An Overlooked Canker Disease in the Landscape

Issue: 
January-February 2010

IN THE LAST COUPLE OF YEARS, we’ve noticed a disease lurking on some of our deciduous trees. A fungal disease that has been misdiagnosed as winter injury. Botryosphaeria (“bot-ree-o-sferia”) are opportunistic fungi that cause cankers and dieback on branches, leaf blights and fruit rots of many woody plants. There are several species of Botryosphaeria, and some with some fairly wide host ranges.

So, why should you care? Because its effects can be fatal. Since I was introduced to this disease by our plant pathologist, Michael Celetti, I’ve been noticing it in a lot of different landscapes. What I had previously thought was a physiological condition, much like winter injury, has more than a few times turned out to be an infestation of Botryosphaeria. Mike says that in most cases, the condition actually did start out as winter injury or a wound from some form of damage such as hail, insects or mechanical. Botryosphaeria then colonizes the wounded tissue and then spreads much further down the branch, killing the sapwood as it goes.

On Malus (apple, crabapple), Botryosphaeria obtusa causes stem and branch cankers, leaf spot (“frog-eye”) and fruit rot (“black rot”). Early symptoms of black rot cankers on limbs and trunks are subtle and first appear as reddish or purplish-brown, slightly sunken areas under the bark. Although the sunken areas often remain small and superficial, some can enlarge up to half a metre in length under the bark. As the cankers grow for two or more years, the infected bark and cambium die. As cankers mature, the outer bark becomes papery and peels away from the canker, exposing black diseased tissue underneath. Extensive branch cankers can weaken the branch, causing it to break in a windstorm. Sometimes, the infection will cause the entire branch to die. Cankers on the main trunk, especially on young trees, may girdle the tree resulting in canopy dieback and premature death.

On Malus, Botryosphaeria obtusa can also cause a distinct leaf spot. Leaf symptoms show up 1-3 weeks after petal fall. The first signs of infection are tiny purple flecks which eventually enlarge into circular lesions, about 4-5 mm in diameter. As the lesions enlarge, their margins remain purple while the centre turns tan-brown with a light centre giving the lesion a “frog-eye” appearance. Frog-eye leaf spot is commonly misdiagnosed as apple scab (Venturia inaequalis) in the landscape. Apple scab causes leaf lesions that are more diffuse, sometime initially appearing as green olive in colour and then turning purplish-brown to black. Often the diffuse scab lesion appears to run along leaf veins. Now you know the difference. Both frog-eye leaf spot and scab often occur on the same leaf making identification tricky. Botryosphaeria obtusa also causes developing fruit to die, turn black and shrivel (“mummies”) and persist on the tree. If the disease attacks larger fruit, it will often exhibit dark, concentric rings on the fruit skin.

So we’ve already mentioned that Botryosphaeria have different species with different host ranges. Unfortunately, it seems that just about every woody plant I hold dear is on the host list of various Botryosphaeria species. There is a small grove of Quercus rubra on campus and I’ve been watching branch cankers take over and kill the lower limbs of the trees at the edge. The cankers look very similar to what we see on Malus and also very much like the images of Botryosphaeria quercuum in Disease of Trees and Shrubs (Sinclair, Lyon and Johnson, 2005). I haven’t confirmed the species yet though. Botryosphaeria quercuum is the most common species of this fungus on northern red oak in North America. Symptoms of branch cankers include leaf dieback on the ends of small twigs and roughened bark on swollen sections of branches (where the canker is developing). Eventually there will be tiny, crescent-shaped openings in the bark bursting with black fruiting structures within.

Since Botryosphaeria is very active during the growing season, it is best to remove and destroy diseased tissue each winter. It is very important to remove pruned wood as well as ALL dead wood from the site (including firewood) since this fungus can survive on many types of dead wood. Applications of protectant fungicides should help reduce disease but there are no fungicides labeled for this disease on landscape trees in Canada (that I am aware of). As you already know, pesticide applications on landscape trees in Ontario must be made in accordance with Ontario regulation 63/09 and the Ontario Cosmetic Pesticide Ban.

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.


P: 519-824-4120 ext. 52671 • F: 519-767-0755
jennifer.llewellyn@ontario.ca

OMAF website: http://www.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp

Nursery Landscape Agriphone: 1-888-290-4441

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