Calcium hydroxide is a natural element, which has many uses in orchards. These range from eliminating lichens and mosses to protecting against Nectria cankers on apple trees. Tree spawn limits oviposition of certain insects before flowering (e.g. pear/apple psyllids).
The addition of 0.5% black soap (23%), i.e. approximately 0.5 litres per 10 litres of water, will reinforce the contact of the spawn on the treated surface.