A GREENHOUSE METHOD FOR SELECTING TOMATO SEEDLINGS RESISTANT TO BACTERIAL CANKER

Infiltrations of suspensions of inocula (10(3)-10(8) cfu/ml) into leaflets and stems were used to evaluate resistance to Clavibacter michiganensis subsp. michiganensis in 4- to 5-wk-old tomato seedlings. Disease increased at slower rates in infiltrated tissues of Heinz 2990 than in Floradade and Morden with all concentrations except 10(8) cfu/ml in leaflets and 10(3) and 10(4) cfu/ml in stems. Seedlings with low disease severity were selected from a segregating backcross population derived from Heinz 2990 following infiltrations of leaflets and stems with 10(5) and 10(8) cfu/ml, respectively. In field experiments, plants were inoculated by leaf excision with a scalpel dipped in inoculum of 10(8) cfu/ml. Heinz 2990 developed less disease (P<0.01) than Floradade and Morden. Less disease (P<0.01) also developed on selfed progeny of previously selected backcross plants than on the susceptible parent Grosse Lisse, and segregation for resistance occurred among nonselected backcross plants. Disease severity levels in field-grown plants were higher (P<0.01) in Morden than in Floradade, and fruiting was earlier and more concentrated in time in Morden. The test of seedlings in the greenhouse has potential for rapid screening of lines for partial resistance and in tomato breeding programs.