Scale insects along the main leaf vein of the Bay laurel.
I have a small bay laurel bush planted in a pot. It simply cannot survive the harsh winter weather in the region I live and I have to keep it in house over winter. Bay laurel grown in pot often suffer from an irregular water supply or may be affected by frost and cold if not moved into a house on time. But, none of these is my trouble. For years I have been trying to fight the scale insects on my bay. They are probably the species Coccus hesperidum, Brown Soft Scale insect. They occupied not only the lower parts of the leaves placing themselves along the leaf veins but also the stems. Of course, they suck the sap and so weaken the plant. They are very hard to treat because of their protective shields. I used to remove them manually with a sharp brush, but now the plant is grown bigger and has too many leaves to do it. But, sucking the sap is not the only problem they cause. They excrete 'honeydew' which is full of sugars. It attracts ants and some other insects to feed on it. The drops of this sweet 'honeydew' fall onto the lower leaves coating the leaf surface and causing the secondary infestation - Sooty Mould.
The leaves become blackened and are sticky to touch. This type of fungi are saprophytic so, they do not cause any tissue invasion to the plant itself but may restrict the light and slow the photosynthesis. Besides, they are really not a pretty sight.
The scale insect
I am afraid, both mould and scale insects are very hard to remove and I have to treat them chemically with fungicides and oil/soap based insecticide. I will try to wash the plant with soapy water and a week later wipe the leaves with methylated spirit. If it doesn't help I would have to reach for chemicals.
To end this posts, something much nicer - my favourite late summer performers: Verbena bonariensis and Pennisetum alopecuroides.