Sooty Mould in Olive Trees

At this stage of the season black olive scale can be seen on olive trees. The adult females are very easy to recognise on the olive tree stems. They are dome shaped dark black in colour and 2-5mm in size. Note also the co-responding sooty mould. 

Olive black scale and co-responding sooty mould.

Olive black scale and co-responding sooty mould.

During spring use copper fungicides to reduce the amount of sooty mould.

At this point of the season scale are dormant underneath their protective shell where they lay egg.

During the December and January cream coloured 'crawlers' hatch from these eggs and move up the stems. They usually settle along the veins of  young leaves. At this stage they don't have the impervious shell of the adult and can usually be killed with one or two applications of insecticidal oil about two weeks apart. It puts an oil film over the young 'crawler' and suffocates it. 

Spray oil during December to stop the crawlers.

If the crawlers are allowed to live, they will moult after about one month and then migrate to the young stems and twigs of the tree. Here they will mature and lay more eggs and their protective brown shells will be impervious to sprays.

Olive black scale. Squash the scale between your fingers to see if it is alive. If it is alive, then your fingers will be wet from the juices squeezed out. If it is dead  then your fingers will be dry and dusty.


Bad infestations of live mature scale may need spraying with an insecticide.

Bad infestations of live mature scale may need spraying with an insecticide.

As the scale feeds they excrete is a sweet, sticky, 'honeydew'. A  fungus known as sooty mould feeds on this food and multiplies until the entire tree may be covered with the black sooty mould.

The leaves are coated with the black deposit so the sun's light can't  penetrate the leaves properly. Therefore photosynthesis can't take place efficiently. This results in a stunted  and unhealthy tree with poor crops.

To make the problem worse, sweet 'honeydew' on the leaves also attracts large  numbers of ants. Ants constantly move over the scale, they frighten away the small wasp parasites which in normal cases would  keep the scale under control. 

The good news is that healthy olive trees don't get the scale, sooty mould and ant infestation to any great extent. More good news is that heavily infested  trees are easily fixed.

James HookComment