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Simple Summary: The spider mite is a destructive pest of various crops during warm and dry conditions in tropical countries, including Thailand. The pest is difficult to control despite synthetic acaricide use. In the field, insect populations gradually develop resistance to synthetic pesticides over long-term use. The use of entomopathogenic fungi is more human- and environmentally friendly. We searched and identified a potential fungal isolate from a culture collection, focusing on the genus Metarhizium. Metarhizium sp. BCC 4849 not only controls the spider mite but also plays a significant role as a natural regulator of other insect pests. Here, we investigated its infection process on the mite, optimized a conidial formulation for extended shelf life, and conducted toxicological assays in animals to assess its biosafety in humans. The fungal genome has been sequenced. The genomic data indicated that oxidoreduction proteins; zinc-, heme-, and iron-binding proteins; and transmembrane transporters are abundant in the genome. Five isolates of Metarhizium sp. were evaluated for their pathogenicity against the spider mite (Tetranychus truncatus Ehara) (Acari: Tetranychidae) and Metarhizium sp. BCC 4849 resulted in the highest mortality (82%) on the 5th day post-inoculation (DPI). Subsequent insect bioassay data indicated similar high virulence against five other insects: African red mites (Eutetranychus africanus Tucker) (Acari: Tetranychidae), bean aphid (Aphis craccivora Koch) (Hemiptera: Aphididae), cassava mealybug (Phenacoccus manihoti Matile-Ferrero) (Hemiptera: Pseudococcidae), sweet potato weevil (Cylas formicarius Fabricius) (Coleoptera: Brentidae), and oriental fruit fly (Bactrocera dorsalis Hendel) (Diptera: Tephritidae), at mortalities of 92–99%, on 3rd–6th DPI, and in laboratory conditions. The pathogenicity assay against E. africanus in hemp plants under greenhouse conditions indicated 85–100% insect mortality on 10th DPI using the fungus alone or in combination with synthetic acaricide. Genome sequencing of Metarhizium sp. BCC 4849 revealed the high abundance of proteins associated with zinc-, heme-, and iron-binding; oxidation-reduction; and transmembrane transport, implicating its versatile mode of interaction with the environment and adaptation to various ion homeostasis. The light and scanning electron microscopy indicated that at 24 h post inoculation (PI), adhesion and appressorial formation occurred, notably near the setae. Most infected mites had stopped moving and started dying by 48–72 h PI. Elongated hyphal bodies and oval blastospores were detected in the legs. At 96–120 h PI or longer, dense mycelia and conidial mass had colonized the interior and exterior of dead mites, primarily at the bottom than the upper part. The shelf-life study also indicated that conidial formulation combined with an oxygen-moisture absorber markedly enhanced the viability and germination after storage at 35 °C for four months. The fungus was tested as safe for humans and animals, according to our toxicological assays. [ABSTRACT FROM AUTHOR]

The reducing clade III polyketide synthase genes, including pks15 , are highly conserved among entomopathogenic fungi. To examine the function of pks15 , we used targeted disruption to investigate the impact of Beauveria bassiana pks15 on insect pathogenesis. Southern analysis verified that the Δ pks15 mutant was disrupted by a single integration of the transformation cassette at the pks15 locus. The Δ pks15 mutant had a slight reduction in radial growth, and it produced fewer spores. Our insect bioassays indicated the Δ pks15 mutant to be significantly reduced in virulence against beet armyworms compared to wild type (WT), which could be partially accounted for by its markedly decreased ability to survive phagocytosis. Total haemocyte count decreased sharply by 50-fold from days 1–3 post-inoculation in insects infected with WT, compared to a 5-fold decrease in the Δ pks15 mutant. The mutant also produced fewer hemolymph hyphal bodies than WT by 3-fold. In co-culture studies with amoebae that have phagocytic ability similar to that of insect haemocytes, at 48 h the mortality rate of amoebae engulfing Δ pks15 decreased by 72 %, and Δ pks15 CFU decreased by 83 % compared to co-culture with WT. Thus, the Δ pks15 mutant had a reduced ability to cope with phagocytosis and highly reduced virulence in an insect host. These data elucidate a mechanism of insect pathogenesis associated with polyketide biosynthesis. [ABSTRACT FROM AUTHOR]

The tridax daisy (Tridax procumbens) is a predominant weed in Thai plantations. Management of weeds in the country, both past and present, heavily relies on the use of synthetic herbicides. Here, we demonstrate the use of a phytopathogen for controlling this weed. The fungus Colletotrichum siamense isolate Cs1 was isolated from a diseased false daisy (Eclipta prostrata). Topical application of a conidial suspension of this fungus on Tridax procumbens plants led to 78–100% disease incidence on days 14–18 post-inoculation. Similar testing in the laboratory and greenhouse settings showed that C. siamense isolate Cs1 was non-pathogenic to cassava (Manihot esculenta) and eight other crops. Fluorescence microscopy of green fluorescent protein-expressing C. siamense indicated that the conidia germinated and formed appressoria for penetration on days 1–7 post-inoculation. Scanning electron microscopy showed hyphae growing on the leaf surface as well as invading the host through stomata. Trichomes of weed leaves served as important infection sites for this myco-pathogen. Oval conidia and pseudohyphae were detected inside the leaf trichomes, indicating that these cell types could help spread the fungus throughout the weed. C. siamense sporulated at high rates, at 1.7 × 1010 conidia/g, on a rice-oat mixture, suggesting a potential production recipe for commercialization. These data suggest that this weed-pathogenic fungus has potential as a bioherbicide for biological control of this weed. • Colletotrichum siamense is a mycopathogen of the weed tridax daisy. • C. siamense shows no or low pathogenic effect on 10 important crops. • The fungus can infect the weed through leaf trichomes and stomata. • Oval conidia and pseudohyphae help disperse the fungus in planta. • The fungus sporulates at high rates on rice mixed with oat. [ABSTRACT FROM AUTHOR]

Five isolates of Metarhizium sp. were evaluated for their pathogenicity against the spider mite ( Tetranychus truncatus Ehara) (Acari: Tetranychidae) and Metarhizium sp. BCC 4849 resulted in the highest mortality (82%) on the 5th day post-inoculation (DPI). Subsequent insect bioassay data indicated similar high virulence against five other insects: African red mites ( Eutetranychus africanus Tucker) (Acari: Tetranychidae), bean aphid ( Aphis craccivora Koch) (Hemiptera: Aphididae), cassava mealybug ( Phenacoccus manihoti Matile-Ferrero) (Hemiptera: Pseudococcidae), sweet potato weevil ( Cylas formicarius Fabricius) (Coleoptera: Brentidae), and oriental fruit fly ( Bactrocera dorsalis Hendel) (Diptera: Tephritidae), at mortalities of 92-99%, on 3rd-6th DPI, and in laboratory conditions. The pathogenicity assay against E. africanus in hemp plants under greenhouse conditions indicated 85-100% insect mortality on 10th DPI using the fungus alone or in combination with synthetic acaricide. Genome sequencing of Metarhizium sp. BCC 4849 revealed the high abundance of proteins associated with zinc-, heme-, and iron-binding; oxidation-reduction; and transmembrane transport, implicating its versatile mode of interaction with the environment and adaptation to various ion homeostasis. The light and scanning electron microscopy indicated that at 24 h post inoculation (PI), adhesion and appressorial formation occurred, notably near the setae. Most infected mites had stopped moving and started dying by 48-72 h PI. Elongated hyphal bodies and oval blastospores were detected in the legs. At 96-120 h PI or longer, dense mycelia and conidial mass had colonized the interior and exterior of dead mites, primarily at the bottom than the upper part. The shelf-life study also indicated that conidial formulation combined with an oxygen-moisture absorber markedly enhanced the viability and germination after storage at 35 °C for four months. The fungus was tested as safe for humans and animals, according to our toxicological assays.