There are 4 or 5 phytophthora diseases that commonly infect citrus trees. Without testing, it is difficult to be sure which fungi is significantly causing your tree's problem. An educated guess is that the particular fungus is Phytophthora citrophthora, which causes both gummosis and citrus root rot in Mediterranean climates where seasonal rainfall occurs during the cooler fall and winter months. Therefore, with P.citrophthora problems are especially prevalent and therefore are noticed by the grower during fall and winter. I've never used phosphoric acid, but looking it up it seems to be an acceptable treatment. Anyway, if you look up P. citrophthora disease problems with citrus you should find some good general citrus control information.- Millet
