Diseases and Pests of Hawthorn

Bacterial blight

Symptoms and Cause

Caused by the bacterium Erwinia amylovora, fire blight is a formidable disease. Fortunately, it only affects a few species of Rosaceae, including hawthorns: Japanese quince, cotoneasters, apple and pear trees, as well as pyracantha. Given that this bacterium poses a real threat to the fruit production of apples and pears, certain species susceptible to fire blight have even been banned from planting in France since a 1994 decree. Thus, the sowing of Crataegus is notably prohibited, as well as the multiplication and planting of Crataegus plants from sowing, “except for those intended to serve as rootstocks in production establishments“. If your garden hosts one or more hawthorns, watch for signs of blackening of the branches, wilting or drying of leaves and flowers. Identifying fire blight is not always straightforward, as other diseases can present similar symptoms.

Branch contaminated by fire blight

Treatment

Hawthorn rust

Symptoms and Cause

Easily recognisable by its round orange pustules, rust is a very common disease in the garden. It is caused by the fungus Gymnosporangium sp: small spots appear on the upper side of the leaves while pustules cover the underside. Damage primarily spreads in spring and summer as warm, humid weather favours rust attacks. Hawthorn is among the many plants that can be affected, alongside roses, St. John’s wort, mahonias, as well as pelargoniums, garlic, and leeks. Affected plants are generally not severely harmed but they quickly become unsightly, and premature leaf drop is noted.

Visible signs of rust

Treatment

• Systematic removal of affected leaves will help limit the spread of rust on your hawthorn;
• Treatments based on horsetail decoction, combined with nettle manure, will be effective both as a preventive and curative measure;
• Fungicides such as Bordeaux mixture or sulphur will be effective treatments;
• Finally, as the fungus survives in winter on fallen leaves, they must be collected and disposed of at a waste facility.

[o mildew]

Symptoms and Cause

Often referred to as “white disease“, powdery mildew is extremely common: a white, farinose coating covers the leaves, new shoots, and flower buds. The young leaves affected may become distorted and sometimes perforated. Not demanding in terms of water, powdery mildew can spread throughout the growing season. The affected hawthorn may become weakened and its growth slowed.

Leaf contaminated by powdery mildew

Treatment

Aphids

Symptoms and Cause

Grouped on young shoots in colonies, aphids can be of various colours: black, green, grey, or whitish… these insects suck the sap from your hawthorns to feed, to their great detriment! The attacked shoots and leaves curl up and honeydew flows out, on which sooty mould forms. No plant is spared by aphids, and even though their attacks are spectacular, they are generally not significantly affected. Nevertheless, aphids transmit viruses to them, hence the need to control them.

Leaf infested with aphids

Treatment

Tenthrid larvae

Symptoms and Cause

Tenthredes, related to wasps, belong to the order Hymenoptera, and their larvae closely resemble caterpillars, which they are not, as they do not turn into butterflies. Like caterpillars, they have the unfortunate tendency to voraciously munch on the leaves of many plants: barberry, St. John’s wort, apple, pear, cherry, rose… and of course your magnificent hawthorn! If these larvae adopt the characteristic S position when threatened: there’s no doubt, you are dealing with tenthrède larvae.

Adult and tenthrède larvae

Treatment