Q. What is hemlock rust? How can I treat it?
Hemlock rust (sometimes called hemlock twig rust) is a disease of both the Eastern Hemlock (Tsuga canadensis) and the Carolina hemlock (T. caroliniana). It is caused by the fungus Melamspora farlowii. Unlike other rusts, the hemlock rust is the same for both species.
The first sign of infection with the rust is a yellowing of the new leaves in spring. This is followed by the changes in the shoots at the leaf bases,which turn yellow. This is followed by loss of needles, except at the tips. The cones can also become infected; they become discolored and do not produce seeds. Hemlock rust is particularly a problem following wet, cool springs.
The Cornell University Cooperative office recommends spraying with triadmefon or mancozeb (Pentathlon), once when buds break and twice thereafter at 7 - 14 day intervals. If the infestation is particulalry severe then it is best not to grow hemlocks on the property.
Two juniper rusts (Kabatina juniperi and Phomopsis juniperoora) also infect hemlock but this is more uncommon.
Note that Hemlock rust should not be confused with Hemlock mite rust which is caused by an insect. The symptoms can be somewhat similar but the latter can distinguished by the appearance of mites on the needle (viisible with a hand lens).
Courtesy of NYBG Plant Information Service