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.
For the second straight year, fireblight on certain Callery pears and crabapples was more severe than usual in Ohio, although incidence and severity was not as common or serious as in 2001. Presumably, greater than usual problems with this bacterial disease (Erwinia amylovora) again this year were associated with abnormally warm, wet weather during bloom.
Blossom infections are thought to be more severe when temperatures exceed 60°F for extended periods during bloom, and these conditions were common throughout much of the state in 2002, especially for Callery pears. Throughout Ohio, there were even periods in the mid-80s in mid-April this year.
At OSU's Secrest Arboretum in Wooster, Ohio, there are even several 'Autumn Blaze' Callery pears which will probably die from this disease, which is a very uncommon occurrence for ornamental pears. As in 2001, 'Aristocrat' callery pear was another pear which was reported to be particularly affected by fireblight throughout Ohio.
One interesting influence of fireblight from 2001 was noticed at the National Crabapple Evaluation Program plots at Secrest Arboretum in Wooster in 2002. After prolonged debate, resident crabologists Erik Draper and Jim Chatfield did not prune out the fireblight strikes from last year, with the idea that this would provide a great challenge to the plants in the evaluation plot by leaving plenty of bacterial inoculum in the plot. Fair enough — or not. Anyway, we noticed this spring that frogeye leaf spot is much more severe on trees with last year's fireblight strikes.
What's going on? Well, frogeye leaf spot is caused by the fungus Botryosphaeria obtusa, which also causes black rot cankers on crabapples and apples, and stem dieback on many plants, including a number of viburnums. This fungus is a great opportunist, colonizing decaying and dying branch tissue.
What happened, presumably, is that the fungus colonized the shoots killed by fireblight and then sporulated freely during early wet weather this spring, causing the explosion of frogeye leaf spot occurring on crabapple leaves adjacent to the fireblight strikes. So, by extension, this is a good lesson for all, illustrating the importance of pruning out declining and dead branches on trees. If this frog-eye leaf spot-fireblight connection sounds familiar, it just proves you were an attentive reader earlier in this article and that the authors think this example is particularly instructive!
This discussion reminds all horticulturalists of the importance of good plant problem diagnostics. Many plant problems, resulting in the new growth dying and hooking over like a shepherd's crook, are being incorrectly diagnosed as fireblight. The shepherd's crook symptom is commonly created when new shoots on a plant wilt and then slowly dry down to form that crook shape. We have seen other plants and diseases, such as Botryosphaeria dieback on viburnum, cause that same blighted, shepherd's crook.
The best way to remember if it is fireblight is to ask yourself if the plant is in the Rosaceae family. Only plants in the Rosaceae family can be infected by fireblight. If the plant is not in the Rosaceae family, then you can rest assured that the disease is not fireblight. Of course, the next question will be, what is causing the damage?
For this answer, it will be necessary to send a sample to the C. Wayne Ellett Plant and Pest Diagnostic Center at Ohio State to have the problem identified. However, remember that the answer will only be as good as the sample sent. Don't just send a dead leaf or dead twig or the result will be inconclusive, other than just a dead leaf or twig.
The best samples are those taken from the area of transition, where normal, typical tissue changes to dead infected tissue. The greater the quantity of affected plant tissue that can be realistically sent to the Clinic, the better the chance for an accurate diagnosis.
Whole plant samples are particularly good because the roots can also tell a great story. So, for sample taking, bigger really is better!