Lise Korsten

Iturin A is the principal inhibitor in the biocontrol activity of Bacillus amyloliquefaciens PPCB004 against postharvest fungal pathogens

Aims:  A Bacillus amyloliquefaciens strain, surviving epiphytically on the surface of fruit, was isolated while searching for naturally occurring biological control agents. This bacterial strain was characterized for its antifungal activity against seven selected fungal postharvest pathogens of citrus.

Biological control of postharvest diseases of fruits and vegetables

Publisher Summary Worldwide, postharvest losses of fruits and vegetables are estimated at more than 25%, and much of this is due to postharvest decay. This chapter provides an overview of the potential application, and limitations of microbial antagonists to prolong shelf-life, and control of decay of harvested commodities. Biocontrol products are expected to protect wounds from being infected during processing, deactivate wound infection occurring during the harvesting process, and this protection must last during storage, shipping and retailing phase. If, we are to maximize the biocontrol potential of microbial antagonists, a more fundamental understanding of their mode of action, their ecology, their compatibility with postharvest commercial practices, and the effect of host physiology on their biological activity is needed. Several other promising biological control approaches that include the use of bioactive compounds and induced resistance are available and can be used in combination with microbial antagonists to provide a greater stability and effectiveness than the approach of utilizing a single microbial biocontrol agent.

Genetic Diversity Among Alternaria solani Isolates from Potatoes in South Africa

Genetic diversity among isolates of Alternaria solani, the causal agent of early blight of potato, from various potato-growing regions in South Africa (SA), was determined using virulence assays, vegetative compatibility (VC) tests, and random amplified microsatellite (RAMS) primers. The virulence assays showed low virulence levels for the largest part of the population, but failed to otherwise characterize the population diversity. The VC tests revealed 19 VC groups (VCGs), indicating a relatively high level of diversity among the isolates. There was little correlation between geographic origin of isolates and VCGs. Analysis of RAMS profiles revealed 27% genetic diversity among 46 isolates. This value is relatively high for an asexually reproducing fungus, but is similar to values obtained previously by authors studying A. solani. Distance analysis of the RAMS profiles also provided no evidence for geographical clustering of isolates. VCG and RAMS profiles indicated that isolates are randomly spread across SA. This fact, together with the high diversity of A. solani in SA, indicates that the fungus has a high potential to adapt to resistant cultivars or fungicides. This information can aid in the breeding and deployment of A. solani-resistant potato varieties, and in early blight disease management in SA.

Pectobacterium carotovorum subsp. brasiliensis causing blackleg on potatoes in South Africa

In South Africa during the 2006/2007 potato growing season, outbreaks of blackleg occurred, causing severe economic losses in commercial potato production fields. Symptoms were initially observed on only one stem per plant, on which the top leaves rolled upwards, wilted and became necrotic. As symptoms progressed to the lower leaves with subsequent leaf desiccation, a light to dark brown discolouration of the vascular system at the stem base developed, followed by external darkening. Under prevailing wet and humid conditions stems became slimy and pale. In the stems, the pith became necrotic and hollow. These symptoms were similar to those described in Brazil, where the causal agent was identified as a new subspecies, Pectobacterium carotovorum subsp. brasiliensis (Pbcb). Isolations from plants showing typical blackleg symptoms were made on CVP medium. Sequences and phylogenetic analysis of the partial 16S–23S rDNA intergenic spacer region indicated that the isolates were Pbcb. Comparison of PCR-RFLP patterns of the 16S–23S rDNA of isolates to reference cultures confirmed the identity of the South African blackleg strains as Pbcb, identical to strain 8 isolated in Brazil. This is the first report of Pbcb in South Africa and it appears to be the most important causal agent of blackleg in South Africa. The disease poses a major potential threat to the South African potato industry especially in terms of seed exports, tuber quality and yield.

Microbial Hazards in Irrigation Water: Standards, Norms, and Testing to Manage Use of Water in Fresh Produce Primary Production

Accessibility to abundant sources of high-quality water is integral to the production of safe and wholesome fresh produce. However, access to safe water is becoming increasingly difficult in many parts of the world, and this can lead to the production of fresh produce contaminated with pathogenic microorganisms, resulting in increased risk of human disease. Water, an important raw material in the fresh produce chain, is used in considerable amounts in many operations, including irrigation and application of pesticides and fertilizers, but also as a transport medium and for cooling and washing in postharvest practices. In several reported outbreaks related to uncooked fruit and vegetable products, water has been identified as a likely source of the outbreak. The present study, initiated by the ILSI Europe Emerging Microbiological Issues Task Force in collaboration with 8 other ILSI branches and support of WHO/FAO, was undertaken to review the status of, and provide suggestions for, consideration by different stakeholders on water and sanitation and its impact on food safety and public health. A limited number of guidelines and regulations on water quality for agricultural production are available, and many of them are still heavily based on microbial standards and (debated) parameters such as fecal coliforms. Data gaps have been identified with regard to baseline studies of microbial pathogens in water sources in many regions, the need for agreement on methods and microbial parameters to be used in assessing water quality, the fate of pathogens in water, and their transfer and persistence on irrigated/processed produce.

Advances in control of postharvest diseases in tropical fresh produce

With increased global trade, fresh produce is increasingly being moved over vast distances to foreign markets. This requires effective cold chain and product management practices to ensure premium quality fresh produce with increased shelf life. Product safety has now also become an additional minimum requirement for market entry. Producers should therefore take cognisance of the impact postharvest treatments and practices have on product quality and safety in a more holistic approach. Traditionally, exporters relied heavily on agricultural chemicals to provide effective crop protection. However, with global chemophobia, producers have to be more resilient in terms of alternative postharvest treatments. This paper will deal with examples of new advances in the control of postharvest diseases of tropical produce. Treatments evaluated in our studies included biological, physical and integrated control. These proved effective against a range of postharvest pathogens of citrus and subtropical crops.

Control of citrus green and blue molds with garlic extracts

Water and ethanol extracts of garlic cloves were applied to artificially inoculated citrus fruits to test their efficacy in the control of Penicillium digitatum and P. italicum, the cause of citrus green and blue mold respectively. Extracts were tested either alone, or in combination with vegetable (sunflower) cooking oil or fruit wax at the rate of (0.1% v/v), using two orange cultivars (Valencia and Shamouti), and grapefruit. Treated fruits were stored at 10±1 °C, and 90–95% relative humidity for 30 days. All concentrations of extracts were more effective than the water control in inhibiting the growth and development of both pathogens, but were not as effective as the fungicide treatment (imazalil 500 ppm + quazatine 1000 ppm). A remarkable increase in the activity of garlic extracts was observed when extracts were mixed with oil. Consequently, the treatment comprising 1% extract plus oil was as effective (100% control) as the fungicide treatment in controlling both green and blue molds on Valencia oranges.

Comparison of cross inoculation potential of South African avocado and mango isolates of Colletotrichum gloeosporioides

Three hundred and eighteen Colletotrichum gloeosporioides isolates from stem-end rot and anthracnose infected avocados as well as from stem-end rot/anthracnose and soft brown rot on mango, were compared using fruit inoculations. Isolates could be categorised according to lesion size and both avocado and mango isolates produced larger lesions when inoculated on their own hosts. Cross-inoculation potential of these isolates was also compared on strawberries, peppers, guavas, papayas and citrus. All isolates produced lesions on all hosts except citrus. Factors such as area of origin and symptom type from which original isolations were made, could not be correlated with lesion development on these hosts.

Effect of different post-harvest treatments on overall quality retention in litchi fruit during low temperature storage

Summary Freshly-harvested litchi (Litchi chinensis Sonn., L) fruit, of the cultivar ‘Mauritius’, were subjected to 5 min post-harvest dip treatments in aqueous solutions of: potassium metabisulphite, or Vapogard , or Chitosan or combined treatments of: potassium metabisulphite + Vapogard , or Chitosan + potassium metabisulphite, or Anolyte + Triton X-100 + Chitosan, or ammonia + Carnauba wax. Standard commercial sulphur dioxide fumigation was included as a comparative control. After treatment, fruit were held at 1°C, 95% RH for 21 d to simulate refrigerated sea shipment conditions, then held at 18°C, 80% RH for 2 d. Several parameters were used to evaluate fruit quality. Combined post-harvest dip treatment with potassium metabisulphite + Vapogard retained fruit marketability most effectively, preventing severe browning post-harvest diseases, and retaining fruit firmness. Potassium metabisulphite + Vapogard dip treatment also revealed superior eating quality, with a 19.5% soluble solids concentration, 0.2% titratable acidity and a decline in anthocyanin concentration. Fruit fumigated with sulphur dioxide showed increased weight loss with intensified micro-cracks on the pericarp. Chitosan was most effective in reducing the total microbial fructoplane population compared to other treatments. Penicillium expansum Link. and Cladosporium spp. were identified as the predominant fungi on litchi fruit used in this trial.

A One-Day Sensitive Method to Detect and Distinguish Between the Citrus Black Spot Pathogen Guignardia citricarpa and the Endophyte Guignardia mangiferae

If South African citrus exporters wish to retain their competitive edge in the European market and access new markets such as the United States of America, it is of quarantine importance to distinguish between the citrus black spot pathogen, Guignardia citricarpa, and the harmless endophyte, G. mangiferae. The endophyte is not a sanitary or phytosanitary concern. This paper describes the design of species-specific primers that are able to detect and distinguish between these two Guignardia species. Application of the primer set CITRIC1 and CAMEL2 in conjunction with the ITS4 primer yielded polymerase chain reaction (PCR) amplicons of approximately 580 bp and 430 bp for G. citricarpa and G. mangiferae, respectively. Results obtained with these primers are in accordance with sequence data, and repeated tests verified accuracy and sensitivity. A BLAST search revealed no matches other than G. citricarpa and G. mangiferae, and no positive PCR results were obtained with Colletotrichum gloeosporioides, which is the most common contaminant in black spot lesions. We are, therefore, able to distinguish G. citricarpa and G. mangiferae unequivocally using a PCR-based method. This method was further improved to directly isolate DNA from fruit lesions by means of the DNeasy Plant Mini Kit (Qiagen). This eliminates the prior need for culturing the slow-growing organism, thereby shortening the time required to one day to test for and verify the presence or absence of the pathogenic G. citricarpa in export consignments.