Scanning electron microscopy of Erysiphe graminis

Scanning electron microscopy of Erysiphe graminis

[ 149 ] Trans. Br. mycol. Soc. 59 (I ), 149-178 (1972) Printedin Great Britain NOTES AND BRIEF ARTICLES (With Plates 20-28 and 7 Text-figures) SCA...

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[ 149 ] Trans. Br. mycol. Soc. 59 (I ), 149-178 (1972) Printedin Great Britain

NOTES AND BRIEF ARTICLES (With Plates

20-28

and 7 Text-figures)

SCANNING ELECTRON MICROSCOPY OF

ERYSIPHE GRAMINIS R. T. PLUMB AND R. H. TURNER

Rothamsted Experimental Station, Harpenden, Herts. Infection of cereals with Erysiphe graminis DC shows as wefts of hyphae bearing conidial chains, as first described in detail by Salmon (1900) and Foex (1911-13). Brodie (1942) and Brodie & Neufeld (1942) studied the conidial chains and conidial development and attachment by light microscopy, but scanning electron microscopy permits much high er magnification and resolution and its greater depth of focus also allows spatial relationships to be seen more readily. The fungus studied was E. graminis f.sp. tritici; portions of infected wheat leaves bearing conidial chains were glued to specimen stubs using 'Silver Dag' and either examined immediately or kept in moist, still air for 24 h while chains developed. The specimens were not coated with metal and were examined quickl y at an accelerating voltage of 3 kV. To examine conidia, plants undisturbed for 24 h were shaken over dry specimen stubs, which were gently tapped to separate th e conidia. After coating with 25 nm of gold th ey were examined at an accelerating voltage of 10 kV. All observations were made with the Cambridge Instruments 'Stereoscan' Mark 2 A scanning electron microscope. Plate I illustrates stages in the development of infection. Plate I, fig. I shows a spiny conidium germinating on the leaf surface to produce a smooth germ-tube and flattened appressorium. After penetrating the leaf the fungus grows rapidly, hyphae ramify over the leaf surface and conidial chains, with the characteristic basal bulge, begin to develop (PI. 20, figs. 2, 3). Ridges develop around the chain as it elongates (PI. 20, fig. 4) and several are produced before invagination begins to separate the distal conidia (PI. 20, fig. 5). The conidia continue to separate and, finally, a forest of conidial chains is produced (PI. 20, fig. 6) forming pustules visible to the naked eye. Plate 2 I, fig. 7 shows two conidia still joined together and their connecting link. Both of the conidia are characteristically spiny with a spine-free collar at the region of attachment. The smooth area is separated from the rest of the surface by a ragged ring of spines. This is the stage when the cytoplasm of the two conidia has probably become distinct and violent disturbance separates them. Plate 2 I, fig. 8 may illustrate the result of separation; although these two conidia were not necessarily joined to

Transactions British Mycological Society each other, the one on the right has part of its contiguous conidium still attached and that on the left has a smooth cushion with a slight central indentation. Each conidium in PI. 21, fig. 9 has only tenuous connexions through a distal terminal papillum and a proximal terminal cushion (Brodie, 1942) that holds the chain together in still air although there is no physical connexion between the conidia. When chains bearing mature conidia are kept in moist air, several smooth-walled germ-tubes are produced (PI. 21, fig. 10). Internal development in the later stages of maturation is probably well reflected by externally visible changes but early chain formation is not. The conidial wall has two layers of which only the inner invaginates to cause separation (PI. 21, fig. 11). The process of separation begins early during chain development and the invagination of the inner layer may contract the outer one to produce ridges at the zones of invagination (PI. 20, figs. 4, 5). As the conidia develop and the cytoplasm of contiguous ones separates, the spiny, outer layer of the wall breaks, revealing the smooth, inner one, visible externally as the collar (PI. 21, fig. 7). Cytoplasmic separation probably occurs when the outer layer of the wall breaks. We thank Dr A. Bainbridge of this department for permission to use figs. 1 and 3 in Plate 1. REFERENCES

BRODm, H.J. (1942). Protoplasmic continuity in the powdery mildew Erysiphe graminis DC. Canadian Journal ofResearch C 20, 595-601. BRODIE, H.J. & NEUFELD, C. C. (1942). The development and structure of the conidia of Erysiphe polygoni DC and their germination at low humidity. Canadian Journal of Research C 20, 4 I -6 I. FOEX, E. (1911-13). Miscellanees. Annales de l'Ecole Nationale d'Agriculture de Montpellier 11-12, 246-265. SALMON, E. S. (1900). A monograph of the Erysiphaceae. Memoirs ofthe Torrey Botanical Club 9, 1-292. EXPLANATION OF PLATES 20 AND 21

Fig. Fig. Fig. Fig. Fig. Fig.

PLATE 20 Germinating conidia with appressoria. 2. Ramifying hyphae on the leaf surface and the initiation of conidial chains. 3. Early stage in conidial chain formation. 4. Ridges on newly formed conidial chains. 5. Ridges and invagination between the distal conidia. 6. Conidial chains on the leaf surface forming a pustule.

I.

PLATE 21 Fig. 7. Link between two spiny conidia showing the collar region. Fig. 8. The conidium on the right shows the possible results of violet separation and bears part of its contiguous conidium, that on the left shows the cushion. Fig. 9. Part of a conidial chain showing the distal papillum and proximal cushion on each conidium. Fig. 10. Infected leaf with mature conidia germinating while still linked in chains. Fig. II. T.S. of conidial chain showing invagination of the inner layer.

Trans. Br. mycol. Soc. 59 (I), (1972). Printed in Great Britain

T rans. Br. mycol. Soc.

Vol. 59.

Plate

20

(Facing p. 150)

Vol. 59.

Plate

2I