Canker

GardenWeb - Posted by Jean Zone 7/TN (My Page) on Tue, Jul 12, 05 at 21:18
I have seen a number of posts regarding canker that are passing on some not entirely accurate information. Hopefully, this will help clear up some of the confusion. Canker is a broad name for a rose disease caused by a group of fungi that cause "cankers" to form on canes. Canker is really not so much a single disease as it is a descriptive symptom that can be caused by several different fungi.

A canker is by definition a necrotic area on a cane caused by either fungi or bacteria. Typically, the canker will start out as a small dark red or purplish blotch on a cane. As the canker develops, it will form a depression in the cane that when closely examined with a magnifying glass reveals fruiting spores (if fruiting spore is confusing, think of them as tiny evil fungi seeds ready to hit the wind and spread further). As the canker grows, it may "girdle" the cane (i.e. - completely grow around the entire cane). At that point, the cane above even if healthy and green, is not long for this world. The cane will die, perhaps not immediately, but eventually.

Depending on the canker and how virulent it is, the canker can march down the cane and kill the entire cane. Certain kinds of canker are capable of killing a plant. Some cankers are less troublesome and will not completely girdle the cane, leaving the cane above viable. While I would probably advise cutting it off, some people have successfully left the cane alone and it has survived. Others have cut out the canker, bandaged the rest of the cane and the cane has gone on to lead a healthy life. (I confess I am way too lazy for this but if I had a canker down low on an established rose that does not throw new basal growth frequently, I might try to find the motivation). Cankers are always going to start where there is a wound. Freeze damage is a favored spot, as are prunung cuts, places where thorns were stripped, insect damage and any other wound where an opportunistic pathogen can invade.

There are four types of canker that are seen. One is caused by botryris cinerea aka gray mold and more commonly referred to generically as botrytis. This will commonly cause cankers to form in areas with cool wet winters and especially where winter protection is used to cover the rose in such climates. Botrytis left unchecked over the fall can overwinter and cause serious damage to a rose. Fortunately, simple rose hygiene will keep it in check along with a dormant spray of lime sulfur or Daconil. I have not found other fungicides to be as useful with botrytis, but others may differ. If you opt for the lime sulfur spray, suit up. It may be 'organic', but it's nasty and it will burn.

Common canker aka stem canker is caused by the fungus Coniothyrium fuckelii (yes, that really is it's name). This is the most common of the cankers. It starts out as reddish spots on the canes, and as it grows, you will see a dead spot on the cane with a dark brown or purple margin (edge) all around. This is one I find commonly at the bud union on grafted plants and is the canker I most commonly see on bareroots when I get them and likely traveled with the plant into cold storage. This one is relatively easily controlled with pretty much any fungicide and is not all that destructive.

Here's a picture of common canker run amok:



Brown canker is caused by cryptosporella umbrina. It starts out as reddish purple small spots which grow over time into a really ugly tan patch with whitish spots and a vivid margin, usually red or purple. This is a destructive form of canker and needs to be treated quickly. It typically results from wet mulch or soil mounded over roses. Daconil works well in preventing this canker from really getting going, but it's usually avoidable by simply not mounding materials over your plants if you're not in an area where the ground stays good and frozen all winter.

Here's a nice picture of brown canker:



Brand canker (caused by Coniothyrium wernsdorffiae) is the last kind and fairly rare in my garden. I occasionally see it on my tender noisettes with freeze damage. It is characterized by small reddish blotches that have light brown centers with vivid reddish to purple margins. As it gets established, the cankers cause splits to form in the canes which then look black from the fruiting spores. The same rules for addressing canker of any kind apply - fungicide and judicious pruning.

This is a nice picture of Aimee Vibert with brand canker in the fruiting stage:



In general, canker can be controlled by not winter protecting where you don't really have solid below freezing temperatures for the whole winter and by maintaining good hygiene. A preventative spray of lime sulfur or Daconil over the winter will go along way to controlling it, particularly the botrytis-induced canker.

Finally, if you have cankers, cut well below the canker and clean your pruners. Canker can be spread by pruning shears.