N. K. B. Adikaram

Biocontrol activity and induced resistance as a possible mode of action for Aureobasidium pullulans against grey mould of strawberry fruit

Efficacy and mode of action were investigated fox Aureobasidium pullulans, a potential biocontrol agent for grey mould on strawberry fruit. Wound inoculation of detached green strawberry cv. Elsanta fruit with Aureobasidium pullulans prevented grey mould rot on fruit inoculated 2 days later with Botrytis cinerea. Treatment of white, pink and red-ripe fruit did not, however, control grey mould. Treatment of wound sites on green fruit with both live and heat-killed A. pullulans cells reduced B. cinerea infection compared to controls. Dip-inoculation of unwounded green strawberry fruit with A pullulans when still attached to the plant delayed the development of grey mould after harvest at the fully ripe stage. The (i) efficacy of A. pullulans on green but not on ripening fruit, (ii) partial inhibitory effect of both live and heat-killed A. pullulans cells and (iii) absence of evidence for antibiotic production from in vitro competition tests suggest that control of grey mould on green fruit is at least partly due to a mechanism other than antagonism and / or competition. Bioassays showed that skin tissue from green fruit treated with A. pullulans had greater antifungal activity than control tissue. Thus, enhanced natural disease resistance in green strawberry fruit contributed to grey mould rot suppression by A. pullulans.

Attack on tea by Xyleborus fornicatus: inhibition of the symbiote, Monacrosporium ambrosium, by caffeine.

Abstract Methylene chloride extracts of tea ( Camellia sinensis ), stems from two clones, TRI 2023 and TRI 2025, inhibited growth of the fungus, Monacrosporium ambrosium , the symbiote of the shot-hole borer beetle, Xyleborus fornicatus . Activity-guided fractionation of the extracts yielded caffeine as the major antifungal compound. The caffeine content of healthy pencil-thick stems of the two clones was estimated and compared with the caffeine content of pencil-thick stems infected by the beetle. Healthy stems of clone TRI 2023 had a very low caffeine content, but accumulation of caffeine was found to have occurred to a greater extent in infected samples of stems from this clone. Infected stems from both clones had a higher caffeine content than samples of healthy stems. The effect of caffeine on mycelial growth, sporulation and spore germination of M. ambrosium in liquid and agar media was also studied. Inhibition of mycelial growth was observed with 5000 ppm of caffeine, while 500 ppm resulted in 21% inhibition in the liquid medium. In the agar medium, colony-size was reduced by 500 ppm of caffeine, while no growth took place with 5000 ppm of caffeine. Inhibition of sporulation was observed with 2000 ppm of caffeine. Germination of conidia was inhibited completely with 3750 ppm of caffeine; inhibition was also observed with 500–2500 ppm of caffeine.

An antifungal chromene from Eupatorium riparium

Abstract Methylripariochromene A (6-acetyl-7,8-dimethoxy-2,2-dimethylchromene), a root constituent of Eupatorium riparium , displayed antifungal activity against five of the seven fungal species tested. The chromene showed a toxicity to the fungus Colletotrichum gloeosporioides , a tropical pathogen, comparable to that of a commercial fungicide, although the chromene appeared to be fungistatic.

Ethylene production by Colletotrichum musae in vitro

Seven isolates of the pathogen Colletotrichum musae (Berk & Curt.) v. arx. were isolated from banana fruit. These isolates produced ethylene to varying degrees in methionine-amended Czapek Dox liquid medium as both shake and static cultures. Rates of ethylene production by C. musae were positively associated with the concentration of methionine in the growth medium. C. musae did not produce ethylene on basal medium containing L-glutamate, alpha-ketoglutarate or L-cysteine. Isolate CM 100 produced the highest cumulative amount of ethylene (2-27 muM g(-1) dry wt) over 12 days on 35 mm methionine-amended shake cultures of basal medium. In the presence of methionine, ethylene biosynthesis by C. musae occurred via 2-keto-4-methylthiobutyric acid (KMBA). The capacity of C. musae to produce ethylene may have a role in its pathogenicity on climacteric banana fruit

Heat-induced tolerance to internal browning of pineapple (Ananas comosus cv. ‘Mauritius’) under cold storage

Summary Pineapple (cv. ‘Mauritius’) fruit, under low temperature (10°C) and 85% RH storage, develops internal browning symptoms. Fruit acidity, polyphenol oxidase (PPO) and peroxidase (POD) enzyme activities increased sharply, and phenylalanine ammonia-lyase (PAL) activity increased slightly during low temperature storage, concurrent with the development of internal browning. Harvesting fruit early reduced both the incidence and severity of internal browning. Post-harvest heat-treatment, in the form of a hot water dip, induced pineapple fruit tolerance to cold injury and, in turn, reduced internal browning during prolonged low temperature storage. Several temperature-time combinations were effective, but the best was 38°C for 60 min. Pineapple fruits treated at 38°C for 60 min developed 70% and 45% less browning than untreated controls in the flesh and core regions, respectively. The results also indicate that an internal tissue temperature of 37°C is a prerequisite for the induction of cold tolerance. Heat treatment, however, slowed down fruit ripening and increased water loss. Wrapping heat-treated fruits in polythene, exposing only the crown, prior to cold-storage reduced internal browning by a further 10% and water loss by 8.5%, giving fruit a better appearance.

Cultivar differences in antifungal activity and the resistance to postharvest anthracnose and stem-end rot in mango (Mangifera indica L.)

The study investigated the possible role of the three classes of constitutive antifungal substances in mangoes in the differential cultivar resistance to anthracnose and stem-end rot (SER). Fruits of eleven mango cultivars were screened for anthracnose and SER development during ripening. Basal level of antifungal activity due to constitutive gallotannins, resorcinols and chitinase in the fruit peel/latex was determined in six of these cultivars. The results showed a clear variability in the resistance of fruits to anthracnose and SER among cultivars. The cultivars, ‘Gira’ and ‘Karutha Colomban’, were among the most resistant to anthracnose disease but were susceptible to SER. Gallotannin activity was high in unripe and ripe fruit peel of all except ‘Willard’, the cultivar most susceptible to anthracnose but resistant to SER. There was a significant negative correlation between the level of gallotannins in the fruit at ripe stage and anthracnose development among six cultivars tested. ‘Gira’ and ‘Karutha Colomban’ that are among the most resistant cultivars to anthracnose showed higher levels of total resorcinols in the latex and 5-(12-cis-heptadecenyl)resorcinol in the unripe fruit peel. Chitinase activity was highest in ‘Rata’ and ‘Kohu’ that are resistant to SER and lowest in cultivars ‘Gira’ and ‘Karutha Colomban’ that are susceptible to SER. The results suggest that mainly the gallotannins and also 5-(12-cis-heptadecenyl)resorcinol contribute to the cultivar resistance of mango fruit to anthracnose pathogen, Colletotrichum gloeosporioides, while chitinases appear to contribute more to the fruit resistance to the SER pathogen, Botryodiplodia theobromae.

The Role of Pre-formed Antifungal Substances in the Resistance of Fruits to Postharvest Pathogens

Plants contain secondary metabolites with antifungal properties. In fruits they are mostly concentrated in the peel at immature stage and decline during ripening in coincidence with fungal rot development. The information on antifungal systems in immature avocado and mango, reviewed here, suggests that they play a role in natural disease resistance. Immature mangoes have evolved a formidable antifungal system comprising several resorcinols, gallotannins and chitinases. Resorcinols and gallotannins are inhibitory to major postharvest pathogens, Colletotrichum gloeosporioides causing anthracnose and Botryodiplodia theobromae causing stem-end rot. Their levels are generally higher in resistant cultivars than in susceptible ones. Mango latex, distributed in a fine network of canals in the fruit peel, contains chitinases which have the ability to rapidly digest conidia of C. gloeosporioides. Gallotannins and resorcinols decline progressively during ripening and the latex disappears when ripe rot development begins. Retention of latex in the harvested fruit reduces anthracnose and stem-end rot development during ripening. Treatment of harvested fruit with CO2 or inoculation with certain non-pathogenic fungi increased antifungal resorcinol concentration. Immature avocado fruits possess a pre-formed antifungal system comprising at least five antifungal compounds. The quiescence of C. gloeosporioides in the immature fruit has been attributed to the pre-formed antifungal activity of the peel. Lipoxygenase activity increases during fruit ripening, while epicatechin levels decline, suggesting that these events are linked to the decrease in di-ene concentrations. Inhibition of lipoxygenase activity results in retention of antifungal di-ene during ripening increasing fruit resistance. In freshly harvested avocados, the di-ene concentration can be further enhanced by treatment with biotic and abiotic agents.

Pathogen-produced ethylene and the Colletotrichum musae-banana fruit pathosystem

Colletotrichum musae isolate CM100 is capable of producing ethylene in vitro on methionine-supplemented basal medium. This isolate also produced ethylene on peel extracts of banana fruit that contained methionine at 0.31–0.42 μmol/g fresh weight. Ethylene production rates by fresh banana peel strip explants and by whole fruit were not significantly increased after C. musae infection compared with non-infected control tissues. Moreover, exogenous application of methionine did not increase either ethylene production or disease severity on banana peel strips or on whole fruit. Overall, these findings suggest that C. musae could not utilise methionine in the host tissue for ethylene production. Autoclaved banana peel strips produced negligible amounts of ethylene compared with non-autoclaved tissues irrespective of C. musae infection. This observation suggests that ethylene evolved by C. musae-infected banana tissues is of host origin. Considered collectively, the results of this study indicate that the ability of C. musae to produce ethylene in vitro does not play a role in its pathogenicity in planta.

Enhanced cold-tolerance in pineapple (Ananas comosus 'Mauritius') by combined cold- and heat-shock treatments or intermittent warming

Summary Internal browning (IB) is a physiological disorder that develops in harvested pineapple (Ananas comosus) ‘Mauritius’ fruit during prolonged periods of cold storage. A brief exposure of fruit to low temperature (i.e., 4°C for 60 min) prior to cold storage at 10°C reduced the incidence of IB by 40% in both core and flesh tissues. When subjected to 4ºC for 60 min before or after a heat-shock (38°C for 60 min), pineapple fruit developed 88% and 40% less IB in the flesh and core areas, respectively, than in control fruit which were not subjected to 4ºC for 60 min. Intermittent warming (IW) involving the exposure of pineapple fruit to 28° – 30°C for 8 h every 6 d during 21 d of storage at 10°C and 85% RH, reduced the incidence of IB by ≥ 80% in the core, and by ≥ 50% in flesh tissue. Fruit subjected to IW showed only isolated areas of IB in their flesh tissue, and this was found in only 40% of the total fruit treated. Heat-shock (38°C for 60 min) before or after a low temperature (i.e., 4°C for 60 min) treatment, slowed fruit ripening slightly, but this effect was not observed in fruit subjected to IW. Cell damage was less in fruit tissues showing no symptoms of IB when fruit were given a heat-shock treatment, before or after a low temperature treatment.