Generally Recognized as Safe Salts for a Natural Strategy to Managing Fungicide-Resistant Penicillium Strains in the Moroccan Citrus Packinghouse

The extensive application of fungicides in citrus packinghouses to mitigate economic losses has resulted in the emergence of fungicide-resistant biotypes of Penicillium spp. Furthermore, many countries have implemented strict monitoring of fungicide residues to protect consumer health and the ecosystem. Maximum residue limits (MRLs) have been established in accordance with Codex Alimentarius standards, which present challenges for exports, as exceeding MRLs may restrict market access. This study aimed to identify fungicide-resistant strains of Penicillium spp. in a Moroccan citrus packinghouse and to assess the efficacy of GRAS (Generally Recognized As Safe) salts as eco-friendly alternatives for controlling these resistant strains through in vitro and in vivo tests. A total of 31 Penicillium isolates, labeled H1 to H31, were collected; 10 were identified as P. digitatum and 21 were identified as P. italicum. Resistance to thiabendazole (61.3%) and imazalil (58.1%) was notable, with some isolates showing dual resistance. In vitro, potassium sorbate, sodium benzoate, and sodium tetraborate salts were highly effective at inhibiting the mycelial growth of resistant isolates, at a concentration of 0.3% (p < 0.0001). In vivo tests on ‘Nadorcott’ fruits demonstrated that 2% and 4% salt solutions effectively prevented the development of green and blue molds caused by Penicillium spp. and showed strong curative effects, resulting in nearly 100% inhibition of most fungal isolates. Additionally, preventive salt treatments increased the accumulation of phenolic and flavonoid compounds, while in fruits treated with sodium benzoate, chitinase and peroxidase activities were significantly enhanced.