Overcoming lemon postharvest molds caused by Penicillium spp. multiresistant isolates by the application of potassium sorbate in aqueous and wax treatments.

Penicillium digitatum and Penicillium italicum are the main causal agents of postharvest diseases in lemon. Over the last decades, the appearance of isolates resistant to the main commercial fungicides has been considered one of the most serious problems for the citrus industry. In this work, potassium sorbate (KS) was evaluated as an alternative to chemical fungicides to control postharvest diseases caused by Penicillium isolates resistant to imazalil, thiabendazol, and pyrimethanil. In vitro assays showed that 1% KS inhibited conidia germination and mycelial growth of sensitive and resistant P. digitatum and P. italicum isolates, being this effect stronger at pH 5 than at pH 9. In curative treatments, the immersion of inoculated lemons in 1% KS aqueous solution for 30 s reduced green and blue molds incidences by around 80%. No wound protection effect was observed when wounded lemons were immersed in 3% salt solution before inoculation. Noteworthy, the inclusion of KS in a commercial wax coating effectively controlled green and blue molds, even in decays caused by fungicide resistance isolates. Together, results encourage the use of KS in lemon postharvest treatments to contribute to the management of resistant strains, which represent a major challenge in packinghouses worldwide. PRACTICAL APPLICATION: The use of KS in citrus postharvest treatments would help producers to reduce spoilage caused by Penicillium fungicide-resistant strains. The inclusion of this generally recognized as safe compound in wax coatings improves its persistence on the fruit surface, keeping product quality during long-term overseas transport. In sum, KS constitutes an affordable and eco-friendly option for controlling postharvest molds in lemon fruit.

Inhibition of the lemon brown rot causal agent Phytophthora citrophthora by low-toxicity compounds.

BACKGROUND: Phytophthora spp., soil-borne oomycetes, cause brown rot (BR) on postharvest lemons. The management of this disease is based on cultural practices and chemical control using inorganic salts of limited efficacy. In the search for new alternatives, the aim of this work was to evaluate the effect of low-toxicity compounds to inhibit the growth of P. citrophthora and to control BR disease on lemons. Sodium bicarbonate, potassium sorbate, polyhexamethylene guanidine, Ascophyllum nodosum extract and a formulation containing phosphite salts plus A. nodosum (P+An) were evaluated. RESULTS: All tested products inhibited mycelial growth, sporangia formation and zoospore germination of P. citrophthora in vitro. In postharvest applications on artificially inoculated lemons, only P+An exhibited a BR curative effect, with incidence reduction of around 60%. When this formulation was applied in field treatments, BR incidence was reduced by 40% on lemons harvested and inoculated up to 30 days post application. CONCLUSION: Our results demonstrate the in vitro direct anti-oomycete effect of low-toxicity compounds and the in vivo efficacy of P+An formulation to control BR, encouraging the incorporation of the latter in the management of citrus BR. © 2020 Society of Chemical Industry.

Polyhexamethylene guanidine as a fungicide, disinfectant and wound protector in lemons challenged with Penicillium digitatum.

Citrus green mold, a postharvest disease caused by Penicillium digitatum, provokes important economic losses on lemon production. Here, the effectiveness of polyhexamethylene guanidine (PHMG) to inhibit P. digitatum growth and to control green mold on artificially infected lemons was evaluated. At sublethal concentrations, PHMG inhibited conidia germination and infectivity (5 mg L-1), and mycelial growth (50 mg L-1). Viability of conidia was completely suppressed by treatment with 500 mg L-1 PHMG. In this condition, membrane integrity loss, cell wall disruption and ultrastructural alterations were detected, as well as conidia distortion, deformation and collapse. In artificially inoculated lemons, a 30 s-immersion in 500 mg L-1 PHMG completely inhibited green mold. PHMG also exhibited a high disinfectant activity, even in the presence of 1% organic matter, with a better performance than the standard NaClO disinfectant. In addition, 500 mg L-1 PHMG protected wounds against infection. Taken together, our results indicate that PHMG is a promising fungicide for the postharvest control of green mold in lemon packinghouses.

Native Killer Yeasts as Biocontrol Agents of Postharvest Fungal Diseases in Lemons.

Economic losses caused by postharvest diseases represent one of the main problems of the citrus industry worldwide. The major diseases affecting citrus are the "green mold" and "blue mold", caused by Penicillium digitatum and P. italicum, respectively. To control them, synthetic fungicides are the most commonly used method. However, often the emergence of resistant strains occurs and their use is becoming more restricted because of toxic effects and environmental pollution they generate, combined with trade barriers to international markets. The aim of this work was to isolate indigenous killer yeasts with antagonistic activity against fungal postharvest diseases in lemons, and to determine their control efficiency in in vitro and in vivo assays. Among 437 yeast isolates, 8.5% show to have a killer phenotype. According to molecular identification, based on the 26S rDNA D1/D2 domain sequences analysis, strains were identified belonging to the genera Saccharomyces, Wickerhamomyces, Kazachstania, Pichia, Candida and Clavispora. Killers were challenged with pathogenic molds and strains that caused the maximum in vitro inhibition of P. digitatum were selected for in vivo assays. Two strains of Pichia and one strain of Wickerhamomyces depicted a significant protection (p <0.05) from decay by P. digitatum in assays using wounded lemons. Thus, the native killer yeasts studied in this work showed to be an effective alternative for the biocontrol of postharvest fungal infections of lemons and could be promising agents for the development of commercial products for the biological control industry.