riverdale full sun crabapple(?)
photo 0f the other side of the leaf
Thank-you for contacting Toronto Master Gardeners with your concern.
It looks like the leaves in your photos belong to a crabapple that is suffering from apple scab. The following information was given in answer to a similar question in the past. I have given the link to the piece at the end. Note as well that if you go to the answer on the website, there are additional posted comments from Toronto Master Gardeners. I have added links to a couple of other resources at the end.
Apple scab is caused by the fungus Venturia inaequalis. The severity of the disease varies from situation to situation, and although it can cause severe leaf drop, it rarely kills a tree. However, if severe defoliation occurs for several years, the tree could be weakened, making it vulnerable to other problems. Apple scab can also infect hawthorn and firethorn.
Fungal spores overwinter on infected fallen leaves and fruit. Spring wind and rains carry the spores on to the young leaves. Cool, moist springs favor infection. Ten to twenty days after infection, new spores are released which infect new leaves. These secondary infections can continue throughout the summer until conditions become too hot and dry. Wet leaves are needed for the fungal spores to germinate and infect the leaf.
Integrated Pest Management Strategies
- Live with the disease. Apple scab rarely kills trees and the severity of infection will vary from year to year depending upon climatic conditions. Give more attention to young trees that may not establish well if prematurely defoliated. Also take note of any trees that become weakened through several years of defoliation and act accordingly.
- Keep plants healthy. A tree that is watered during dry periods will be better able to withstand periodic defoliation.
- Clean up and destroy infected leaves. The fungus overwinters on infected leaves; therefore, it is important to rake up and destroy leaves in the fall. Since the spores also travel on the wind, cleaning up leaves may be only partially effective. Do not compost leaves or infected plant material.
- Prune trees to allow good air circulation. Allowing for better air movement will allow leaves to dry more quickly and help prevent infection from occurring. Do not use overhead watering.
- Apply fungicidal sprays to susceptible trees if clean foliage is necessary or if defoliation reduces fruiting on eating apples. Sulfur can be used but it must be applied before the fungal spores germinate. Fungicidal sprays must be applied before infection as a preventive to be effective. Please note that apple scab resistance to some fungicides has been found in Ontario. For more information on this, refer to the Government of Ontario Ministry of Agriculture, Food and Rural Affair’s webpage on Managing Apple Scab and Powdery Mildew:
- Replace the tree with a variety resistant to apple scab. Resistant varieties of eating apples include: ‘Florina’, ‘Freedom’, ‘Gavin’, ‘Jonafree’, ‘Liberty’, ‘Macfree’, ‘Nova Easygro’, ‘Novamac’, ‘Priam’, ‘Prima’, ‘Priscilla’, ‘Redfree’, and ‘Sir Prize’.
Strategies 1, 2, 3, 4, and 6 are organic approaches.
Link to the Toronto Master Gardener answer: