Fungus on a Washington Hawthorn

Washington Hawthorns

• Used primarily as flowering landscape trees, mature Washington hawthorns usually range from 25 to 30 feet in height with similar spreads. The thorny stems bear glossy, deep green leaves that turn striking red and orange shades in autumn. The showy flower clusters appear in late spring, featuring fragrant white blossoms that give way in the summer to edible fruits called haws. The glossy, bright red haws last through the winter, adding color and interest to your landscape if your feathered friends don't eat them first.

Powdery Mildew

• Powdery mildew is one of the most common fungal diseases found on Washington hawthorn trees. The Phyllactinia guttata pathogens cause a white, powdery substance to appear on the foliage in late summer or early autumn. These powdery spots sometimes merge to form fuzzy white growths that completely cover infected leaves. The fungus robs the hawthorn of nutrients, causing a decline in overall health and stunted plant growth. Fortunately, this fungus rarely causes long-lasting damage to hawthorn trees.

Rusts

• Caused by the Gymnosporangium globosum fungus, symptoms of cedar-hawthorn rust include deformed foliage scattered with yellow-orange spots. Severe rust infections cause premature defoliation. Warm, wet spring weather promotes the growth of rust-colored, gelatinous galls on the foliage. These growths sometimes girdle and kill small branches and twigs. Infections lasting several growing seasons reduce a tree's vigor and increase its risk of suffering from insect damage and other disease problems. Cedar-hawthorn rust requires a second host from the juniper species (Juniperus spp.) to be growing within a mile of a Washington hawthorn tree, or the pathogen cannot exist.

Leaf Spots

• Humid weather often encourages the growth of Entomosporium mespili or Entomosporium maculatum fungi, both of which cause leaf spot diseases on Washington hawthorn trees. Early symptoms include tiny, red-brown to gray spots appearing on infected leaves. Yellow or brown borders sometimes surround the spots. The affected areas eventually enlarge and merge, with visible fungal spore-producing bodies appearing as dark spots in the center of the affected areas. Severely infected foliage turns yellow and falls from the tree prematurely. The twigs sometimes develop irregular, brown spots as well.

Preventing Fungal Diseases

• Fungicides need such frequent applications that they aren't typically recommended for home use, according to the University of Florida IFAS Extension. Since you can't treat the diseases, it's best to prevent fungal problems from occurring in the first place. Fungus thrives in humid environments, so avoid using overhead irrigation methods that promote spore germination. Don't overcrowd your tree because tight growing conditions also promote humidity. Use the tree's mature size as a spacing guide when planting. The University of California Statewide Integrated Pest Management Program website suggests taking out any groundcover plants growing beneath your hawthorn and applying a 2- to 3-inch layer of organic mulch instead. Prune your hawthorn to improve airflow and light penetration throughout the entire canopy. Perform annual maintenance pruning and raking chores in late summer to prevent the fungi from overwintering in plant debris around your tree. However, remove infected plant tissue as soon as you notice it to prevent spreading fungal pathogens to other Washington hawthorns in your landscape.