Tag Archives: Pear

Apple Scab

What is apple scab?  Apple scab is a potentially serious fungal disease of ornamental and fruit trees in the rose family.  Trees that are most commonly and severely affected include crabapple, hawthorn, mountain-ash, apple and pear.  In ornamental trees, leaf loss caused by apple scab can make trees unsightly and aesthetically unappealing.  In fruit trees, leaf loss can reduce fruit yield.  In addition, the presence of apple scab on fruits can make the fruit difficult, if not impossible, to market.

Leaf spots typical of apple scab on apple. Typical apple scab leaf spots (left) and fruit lesions (right). The disease can cause total defoliation and make fruit unmarketable.
Leaf spots typical of apple scab on apple. Typical apple scab leaf spots (left) and fruit lesions (right). The disease can cause total defoliation and make fruit unmarketable.

What does apple scab look like?  Apple scab lesions (diseased areas) are often first noticed on leaves, where they most commonly occur on the upper leaf surface.  Fruits are also very susceptible to infection.  Lesions on both leaves and fruits are roughly circular with feathery edges and have an olive green to black color.  Lesions can be as small as the size of a pinhead or as large a ½ inch in diameter.  When disease is severe, lesions can merge and cover a large portion of the leaf or fruit surface.  Defoliation of a tree (i.e., extensive leaf drop) often follows.

Where does apple scab come from?  Scab is caused primarily by the fungus Venturia inaequalis.  Other species of Venturia can be involved in diseases similar to apple scab.  In particular, Venturia pirina causes a very similar disease (called pear scab) on pear.  Venturia inaequalis and its relatives survive the winter in leaf litter from infected trees.  Scab is most severe in cool, wet years.

How do I save a tree with apple scab?  Apple scab is not a lethal disease, even when trees totally defoliate.  Once symptoms are visible, it is too late to treat a tree.  Proper long-term management of apple scab is important however.  If left unchecked, defoliation due to apple scab year after year can stress a tree and make it more susceptible to other, more serious and lethal diseases and insect pests.

How do I avoid problems with apple scab in the future?  If your crabapple, apple or pear tree has a history of severe scab, consider replacing it with a resistant variety.  See UW Bulletins A2105 (Apple Cultivars for Wisconsin), A2488 (Home Fruit Cultivars for Northern Wisconsin), and A2582 (Home Fruit Cultivars for Southern Wisconsin) for recommendations.  These publications are all available at https://learningstore.extension.wisc.edu/.

If you have a susceptible tree that you want to maintain, carefully collect up and discard fallen, infected leaves each autumn.  These leaves are a major source of spores that can infect leaves the following growing season.  Also, be sure to routinely thin your trees to open up the canopy and allow better airflow.  Thinning will promote more rapid drying of leaves, which is less favorable for apple scab development.

Even with proper fall leaf clean up and thinning, you may have to consider applying fungicide treatments to susceptible trees, particularly when the weather is cool and wet.  Captan, chlorothalonil, mancozeb, myclobutanil, propiconazole, or thiophanate methyl, are available for apple scab control, although not all of these active ingredients can be used on trees where fruit will eventually be eaten.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the correct fungicide(s) in the safest and most effective manner possible.  Typically, you will need to treat every seven to 14 days from bud break until wet weather subsides.  DO NOT use myclobutanil, propiconazole, or thiophanate methyl as the sole active ingredient for all treatments.  If you decide to use one of these active ingredients, alternate its use with at least one of the other active ingredients listed above to help minimize problems with fungicide-resistant strains of the apple and pear scab fungi.

For more information on apple scab:  See UW-Extension Bulletins A2173 (Crabapple Disorder:  Scab) and A2594 (Mountain Ash Disorder:  Scab), both available at https://learningstore.extension.wisc.edu/, or contact your county Extension agent.

Bacterial Blight

Death of lilac branch tips and leaves due to bacterial blight.
Death of lilac branch tips and leaves due to bacterial blight.

What is bacterial blight?  Bacterial blight, also known as blossom blight or shoot blight, is a common and often serious disease of Chinese, Japanese, Persian and common lilac, as well as walnut, apple, pear, plum and cherry.  White flowering varieties of common lilac are most susceptible to the disease.

What does bacterial blight look like?  Initial symptoms of bacterial blight may include dark brown necrotic (dead) leaf spots with yellow halos.  If leaf spots develop before leaves are fully expanded, leaf curling and twisting may result.  More advanced symptoms include necrotic blotches starting at the leaf margins and advancing inward, as well as black streaking on twigs.  In its most severe form, bacterial blight can result in the death of branch tips, leaves and blossoms.

Where does bacterial blight come from?  Bacterial blight is caused by the bacterium Pseudomonas syringae pv. syringae (Pss), which survives in diseased stem tissue (cankers), plant debris, and soil.  Pss can be spread by insects and on pruning tools but is more commonly spread by wind and rain.  Often Pss is found on the surface of healthy plants and does not cause disease.  Infections can occur when the bacterium enters tissue through natural openings or through wounds caused by insects, pruning, wind damage or hail.

How do I save a plant with bacterial blight?  Prune diseased twigs 10 to 12 inches below the point of visible symptoms, and dispose of the branches by burning (where allowed by local ordinance) or burying them.  Always prune in dry weather, and disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

How do I avoid problems with bacterial blight in the future?  When planting lilacs, provide adequate spacing between shrubs.  Thin individual shrubs each winter to promote good air circulation (see University of Wisconsin Garden Facts XHT1015, Pruning Deciduous Shrubs, for pruning tips).  Properly water, fertilize and mulch shrubs to avoid stress that may predispose them to disease.  Avoid overhead watering that may keep leaves wet.  If you have had chronic problems with bacterial blight, you may want to use a combination of copper and mancozeb-containing fungicides for control.  Apply fungicides two to three times at seven to 10 day intervals as leaves emerge, but before symptoms develop.  Read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.

For more information on bacterial blight:  Contact your county Extension agent.