Rachid Lahlali

A Box-Behnken design for predicting the combined effects of relative humidity and temperature on antagonistic yeast population density at the surface of apples.

The objective of this work was to develop models predicting the combined effects of relative humidity (RH, 75-98%), temperature (5-25 degrees C), and initial applied yeast concentration (10(4)-10(8) CFU/ml) on the apple-surface population densities of two biocontrol agents fused against postharvest diseases; the antagonistic yeasts Pichia anomala strain K and Candida oleophila strain O. Experiments were carried out according to a Box-Behnken matrix. Multiple regression analyses showed that both models yielded a good prediction of yeast density. The effect of relative humidity appeared greater than that of temperature. The number of yeast colony-forming units per square centimeter of apple fruit surface increased with increasing relative humidity, temperature, and initial applied yeast concentration. The models predict that under optimal growth conditions (25 degrees C, 98%), strains O and K should reach a density of 10(4) CFU/cm2 when applied initially at 2 x 10(7) (strain O) or 10(7) CFU/ml (strain K). The model results suggest that rainfall was likely the principal cause of the variability of yeast efficacy reported for previous preharvest orchard trials spanning two successive years. Temperature may also contribute to this variation. The models developed here are important tools for predicting population densities of both strains on the apple surface within the experimental limits. The use of these results should contribute to achieving yeast densities of 10(4) CFU/cm2 on apples by controlling yeast application and environmental factors such as relative humidity and temperature. The results of this study also confirm our previous in vitro findings that water activity has a greater effect than temperature on yeast population density.

Response surface methodology study of the combined effects of temperature, pH, and aw on the growth rate of Trichoderma asperellum

Aims:  To evaluate the influence of environmental parameters (water activity aw, temperature, and pH) on the radial growth rate of Trichoderma asperellum (strains PR10, PR11, PR12, and 659‐7), an antagonist of Phytophthora megakarya, the causal agent of cocoa black pod disease.

Production and oil-emulsion formulation of Cadophora malorum and Alternaria jacinthicola, two biocontrol agents against Water Hyacinth (Eichhornia crassipes)

Cadophora malorum isolate Mln715 and Alternaria jacinthicola strain MUCL 53159 are under development as biocontrol agents against Water Hyacinth (Eichhornia crassipes) in Mali. Production of spores of these agents on locally available substrates (Water Hyacinth, powdered paddy rice chaff, wheat semolina) was assessed with a view to mass production. The C. malorum isolate sporulated best on Water Hyacinth (4.08 × 10 7 spores ml -1 ), followed by wheat (1.06 ×10 7 spores ml -1 ), whereas A. jacinthicola produced more spores on paddy rice chaff and wheat (0.24 x 10 7 spores ml -1 ). The severity of the damage caused by each pathogen was evaluated in the greenhouse and in the field. Under both greenhouse and field conditions, the biocontrol efficacy of the fungal isolates was improved with (unrefined) Carapa procera (L.) oil or (refined) palm oil, supplemented with soybean lecithin and Tween 20. When such a formulation was used, the incubation time was 4 to 5 days in the greenhouse and 7 to 9 days on the field, and the damage severity (DS) recorded 6 weeks after treatment varied from 87.02 to 93.13% in the greenhouse and from 59.11 to 63.00% in the field. For unformulated C. malorum and A. jacinthicola respectively, the incubation times were longer and the DS values were only 22.11 and 29.05% in the greenhouse and 12.05 and 15.15% on the field. Our results highlight good substrates for mass production of these mycoherbicides and demonstrate the ability of vegetable oil formulations to improve their efficacy.

A new semi-selective agar medium for recovery and enumeration of antagonistic yeast, Pichia guilliermondii strain Z1 from orange fruit surface

The aim of this work was to develop a semi-selective medium for recovery and enumeration of Pichia guilliermondii strain Z1, a reliable biocontrol agent against postharvest pathogens of citrus fruit, and to assess the population dynamic of this antagonistic yeast on orange fruit in relation to incubation temperature and time of incubation. PDA is the basal medium used in this study which allows the antagonistic strain Z1 source of carbon and nutrients. Different chemicals (thiophanate-methyl, thiabendazole, thiram and imazalil) and antibiotics (hygromycin, tetracyclin, ampicillin and chloramphenicol) have been tested individually and based on the plating efficiency up to 90%, 40 combinations have been assayed between antibiotics and fungicides. Ten combinations proved to be highly selective against citrus pathogens and laboratory microflora, but only one consisting of tetracyclin (1 g/L) and thiabendazole (60 mg/L) was retained based on plating efficiency up to 99%, total selectivity against laboratory microflora, epiphytic microflora from washed orange fruit and lower cost. The semi-selective medium TET-TBZ-PDA has been used to assess the impact of temperature and incubation time on the survival of strain Z1. It appears that population density was significantly influenced by both factors and the highest population size was recorded at 25°C followed by 5 and 35°C, respectively. This strain required a time of adaptation before entering the exponential growth phase with a maximum growth observed at 25°C relative to others. The semi-selective medium TETTBZ-PDA could be an efficient and valuable way to track the population density of this strain on the surface of orange when applied pre-or postharvest. This semi-selective medium may also aid in reaching a population density allowing a better efficiency in relation to environmental conditions.