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Oxidizing compounds such as sodium hypochlorite (NaCIO) and hydrogen peroxide (H2O2) are widely used in food sanitization because of their antimicrobial effects. We applied these compounds and metals to analyze their antifungal activity against Penicillium digitatum, the causal agent of citrus green mold. The MICs were 300 ppm for NaClO and 300 mM for H2O2 when these compounds were individually applied for 2 min to conidia suspensions. To minimize the concentration of these compounds, we developed and standardized a sequential treatment for conidia that resulted in loss of viability on growth plates and loss of infectivity on lemons. The in vitro treatment consists of preincubation with 10 ppm of NaClO followed by incubation with 100 mM H2O2 and 6 mM CuSO4 (cupric sulfate). The combination of NaClO and H2O2 in the presence of CuSO4 produces a synergistic effect (fractional inhibitory concentration index of 0.36). The sequential treatment applied in situ on lemon peel 24 h after the fruit was inoculated with conidia produced a significant delay in the fungal infection. The in vitro treatment was effective on both imazalil-sensitive and imazalil-resistant strains of P. digitatum and Geotrichum candidum, the causal agent of citrus sour rot. However, this treatment inhibited 90% of mycelial growth for Penicillium italicum (citrus blue mold). These results indicate that sequential treatment may be useful for postharvest control of citrus fruit diseases.

Cerioni, L., Rapisarda, V. A., Doctor, J., Eikkert, S., Ruiz, T, Eassel, R., and Smilanick, J. L. 2013. Use of phosphite salts in laboratory and semicommercial tests to control citrus postharvest decay. Plant Dis. 97;201-212.Potassium phosphite (KP) concentrations that inhibited the germination of 50% of Pénicillium digitatum conidia were 229, 334, 360, 469, 498, or 580 mg/liter at pH 3, 4, 5, 6, 7, or 8, respectively. Increasing phosphate content in media reduced phosphite toxicity. To control green or blue mold, fruit were inoculated with P. digitatum or P. italicum, then immersed 24 h later in KP, calcium phosphite (CaP), sodium carbonate, sodium bicarbonate, or potassium sórbate for 1 min at 20 g/liter for each at 25 or 50°C. Mold incidence was lowest after potassium sórbate, CaP, or KP treatments at 50°C. CaP was often more effective than KP but left a white residue on fruit. KP was significantly Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture (USDA). USDA is an equal opportunity employer.

Phytopathogenic fungi responsible for post-harvest diseases on fruit and vegetables cause important economic losses. We have previously reported that harmol (1-methyl-9H-pyrido[3,4-b]indol-7-ol) is active against the causal agents of green and gray molds Penicillium digitatum and Botrytis cinerea, respectively. Here, antifungal activity of harmol was characterized in terms of pH dependency and conidial targets; also photodynamic effects of UVA irradiation on the antimicrobial action were evaluated. Harmol was able to inhibit the growth of both post-harvest fungal disease agents only in acidic conditions (pH 5), when it was found in its protonated form. Conidia treated with harmol exhibited membrane integrity loss, cell wall disruption, and cytoplasm disorganization. All these deleterious effects were more evident for B. cinerea in comparison to P. digitatum. When conidial suspensions were irradiated with UVA in the presence of harmol, antimicrobial activity against both pathogens was enhanced, compared to non-irradiated conditions. B. cinerea exhibited a high intracellular production of reactive oxygen species (ROS) when was incubated with harmol in irradiated and non-irradiated treatments. P. digitatum showed a significant increase in ROS accumulation only when treated with photoexcited harmol. The present work contributes to unravel the antifungal activity of harmol and its photoexcited counterpart against phytopathogenic conidia, focusing on ROS accumulation which could account for damage on different cellular targets.

Aim:  To investigate the cellular damage on Penicillium digitatum produced by a sequential oxidative treatment (SOT), previously standardized in our laboratory, to prevent the conidia growth. Lethal SOT consists of 2‐min preincubation with 10 ppm NaClO followed by 2‐min incubation with 6 mmol l−1 CuSO4 and 100 mmol l−1 H2O2 at 25°C. Methods and Results:  After the application of lethal SOT or sublethal SOT (decreasing only the H2O2 concentration), we analysed several conidia features such as germination, oxygen consumption, ultrastructure and integrity of the cellular wall and membrane. Also, we measured the production of reactive oxygen species (ROS) and the content of thiobarbituric acid‐reactive species (TBARS). With the increase of H2O2 concentration in the SOT, germination and oxygen consumption of conidia became inhibited, while the membrane permeability, ROS production and TBARS content of conidia increased. Several studies revealed ultrastructural disorganization in P. digitatum conidia after lethal SOT, showing severe cellular damage without apparent damage to the cell wall. In addition, mycelium of P. digitatum was more sensitive than conidia to the oxidative treatment, because growth ceased and permeability of the membranes increased after exposure of the mycelium to a SOT with only 50 mmol l−1 H2O2 compared to a SOT of 100 mmol l−1 for these effects to occur on conidia. Conclusion:  Our insights into cellular changes produced by the lethal SOT are consistent with the mode of action of the oxidant compounds, by producing both alteration of membrane integrity and intracellular damage. Significance and Impact of the Study:  Our results allow the understanding of SOT effects on P. digitatum, which will be useful to develop a reliable treatment to control postharvest diseases in view of its future application in packing houses.

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