We live near London, Ontario. We have a large Bosc pear tree that was already in the yard when we moved in 12 years ago. The fruit the tree bears is deformed or has dimples with a woody piece under the dimple leading to the core. I assume this is some sort of pest. When, how and with what should I treat this tree? I have tried lime sulphur in the spring before, but my timing may have been off.
From your description, it seems your Bosc Pear tree (Pyrus communis) has Pear Scab (Venturia pirina), which is a fungal disease. To some extent, for home gardeners, this is a cosmetic defect and can be mostly overlooked but it will affect long term storage of the fruit. More seriously, this disease can cause premature fruit drop, early defoliation, weakened trees and reduced fruit-bud development for the next year’s crop, if it is left untreated. This fungal disease is spread by spores that land on the new green leaf tips during wet weather in early spring.
Scab is difficult to combat with sprays – commercial orchardists may spray fungicides seven or eight times during the growing season. If you decide to spray, you can use Bordeaux mixture, which has two active ingredients, lime and copper sulphate.
To make Bordeaux mixture, add 6 tablespoons of spray lime and 2 tablespoons of copper sulphate to a gallon of water for a weak mixture and 8 tablespoons of both for a strong mixture. Put the copper sulphate in the water first and dissolve completely, then slowly add the lime, continuously shaking. The mixture should also be agitated while spraying.
Apply the strong mixture, combined with dormant oil, when the green tips of the leaf buds appear, and spray the weak mixture during bloom. Leaves and flowers must be covered with the spray. If it rains within 48 hrs, the fungicide must be reapplied. If you have just one or a few trees to do, a pump sprayer will work well. These products can be found at most garden centers and anytime they are used, protective gloves, eye-wear and mask should be worn and spraying instructions followed exactly. Repeated treatments may be needed.
Cleanliness around your tree is also vitally important. The fungus overwinters in infected leaves, dead branches or fruit left on the ground. The spores from the fungus start developing in late winter or early spring and are released into the air to be spread by rain, wind, insects or birds. They germinate on the leaves when the surface is wet or humid, and begin creating the spots. Heavy infections cause the leaves to turn yellow and drop off. Clean up fallen leaves, branches and fruit regularly and put them in the city compost bags. Don’t try to compost these materials in your home compost as it will not get hot enough to destroy the fungal spores.
As the fungal spores can travel distances, have a look around your neighbourhood to see if someone else has a pear tree that might also be infected. If so, you could approach that neighbour and try to combat the problem together, by cleaning up & disposing of any leaf, twig or fruit debris and spraying both trees at the same time. This way there will be less chance of re-infection.
Without seeing your tree, it is impossible to know for sure, but it is probably salvageable, however it will take a bit of work followed by regular maintenance. If you feel the problem is beyond your capabilities, then calling in a professional arborist who specializes in fruit trees would be a good idea. Below, you will find a link to a Ontario Ministry of Agriculture, Farm & Rural Affairs (OMAFRA) website article on Fruit Trees in the Home Garden.
https://www.omafra.gov.on.ca/english/crops/facts/fruit_home.htm