University of Illinois Extension

Care of Crabapples and Hawthorns

Disease can threaten crabapples and hawthorns, but prudent precautions can prevent this, said James Schuster, U of I Extension plant pathology specialist.

"Some crabapple trees may have developed brown spots on their leaves, and both crabapples and hawthorns may have yellowish gold spots on the leaves," said Schuster. "Some crabapples are very susceptible to the foliar disease called apple scab. This disease attacks any time after the buds have started to open and there is adequate moisture."

The disease requires freestanding moisture for germination and infection of the foliage. The moisture can be from rain, dews or overhead watering. From infection to the first visible sign of a spot can take between six and 18 days, he noted.

"Temperature affects how fast the spots develop after infection. Scab can infect during the entire growing season as long as there is adequate moisture," he said. "The first line of defense against this disease is to buy highly resistant crabapple varieties. If you do decide to cut down the susceptible crabapple and replace it with a resistant variety, remember that 'apple' wood is some of the best burning fireplace wood once it has been properly dried." Rust

If removal and replacement is not an option, then spraying for the disease is another possibility. The first fungicide treatment should be applied at bud break. Repeat sprays are done at seven- to 14-day intervals depending on severity of rains, dews and overhead watering.

"The more the leaves stay wet, the more frequently the fungicide applications need to be applied," said Schuster. "The final option is to lower your aesthetic values and live with the tree's sickly appearance. The disease is not directly fatal to the tree even though it weakens or stresses the tree."

On hawthorns and some crabapples, there are rust diseases that affect the foliage and sometimes the twigs and fruit. The apple and hawthorn rusts (above) can cause yellowish gold spots on the leaves.

"These spots will develop small black pustules which rupture to release spores for additional infection," he said. "While this is going on, the disease will grow down through the leaves and produce short hornlike structures that send spores out to infect several types of junipers. Some times, some of the infected crabapple and hawthorn leaves die and fall off by mid- to late summer."

"Quince rust often infects the twigs and fruit of hawthorns rather than the leaves. There are no spots. Instead the twig develops a swollen area. After the rust releases its spores from these galls, the rust dies and so does the twig from the gall out to the branch tip."

Preventive fungicide treatments need to start generally at flower bud break and continue till about mid-June. The fungicide is applied every seven to 14 days, depending on the amount of freestanding moisture on the plant material, Schuster said.