Q. I have two chaste trees in my back yard and for a couple of years now there is a black appearance taking over the trees.   It started at the root or base of the trees and has been traveling up the tree onto the branches.  It is a slow process but somehow this does not seem to be a healthy appearance to me – it looks like black soot although i have not touched it with my hands.  I have asked several people about this and they say the trees are getting old – but i have only had them for about six years and they are maybe 9 or 10 feet, small trees. Can you tell me what to do about this? A. I have not yet seen any disease or insect problems on chaste tree here in our climate and a quick look on the internet I have not seen any reports from reliable sources. This observation is a new one for me.            Vitex winter form. Aphids have been reported to be a problem on chaste trees in other places. If aphids are present you would see them in the spring and fall. Leaves might be cup-shaped due to their feeding.             Excretions from aphids are full of sugar and will cause a blackening of limbs and leaves due to a black mold that grows on the sugar. Even though extracts from seeds of related trees have been reported to kill aphids. I will take a closer look at some here in town and see if I see anything but so far I would have to agree with others. This is a good small tree for the desert with few pest problems except when people water it too often.             If there are no signs of the tree being sick other than this black soot on the trunk, I would guess it is natural aging of the trunk. We do see this type of thing happening on other trees as well.             Diseases like sooty canker you would see a decline in the tree, bark splitting or cracking and the “soot” (under cracked bark) would rub off on your fingers exactly like heavy soot from an active chimney. Sooty canker disease has black spores just under the bark that rub off on your finger like chimney soot.