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The concept of soil infectivity, widely used for soil-borne plant pathogens, is applied to the fungi forming vesicular–arbuscular endomycorrhizae. The authors propose a method for the determination of the mycorrhizal soil infectivity (MSI) using leek, a highly mycotrophic plant, for the bioassays. Under controlled conditions, populations of leek plants are grown in a range of concentrations of a natural soil mixed with autoclaved volumes of the same soil. The relationship between the percentage of plants forming mycorrhizae and the soil concentration is used as the basis for the determination of the quantity of soil required to obtain mycorrhiza formation on 50% of the host plant population; the results are defined in terms of MSI units and are expressed as MSI50 for 100 g of soil. The features of the method are illustrated in a comparative study of four agricultural soils from France.

. There are many records of mycorrhizae, particularly ectomycorrhizae, protecting roots against soil‐borne plant pathogens, though fungal or viral diseases of the green parts are usually more severe in mycorrhizal plants. There are various mechanisms by which mycorrhizae protect themselves against soil‐borne diseases. The occurrence and ecological significance of these mechanisms are discussed in relation to environmental factors. Mycorrhizae will probably become a very effective means of biological control once we have learned how to manage them.

Sensibilite des jeunes rameaux de Peuplier au M. brunnea 199 Pustules tend to be crowded at the ba.ses of ttie shoots and they seem to be independent of late infection on foliage. Ttiis suggests that shoots have a characteristic pattern in the development of .susceptibility during development. ZusammenfassungDie Anfdlligkcit junger Pappeltriebe gegen Marssonina brunnea (Ell. et Ev.) P. magn. Registriert wurde die Anzahl von Marssonina brunnea-Pusteln an Trieben versehiedener Pappelklone. Eine diesbeziigliche Rangfolge unter den Klonen scheint sowohl zeitlich wie raumlich zu bestehen. Die Methode durfte eine grobe Absehatzung der Anfiilligkeit von Klonen zulassen, es kommen allerdings Unterschiede zwischen Klonen mit gleieher Intensitat des Btattbefalts vor. An der Triebbasis treten die Pusteln nieist gehiiuft auf; mit Spatinfektionen der Blatter scheinen sie nicht zusammenzuhangen. Das la(5t vermuten, da(! Triebe spezifische Auspragungen in der Anfiilligkeit wahrend ihrer Entwicklung durchlaufen. Bibliographie CASTELLANI, E., 1966: Influence des facteurs climatiques sur les infections des Peupliers euramericains par Marssonina brunnea (Ell. et Ev.) P. Magn. Phytopath. mediterr. 5, 41-52. CASTELLANI, E.; CELLERINO, G. P., 1964: Una pericolosa nialattia dei pioppi euroamericani determinata da Marssonina brunnea (Ell. et Ev.) P. Magn. Cellulosa e Carta 15, 8, 3-17. --1969: Cinque anni di osservazioni sul eomportamento di vari ctoni di pioppo verso la Marssonina brunnea. Cellulosa e Carta 10, 4, 3-16. CELLERINO, G. P.; ANSELMI, N.; PINON, J., 1978: Influence de l'age des feuilles de Peuplier sur la sensibilite a Marssonina brunnea. Eur. J. For. Path., 8, 273-279. PINON, J., 1974: Influence de la concentration de l'inoculum sur la sensibilite des peupliers cultives a Marssonina brunnea (Ell. et Ev.) P. Magn. Eur. J. For. Path. 4, 54-59. PINON, J.; POISSONNIER, M., 1975: Etude epidemiologique du Marssonina brunnea (Ell. et Ev.) P. Magn. Eur. J. For. Path. 5, 97-111.

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