Robinson, R.W. and J.E. Hunter

Mutations inhibiting fruit skin pigment influence susceptibility to early blight.

A mutation for pink fruit color, due to absence of yellow pigment in the fruit epidermis, was induced in ‘New Yorker’ by seed treatment with ethyl methane sulfonate. A similar mutant occurred spontaneously in a breeding line derived by backcrossing og^c into ‘New Yorker’.

            The fruit and foliage of both the induced and the spontaneous mutant were affected much more severely than ‘New Yorker’ or any other cultivar by early blight (Alternaria solani). All ripe fruit of the mutants were blackened by Alternaria spores. The vines were completely defoliated early in the season, despite a good fungicide program and good disease control with other cultivars.

            It seems likely that the increased susceptibility to early blight is associated with the ‘New Yorker’ gene background. It may be related to the very compact, open habit of ‘New Yorker’, exposing fruit to the sun. Sunburn injury often occurred on fruit of the new mutants, much more than on normal ‘New Yorker’ fruit, and this may have predisposed them to infection with early blight. Evidently the pigment in the fruit epidermis provides some protection from the damaging rays of the sun. Sunburn injury and early blight were not severe on cultivars and genetic stocks homozygous for y, thus lacking pigment in the fruit epidermis, but these lines had sufficient foliage cover to prevent sunburn.