Powdery mildew on a Japanese maple doesn’t happen by chance and is usually indicative of the plant experiencing moisture stress. This is normally caused by a number of factors such as over or under-watering, with related damage to the root system. It can also be the first sign that your Japanese maple is being attacked by vine weevil or other root-eating grubs.
Typically, powdery mildew is most commonly seen on potted plants; it’s rare to see it on plants in the ground as stress to the root system develops over a longer period. The term ‘powdery mildew’ would also suggest that it is a consequence of a damp atmosphere or weather but it is usually at it’s most aggressive in late summer when dry conditions are more prevalent.
Powdery mildew on new shoots
It’s a fairly simple matter to cure, but whatever is used will leave unsightly marks on the foliage after the powdery mildew has been killed but no long term detrimental effects on the plant. Slightly more serious is when your Japanese maple is producing a second flush of growth and powdery mildew establishes itself on the growing tips. These, even if the infection is eradicated, will kill the growing tips and the shoots are best cut back to the earlier spring growth.
Any contact or systemic fungicide will be effective, as well as Neem oil and sterilizing tablets used for baby feeding utensils or home brew kits. The latter should be made up using the weakest possible dilution according to the manufacturer’s instructions and tested on a small area first. Horticultural grade quaternary ammonium compounds used for surface sterilizing grafting knives and secateurs are also effective.
Prevention rather than cure is always going to be more effective, particularly if infections are occurring every year, and attention should be focused on the root system. Any of the following conditions will reduce your Japanese maple’s resistance to powdery mildew and should be corrected as soon as possible. Under-potting, where the majority of the roots are crowded around the edge of the root ball leading to excessive heating of the roots in hot weather. Over-potting coupled with poor drainage, causes the fine feeding roots to suffocate and die. Excessive watering coupled with poor drainage again, in the belief that your plant needs a lot of heavy watering in the summer. Where vine weevil or other root-eating grub damage is suspected, chemical or nematode treatment should be applied immediately.
To sum up, powdery mildew can be disfiguring but is only likely to be fatal on small, weak growing plants but if you have a healthy root system on your Japanese maple, the likelihood of infection is not going to arise.