Impact of Monosporascus cannonballus on root growth of diverse melon varieties and their F1 progeny in the field

Genetic variation for resistance to Monosporascus cannonballus in melon has been demonstrated in several greenhouse experiments. The evaluation of eight diverse melon lines and F1 progeny for resistance to this pathogen under field conditions was carried out over two successive seasons in South Texas. Roots were excavated and rated on a 1 to 5 scale with 1 being disease free and 5 being dead and withered. Lateral root numbers were counted and narrow-sense heritability (h 2 ) estimates for all traits were calculated. ‘Deltex’ and its progeny exhibited low damage ratings corresponding well with greenhouse results. ‘PI 403994’ performed much better in the field than in the greenhouse tests and ‘Creme de Menthe’ x ‘Caravelle’ performed much poorer. Heritabilities for the inverse of the root damage rating were high in both seasons as was the h 2 for lateral root number. Additive genetic variation for these traits appears to be important. Additionally, there was a strong negative correlation (r = -0.71) between lateral root number and root damage rating. Improvement of both root structure and M. cannonballus resistance should be possible in a reasonable timeframe.