Seeking Answers from the Plant Disease Clinic

Blights and spots and leaves that drop
After too much rain and that hard bitten frost
Dieback or scales and all these entail
What's ailing this plant, what's making it frail

A garden in turmoil, some pathogen unknown
The gardener frets but does not stand alone
Through root rot and cankers and mildews and rust
Pathogens aplenty! Understand them we must

Is it viral, bacterial or insect pest
Who can help with such quandaries, who will run the right tests

Take heart dear gardener, you are not forgotten
Take action! Follow a path well-trodden

No matter the malady nor atrocious the necrosis
The Plant Disease Clinic will provide a diagnosis

In the woods close to my house sits a grove of spicebush (Lindera benzoin). While spring blooms are modest, I've always appreciated its tangle of shrubs settled along the side of a steep slope. They are native and an important host plant for the spicebush swallowtail butterfly (Papilio troilus). A couple of spicebush shrubs grow near the house, and last spring, I noticed they weren't leafing out like they normally do. Upon closer inspection, leaf spot was evident as well as dieback along many of the stems. This appeared true for at least some of the shrubs growing down the slope as well, at least from what I could make out.

Spicebush is a member of the laurel family (Lauraceae). A little research on the Internet suggested laurel wilt might be the culprit, but after viewing a webinar on this disease as part of a Master Gardener diagnostic, I was disabused of the laurel wilt idea. Dr. Calvin Norman, a forestry professor at Penn State, explained that sassafras (Sassafras albidum) trees in Southern states are one of the more likely victims of this fungal disease carried by an invasive beetle, the female redbay ambrosia beetle (Xyleborus glabratus). No beetle frass was evident, and although the disease is creeping into Pennsylvania, I'm located in the northeast corner of the state. I ruled out that suspect.

Through the webinar's Q&A, however, it was suggested I seek out the Plant Disease Clinic. Of course! Master Gardeners who help with garden hotlines all over the state rely on the clinic to identify all manner of plant maladies.

It doesn't matter if you are a farmer, commercial grower, professional landscaper, or homeowner; since 1970, the Plant Disease Clinic has helped Pennsylvania residents struggling to understand what is happening with distressed plants. The service is free to residents in the commonwealth.

As part of Penn State's College of Agricultural Sciences in the Department of Pathology and Environmental Microbiology, the Plant Disease Clinic receives approximately 2,000 samples each year. Dr. Sara May manages the clinic and has been the chief plant diagnostician and pathologist since 2009. A big slice of the clinic's work comes through county extension offices, which includes the Garden Hotline.

"The Master Gardener hotlines are a really good resource for homeowners because they can answer questions often faster than we can and handle some common problems. And they can help the client to collect a good sample to be forwarded to our lab," she said.

Ailments concerning trees and shrubs, herbaceous plants, and turf grass are some of the more common health issues the clinic diagnoses. When it comes to homeowners, she noted that the clinic processes mostly problems with ornamentals. The request to diagnose the spicebush was unusual.

I initially decided to send a virtual sample of the spicebush, so I emailed six photos, a mix of both wide shots and close-ups, along with a completed Specimen Information Form. A detailed form provides clues for the diagnostician. For example, I explained that I thought more than one specimen was affected. A response to my inquiry occurred within two days.

"Hi Joan,

There are symptoms of foliar disease in the photos, specifically leaf spots and dieback. There could be an anthracnose-type fungus involved. Defoliation and twig dieback is common with anthracnose-type diseases. We can check for anthracnose and other diseases that could cause these symptoms if you submit a physical sample of the foliage (twigs with attached symptomatic leaves). Include live, symptomatic branches that are at least ½ - 1 inches in diameter and at least 6 inches long for a vascular wilt disease evaluation. If you include roots, we can also test for root rot pathogens. More instructions on collecting and submitting samples can also be found on our website."

A video on the clinic website lays out step-by-step instructions for sending samples. Questions about how to prepare a specific sample are likely to receive a quick response through email.

The methods for packing and wrapping plant materials are important. I gently wrapped leaves, twigs, and branches in plastic and placed a root sample in a plastic bag, which was everything that was requested. Depending on the type of plant, a variety of samples is always helpful, especially those showing the margins between healthy tissue and areas that are diseased. Samples should be alive and as fresh as possible. I made a point to send my samples overnight on a Monday so that there would be absolutely no possibility they would sit unopened over a weekend. USPS packages are routed through the University and thus are slow to arrive, so the clinic recommends FedEx or UPS.

To diagnose the leaf spot and dieback, the lab grew cultures from the samples they received, which took time. It generally requires between 2–3 weeks to process materials, depending on the various tests the clinic might need to run. Keep an eye on your email inbox. I made sure to enter the email address of the clinic in my "contacts folder" so their response would not end up as "spam."

From my spicebush samples, the clinic's report identified two fungal pathogens: anthracnose - Colletotrichum leaf spot (Colletotrichum spp.) and Cytospora canker (Cytospora spp.). Both diseases affect many other woody species, and both take advantage of plants "weakened by various factors including poor subsurface drainage, drought stress, cold injury, mechanical injury, root diseases, etc." The root sample tested negative for any pathogens.

While a diagnosis is not a cure, the clinic was able to make general recommendations. The diagnostician advised pruning and removing dead branches as well as destroying any debris, such as leaves, twigs, and branches, "to reduce overwintering inoculum." I learned that the Colletotrichum fungus has not been studied extensively, but it does not appear to be "wiping out plants." Under proper environmental conditions, the spicebush shrubs can recover from the anthracnose. In fact, by late summer, some new leaves were pushing out on the shrubs near the house. Removal of tissue infected by the Cytospora canker and improving environmental conditions might prevent the demise of these spicebush shrubs. For the spicebush along the slope in the forest, come spring, I'll have to keep an eye out and observe how they fare in their natural environment.