Ornamental Plants
Annual Reports and Research Reviews 2002

Special Circular 189

Infectious Disease Problems of Ornamental Plants in Ohio: 2002

James A. Chatfield, Ohio State University Extension, Northeast District/Horticulture and Crop Science; Nancy A. Taylor, Plant Pathology, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Ohio State University; Erik A. Draper, Ohio State University Extension, Geauga County; and Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District.

Horticulturalist, Cleanse Thyself

Sanitation practices are an essential part of good horticulture when it comes to disease management. How so? Let us count a few ways.

1. The obvious example is a disease like rose black spot. The fungus makes it from one year to the next (overwintering) on black-spotted leaves, canes, and other plant parts. So cleaning up diseased plant tissue at the end of the season, in the fall, or before the next one starts the following spring, helps control the disease.

   Of course, this sanitation effort is not a complete control. Microscopic spores may blow in from other people's roses to infect your crop, and preventive fungicides may be needed, but sanitation helps limit disease significantly.

   Since the rose black spot fungus also has repeating cycles of spore production during the growing season, it is also important to clean up spotted leaves to the extent possible. Get them off the plants if they become infected to reduce the amount of new infections as the season progresses.

2. Sometimes the effects of good sanitation are less obvious. One example is the relationship on crabapple of the diseases frogeye leafspot, black rot, and fireblight. Frogeye leaf spot and black rot (which cause branch cankers and dieback on crabapple) are caused by the same Botryosphaeria fungus. This "Bot rot" fungus is sometimes termed an "opportunistic" pathogen on stem tissue, meaning that it is best at colonizing already declining, dying, or dead stems. Here is where fireblight comes into the picture.

   Fireblight is caused by a bacterium that causes blossom, leaf, and stem tissue to die back. Guess what? After this stem tissue dies (leaving dead shoots with characteristic "shepherd's crook" symptoms), this stem tissue often becomes colonized by the opportunistic Botryosphaeria fungus.

   And if these fire-blighted shoots are left on the plant, not only do the fireblight bacteria overwinter and provide bacteria to infect other blossoms and shoots the next season, but also the black-rotted stem tissue provides spores that cause much more than usual frogeye leaf spot the next year. You can see it clearly, as the leaves next to the fireblight strikes have many more leaf spots (infections) than the other foliage on the tree.

3. Sanitation is critical at all levels of horticulture, from orange rust on brambles which is so hard to control with fungicides that the best approach is to simply remove (rogue out) all of the affected brambles (both cultivated and wild), to late blight of potato (a big contributor to the Irish potato famine), in which potato farmers soon learned that the fungus survived from year to year in cull piles of black-rotted potatoes that were left behind in the fields.

   In fact, one of the pithiest plant pathological sayings from around the turn of the century (from the 19th to the 20th Century) was about sanitation, when Antonin Woronin intoned: "The only cure for cabbage hernia is fire!" Say what? To decode: cabbage hernia is the old name for what we now call club root of cabbage, a serious disease and the only control at that time was to burn the crop residue with its infested leaves, depriving the pathogen of a place to survive over the winter. They went after the pathogen where it lived.

   So, if you see crown gall growths on stems of the rose or euonymus you are about to purchase don't! If Botrytis gray mold develops on geranium flowers deadhead. Remember, when planting, it is: Location. Location. Location. For plant health maintenance, it is often: Sanitation. Sanitation. Sanitation.

   Now, lets turn to some profiles of a few of the diseases noted in 2002.