Plant pest panel discusses disease puzzles

“The important thing is not to stop questioning. Curiosity has its own reason for existing.” So said Albert Einstein – and it’s imperative for growers to remain curious.

A nursery-focused disease panel at the most recent Cultivate event took place solely so those in attendance could query about the issues that most concerned them. The panel featured some true experts – Janna Beckerman, Ph.D., Envu Green Solutions Team specialist; Francesca Hand, Ph.D., Ohio State professor of ornamental pathology; George Grant, Ph.D., technical services manager at Griffin Greenhouse Supplies; and Shannon Carmody, plant pathologist lead for Ball Horticultural Company.

The first issue to be brought up was Pseudomonas, a bacterial genus with 348 different species. Most well-known in the green industry are the plant pathogen P. syringae, the soil bacterium P. putida and the plant growth-promoting P. fluorescens, P. lini, P. migulae and P. graminis.

“Bacterial diseases always big issues – they have oily, wet, dark spots (vs. fungal, which are round and dry out),” Grant said. “Don’t be afraid to throw things away or cut back material. Get it out of the greenhouse and then spray.”

He noted that copper is still the go-to with Pseudomonas, but “CleanGro is very versatile and powerful.”

Beckerman backed him up, adding, “ Pseudomonas has copper resistance, so rotate” your bacterial killers.

She said the sanitation step is critical is critical in its control too. “Sanitize tools between pruning, even knowing it will likely rust your pruner,” she said. “You can throw diseased material out, or I suggest burning diseased plants to the ground. Bleach is good, but change your solution often. ZeroTol is also very good.”

Hand reminded attendees “that plant diseases are not stuck in time – their symptoms will change and evolve. You cannot diagnose based on symptoms. It needs to go to the lab to be tested. There are some home tests, but not for many diseases.”

Following this line of thought, the next question was “Are there agricultural antibiotics for bacterial diseases?”

Beckerman replied that there are a number of products available for specific times and issues, but they only last a few days. As with any antibiotic, there is the risk of resistance. Be sure to rotate.

“Some sprays may only work once – don’t bet the farm on it,” she said. “Bacteria are slutty little organisms. They will hook up with others to create new disease issues.” Hand added that bacteria also reproduce at a very fast rate, and thus resistance spreads quickly.

When it comes to crown gall and leafy gall, Grant remarked, “This is a see-and-throw-away situation. Ninety percent of the time it’s coming from the stock.”

Dogwood anthracnose is caused by the fungal pathogen Discula destructiva, and it’s a serious disease that affects various dogwood species. All varieties of the native flowering dogwood (Cornus florida) and Pacific dogwood (C. nuttallii) are susceptible. It was inadvertently introduced into eastern North America from Asia in the late 1970s and quickly spread.

“As things get hotter and wetter, we will see more anthracnose issues,” Beckerman said. With dogwoods, be on the lookout for tan spots with irregular purple margins appearing on the leaves in lower crown in spring, progressing up the tree if wet spring weather persists.

Boxwood blight, which continues to be a problem, was discussed as well. Beckerman said in the last year, it’s made a “huge jump to Oregon, but the biggest issue is in the Mid-Atlantic, and it’s spreading slowly to the Midwest.” The ‘Green Velvet’ cultivar of boxwood is the most susceptible variety.

Clorothalonil remains the weekly go-to for dealing with boxwood blight, but other options are being developed.

“Boxwood tree moth is still expanding its range in the Northeast, so scouting is important,” Hand noted. “[Fungicide] coverage can be tough due to tight, waxy leaves, so the IPM approach is critical to boxwood propagation.”

And finally, an ongoing issue was covered again: Cercospora on leafy plants.

“In the field, it’s difficult to differentiate between leaf spots, and sometimes you don’t have time to send to a lab,” Grant admitted. Because of that, Beckerman suggested using a 7-11 (a powerful, broad-spectrum, pre-mix fungicide combining FRAC Group 7 (SDHI) and Group 11 (QoI/Strobilurin) modes of action to control major diseases like Botrytis, Rhizoctonia, powdery mildew, and leaf spots) and then a FRAC-3. She said Avelyo is also a strong fungicide.

“It’s much, much cheaper to do something preventative versus fighting after the disease sets in and you’re having to throw out inventory,” Grant concluded.

by Courtney Llewellyn