Friday, May 14, 2010

Apple Trees and Cedar-Apple Rust



For all of those who are planning an orchard, there are a few facts about apple trees, junipers, and cedars which are odd, interesting and important to review before the purchase. The combination of any of the aforementioned may result in the formation of Cedar-Apple rust, which is a most interesting fungus. It is necessary for the rust to have both the apple and the cedar to complete its life-cycle so purchase of resistant apples is paramount considering the numbers of cedars infecting our environment.

In these warm days of early spring, the galls associated with the rust appear on infected Cedar trees following a rain. The galls are golf ball size, bright orange, and any kind of moisture will cause the formation of tendrils which secrete a gooey gelatinous substance that actually drips from the tree. Our patio Cedar was infected after we planted several Jonathan and Golden Delicious apples trees in the orchard, both of which are highly susceptible to the fungus. As the galls grew on the Cedars and began to drip, the children often complained of being 'slimed'. The slime secreted is actually a fungal spore which can travel up to two miles on the wind looking for a susceptible apple or crab apple host.

Upon arrival on the apple cultivar, the spore settle in and the apple becomes infected. The first sign of infection is the formation of small yellow spots which appear rather suddenly in the uppermost branches of the apple tree shortly after flowering. The spots begin to enlarge and turn a vivid orange making the condition easy to identify. In late summer, small tube-like structures appear on the underside of the leaves spores from these tubes are relaesed into the wind and settle on susceptible cedars or junipers thus completing the cycle. Oftentimes as the disease progresses, the apple trees lose almost all of their leaves making their appearance quite pitiful.

Fortunately, there are new disease resistant varieties of apples which are readily available. Redfree, Liberty, William's Pride, and Freedom are extremely disease resistant and provide ample fruit. Additionally, they show resistance to powdery mildew, apple scab, and fire blight. Crabapples which are resistant include Indian Summer, Prairie Fire, White Angel, Adams, David, and Donald Wyman which are excellent choices. Avoid planting Vangard, Hopa and Radiant as they are very likely to become sickly and succumb to the disease considering the number of Cedars in our environment. Whenever you choose a tree it is wise to check for the latest varieties which are disease resistant. Your local Extension office has a list of the latest, healthiest, and most productive varieties available for your zone.

*Photo credit Cornell Research