During the study of the effectiveness of resistance genes to Fulvia fulva (= Cladosporium fulvum), we observed the highly similar reaction of 2 genotypes received from E.A. KERR in 1980 to approximately a hundred of isolates of the fungus. These two genotypes were ONTARIO which, after artificial inoculation, generally takes the form of white areas composed of mycelium with or without sporulation, depending upon the agressivity of the isolate of Cladosporium and the medium conditions.
As these observations suggested an allelic relationship between the resistance factors of these 2 resistance genotypes, we carried out an allelism test, the results of which are given in Table 1. The inoculated Cladosporium isolate belongs to race O and was isolated in 1986 from open field grown tomatoes at Sarrians, in South-East France. This isolate is highly sporulant. Three pairs of ONT. 7816 and ONT. 7811 plants served as the basis of the test which can thus be considered as covering 3 repetitions. Not one susceptible plant was found among the 174 F2 plants (ONT 7816 x ONT 7811), nor among the 181 plants which resulted from the crossing of the F1 (ONT 7816 x ONT 7811) with the susceptible variety, Monalbo. Thus, in spite of the fluctuation of the response of partially resistant plants, the allelism of the Cf-16 and Cf-21 genetic factors can be concluded and until there is proof to the contrary, it can be considered that Cf-21 is a reoccurrence of Cf-16.
Table 1: Response of plant populations and lines after inoculation with a race 0 Cladosporium isolate.
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Populations or Lines Number of plants by class
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I B BS S SS SSS Total
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F1 (ONT7816xONt7811) 1 2 29 9 10 - - 50
F1 (ONT7816xONt7811) 2 2 42 13 3 - - 60
F1 (ONT7816xONt7811) 3 1 45 10 4 - - 60
F2 (ONT7816xONT7811) 1 - 34 14 12 - - 60
F2 (ONT7816xONT7811) 2 1 24 19 16 - - 60
F2 (ONT7816xONT7811) 3 - 22 18 14 - - 54
F1 (ONT7816xONT7811) 1 - 42 11 7 - - 60
x Monalbo
F1 (ONT7816xONT7811) 2 1 38 15 7 - - 61
F1 (ONT7816xONT7811) 3 - 35 21 4 - - 60
ONTARIO 7816 (Cf-16) - 2 8 6 - - 16
ONTARIO 7811 (Cf-21) - 2 8 6 - - 16
Monalbo (susceptible line) - - - - - 16 16
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Class of attack: I = no symptoms
B = white specks with abundant mycelium
BS = the same as B but with a few sporulation
S = moderate sporulation
SS = heavy sporulation
SSS = very heavy sporulation