The discolored area is a bacterium. Bacterium spread very quickly. You need to remove the infected tissue as soon as possible. Use a clean disinfected sharp pair of pruning shears or scissors and cut away the diseased tissue by cutting into the green tissue. The green tissue will heal.
To prevent further infection:
Prevent damage to leaves. Damage creates and injury where bacterium and fungus can enter the plant. Many bacteria and fungus organisms become pathogenic on damaged plant tissue. Orchid leaves while growing, elongating are very easy to damage because the leaves tissue is so tender. Once the leaf has finished growing, the leaf hardens off and is not so tender.
Water and fertilize only the roots. Bacterial and fungal spores splash and flow with water and are blow by air. Air circulation actually helps control so the spores blow away.