Q. Help identifying apple tree disease
Thank you for your photos and your question regarding apple tree disease.
We are having some difficulty in making a diagnosis from your scans - they are very clear but lack the three dimensionality we need to examine texture and look for spores and fungal fruiting bodies. That said, both sets of photos appear to show fungal disease; the first looks more like scab and the second more like rust. Let me describe both diseases so you can make an examination on site and see if they match what you are observing.
Apple scab is first noticeable as olive green spots with unclear border on fruit and leaf. The spots enlarge, darken to green-brown with a fuzzy texture, and may extend to twigs. A severe infection can cause leaves to drop prematurely and weaken the tree in ensuing seasons. Fruit, if affected, will malform and crack, and may form corky patches and drop.
This is a fungal disease caused by Venturia inaequalis, which produces black spores on fallen apple leaves around the time in spring that the flower buds develop. The infection is apt to be worst when wet, cold conditions persist in the spring as the spores are carried by the wind and splashing water. Moisture is also required to be present for several hours for the spores to germinate. Further spores erupt on infected leaf buds and emerging fruit, and a secondary transmissions take place through rain and wind. The fungus resides on leaf litter over the winter.
The most important control measure is the placement of trees in an area where sun and air circulation help control moisture. The disposal of fallen leaves in autumn is essential. A preventative spraying regimen requires at least four, carefully timed, treatments. More information on products registered for the treatment of Apple Scab in New York State and their use, refer to the Horticulture Diagnostic Laboratory Fact Sheet from Cornell Cooperative Extension of Suffolk County. Organic sulphur products are available but less effective than other fungicides.
Cedar-apple rust disease, (Gymnosporangium juniper-virginianae), and several closely related rust diseases, require two hosts to complete the life cycle of the disease. For cedar-apple rust, that means that both an apple or crabapple and eastern red cedar (or other juniper) must be present within a distance that allows the spores to travel from one plant to the other.
The first symptoms noticeable on an apple tree are yellow leaf spots that appear shortly after bloom. A wet spring, particularly just after leaf buds break, will increase the severity and spread of the infection. Sometimes oozing, orange bumps will form under the leaf spots and then become black. Later in the season, clusters of thread-like fruiting bodies grow from the underside of the leaf spots, as well as twigs and fruit. Spores that infect the needles of the alternate (cedar) host are transmitted in warm, damp weather.
While the cedar host may not be present on the same property (spores can travel for up to 5 miles), the infection on that plant can be identified by the emergence of a gall six months after spores reach it. About 18 months later, brown telial horns emerge from depressions on the gall, elongate and become bright orange and gelatinous in the spring rain. They dry up after releasing spores. Leaves may fall from the tree, especially in dry weather.
This disease rarely kills its hosts but it can make them unsightly and gradually weaken them in succeeding years of early leaf fall. While growing the two plants in close proximity should be avoided, removal of the cedar host may not be an effective solution because the spores can travel well beyond the confines of a single property. An important control measure is the placement of trees in an area where sun and air circulation help control moisture. Use of fungicides on apples and crabapples can be effective if timed with the release of spores by the cedar host while the gall is orange and gelatinous. Reapplication may be necessary if the weather is rainy but the risk to the apple tree passes about a month after bloom.
It is simply not possible for us to be certain about diagnosing your plants without greater examination of plant materials (photos) and some additional information about the appearance of symptoms. You can contact us by email (firstname.lastname@example.org) if you would like to share additional photos or with future questions. You can also send your plant materials to a Cornell Cooperative Extension office for disease analysis. Follow this link for more information on how to do that.
Courtesy of NYBG Plant Information Service