In 1975 we found and reported high level of resistance to powdery mildew caused by Leveillula taurica in progenies derived from crosses with L. chilense (Tomato Genetics Cooperative Report No. 25, 1975). It is based on hypersensitivity. Under condition of sever natural infection with L. taurica on the top leaves of the resistant lines a few small chlorotic or necrotic spots can be observed (Tomato Genetics Cooperative Report No. 37, 1987).
After additional genetic study it is shown that resistance of the best line - Laurica, is controlled by a single dominant gene. The segregation in F2 generation fits the ratio of three resistant plants to ne susceptible plant. The resistance of the line Laurica have been confirmed not only in Bulgaria, but in other countries too. We propose the symbol Lv for dominant gene controlling high level of resistance to powdery mildew caused by Leveillula taurica.
During the last summer under conditions of heavy natural infection with new powdery mildew in the field we observed white sporulation on the upper side of the leaves of 20 tomato lines resistant to L. taurica. This phenomenon made us inoculate artificially the above mentioned lines together with spore suspension of the new powdery mildew.
We used the same method for artificial inoculation applied by Dr. Pim Lindhout from IVT - Wageningen (Holland). Ten days after inoculation we observed abundant white mycelium on the upper side of the leaves of all tomato lines known for their resistance to L. taurica. Two wild species offered by Dr. Lindhout as resistant to the new powdery mildew didn't show any symptoms. It is clear that the two diseases - the one caused by L. taurica and the other caused by a fungus belonging to family Erysiphaceae - have different gene control. This fact obliges us to incorporate into commercial tomato varieties different genes for resistance - to old and to new powdery mildew on tomatoes.
Literature cited:
Yardanov, M., L. Stamova and Z. Stoyanova 1975 Leveillula taurica resistance in the tomato. Tomato Genetics Cooperative Report 25, 24.
Stamova, L. and M. Yordanov 1987 Resistance to Leveillula taurica, Tomato Genetics Cooperative Report 37, 73.