. polystroma was first reported from Hungary and Europe on apple shoots and fruits. M. fructigena was identified on tea-rose hybrid pseudofruits, which is the first occurrence of this pathogen on this host. M. laxa causes brown rot of grapes, which has only been reported in New Zealand. Substitutions and insertions were detected when comparing M. laxa, M. fructigena, and M. polystroma sequences. In the genomic sequence with unknown function, three repetitive sequence motifs were identified in different numbers, depending on species and isolate. On the phylogram produced in this analysis, the Hungarian M. polystroma isolate (UFT) and M. polystroma reference isolates localized at a different branch than the closely related M. fructigena isolates and other Monilinia species.     

Antifungal Vapour‐phase Activity of a Combination of Allyl Isothiocyanate and Ethyl Isothiocyanate Against Botrytis cinerea and Penicillium expansum Infection on Apples

The potential use of allyl isothiocyanate (AITC) and ethyl isothiocyanate (EITC), singly and in combination, was tested in in vitro and in vivo trials for their effect on Penicillium expansum Link and Botrytis cinerea Persl. infection on apple when used as a fumigant. A 3 : 1 ratio of AITC : EITC was more efficient at reducing in vitro spore germination of P. expansum and B. cinerea than were other combinations or either AITC or EITC alone. The optimized combination showed the lowest EC₅₀ values, at 0.08 and 0.14 μg/ml air, for P. expansum conidial germination and mycelial growth, respectively, and 0.07 and 0.12 μg/ml air for B. cinerea conidial germination and mycelial growth, respectively. In in vivo trials, artificially infected apples were exposed for 4 days to an ITC‐enriched atmosphere. Among the ITCs tested, AITC, EITC and their combinations reduced incidence by more than 85% after 3–4 days of apple incubation at 20°C. Although further studies are necessary to evaluate any detrimental effects on apple quality, the evidence from this study supports the use of fumigation based on ITCs, and in particular a 3 : 1 combination of AITC and EITC, for control of postharvest mildew in apple fruit.     

Biocontrol of Postharvest Grey Mould on Tomato by Yeasts

The fungal pathogen Botrytis cinerea causes severe rots on tomato fruit during storage and shelf life. Biological control of postharvest diseases of fruit may be an effective alternative to chemical control. Yeasts are particularly suitable for postharvest use, proving to be highly effective in reducing the incidence of fungal pathogens. Yeast fungi isolated from the surface of solanaceous plants were evaluated for their activity in reducing the postharvest decay of tomato caused by B. cinerea. Of 300 isolates, 14 strains of Rhodotorula rubra and Candida pelliculosa were found to be strongly antagonistic to the pathogen in vitro and were selected for further storage experiment. The antagonists were evaluated for their effect on the biological control of postharvest grey mould. Artificially wounded fruits were treated by means of a novel technique: small sterile discs of filter paper imbibed separately in suspensions of each yeast and the pathogen were superposed onto each wound. After 1‐week, 11 isolates were significantly effective in reducing the diameter of lesions by more than 60% compared to the control treated with B. cinerea alone. Total protection was obtained with the strain 231 of R. rubra on fruits challenged with pathogen spores. To our knowledge, R. rubra and C. pelliculosa have not been described as biocontrol agents against grey mould caused by B. cinerea. Our data demonstrate that the application of antagonistic yeasts represents a promising and environmentally friendly alternative to fungicide treatments to control postharvest grey mould of tomato.     

The Potential Efficacy of Glycyrrhizic Acid and Its Nanostructure Against Brown Rot of Peach fruits

Production of peaches (Prunus persica (L.) Batsch) for both local market and export is increasing each year in Egypt. Brown rot disease, caused by Monilinia laxa and Monilinia fructigena, is considered one of the most important postharvest rots affecting peaches in Egypt and economic losses are increasing. Antifungal activity of glycyrrhizic acid nanoparticles (GA-NPs) and glycyrrhizic acid (GA) at 0.2 and 0.4 mmol/L was investigated as a control for both these brown rot pathogens on peach fruits in both in vitro and in vivo studies. In the in vitro studies, GA-NPs were the most effective as shown by the ability to decrease linear growth of both brown rot pathogens in potato dextrose agar (PDA) amended with 0.4 mmol/L GA-NPs. Micrographs of M. fructigena exposed to 0.4 mmol/LGA showed mycelial deformations, nodule formation, detachment of the cell wall, shrinkage and inhomogeneous cytoplasmic materials with large vacuoles. Mycelium of M. laxa exposed to 0.4 mmol/ LGA-NPs resulted in thinner and distorted hyphae, nodule formation, cell wall thinning, and swellings. The GANPs and GA treatments improved fruit quality by maintaining firmness and total soluble solids (TSS). GA-NPs were more effective in decreasing decay incidence than their bulk material. The 0.4 mmol/L GA-NPs completely inhibited the disease on naturally infected peach fruits for both seasons of 2018 and 2019. Furthermore, 0.4 mmol/L GA-NPs reduced the disease incidence in inoculated fruits by 95 (M. laxa) and 88% (M. fructigena) in 2018 season and 96 (M. laxa) and 85% (M. fructigena) in 2019 season. In conclusion, GA-NPs could enhance the resistance of peaches against brown rot caused by M. laxa and M. fructigena.     

Overwintering of Monilinia fructicola in Stone Fruit Orchards in Northern China

Survival of Monilinia fructicola on the surface of mummified fruit and in peduncles and shoots of stone fruit trees infected by M. fructicola in the previous season was studied from 2003 to 2006 in orchards in the agricultural region of Beijing. Viable conidia of M. fructicola were consistently detected on fruit mummies from mid‐March to the end of April. During flowering (in mid‐April), studies in five peach orchards showed that 33–87% of mummified fruit bore viable conidia. The germination rate of conidia on diseased fruit was about 64% in autumn. It decreased to 24% in mid‐winter when the fruit was completely mummified, and in the following year to 2–4% in early spring. Viable M. fructicola was consistently detected in peach and nectarine shoots collected in winter and spring. In general, viable M. fructicola in peduncles was detected from mummified fruit of 11–27% branches and from asymptomatic plant tissues of 3–20% branches. Sporulation of M. fructicola was observed on peduncles in seven of eight surveys, and the percentage of branches containing viable M. fructicola in peduncles in contact with mummified fruits ranged from 18% to 40%. This study demonstrates that the tree‐borne mummified fruit and the peduncles could be the parts of trees where M. fructicola can survive the winter in orchards in suburban Beijing.     

Chitosan and hot water treatments reduce postharvest green mould in ‘Murcott’ tangor

This study aims to evaluate the efficacy of hot water and chitosan treatments to control green mould caused by Penicillium digitatum in 'Murcott' tangor. P. digitatum conidial germination and mycelial growth were evaluated in assays in vitro to verify whether chitosan (0.5, 1 and 2%) or hot water (45, 50, and 55°C, for 30 s, 1, 2, and 5 min) acts directly on fungus development. In vivo assays consisted of inoculating the fruit with P. digitatum (10⁵ conidia ml⁻¹) 4 hr before the chitosan and hot water treatments. Subsequently, green mould incidence and severity were evaluated in fruits stored at 25°C/80% RH for 4 days. Also, treatments combining chitosan and hot water were investigated for controlling green mould and the effect on postharvest quality of fruit stored at 5°C/90% RH. The results showed that P. digitatum conidia germination and mycelial growth were significantly reduced by the hot water treatments especially at 50°C/5 min and 55°C/2 or 5 min in the first case and 50 and 55°C/5 min in the second. These two treatments, when applied alone, 1 min dipping in 2% chitosan or hot water at 50°C/5 min, significantly reduced green mould development in fruit kept at 25°C/80% RH or refrigerated. However, the hot water dip combined with chitosan did not improve green mould control on ‘Murcott’ tangor at room temperature or under refrigeration. Besides, chitosan and hot water did not impair fruit quality. Thus, chitosan and hot water could be an alternative to synthetic fungicides to control green mould in citrus while also contributing to a decrease in the postharvest losses of ‘Murcott’ tangor.     

Control of green mold and sour rot of stored lemon by biofumigation with Muscodor albus

Control of postharvest lemon diseases by biofumigation with the volatile-producing fungus Muscodor albus was investigated. In vitro exposure to M. albus volatile compounds for 3 days killed Penicillium digitatum and Geotrichum citri-aurantii, causes of green mold and sour rot of lemons, respectively. Lemons were wound-inoculated with P. digitatum and placed in closed 11-L plastic boxes with rye grain cultures of M. albus at ambient temperature. There was no contact between the fungus and the fruit. Biofumigation for 24–72 h controlled green mold significantly, even when treatment began 24 h after inoculation. Effectiveness was related to the amount of M. albus present. In tests conducted inside a 11.7-m³ degreening room with 5 ppm ethylene at 20 °C, green mold incidence on lemons was reduced on average from 89.8 to 26.2% after exposure to M. albus for 48 h. Ethylene accelerates color development in harvested citrus fruit. M. albus had no effect on color development. Biofumigation in small boxes immediately after inoculation controlled sour rot, but was ineffective if applied 24 h later. G. citri-aurantii may be less sensitive to the volatile compounds than P. digitatum or escapes exposure within the fruit rind. Biofumigation with M. albus could control decay effectively in storage rooms or shipping packages.     

Fluidised-bed spray-drying formulations of Candida sake CPA-1 by adding biodegradable coatings to enhance their survival under stress conditions

The biocontrol agent Candida sake CPA-1 has demonstrated to be effective against several diseases on fruit. However, for application of CPA-1 under field conditions, it was necessary to mix it with a food coating to improve survival under stress conditions, as well as adherence and distribution on fruit surfaces. The objective of this study was to obtain a more competitive formulation under field conditions to be applied independently of any product. To achieve this purpose, the drying process of CPA-1 by a fluidised-bed spray-drying system together with biodegradable coatings was optimised. This approach is novel for the drying system used and the formulation obtained which was able to form a film or coating on fruit surfaces. Several substances were tested as carriers and binders, and drying temperature was optimised. The addition of protective compounds was also tested to improve survival of CPA-1 during the dehydration process. Product shelf life, biocontrol efficacy on grapes against Botrytis cinerea, and the improvement of C. sake behaviour under stress conditions were tested. The optimal temperature of drying was 55 °C and two formulations that were able to develop a coating on fruit surfaces were obtained. One of the formulations was created by using a combination of native and pregelatinised potato starch; the other formulation was obtained using maltodextrin and by adding skimmed milk and sucrose as protectant compounds. The formulated products reduced the incidence and severity of B. cinerea, and CPA-1 survival rate was increased under stress conditions of temperature and humidity.

     

Chlorine Dioxide for Reduction of Postharvest Pathogen Inoculum during Handling of Tree Fruits

Alternatives to hypochlorous acid and fungicides are needed for treatment of fruit and fruit-handling facilities. Chlorine dioxide was evaluated and found effective against common postharvest decay fungi and against filamentous fungi occurring on fruit packinghouse surfaces. In vitro tests with conidial or sporangiospore suspensions of Botrytis cinerea, Penicillium expansum, Mucor piriformis, and Cryptosporiopsis perennans demonstrated >99% spore mortality within 1 min when the fungi were exposed to aqueous chlorine dioxide at 3 or 5 μg · ml^-1. Longer exposure times were necessary to achieve similar spore mortalities with 1 μg · ml^-1. Of the fungi tested, B. cinerea and P. expansum were the least sensitive to ClO₂. In comparison with the number recovered from untreated control areas, the number of filamentous fungi recovered was significantly lower in swipe tests from hard surfaces such as belts and pads in a commercial apple and pear packinghouse after treatment of surfaces with a 14.0- to 18.0-μg · ml^-1 ClO₂ foam formulation. Chlorine dioxide has desirable properties as a sanitizing agent for postharvest decay management when residues of postharvest fungicides are not desired or allowed.     

Gamma irradiation‐induced disease resistance of pear (Pyrus pyrifolia “Niitaka”) against Penicillium expansum

In this study, the effects of gamma irradiation on the resistance of pear fruit against Penicillium expansum, the causal agent of blue mould disease, were investigated. A low dose of gamma irradiation for 14 days increased the disease resistance and firmness of pear fruits. Remarkably, exposure to 200 Gy of gamma irradiation significantly maintained fruit firmness, markedly reduced disease incidence and enhanced the activity of defence‐related enzymes (e.g., β‐1,3‐glucanase, phenylalanine ammonia lyase, peroxidase and polyphenol oxidase) and expression of pathogenesis‐related (PR) genes (e.g., PR‐1, PR‐3 and PR‐4). Therefore, the gamma irradiation‐induced resistance against P. expansum involves both metabolic changes and the induction of expression of defence‐related genes. In addition, scanning electron microscopic analysis revealed that gamma irradiation significantly inhibits the growth of P. expansum. These results suggest that exposure of mature harvested pear fruits to artificial gamma irradiation confers fungal disease resistance; therefore, gamma irradiation represents an important strategy for controlling postharvest diseases in pear fruit.     

Selection of a Suitable Medium to Determine Sensitivity of Monilinia fructicola Mycelium to SDHI Fungicides

Succinate dehydrogenase inhibitor (SDHI) fungicides are important tools to control preharvest and postharvest brown rot of stone fruits. In this study we determined the mycelial growth rate of Monilinia fructicola isolates on various media and picked the three that allowed the fastest growth to assess the sensitivity of mycelium to SDHI fungicides boscalid, fluopyram, and penthiopyrad. Minimal medium (MM) supported mycelial growth the best and yielded lowest EC₅₀ values for three SDHI fungicides. EC₅₀ values corresponded with disease incidence data obtained from detached fruit assays. Penthiopyrad had significantly greater intrinsic activity in vitro compared to fluopyram at the α = 0.05 level and compared to boscalid at the α = 0.1 level. However, detached fruit assays revealed that this ‘advantage’ did not carry over in vivo. In conclusion, MM appears to be the best medium currently available to assess the sensitivity of M. fructicola mycelium in vitro.