Garden Plant Pest

[Alice Conroy]

Each year I have to remove several climing plants because they start dying back, ( I think they are a type of Allamand. but the white flowered one, and the leaves are not as leathery ) The stems and near new growth are covered in what looks like a small fur of white coating.They squash very easily, and are sticky.I have looked very closely and I don't think that they are an insect, but i might be wrong.
When I cut the branches off the white easily floats off into the air, yet I seem unable to wash them off the plants.
I have tried white oil numerous times, and also confidor.
White oil only seems to make the stems even softer.When I dug one of the plants up it seemed to have nematodes on the roots. I have just replanted in the past, but it just re-occurs. Before I plant any more ,can anybody please tell me what they are? why do these same plants keep getting them, and why can't I get rid of them?