Infestation on china girl/boy hollies
This infestation is by a white ‘grub?’ On the underside of the leaf and then a black ‘mold’ on the upper side of the leaf. It is devastating all our hollies and I believe it is now spreading to the Japanese’s hollies. We sprayed them with Safari last year but notice that the new leaves are now showing signs of infection. We do not like to use products like safari. We are considering removing all the plants, we must have over a hundred of them. If we do remove them, will we need to treat the soil ? When would it be safe to replant with alternative plants? Would it be possible to cut the plants back to about 2or3 inches and treat the whole area while waiting for them to resprout. Or would that be a waste of time and effort. Any suggestions you could give me would be very helpful. Thank you. Kate McNeilly
Hollies are wonderfully resistant to many pests, but yours have a bad case of sooty mold. The reason the mold is growing so well is that some sort of sucking insects, like aphids or scale insects, have managed to attack your plants. The answer to this is not as drastic as you might think. You can spray weekly with an organic horticultural oil, such as neem oil, to get rid of the insects. (Follow the label instructions.) The sooty mold will gradually disappear once it doesn't have oozing sap to live on. Once your plants look healthy again, you may not need to spray any more.
That said, usually a bad infestation occurs when plants are stressed. It may be useful to consider whether there is anything you can do to lessen their stress. If they suffer drought, regular watering may help. If they are in heavy clay soil, better drainage may be an improvement. If the soil is alkaline, you can add soil sulfur as a top dressing to lower the pH. If they are in a lot of shade, you may be able to thin out some foliage to give them more sun.
Good luck with restoring your hollies to good health!