Field and glasshouse observations of Lolium spp. grasses indicated that the lower, abaxial, leaf surface was rarely infected by powdery mildew (Erysiphe graminis) even when the upper, adaxial, surface was densely colonized. Experiments showed that conidia of two strains of E. graminis, one from Lolium and one from Avena, germinated equally well on both surfaces of Lolium and Avena leaves, but that the subsequent growth and development of germlings was impaired on the lower surface of Lolium leaves, so that most formed only multiple short germ tubes or an abnormal long tube, and only c. 25% or fewer formed infection structures. This contributes to the apparent resistance of the lower Lolium leaf surface to powdery mildew and may help to explain why the disease is relatively unimportant in UK ryegrass crops, since infection structures develop at a high frequency on only 50% of the leaf area, i.e. the upper surface. Scanning electron microscopy showed that the epicuticular waxes on the lower Lolium leaf surface form amorphous sheets. This contrasts with the crystalline plate waxes seen on the upper surface of Lolium leaves and on both surfaces of oat leaves. However, when the lower Lolium leaf surface was washed with chloroform to remove epicuticular wax, normal germling and infection structure development was obtained on the wax‐free surface. This suggests that the sheet waxes prevent the pathogen gaining access to features of the cuticular membrane which trigger normal germling development.
Within minutes of spore deposition, enzymes including cutinase are released by conidia of Erysiphe graminis; these may prepare the infection court and facilitate subsequent fungal development. Germination follows with emergence of the primary germ tube, which contacts the host leaf. Extracellular material is secreted beneath the primary germ tube, which adheres to the leaf. The primary germ tube forms a penetration peg that breaches the host surface and gains access to host cell components including water. The primary germ tube also recognises factor(s) present in the host surface, and this stimulates elongation of the second-formed germ tube. The elongated second tube in turn responds to host surface factor(s) by differentiating an appressorium. Extracellular material, secreted beneath the growing appressorial germ tube, is laid down thickly around the appressorial lobe. An inner ring of extracellular material can often be seen surrounding penetration pores revealed by the removal of primary germ tubes and appressoria. The chemistry of the extracellular material is unknown, but in addition to adhesive properties recent studies show that a monoclonal antibody that recognises purified cutinase and, possibly, hemicellulases, binds to secretions from conidia, primary germ tubes, and appressoria. External factors can also influence germling development. Although light has little effect on the early stages of germling development, it has a profound effect in delaying the formation of haustoria by apparently mature appressoria. Understanding the control of early pathogen development could indicate avenues for genetic engineering and breeding for disease-resistant plants. Key words: Erysiphe graminis, powdery mildew, primary germ tube (PGT), appressoria, extracellular material.
Low temperature scanning electron microscopy was used to study the development of Erysiphe graminis DC f.sp. avenae Marchal from germination through infection to sporulation of the fungal colony. To clarify resolution of the fungus–host interface and facilitate interpretation of fungal surface structures, epicuticular waxes were removed from host leaves before inoculation. Whereas conidia were covered in spinelike protrusions or globular bodies, young germ tubes, appressoria, and hyphae were initially smooth walled, but by 15 h after inoculation, wartlike bodies, resembling globular bodies on conidia, were present on first appressorial lobes; these increased in number, eventually covering the appressorium surface and appearing on contiguous hyphae. Wartlike bodies also appeared at junctions of hyphal branches, on hyphal appressoria, and on conidiophore basal cells. Their function, if any, is unknown. The meristematic zone, at the apex of the conidiophore basal cell, remained smooth walled, but globular bodies appeared on the wall of young conidia as soon as the limiting septum had formed. Observations with the fungus in situ revealed the presence of amorphous extracellular material around primary germ tubes and appressorial lobes. Extracellular material was also present beneath appressorial germ tubes and hyphae but it was hidden unless the fungus was displaced. It could not be seen beneath conidia. The extracellular material appeared to be adhesive, sticking the fungus firmly to the host surface. Removal of the fungus showed that the extracellular material was deposited close to the tip of developing germ tubes and hyphae. It was particularly thick around primary germ tubes and appressorial lobes, and a discrete ring of extracellular material was often visible around penetration pores (holes in the leaf surface seen beneath primary germ tubes and appressorial lobes). In addition to its adhesive properties, the extracellular material may act as a matrix in which fungal enzymes are sited and focused for attack on the host. Key words: Erysiphe graminis, low temperature scanning electron microscopy, extracellular material, fungal adhesion, fungal surface morphology.
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