Posts Tagged ‘Fungus’

Wednesday, November 25, 2009

Black Knot Fungus – Scourge of Adirondack Cherries

Nature is constantly at war with itself. Romantics tend to see nature as colorful sunsets, fox pups playing around their dens, and bluebirds feeding their young. People with a more utilitarian outlook see nature as either a source of food (deer, turkeys, blackberries), or something to be conquered at all costs (human needs come first). There is truth in all views, but not one of them is exclusively “correct.” Nature has its warm fuzzy moments, but in reality, it is, as the saying goes, “red in tooth and claw.” This holds true for plants as well as animals.

Sometimes I think it’d be kind of nice to come back as a tree. Trees can live a long time. They provide food and shelter for birds and other wildlife. They help pull pollutants from the air and make oxygen for us to breathe. But they are also food and shelter for insects and fungi and myriad other pathogens. And then there’s the weather: wind storms, ice storms, lightning – these can all take their toll.

Not far from my house there’s an area that was once cleared and is now rapidly returning to a cluttered, tree- and shrub-filled tangle of growth. Certain species dominate the growth, and in the shrub department it is blueberries, chokeberries, and choke cherries.

Choke cherries (Prunus virginiana) are a native shrub that can grow to 30 feet in height. Around here, however, every specimen I’ve seen has been shorter than I, courtesy of the local pruning service: Odocoileus virginiana, the white-tailed deer. The berries are full of antioxidants and are edible by people (best in jams and syrups, where you can counteract their astringency with a good dose of sugar). Keep in mind, though, that the plant is toxic to horses.

Choke cherries, like other cherries and plums, are susceptible to a native pathogen called Black Knot Fungus (Dibotryon morbosum). The patch of choke cherries that I visit weekly is riddled with this fungus. It looks like someone has stuck a bunch of burned corndogs on the branches. This time of year the blackened growths are as obvious as the nose on your face, but when the disease is in its earliest stages, it can be very difficult to detect. If you are growing cherries or plums commercially, or even for your own enjoyment at home, you will want to know how to detect this virulent pathogen as soon as possible.

Black knot begins its colonization when spores are released from the parent fungus. The spores come in two varieties. The first are asexual, called conidia, and they appear as an olive-green, velvety growth on the black knot cankers in their second spring of growth. From early spring to early summer, wind and rain break off the conidia and spread them to new infection sites. The second kind of spores are ascospores, and these are formed sexually through the fruiting structures of the fungus, which are found on knots that are beginning their third year of growth. Like the conidia, they are spread by wind and splashing rain from early spring to early summer.

Once the spores are airborne, either blown or splashed, some will land on young wood, such as twigs and branches. Here the spores settle in for the long haul, either entering the plant via wounds or directly inserting themselves through the bark. Often entry is at the crotch of the twigs and branches. If the weather is wet (wet being a relative term, for it only needs to be wet for a few hours), and the temperature is between 55 and 75 degrees Fahrenheit, conditions are ripe for infection.

So, the spores start to grow. Mycelium snake their way into and all along the wood of the tree/shrub. During the first year of growth, a small brownish blob may appear on the infected stem. It’s not terribly noticeable, which is why the disease is easily overlooked until it is well-established. Year two rolls around, and now the knot grows rapidly. At first it is soft and develops a greenish-brown color: this is the sign that the conidia are developing. As summer number two progresses, the knots, which are now rather large (they can grow up to a foot in length over time), start to harden and turn black.

Eventually the knots can encircle the twig/branch on which they are growing, effectively girdling it. The end result is a dead twig/branch above the knot. And even though the knot is now hard and crusty, its edges can continue to grow. Eventually, the oldest parts of the knot will break down, and this opens them up to invasion by boring insects (not insects that are dull conversationalists, but insects that will chew their way into the woody tissue of the plant, potentially bringing with them a whole new set of pathogens).

Some authorities consider black knot to be a minor disease, while others call it a serious problem. The latter are probably involved with commercial fruit growers, for whom black knot can indeed be a serious problem. But even if you only have a single cherry or plum, you want to know how to deal with this fungus, for if left untreated, it will work its way through the cherry and plum population, eventually killing off all the trees.

The first thing to do is routinely inspect your trees. You want to nail the fungus as early as possible, so know what the first summer’s growth looks like. If you miss it, and you don’t notice the knots until they are well-formed, it is still not too late. Grabbed your pruners and cut off the offending branch(es). You want to cut about eight inches below the canker to ensure that you are getting most of the mycelium inside the wood. Gather all your prunings and burn them. Or bury them deep in the ground.

If cankers have formed on the trunk of your tree (not as common, but still possible), dig them out with a knife and chisel, taking an additional inch of wood all around. If the resulting hole is greater than two inches across, paint it with shellac and cover with tree-wound dressing. You will also want to destroy all affected wild trees/shrubs in the immediate area. The recommended distance is 600 feet. If you have an orchard you need to protect, contact your local extension office to find out what dormant sprays and fungicides are recommended.

It’s a war zone out there. Fungi, insects and other pathogens are attacking trees and shrubs; trees fight back with sticky saps and toxic chemicals. Some plants call in the cavalry, in the form of insects that will attack the offenders (such as ladybugs vs aphids). The next time you go for a walk in the woods, think about this. Take a look around. See if you can find some evidence that all is not as calm as it seems.

Saturday, June 13, 2009

Orange Rust – a Blight on Adirondack Berries

“What is this orange stuff?” I’ve asked this question myself, and I’ve been asked by many other people. Today when I saw it while doing an aquatic studies class, I finally decided to investigate, and this time I spared the good folks over at the Cornell Cooperative Extension Office in Westport by looking it up on-line first. (I’ve developed a name for myself at the Coop. Ext. office, thanks to all the strange samples I send them for ID.)

When faced with a strange thing to identify, it helps to gather as much information as possible ahead of time. For instance, I’ve only ever seen this on raspberries growing along the roadside where I walk the dog. Today we found it along the outlet of Rich Lake. I suspected it was a fungus. So, I fired up the computer and did a search for orange fungus raspberry leaf. Here is what I discovered.

Orange rust is indeed a fungus. Originally labeled Gymnoconia peckiana (although I did find one source that calls it Gymnoconia interstitialis), this fungus has now been subdivided into two forms, based on morphological differences. These differences depend on the species of berry affected (black raspberry vs blackberry). So, now we have Gymnoconia nitens, which is common on blackberries, and Arthuriomyces pekianus, which occurs on black raspberries. While one source I found claimed that orange rust isn’t really a problem for the overall plant, most other sources state that it is a serious disease in the Northeast, affecting wild and even cultivated brambles. So far, red raspberries seem to be resistant.

The good news is that orange rust has no alternative hosts. In other words, its entire lifecycle is dependent on the blackberries and black raspberries. In the winter, the fungus hides out on the new roots underground, just waiting to reappear and spread the following year. You will know your plants have it when in late May and throughout June you find the undersides of your berry leaves coated with bright orange “stuff.” The cure: destroy the infected plants. Rip (dig) them up in the early spring (before the pustules erupt), get thier roots, and destroy them.

UM…pustules? Yes – if you look at the plants in early spring, before the leaves have completely unfolded, you may find glandular bodies on the leaves. These are the pustules (actually, they are called sori, which is the scientific name for a spore-containing structure; ferns reproduce with spores and you can often find their sori on the underside of their fronds). When they mature, they look like black specks and the surrounding tissue is yellowish. After maturing, they erupt, sending their spores out to populate the world; this is the orange “stuff” you see on the underside of your leaf.

The next question that comes to mind is “how” – just how are you supposed to destroy the infected plants. There you are with your pile of dug up infected berries – what are you supposed to do with them? Do you burn them? Bag them up and take them to the dump? I couldn’t find an answer on-line, so I broke down and called Cooperative Extension (I could hear the cringe on the other end of the line). The official word is don’t burn them (it could spread the spores on the smoke); either bag and take them to the dump, or take them to some far away part of your property where there are no berries, put them on the ground and cover with plastic. Anchor the plastic well. The plants will die and compost. With no wind to spread the spores, and no nearby berries to infect, the fungus should die out.

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