Tag Archives: Plant Pathology

Bark Mulch

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June may smell like roses, May smells like Lily of the Valley in my mind, but spring, sweet, sweet spring, smells like bark mulch.

Like many things in Horticulture, mulching really isn’t for the plants, but for us. Meant to imitate the look and ecosystem of a forest floor, mulch does neither, but it is not completely benefit free either. Mulch can help regulate soil temperatures in summer and winter, helps retain soil moisture, and can also help keep weeds out of the space. A little can go a long way, though, and as we’ve learned here, mulch is not something to just put down and forget.

Mulch can be a variety of materials, but let’s focus on tree bark and wood chips. On campus we use a double ground spruce/hemlock mix locally made in Newport, but that’s really all about looks. I used to be one of those people selling bark mulch, and so would steer people away from using (free) wood chips, but I’ve since recanted. The handy excuse I would use is that the as chips break down through normal soil processes they steal nutrients from the soil, and therefore the plants, but this  has not been supported by research. And in fact a good mixed load of chips, twigs, leaves, and needles from the back of a typical arborist’s truck seems to imitate the duff of a forest floor nicely, and is becoming the preferred mulch for some. If you are interested, some good reading on this is published by Dr. Linda Chalker-Scott of Washington State University.

I used to say 3” of bark mulch was the right depth. I now think that’s too much and 2” is probably fine. Mulch too deep can actually inhibit water from reaching down to the roots, and/ or cause shallow rooting of trees. This is easy to see either by a white fungus growing throughout the mulch, or by roots growing right through the mulch.

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White Fungus hiding in bark mulch

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Shallow roots at base of River Birch

Too much of a finely shredded mulch can also decrease oxygen to the soil and roots. Unlike leaves, which make oxygen, roots require oxygen for respiration, and a thick layer of mulch will prevent good gas exchange between the soil and the atmosphere. The white fungus seen above has the added nastiness of turning the mulch hydrophobic, or water repellant. The mulch not only will not allow water to pass through, but will actively repel water right from the top, and let it run off.

Another thing I ignored in the past was the edict of not placing mulch against the trunk of the tree. I’d personally never seen any harm or problems by this practice, and being a busy landscaper, never took the time to push back the couple inch ring of bark mulch from around the trunk.

A gingko tree has recently proven me wrong. We’d noticed this tree declining for several years, and given the pattern of decline, assumed it was something wrong with the roots, like compaction or too abundant moisture. Closer inspection at the root flare (the most critical part of any tree) showed a ring of decay circling the entire tree, preventing all the nutrients and some of the water from its proper flow within the cambial layer. This decay was undoubtedly caused by the mulch against the trunk, and can be directly implicated in the death of the tree.

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Weak tree-notice tip dieback

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Ring of rotted wood at base of tree

Even if the mulch did not cause the bark to rot away, the constant moisture against the trunk, like the roots in the example above, decrease gas exchange. The trunk needs oxygen much like the roots, and can’t do this when it is constantly moist from mulch.

So my new mulching recommendations are these. Mulch is good, about 2”, real shredded bark if you can afford it, wood chips if they are free. Avoid the dyed mulch, which just plain looks tacky. Don’t blindly put a new ½” to 1” of mulch on top of last years’, better to dig down in the old mulch to look and see what is going on. Maybe stir the mulch up some, break up any hydrophobic layers, or add some compost for a biological kick in the pants to break down the existing mulch and closer imitate a forest floor.

And stone pebbles for mulch? Only in a zen garden, please.

Cedar Apple Rust

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Once you see one, you start seeing them everywhere.

Cedar Apple Rust-Juniper Spore Horn

The first warm rain in the spring makes these spores appear on Red Cedar (Juniper) trees, completing part of its life cycle. Red Cedar is a first-growth conifer pretty common in Vermont in abandoned pastures, along roads, and elsewhere, so finding these disgusting things aren’t hard. Sometimes they are as big as your fist, sometimes just a golf ball, but they are always bright orange, and disgustingly slimy and gelatinous. I remember being thrown one by my landscaper employer in Connecticut, and being grossed out for the entire rest of the day, catching it absentmindedly.

This is brought to you by a fungus called Cedar Apple Rust, latin name (brace yourself) Gymnosporangium juniperi-virginianae. The fungus is a ‘dual-host’ fungus, needing two different type plants to complete its life cycle. Part is on the Red Cedar, where small galls sit and lie in wait for the warm rain in the spring to activate. They can be mistaken for cones for those not totally up on their conifer botany. Once activated, spores then alight on the wind, and carry to apple trees (or crabapples), where they infect the twigs and leaves. On this host, it appears like small orange or yellow spots. These spots then produce spores in July or August, and re-infect the juniper.

Because of the ubiquity of Eastern Red Cedar control is very, very difficult. Like many things, the best techinique is prevention, which entails removing all cedar from within a mile (!) of the orchard. Fungicide sprays are effective, but need to be done now, which is another whole host of problems, seeing as the apples and crabapples are starting to bloom, a delicate time to say the least. Apples and crabapples vary in their susceptibility to the disease. I fell in love with a crabapple once, a double flowered variety named ‘Brandywine’, with dark pink flowers like tiny roses (I won’t post a picture and tease you too), only to watch it totally defoliate by the end of July-not a leaf left the rest of the year-all from a severe infection. Resistant varieties can be found, though, and are probably a good idea.

 

Leaf Color

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I’ve been reading quite a bit this fall in various newspapers, web sites, etc. about the science of leaf change, and I thought, well, heck, there goes another blog post. I don’t see the sense to retread ground others are covering.

By now, you’ve probably read that leaf color changes by the shortening of day-length light triggers the tree to begin shutting down the leaves, and that chlorophyll breaks down, and sugar is absorbing into the tree. In a nutshell, the veins connecting the leaf to the tree are closed (abscission layer), and once this is complete the leaf falls.

Weather does play a part in leaf color, and in the color you see in the hills as you visit your children on parent’s weekend here (Hi parents! Your kids are doing fine. They want more money.) Many articles have talked about warm weather delaying fall, cool nights are good, drought bad. It’s easy to understand, though, if you think of it in terms of plant health.

A happy, healthy tree in a good growing season will more than likely have pretty spectacular colors. The factors responsible for bad fall color aren’t good for the tree health either. Drought is bad for fall color, and also bad for the trees. Southern Vermont this year had a pretty bad late summer drought, and when I was on route 4 a week or so ago near Woodstock the leaves were terrible, turning brown and falling off, rather than turning nice colors. Here at Middlebury it’s been a dry September, and then the recent rain storms came at just the right time, and the leaves held on long enough to turn well. Warm fall days and cool nights? Good for sugar production, and the breaking down of chlorophyll in the active leaf. A late spring or a severe summer drought can delay fall color-the tree holding on to it’s leaves as long as possible, storing as much energy as it can before winter.

Another leaf color fact plant geeks have probably noticed is called the Leaf Wave Model. An article at the University of Georgia discusses this: Peak color is an opinion. Different trees turn at different times, and in differing colors. Yellows dominate early, then oranges as both later trees turn, as well as some yellow leaves becoming more orange. Finally reds dominate the landscape, with accompanying orange. Browns come last, generally in oaks. The leaves in Vermont are spectacular because of the forest cover types found here, yellow Ash, orange and red maples, along with splashes of green from Pine and Spruce. By paying careful attention to the mountains in the fall you can watch this leaf color wave happen.

Some other reading I’ve been doing this fall was about the color red, something I’ve never thought about. An interesting question for botanists has been “Why red?”.  As chlorophyll disappears from the leaf, the other colors emerge, such as yellows and oranges provided by Carotenoids. Red, though, is expressed through Anthocyanin, but is not found in a leaf, and must be produced. The question, therefore, becomes why would a plant be producing a compound, expending energy, at a time in it’s life cycle when it is trying to store and conserve? There are two schools of thought, and probably both are correct, some for some plants, some for others.

One theory is that anthocyanin is produced in trees in nitrogen poor soils. In some varieties of trees, as the green chlorophyll breaks down, the leaves are vulnerable to bright sunlight, and this sunlight breaks down the produced sugars, thereby not being absorbed back into the tree as energy storage. The red pigmentation acts like a barrier from the sunlight, allowing the tree to absorb more of the sugars it has produced. Nitrogen poor soils means the tree would have produced less sugars, being weaker growing, so more red pigmentation would conserve more of the valuable food.

The other theory is one of coevolution, that red leaves are a signal to insects as a repellent, a red warning signal to the insects attempting to use the tree as an overwintering site. A study has shown this in aphids and apple trees, that wild apple leaves turn red in the fall, and suffer less aphid predation.

Big Huge Icky Giant Tar Spot

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Giant Tar Spot

The study of plant diseases made me terribly depressed in school, but also made me laugh on occasion. I imagined two plant pathologists walking around, looking at sick leaves. Of course, each of them wouldn’t admit to the other that they didn’t know or recognize some strange disease, so they’re making up names as they go along. Strange fungal spores on a cherry tree? Well, it looks like Black Knot, and the disease is named.

A fall arrival on campus (and occasionally in my inbox as a question) is always the blatetenly named Giant Tar Spot. (“Hey, what’s this?” “You don’t know that? Any idiot knows thats, um, Giant Tar Spot. Yeah, that’s it. Giant Tar Spot”) You guessed it, huge jet black spots on the leaves of primarily Norway Maple.  This, too, is a fungus, Rhytisma acerinum for you latinally inclined. It infected the leaves way back in early summer, but they don’t really show up until now. When it’s too late to treat.

Not that you need to. Late diseases and insects like this look terrible but, let’s face it, the leaves are only around for a couple of more weeks anyway. It’s not a bad year for it this year on campus, we’ve had it much worse. If you’re looking to get rid of it on your tree at home, rake and dispose of the trees at your neighbors…