In some pine species the young, expanding needles are sensitive to HF. This is lodgepole pine (Photo by L.H. Weinstein). Sometimes there is a dark brown line between necrotic and green tissue but not always so it is not a reliable diagnostic feature. Once needles stop expanding they are much less sensitive, so for diagnostic purposes it is vital to know the HF concentrations during expansion. Because needle expansion tends to be uniform on individual trees, injury tends to develop uniformly within that individual. However, if individual trees develop new needles even a few days apart this can alter the degree of symptom development so there can be differences between trees.