Comparison of soil infestation and toothpick methods for Macrophomina species in melon plants

In the northeast region of Brazil, melon production has been affected by root diseases caused by Macrophomina species. This study aimed to analyse the influence of inoculation methods using three Macrophomina species (M. phaseolina, M. pseudophaseolina, and M. euphorbiicola) on the expression of symptoms of root rot and branch decline in melon plants. Two inoculation methods were evaluated in greenhouse experiments: (1) inserting an infested toothpick into the neck of the plants; and (2) infesting the soil with rice grains at concentrations of 1, 3, 6, 9, 12, 15, or 18 grains per 2.7-L pot. Sixty days after transplanting, disease incidence and severity were assessed, along with biometric variables. To obtain disease incidence and severity values comparable to those of the toothpick method, 3 grains were required for M. phaseolina, 12–18 grains for M. pseudophaseolina, and 18 grains for M. euphorbiicola. The highest inoculum concentration (18 grains) increased disease severity and fresh root weight (by 50%, 100%, and 100%) and reduced root length (by 75%, 65%, and 80%), shoot length (by 74%, 74%, and 69%), shoot dry weight (by 75%, 75%, and 60%), and root dry weight (by 67%, 69%, and 48%) for M. phaseolina, M. pseudophaseolina, and M. euphorbiicola, respectively.