Genetic variation in melon root structure and Monosporascus cannonballus tolerance

Vita.Six greenhouse and two field experiments were conducted to quantify phenotypic variation in melon (Cucumis melo L.) root structure and response to Monosporasciis cannonballus (TX 90-25) inoculation. The first experiment revealed highly significant differences among six week old plants o f 14 diverse melon lines for root dry weight, vine dry weight and vine length. The second experiment revealed significantly greater root lengths, areas and dry weights for M. cannonballus tolerant types during the first two weeks o f development but few significant differences by week 4. In experiment 3 the tolerant line ‘D eltex’ was superior to the susceptible line ‘Magnum 45’ for all root traits. In experiment 4 the two tolerant lines ‘Deltex’ and ‘Doublon’ had highly significantly greater root lengths and areas than the susceptible lines, ‘Magnum 45’ and ‘Caravelle,’ when plants were inoculated with 50-60 CFUs o f M.cannonballus (TX 90-25). The fifth and sixth experiments employed Design II and Parent-offspring mating schemes, and ten diverse melon lines, to generate narrow sense heritabilities (h2) for root lengths, root areas, root dry weights, stem dry weights and vine lengths, which were moderate to high for all traits examined. The sixth experiment utilized the same lines and parent-offspring regression to calculate h2 values for the same traits. Experiment 6 also evaluated heritabilities for M. cannonballus tolerance based on reduction o f the root traits among inoculated plants. These h2 values were all high except root tip number and small diameter (0.5-1.0mm) roots. These experiments provided evidence that additive genetic variation is important in melon root development and M. cannonballus tolerance among these diverse lines. The final two experiments were conducted in a field which was heavily infested with the pathogen. Root disease damage ratings over two seasons were lower for tolerant types such as ‘Deltex’ and ‘PI 124111 x TDT compared to susceptible types such as ‘Caravelle’ and ‘Perlita. The excellent field tolerance exhibited by PI 403994, which was highly susceptible under greenhouse inoculation, provided support that great root vigor can act as a tolerance mechanism in the field, where inoculum levels are not excessively high. Other mechanisms, possibly biochemical, may be present in tolerant lines such as ‘Deltex’ and ‘Doublon.