Citrus diseases caused by fungal pathogens drastically decreased the yield and quality of citrus fruits, leading to huge economic losses. Given the threats of chemical pesticides on the environment and human health, biocontrol agents have received considerable attention worldwide as ecofriendly and sustainable alternative to chemical fungicides. In the present study, we isolated a Bacillus velezensis strain TZ01 with potent antagonistic effect against three citrus pathogenic fungi: Diaporthe citri, Colletotrichum gloeosporioides and Alternaria alternata. The culture supernatant of this strain exhibited remarkable antifungal activity on potato dextrose agar plates and detached leaves of five citrus varieties. Treatment with TZ01 culture supernatant obviously affected the hyphal morphology and caused nucleic acid leakage. The crude lipopeptides (LPs) extracted from the culture supernatant were found as the major active ingredients, and could maintain the activity under a wide range of temperature and pH and ultraviolet radiation. Furthermore, the type of LPs, produced in vitro, were explored. Whole-genome sequencing of TZ01 revealed secondary metabolite gene clusters encoding synthetases for non-ribosomal peptides and polyketide production, and gene clusters responsible for the synthesis of three important LPs (surfactin, iturin, and fengycin) were identified in the genome. The liquid chromatography-mass spectrometry analysis confirmed the presence of various homologs of surfactin A, bacillomycin D, and fengycin A in the extracted LPs. Taken together, these results contribute to the possible biocontrol mechanisms of B. velezensis strain TZ01, as well as providing a promising new candidate strain as a biological control agent for controlling citrus fungal pathogens.

The Asian citrus psyllid, Diaphorina citri Kuwayama, is one of the most destructive pests of citrus and the primary vector of Candidatus Liberibacter spp., which cause citrus Huanglongbing (HLB). Microbial insecticides based on entomopathogenic fungi are safer alternatives to chemical pesticides. They can mitigate the spread of HLB by suppressing D. citri populations in citrus groves. Purpureocillium lilacinum strain ZJPL08, previously isolated from D. citri cadavers, is highly pathogenic to D. citri. We determined the virulence of strain ZJPL08 conidial suspensions and the efficacy of a wettable powder formulation. The LC₅₀ of strain ZJPL08 against D. citri at 7 d post-inoculation was 2.11 × 10⁴ conidia/mL, and the LT₅₀ of a 1.0 × 10⁸ conidia/mL suspension was 3.14 d in the laboratory. Under the same conditions, the LC₅₀ of the wettable powder was 1.39 × 10⁴ conidia/mL, and the LT₅₀ was 2.64 d. The corrected mortality caused by the 1500-times diluted solutions of strain ZJPL08 wettable powder in D. citri reached 86.67% at 7 d post-treatment and 100% at 14 d post-treatment under field conditions. Our study revealed that strain ZJPL08 infected D. citri by hyphae penetrating directly through the cuticle and into the body cavity and tissues. These findings show that P. lilacinum strain ZJPL08 has potential for the effective control of D. citri.

The most serious citrus postharvest diseases are citrus green mould caused by Penicillium digitatum and citrus blue mould caused by Penicillium italicum. Marine microorganisms have become a new measure in the control of these diseases. In this study, we isolated 56 marine actinomycetes, and strain AM-4 isolated from Drupa granulata exhibited the strongest antagonistic activity against P. digitatum. Molecular, morphological, cultural and physio-biochemical analysis identified strain AM-4 as Streptomyces chumphonensis. Strain AM-4 also exhibited antagonistic activity against 11 other phytopathogens, with an inhibition zone width ranging from 0.65 ± 0.04 to 2.35 ± 0.02 cm. The mycelial growth inhibition rates of the fermentation liquid of strain AM-4 against P. digitatum and P. italicum were 66.23% and 61.42%, respectively, and the conidial germination inhibition rates were 65.77% and 62.13%, respectively. The relative control efficacy of the gradient-diluted fermentation extracts of strain AM-4 against citrus green mould caused by P. digitatum ranged from 77.95% to 92.91% at 5 days post inoculation and 56.30% to 87.22% 10 days after inoculation. The relative control efficacy of the gradient diluted fermentation extracts of strain AM-4 against citrus blue mould caused by P. italicum ranged from 61.32% to 84.91% 5 days after inoculation and from 51.34% to 80.92% 10 days after inoculation. The relative control efficacy of the fermentation extracts of strain AM-4 was even better than prochloraz against citrus green mould caused by prochloraz-resistant P. digitatum. Therefore, as a biological control strain, AM-4 has a good prospect for development and utilization potential.

To obtain efficient biocontrol fungi against Panonychus citri, this study systematically identified the highly pathogenic strain HYC2101 through an analysis of its cultural characteristics, physiological and biochemical properties, and molecular identification. The isolate was identified as Penicillium oxalicum. In laboratory observations using stereomicroscopy and scanning electron microscopy, the infection process of P. oxalicum HYC2101 in P. citri was documented, and its pathogenicity against female adults and larvae was determined. The optimal temperature range for the mycelial growth and conidial production of P. oxalicum HYC2101 was found to be 25–35°C, with the highest sporulation on SDAY (sabouraud dextrose yeast extract agar medium). Observations of the infection process revealed that conidia easily attached to the cuticular folds and setae of the mite. After 24 h, the spores germinated and penetrated the cuticle. By 48 h, the hyphae had invaded the mite’s interior through the cuticle, mouthparts, and anus. At 96 h, the mite’s body was fully covered with hyphae and a large number of spores, ultimately leading to the death of the host. The results of the pathogenicity tests indicated that strain HYC2101 was significantly pathogenic to both female adults and larvae, with greater pathogenicity against female adults. The LC₅₀ values after 7 days of infection were 5.92 × 10⁴ and 9.22 × 10⁵ spores/mL for female adults and larvae, respectively. Under the highest spore concentration of 1 × 10⁸ spores/mL, the LT₅₀ values for female adults and larvae were 2.80 and 4.79 days, respectively. In conclusion, the highly pathogenic P. oxalicum strain HYC2101 shows significant potential for use in the green prevention and control of citrus red mites and warrants further development as a biocontrol resource.

Citrus tatter leaf virus (CTLV), a major pathogen threatening global citrus production, remains poorly characterized in terms of its regional genetic diversity and evolutionary dynamics. To address this gap, we conducted a comprehensive population genetic analysis of CTLV in Zhejiang Province, China, using 181 coat protein (CP) gene sequences—the largest regional CTLV dataset to date. Our analyses uncovered substantial genetic diversity among Zhejiang CTLV isolates. Phylogenetic reconstructions revealed that these isolates span multiple clades, closely aligning with global CTLV population structures, indicative of recurrent viral introductions and extensive regional circulation. Population structure analyses revealed significant genetic differentiation driven by geography, with Jinhua isolates forming a distinct cluster, and by host species, with Citrus reticulata ‘Criton’ isolates diverging from those in other citrus varieties. Selection pressure analysis indicated that while most CP polymorphic sites were under purifying selection, several clade-specific codons showed signatures of positive selection. These results offer new insights into CTLV’s population structure and localized evolutionary trajectories, enhancing our understanding of its regional adaptation and informing strategies for disease management and control of this globally significant pathogen.