Over the last couple of months, most of my trees have been getting strange rusty coloured leaves, whereas before they were green. This includes a magnolia tree and a bay leaf tree - even though they are not close together. Can you tell me what it is, and how I can fix it?
From your photos it looks like your trees are heavily infected by thrips, with a secondary infection of sooty mould. Thrips are very, very small insects that attack plants en masse and suck the sap out of leaves. This produces a sticky, sweet substance which attracts the growth of sooty mould.
The symptoms of thrips are visible on the upper side of a leaf by a grey, silvery colouring, while on the underside of the leaf, a brown and sometimes-sticky appearance is very obvious.
Spray your trees with a good quality horticultural oil, especially the undersides of leaves. Once the insects are dead and there is no more “food” for sooty mould to grow on, it will begin to disappear. However, while spraying kills the thrips, unfortunately the symptoms will remain on your tree’s leaves until they fall and are replaced in the growing season.
Thrips infections can be limited by maintaining the plants’ optimum growth; watering, fertiliser applications and mulching over the summer months. To avoid reinfection next growing season, ensure you remove and burn all infected leaves as they fall off the magnolia. With the bay tree; fertilise heavily starting early this spring with Daltons Premium Tree & Shrub Fertiliser. Heavily infected leaves will probably fall off the bay tree during the winter months, even though the plant is evergreen.