Each spring we discuss cedar–apple and related rusts. In the landscape, we are especially concerned with controlling cedar–quince rust on hawthorn (issue no. 3, 2008) because it causes stem cankers and fruit infection. The cankers cause stem tips to die. The fruit infection is attractive only to a plant pathologist. Repeated rain events this spring allowed repeated waves of spore movement and germination from the alternate host (junipers) to the susceptible hawthorns. Secondary spread is by mycelia only, and that growth is very limited. In many cases, hawthorn fruits have been covered with the aecial fruiting bodies of this fungus, as in the image. You can see the aecial spores which have washed or blown onto the foliage. These spores will be moved by air to nearby juniper hosts.
There is nothing you can do to help an infected tree now. Still, look for these infections now so that you can remedy the situation next spring. Fungicides may be used to prevent infection for the season. Fungicide applications begin at flowering and continue until 1 to 2 weeks past petal fall. Mark you calendar now to be ready to spray when hawthorns flower in May 2009. The Illinois Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide lists many registered product options. At the end of each disease chapter is a list products that are recommended in that chapter, by active ingredient, trade name, company name, and mobility. Take advantage of these tables when deciding on a product.
If you are considering a new planting of hawthorns, look into ones with disease resistance. Thornless cockspur and Washington hawthorns are very susceptible to cedar–quince rust. Interestingly, this rust also infects apple, mountain–ash, cotoneaster, pear, quince, serviceberry, and many other members of the Rosaceae family. It seems to cause most damage, however, on hawthorn.