Monilinia laxa is a fungus that causes brown rot in stone fruit. Immature green fruits in the first stage of fruit development (stage I) are generally susceptible. To investigate the relationship between the physical characteristics of immature fruit and susceptibility to M. laxa, we characterized the progeny, derived from a clone of wild peach (Prunus davidiana) crossed with two commercial nectarines (Prunus persica) varieties, through laboratory infection, transpiration monitoring and stomata counting. Two types of fruit infections were observed - ‘classic’ brown rot and ‘clear spot’ symptoms - which have not previously been described in the literature. The number and density of stomata did not explain the observed variability of infection in the progeny. However, surface conductance was positively correlated with infection level. This study provides experimental evidence partially linking physical fruit characteristics to brown rot infection at the immature fruit stage. The role of delayed cuticle deposition in susceptibility to brown rot of immature fruit is discussed.