Tolerance to a whitefly-transmitted virus causing muskmelon yellows disease in Spain.

Muskmelon yellowing disease was one of the most serious problems affecting muskmelon crops along the south-east coast of Spain throughout the 1980s. The causal agent of this disease is a virus that we call muskmelon yellows virus (MYV); MYV is transmitted by the greenhouse whitefly Trialeurodes vaporariorum Westwood. It has proven impossible to find sources of resistance to MYV within a wide collection of Spanish muskmelon landraces and exotic varieties. However, 'Nagata Kin Makuwa' and PI 161375, lines of Asiatic origin, show tolerance to this disease. These two lines were studied together with two others ('Galia' and 'Piel de Sapo' type) that are very susceptible to MYV. The crosses between them (susceptible x tolerant) and the segregant generations derived from these crosses were also investigated. The studies were carried out in two different places and years. The expression of tolerance is influenced by the environment. A parabolic type relationship exists between the average value of percentage of tolerant plants and their variation. This allowed us to quantify the expected response in the segregant generations. The results observed in these generations agreed with a simple genetic control of tolerance. This tolerance, combined with protective measures which delay the infection, can contribute notably to mitigating the effects of MYV.