kostheos

I would like to know what kind of disease is this one. The story is like this: This banana clump has been growing for almost 20 years without any problem. But this summer I have observed damage on leaf petioles and blades on one of the pseudostems. So I decapitated the damaged pseudostem and the next bigger one as well and treated cut points with insecticide (abamectin) in two applications within 15 days. I have inspected also removed crowns through dissection but I have found no signs of bugs (last three pictures). During the rest of summer new crowns developed from those pseudostems and the bigger one has also fruited but the stem has cracked afterwards. During past October I have observed again same damage signs on new leaves (first three pictures). Then I have beheaded all pseudostems plus I cut down the fruiting cracked one, whose base is absolutely white and healthy, and performed dissection on all crowns with following finding: none.. or almost none! I say 'almost' because I think I have seen twice with the angle of my eye by the very recent time after removal of the crowns a very tiny and very thin white thing moving rapidly through the healthy spongy tissue inwards from the exposed point. It was so thin and so white,that I could not find it through some sample dissection, because it could have been very well hidden among every inch of healthy, spongy tissue. If what I think I saw really exists, I am absolutely sure that it was not a larva of a weevil. I know how such larvae look like and I assure you that this thing looked rather like a microscopic worm! Please any help in the diagnose and cure will be immensely appreciated!