biologiques et ecologiques. La determination du r81e de ces agents pathogenes est gen6ralement moins simple que dans le cas des maladies des parties aeriennes des plantes et une identification complke necessite souvent le recours i des inoculations sur differentes plantes dans des conditions dkterminkes. L‘Ctude des formes spkciales et des races de F. oxysporum est donnee comme exemple.

La connaissance des caracteristiques biologiques et ecologiques des champignons phytopathog6nes existant dans le sol peut &re grandement amelioree par des re- cherches quantitatives sur leurs populations. Par des methodes de detection, on cherche B determiner si ces champignons sont presents ou non dans un sol donne et, par con& quent, B savoir s’il existe un risque pour la culture d’une plante sensible faite sur ce sol.

Les autres methodes, B but plus directement quantitatif, sont des analyses micro- biologiques. Elles ont pour dessein principal d’etudier la dynamique des populations de champignons phytopathogenes en fonction des facteurs du milieu (Ccologie dans le sol) et d’Ctablir s’il existe des correlations entre les quantids de germes et la gravitk des maladies. De nombreux facteurs concernant les champignons pathogdnes, le sol et les plantes entrent en jeu dans les processus infectieux. Dans ces processus, la densite d’inoculum est un composant essentiel, mais pas unique. Diff krents exemples montrent qu’il n’y a pas toujours une corrklation directe entre le nombre de germes prksents dans l’ensemble d’un sol et l’importance de la maladie qu’ils peuvent provoquer. Un cas particulier est dktaillk : par incorporation de differentes substances organiques dans un sol infect6 par F. oxysporum f. sp. melonis, on peut faire varier considerablement le nombre de germes de ce champignon (multiplication par plus de 60 ou disparition des 4/5 des germes), mais on ne trouve pas une correlation etroite entre le nombre de germes et le nombre de plantes malades.

L‘influence des differents facteurs qui entrent en jeu dans les maladies provoquees par les champignons phytopathogenes telluriques est encore insuffisamment connue. Mais les etudes qualitatives et quantitatives nous permettent de prendre conscience de l’im- portance de certains de ces facteurs et elles doivent aboutir i de nouvelles methodes de lutte selectives reposant sur des bases biologiques prCcises.

OEPP/EPPO Bull. No 7 : 5-15 (1972)

RESUME

Significance of Qualitative and Quantitative Studies of Phytopathogenic Soil Fungi

by J. LOUVET

Station de recherches sur la flore pathogene dans le sol [INRA), Dijon [France)

For efficient control, pathogenic soil fungi must be identified precisely and their role as pathogens and their parasitic specialization must be determined, often by the inoculation of various plant species under special conditions. For instance, when Fusurium oxysporum (Schlecht.) Sn. et H. is found in melons (cf. table 1) it must be ascertained whether it is acting as a saprophyte, or as a pathogen causing rots or

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vascxilar diseases, which then indicates the presence of its special form nzelolzis : but before an efficient, resistant melon variety can be chosen, further experiments must reveal which physiological race of melonis is involved.

Control must also be based on a knowledge of the biology and the ecology of rdch of the fungi in the soil ; those with a high saprophytic competitivity can easily re-infect physically or chemically treated soils and control will depend on whether the fungi are present in the upper or lower layers of the soil. The quantitative studies needed to solve these problems have 3 main aims :

a) To determine whether or not the fungi are present in a given soil. To this end, pieces of plant tissue or preferential cultural media can be placed in the soil, or especially susceptible crops can be grown ; such techniques are useful for the detection of Rhizoctonia solani Kiihn, Thielaziopsis basicola (Berk. et Br.) Ferr., and fungi causing the damping off of seedlings or diseases in young plants, but a negative result is not necessarily conclusive. Microbiological analyses can also be made, either by direct observation of the soil, or by using cultural media, especially if made selective by the addition of substances which favour the fungus concerned while repressing others IMENZIES. 1963, DOMSCH and SCHWINN, 1965, BLOSS, 1970). Even then plant inocula- tion may be needed.

b) To assess population dynamics in relation to environmental factors. This is done by a quantitative analysis of very small soil samples (BAKER, 1968, MENZIES, 19681, but the uneven distribution of the pathogens makes representative sampling difficult, while the analysis usually does not reflect the distribution on the microscopic level which can be of great importance for the triggering off of a disease (HENIS, 1968, NASH, 1968, POWELSON, 1968, SCHMITTHENNER, 1968, BAKER, 1968). Nevertheless, good results, allowing measures to be devised for reducing the amounts of inoculum and for assessing the efficacy of direct methods of control, have been obtained.

c) To establish whether or not there is a relationship between the number of pathogens present and the intensity of attack. Results in this field have often been contradictory since many factors are involved. The populations of many fungi increase during the growing season either by saprophytic colonisation of organic matter in the soil or under the influence of the root secretions (as in R. solani and F. solani [Mart.) Appel et W r . , cases in which a general soil analysis does not really indicate the infec- tious potential). On the other hand, the sclerotia of Verticilliurn dahliae Kleb., Sclerotinia niinnr Jagger, S. sclerotiorum (Lib.) de By and Sclerotium cepiz*orum Berk. remain inac- tive until a susceptible plant becomes available, so that a better correlation between number of pathogens and disease incidence is recorded. In F. oxysporum the infection of a few or even one root can kill a plant, although accumulations of chlamydospores may be needed for this, but these are also detected by modern methods. A similar case is L7. alho-atrwn Reinke et Berth.

Environmental factors can still change the picture and it is therefore not SW- prising that the incidence of diseases is often not proportional to the quantity of patho- gens recorded. In regions with a semi-arid climate, chlamydospores of F. so lmi f. sp. p h d ~ r o l i (Jones) Sn. et H. can be very numerous but damage to beans is more serious in humid soils, with low C/N figures, even if the number of chlamydospores is ten rimes less (NASH, 1968). Another example is reproduced in table 2 which shows the relationship between F. oxysporum f . sp. melonis Sn. et H. in soils to which various organic substances had been added and the ensuing attacks on melons 50 days later : the substances added caused changes in the multiplication rate of the fungus varying

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from a 60-fold increase to a n 80 96 decrease, while the disease rate remained unchanged in some plots but was 5 times higher in others.

The relationships within the soil are complicated and render the assessment of the risks of disease more difficult than for disorders in aerial plant parts, but recent investigations have elucidated the so far unknown importance of certain factors. Hopefully, they will enable more precise prognoses to be given, in due course.

BIBLIOGR APHIE

BAKER, R.R. (1965). The dynamics of inoculum. In BAKER K.F., SNYDER, W.C. (ed.) : Ecology of soil-borne plant pathogens. Univ. California Press, Berkeley : 395-403.

-- ( 1968). Mechanisms of biological control of soil-borne pathogens. Annu. Reu. Phytopath.

- (1970). Fungal populations and incidence on disease. I n Nature of the influence of crop residues on fungus-induced root diseases. Wash. Agr. Exp. Stat. Bull. 716 : 8-11.

BLOSS, H.E. (1970). Techniques for measuring populations of fungal pathogens in soil. In Nature of the influence of crop residues on fungus-induced root diseases. Wash. Agr. Exp. Stat. Bull. 716 : 6-8.

DIMOND, A.E. & J.G. HORSFALL (1965). The theory of inoculum. I n BAKER K.F., SNYDER, W.C. (ed.) : Ecology of soil-borne plant pathogens. Univ. California Press, Berkeley : 404-418.

DOMSCH, K.H. & F.J. SCHWINN (1965). Nachweis und Isolierung von pflanzenpathogenen Bodenpilzen mit selektiven Verfahren. Zentbl. Bakt. Parasitkde. Abt. I. Suppl. I : 461-485.

HENIS, Y . (1968). Significance of population level of Rhizoctonia solani in soil. 1" Congr. Phytopath. Londres. Abstracts, 83.

MENZIES, J.D. (1963). The direct assay of plant pathogen populations in soil. Annu. Rev. Phytopath. 1 : 127-142.

- (1968). Factors influencing pathogen populations in soil. 1" Congr. Phytopath. Londres. Abstracts, 128.

NASH, Shirley M. (1968). The significance of Fusarium populations in soil. 1" Congr. Phytopath. Londres. Abstracts, 186.

POWELSON, R.L. (1968). Significance of population level of Verticillium in soil. 1" Congr. Phytopath. Londres. Abstracts, 154.

ROUXEL, F. & D. BOUHOT (1971). Recherches sur l'ecologie des champignons parasites dam le sol. IV. Nouvelles mises au point concernant l'analyse selective et quantitative des Fusarium oxysporum et Fusarium solani dans le sol. A y . Phytopathol. 3 (2) : 171-188.

SCHMITTHENNER, A.F. (1968). Significance of populations of major plant pathogens in soil : Phytophthora and Pythium. 1" Congr. Phytopath. Londres. Abstracts, 173.

6 : 263-294.

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