Controlling “Leaf Spot” fungal disease

Credit: iStock/Mike Bentley

Q: Two seasons ago, I planted a Paul’s Scarlet. Magnificent display of flowers the second year but alas, as with the first season, the leaves contracted leaf spot (which I now know it is susceptible to), turned yellow/brown and fell prematurely. Not sure how this third season will play out. I believe I will have to spray the tree, which I am hesitant to do so. The less fungicide, etc., the better however.

Do you have any recommendations as to what product I might spray the tree with or other means of intervention? Otherwise, I am thinking or replacing the tree with a less problematic species.

The fungal disease is called “Leaf Spot (Fabrea Blight)” Diplocarpon mespili (Entomosporium mespili) and is very common here on the West Coast especially on Crataegus laevigata ‘Paul’s Scarlet’. Our wet spring weather contributes to the disease development.

Initially symptoms appear as small, angular, red spots on the upper leaf surface in spring and early summer. Eventually, when the spots coalesce, the leaf turns yellow and drops off. It’s not unusual by mid-summer to see an entire tree completely defoliated.

Established trees usually recover. Besides hampering aesthetic looks, this problem does not seem to kill susceptible host plant.

Control measures include:

1. Rake up and dispose of diseased leaves.

2. Dormant winter spray with lime-sulphur as directed on package.

3. Copper spray at 4 milliletres per litre (1 level teaspoon per quart) every 7–14 days during wet weather as a protectant.

4. Increase air circulation around the tree to keep leaves drier.

5. Resistant species or cultivars are available, such as Crataegus phaenopyrum “Washington Thorn” or C. crusgalli ‘Cockspur Thorn’, but most tend to have white flowers.