A day in the life of a plant pathologist

Other than, "How tall are you?, " the question I get asked most is, "What is a plant pathologist?".

I guess that "plant pathologist" conjures up images of a person in a white lab coat dissecting a dead plant on a cold, metal table. A forensic pathologist....only for trees.

Most of the time, I'm not playing the role of a coroner (sometimes, though!) but rather studying the diseases and insects that interact with and affect the health of landscape plants. As a field plant pathologist, most of my time is spent at nurseries or in landscapes looking for signs and symptoms of fungi, bacteria, viruses and insects.

It is a lot like CSI most of the time. I have to asked a lot of questions to find out what happened prior to illness. How often was it watered? When was the last time it was fertilized? When did you notice it was in decline? Have you done anything to this plant recently?

CASE STUDY
A few weeks ago, I started seeing this on the Camellias at a few nurseries and in gardens:I had been told a few years ago by another pathologist that it was a virus called Camellia Ringspot Virus. Although the leaf spots look very viral, something about it didn't add up (you have to go with your gut a lot), so I took a second look when it appeared this winter.

The circular leaf spots occur on older foliage following cold weather. As soon as the new leaves emerge in the spring, these diseased leaves fall to the ground.

As it turns out, this ring-spotting is caused by a fungus. And this fungus overwinters on the infected leaves that fall to the ground beneath the Camellias. After a week or two on the ground, a large brown lesion will develop on the leaf.
If conditions are right, the fungus will sporulate and fruiting bodies will become evident on the leaves. This is an image of the fruiting bodies though a dissecting microscope:The black squiggly stuff is actually masses of spores that are being pushed out of the fruiting bodies. Plant pathologists call these spore tendrils. It's kind of like a really bad pimple.

When you look at these spore tendrils under a microscope the spores are visible.

This is how I am able to diagnose the disease that is affecting a plant- by the spores. Just like a flower is the definitive way to determine the species of a plant, spores are the way you determine the species of a fungus. In this case, Phyllosticta (the asexual phase of Guignardia).

Case closed!

And that's what a plant pathologist does (sort of). Wow, right?

If you are a bit geeky, this article has all the information you'll ever need: A New Leaf Spot Disease of Camellias caused by the Fungus Guignardia.

Tar Spot on Maple

I love when I find this disease on Red Maple (Acer rubrum).
It doesn't show up until late summer or early fall and it usually only affects a small portion of the leaves. But the spots are large and very distinctive- and they look like tar, hence the common name.

The fungus is Rhytisma acerinum. The specific epithet indicates that the fungus will only infect the leaves of Maple.

When you run your fingers over the leaf, it feels rubbery, as if a dollop of tar dried on the leaf.

Tar Spot can cause premature defoliation, but unless the infection is bad you won't notice.

If your Maple has this disease, the best way to manage it is to rake up the leaves in the fall so they can't cause reinfection the next year. I wouldn't bother unless the trees are being grown for sale.

Wow, right?

Plant Pest: Powdery Mildew

It's mildew weather- Powdery Mildew weather.

While Powdery Mildew will not usually kill a plant, it does distort growth, reduce flowering, discolor foliage and impact vigor. Not desirable for garden or nursery.

Before treating for Powdery Mildew, make sure that you have properly identified the disease.

Powdery Mildew is commonly confused with Downy Mildew. While they are two very different fungi, I understand why they are often misidentified: They both have "mildew" in their names and they are characterized by a white growth on the leaves. In addition, Powdery and Downy Mildews tend to occur in the spring and fall when temperatures are lower and humidity is high.

HOW TO IDENTIFY
If you think you have Powdery Mildew, look to see which side of the leaf is covered with the white growth. If it is on the upper side of the leaf, it is probably Powdery Mildew (Downy Mildew sporulates on the underside of the leaf).

Then look at the growth pattern. Powdery Mildew has diffuse margins while Downy Mildew creates angular leaf spots. This is illustrated in the image of the Japanese Magnolia below:
The white growth is the reproductive phase of the fungus, Oidium. The vegetative portion of the fungus is actually inside the leaf, taking nutrients directly from the leaf cells. The spores only appear when it is cool and humid.

See how the Dogwood leaf is purpling in the areas where there are spores on the surface? Those are the areas where the fungus has taken nutrients from the leaf. The premature fall coloration occurs because these areas of the leaf are not photosynthesizing properly.

MANAGING POWDERY MILDEW
The best way to prevent Powdery Mildew is to use resistant species and cultivars. Resistance has been bred into certain Crape Myrtles, Hydrangeas, Phlox and other plants.


If that's not an option and you have an infection:

• Apply lightweight paraffinic oil as a coating to the leaves to provide good management. Spray the leaves on a weekly basis (when daily temperatures are below 80F) with a 1-2% solution.

• Increase air circulation in the growing area to reduce the humidity level.

• Grow where the plant gets at least partial sun during the day.
  

• Rake up fallen leaves during the dormant season. Powdery Mildew overwinters as cleistothecia (survival spores) on these fallen leaves and will reinfect plants the following spring when the spores splash back onto the plants.

UP CLOSE
The powdery growth is made of these chains of round spores. These spores (conidia) are spread to new leaves and plants by air currents and the wind.*Above image from Cornell University Department of Plant Pathology.

This is so typical II

I don't have TV anymore because I watched (loved) it too much. Since I canceled cable ten months ago, I haven't paid much attention to weather forecasts. And so far, it hasn't been much of a problem.

Until this week.

I left the house early Wednesday morning for a 2-day scouting trip. Uncharacteristically, I loaded the car the day before with every thing I thought I would need.

Microscope? Check. Pruning shears? Check. Toothbrush? Check. Long pants? Nope.

I didn't realize I had forgotten to grab a pair of jeans out of the dryer until I was heading north on the interstate. I really didn't think it would be a problem because I could simply wear my shorts both days.....but then the rain started. And the temperature quickly dropped from a balmy 70 degrees to a terrifying 49 degrees. Since I haven't checked a weather forecast since 2008, I didn't know how much colder it would get. But I knew I was not dressed appropriately.

I was going to look like a fool if I didn't get some pants, so I stopped at an outlet mall just off the interstate and bought the first pair of jeans that I found that were long enough. Wearing them out of the store, I thought, "Ah, crisis averted."

It rained the whole time I was on my trip. The constant chill and dampness made the palms of my hands take on a bluish tint.

Only my hands weren't cold.

In reality, my new, dark denim pants were wet and were staining everything with blue dye. Not only were my hands blue, but now my car interior is blue, too! Nice.

I did remember to pack my boots though!
I think there are two lessons that can be gleaned from this experience:
1. Always wash your jeans before you wear them.
2. Always pack at the last minute so you don't forget anything important. (ha!)
***********************************************************
Other than staining the light gray interior of my car, it was a good two days. As luck would have it, rain makes it easier for me to diagnose fungal diseases like Downy Mildew.

The white fluffy stuff on the underside of this rose leaf are the spores of Downy Mildew, caused by the fungus Peronospora sparsa. You can see why it has the specific epithet "sparsa":

Horticultural Blights and Plagues

When a plant dies in a garden or nursery, my clients are often concerned that it will spread to all other plants in the area.

And this can happen in certain conditions. For example, the fungus Botrytis can blight many types of bedding plants in a greenhouse if the humidity is high and the leaves are staying wet.

Thankfully, indiscriminate blights are not usually a problem. Plant pathogens tend to be more host-specific. Take a look at this propagation greenhouse:Notice first that this greenhouse is well-maintained. There are no weeds, the plants are all the same age and it is organized. This is a good growing environment.

The Oakleaf Hydrangeas (on the right) have collapsed and are infected with a fungal pathogen.

It is significant that the Hydrangea macrophylla crop (with bright green leaves) is very healthy. They are growing right next to the diseased crop of Oakleaf Hydrangea (Hydrangea quercifolia) and yet are showing no signs of disease.

WHAT DOES THIS MEAN?
The take-home message is that if you are having a disease problem in your garden or nursery, don't fear that it will destroy everything.

If a large area of your garden, nursery or greenhouse is in decline, have the problem properly diagnosed. Then ask the following questions:

• Are the plants being over- or under-watered?
• Does the soil drain properly?
• Are the plants in the right amount of sunlight?
• What is the soil pH?
• Is there too much mulch around the bases of the plants or are they planted too deep?
  
• Is the irrigation water salty?

A fungus or bacterium may be the primary problem. But in my experience, if many different species of plants are affected, the growing conditions are generally to blame.