We have reported about the reaction of the plants Pto/Pto and Pto/+ after spraying with Lebaycid insecticide (Laterrot, 1985). Later we verified that this reaction is caused by the active ingredient Fenthion contained in the commercial product (Lebaycid liquid). Up to the present, we have encountered no exception to the necrotic reaction to Fenthion in plants which are resistant to Pseudomonas tomato. We use this reaction in our breeding programs for resistance to this bacteria. Furthermore, thanks to this reaction, we have come upon fresh resistance sources (H. Laterrot and J. Philouze, 1986).
Recently, exceptions t this linkage have been reported (Lapushner et al., 1988). These are the Farthest North and Oregon Cherry varieties which, while given as being resistant to P. tomato (Pitblado and Kerr, 1980), did not show the necrotic reaction to Fenthion. These findings are surprising for Farthest North which is probably the original source of resistance in Ontario 7710 (Pitblado and Kerr, 1979), which suffers from necrosis. The situation differs in the case of Oregon Cherry: in 1979, using seeds received from Oregon State University in 1978, we found this variety to be susceptible to the bacteria in our tests using 3 French isolates of P. tomato.
As the above findings prompted further study, we observed the behavior of these 2 varieties, on the one hand, after inoculation in two separate tests with a French and then with a Turkish isolate of P. tomato, and on the other hand after application of Fenthion. Results are given in Table 1. Farthest North is resistant to P. tomato and has a necrotic reaction to Fenthion like Ontario 7710: this is contrary to the results presented by Lapushner et al. (1988). Oregon Cherry is susceptible to the bacteria and has not got a necrotic reaction to Fenthion, like Monalbo. Our results do not agree with these presented by Pitblado and Kerr (1980), who quote this variety as being resistant to P. tomato.
Our results indicate that the linkage between Pto and a necrotic reaction specific to Fenthion is true in all cases. Until such time as it may be established the resistance to P. tomato and the necrotic reaction after treatment with Fenthion are controlled by the same mechanism, we suggest that the symbol Fen be used to indicate necrotic reaction to this molecule.
Table 1. Response of the Farthest North and Oregon Cherry lines to inoculation with P. tomato (using 2 distinct isolates) and to the spraying of leaves with Fenthion insecticide (product marketed by BAYER in the form of Liquid Lebaycid); comparison with the usual control lines Ontario 7710 and Monalbo.
_____________________________________________________________ Varieties P. tomato Fenthion (sources of seeds used) _____________________________________________________________ Farthest North Resistance Necrosis (Simcoe Station, CANADA, via Volcani Center, ISRAEL) Oregon Cherry Susceptibility No necrosis (Oregon State University, USA, via Volcani Center, ISRAEL) Ontario 7710 Resistance Necrosis (Simcoe Station, CANADA) Monalbo Susceptibility No necrosis (INRA, France) _____________________________________________________________Literature Cited:
Lapushner, D. et al. 1988 TGC Report 38:23.
Laterrot, H. 1985 TGC Report 35:6.
Laterrot, H. and Philouze, J. 1986 TGC Report 36:23.
Pitblado, R.E. and Kerr, E.A. 1980 Acta Horticulturae 100:379-382.
Pitblado, R.E. and Kerr, E.A. 1979 TGC Report 29:30.