Resistance to Oidium lycopersicum : allelism test between Lycopersicon hirsutum G1 . 1560 and L . hirsutum PI247087 Moretti, A . and Caranta, C . INRA, Genetics and Breeding of Fruits and Vegetable, Dom . St Maurice, BP94, 84143 Montfavet cedex, France .   E - mail : Carole . Caranta@avignon . inra . fr Resistance to powdery mildew ( Oidium lycopersicum ) in Lycopersicon hirsutum G1 . 1560 was shown to be controlled by an incompletely - dominant gene, Ol - 1 , mapped on chromosome 6 in the vicinity of the resistance genes Mi / Meu - 1 , Cf - 2 / Cf - 5 (van der Beek et al . 1994) .   Among the accessions of wild species frequently used at INRA, a very high level of resistance to O . lycopersicum was identified in L . hirsutum PI247087 . The intermediate levels of resistance observed during the introgression of PI247087 resistance into elite tomato lines attested to the oligo - or polygenic nature of this resistance . Moreover, fruits from all the plants with the higher level of resistance were yellow . An allelism test between these plants and a tomato line homozygous for the r gene (yellow flesh) was performed and revealed a linkage between r and resistance to O . lycopersicum (Laterrot & Moretti, unpublished data) . To determine whether a part of PI247087 resistance to O . lycopersicum is due to the gene Ol - 1 , an allelism test was performed between PI247087 and L . hirsutum G1 . 1560 . Resistance of L . esculentum Momor, L . hirsutum PI247087 and G1 . 1560, the F1 hybrids (Momor X PI247087) and (PI247087 X G1 . 1560) and the F2 progeny (PI247087 X G1 . 1560) to O . lycopersicum was evaluated during two independent tests . Plants were inoculated three weeks after sowing by spraying with a suspension of 10 4 conidia ml - 1 . Inoculum was prepared from freshly sporulating leaves of heavily diseased tomato plants . After inoculation, plantlets were transferred into growth chamber with 100% relative humidity, 24°C day/18°Cnight and 12 hours light . Disease was evaluated 18 days after inoculation according to the following index : 0=no visible sporulation (healthy plants), 1=very few powdery mildew spots surrounded by a necrosis (stopped sporulation), 2=moderate number of spots (moderate sporulation), 3=very high number of spots (strong sporulation) . Plants classified 0 or 1 were considered resistant and plants classified 2 or 3, susceptible . Results are presented in Table 1 . All plants could be classified unambiguously . L . esculentum Momor was completely susceptible with most of the plants with a high number of “sporulation spots” . L . hirsutum PI247087 and G1 . 1560 were resistant with most of the plants without sporulation . A small number (3/15) of the F1 (Momor X PI247087) showed some sporulation . The same observation was made by van der Beek et al . (1994) on the F1 hybrid between G1 . 1560 and a susceptible L . esculentum line . On the contrary, all