Pre-penetration events affecting host specificity of Verticillium lecanii

doi:10.1017/S0953756296002808

Mycological Research

Volume 101, Issue 5, May 1997, Pages 535-541

https://doi.org/10.1017/S0953756296002808 Get rights and content

The pathogenicity of two isolates of Verticillium lecanii towards non-target invertebrates and six species of aphid was tested in bioassays. There was no evidence of infection in any of the 20 non-target invertebrates. All target aphid species were susceptible to at least one isolate. The level of susceptibility varied between aphid species, with Myzus persicae and Brachycaudus helichrysi showing the greatest susceptibility.

Studies on initial spore deposition, retention and germination on two susceptible aphid species and four resistant non-target insects revealed differences in the behaviour of the pathogen between the two groups. Spore density mm^-2 was greater on susceptible insects both immediately post-inoculation and following 24 h incubation. There was no loss of spores from M. persicae over 24 h. All other species showed more than 50% spore loss in the same period. However, even after 90% spore loss, large numbers of spores remained on the bigger insects such as Agonum dorsale. Germination and germ-tube growth were possible on such resistant non-target insects, indicating that resistance to infection occurs after this stage. There was little difference in rate of germination on the two groups. However, differences in germ-tube growth were evident, with the production of longer and narrower germ-tubes on the resistant insects.

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Citation Excerpt :
The replication of our germination experiment on live C. zealandica larvae demonstrated that the presence of an active immune system, which is absent in the excised cuticle (Gillespie et al., 2000; Vilcinskas and Gotz, 1999), does not influence the outcome. Similar results were shown by Sitch and Jackson (1997) for Lecanicillium (=Verticillium) lecanii Zare & Gams conidia on the cuticle of host aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) and non-host adult Bembidion obtusum Audinet-Serville (Coleoptera: Carabidae), although retention of conidia was greater on the host insect compared to non-host insects. Their findings indicated that for many interactions between entomopathogens and insect hosts the resistance mechanisms in non-host species must intervene after fungal germination.
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Attachment and germination of Entomophaga maimaiga conidia on host and non-host larval cuticle
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The lepidopteran-specific fungal pathogen Entomophaga maimaiga is highly virulent against Lymantria dispar (gypsy moth) larvae, and other members of the family Lymantriidae. Numerous species in the subfamily Cuculliinae (Family Noctuidae) are not susceptible to E. maimaiga due to the inability of this fungus to penetrate the larval cuticle. Conidial attachment and germination were compared among five cuculliine species and L. dispar using bioassays and scanning electron microscopy. Although conidia were showered evenly across larvae during bioassays, on L. dispar conidia were most abundant on segments, where they adhered well to the cuticle and germinated at high percentages. Conidia on cuculliine cuticles were predominantly found in large, loose aggregations in intersegmental areas. Few conidia on cuculliine cuticle germinated and scanning electron microscopy revealed a thick film of mucous enveloping conidia. We hypothesize that the conidia on cuculliines become coated by this film and were only loosely attached to the larval cuticle. No such film was seen on L. dispar larvae where individual conidia appeared well attached. On L. dispar larvae many conidia also adhered to setae. To determine if hydrophobicity affected the ability of E. maimaiga conidia to attach and germinate on a substrate, a goniometer was used to determine relative hydrophobicity of larval cuticles. L. dispar cuticle was more hydrophobic than cuculliine cuticle, suggesting that a high level of hydrophobicity could be a required characteristic for hosts. Cuticles from four cuculliine species and L. dispar were sequentially extracted using hexane, chloroform, and methanol. Conidia were showered onto glass slides coated with the different extracts and germination was quantified. Methanol extracts of cuculliine cuticle consistently decreased germination, compared to all extracts of L. dispar cuticle. For all L. dispar extracts, the majority of conidia produced germ tubes, which is a normal prerequisite for cuticular penetration. For the cuculliines, conidia exposed to hexane and chloroform extracts produced secondary conidia as did all controls, but the conidia exposed to cuculliine methanol extracts that germinated produced germ tubes. These studies demonstrated that a range of factors act in concert to prevent E. maimaiga infection of the cuculliine species investigated.
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Pre-penetration events affecting host specificity of Verticillium lecanii

Pedobiologia

Journal of Invertebrate Pathology

Journal of Invertebrate Pathology

Journal of Invertebrate Pathology

Mycological Research

Transactions of the British Mycological Society

Journal of Invertebrate Pathology

Journal of Invertebrate Pathology

Mycological Research

Mycological Research

Journal of Invertebrate Pathology

Laboratory studies on the fungus Verticillium lecanii, a larval pathogen of the large elm bark beetle (Scolytus scolytus)

Annals of Applied Biology

Pathogenicity of the entomogenous hyphomycete fungus, Metarhizium anisopliae, against the chrysomelid beetles Psylliodes chrysocephala and Phaedon cochleariae

Biocontrol Science and Technology

Germination of the entomopathogenic fungus Verticillium lecanii on scales of the glasshouse whitefly Trialeurodes vaporariorum

Biocontrol Science and Technology

Physiological and destructive pathogenesis in insects by fungi: a speculative review.

Germination and effect of reduced humidity on expression of pathogenicity in Verticillium lecanii against the glasshouse whitefly Trialeurodes vaporariorum

Annals of Applied Biology

Investigations on the potential of a parasitic fungus (Verticillium lecanii) for biological control of the greenhouse whitefly (Trialeurodes vaporariorum)

Swedish Journal of Agricultural Research

Adhesion of the fungal spore to the insect cuticle in relation to pathogenicity.