Hawthorn Leaf Spot

Hawthorn Leaf Spot, Hawthorn leaf spot, caused by the fungus Entomosporium mespili Diplocarpon mespili, is common on Paul’s Scarlet hawthorn, and occurs on other hawthorns. Apple, pear, quince, mountain- ash, Cotoneaster, and Photinia are some of the more than 60 species of plants attacked by this fungus. However, studies have shown different strains of the fungus occur; thus, it is not expected that the strain infecting hawthorn will also attack all of these plant species.

Red to purple, round or somewhat angular spots develop on the leaves, leaf petioles and succulent stems. Spots are 1/32-1/16 inch diameter and may coalesce to form blotches. The center of each spot is black, but may become light-colored and somewhat shiny when numerous spores are present. Smaller spots are usually uniform in color. There are usually many spots per leaf. Severe leaf drop may occur, and usually happens in early to mid-summer but leaf spotting and drop can occur soon after the leaves emerge.

Leaf spot is a fungal disease which can infect hawthorn, photinia, pear, pyracantha, crabapple, and related species. Initial infections are small reddish-brown to grayish spots on the upper or lower leaf surface. The spots may enlarge and coalesce on very susceptible species. On hawthorn, the spots are followed by yellowing and premature drop of the affected leaves.

A “green island” effect can arise when the leaf turns yellow, as the areas around the fungal lesions may remain green. The disease is favored by wet weather and is easily spread from infected tissues by rain and splashing water. It overwinters mainly on fallen infected leaves. Succulent new growth is particularly susceptible to infection.

Disease Cycle : Hawthorn leaf spot survives the winter in lesions on green twigs and fallen leaves. In the spring, rainfall splashes spores from fruiting structures onto developing leaves. Cool rainy conditions maintain leaf wetness and favor spore germination and penetration of leaves and green shoots. Fruiting structures arise in leaf spots and repeating cycles of spore production occur throughout the spring and summer.

Control: Collect fallen leaves to reduce the amount of inoculum available to initiate subsequent spring infections. Prune and space trees well to increase air circulation and sunlight penetration so foliage dries quickly. If irrigating the trees avoid wetting the foliage and water early in the day so leaves dry rapidly in the afternoon.

Where hawthorn leaf spot is a persistent problem, grow resistant hawthorn species such as Crataegus crusgalli cockspur hawthorn and Crataegus phaenopyrum Washington hawthorn. Fungicides protect developing leaves and twigs in the spring on susceptible high value trees. Begin applications when buds break open and repeat at labeled intervals as long as the weather is cool and wet.

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