In the last decade, the emergence of leaf disease caused by the fungus Mycosphaerella sp. is jeopardizing the development of some species of the genus Eucalyptus at its juvenile life stage. The development of forest species for energy purposes, namely to generate biomass in shorter periods, must take into account this circumstance at the time of measuring the energy density generated by these species. This paper defines a method which relates the measure of the severity of the disease with the energy lost per Ha. First, it has been characterized from the energy point of view, thus, it has quantified the calorific value of the biomass of Eucalyptus globulus decomposing it in their respective fractions at certain moisture. Then, depending on the degree of severity of the disease, it has been estimated the amount of dry biomass per tree at the age of 27 months and crossing it with the calorific values it has been determined the energy loss per unit of area according to the overall damage index. Finally, a brief economic analysis considering the consequences of this disease in a stand of Eucalyptus globulus for energy production, has been performed.