Gardening with Verticillium Wilt

A branch from a Magnolia globosa showing dead cambium and dark staining in the wood, a common sign of verticillium wilt. The stain in magnolias shows a little differently than in other trees like maples.

Pests and diseases are a fact of life in every garden. Some are easier to deal with than others, but working with what we have is a big part of gardening. This spring, conditions in the Pacific Northwest were such that many gardeners in the region have noticed damage caused by verticillium wilt, or lost plants entirely to the fungal pathogen. I've removed three young trees in my own garden due to varying degrees of wilt, all showing the dark stain in the wood characteristic of verticillium. Removal is not strictly necessary. Woody plants can survive verticillium, compartmentalizing the infection and growing new vascular tissue over the infected wood. However, there is always a chance of re-infection or of the original infection escaping the compartmentalized section. Having neither the funds nor the desire to fumigate the soil with expensive chemicals to reduce (but not eliminate) the fungus, and not wanting to watch a slow death or worry about re-infection, I chose to remove my infected trees and replace them with resistant varieties.

Magnolia globosa, struggling to leaf out due to infection.

Cross-section of the trunk of Magnolia 'Silk Road' x insignis, showing dead cambium all the way around and some staining.

Other pathogens, such as fusarium, as well as cultural conditions and physiological factors, can also cause staining of the wood, so for positive identification, a culture test is recommended. You can contact your local extension agency to find out where you can send a sample for laboratory testing, usually for a small fee. I did not have any of the trees I removed tested, and I may come to regret that later, if the replacements develop similar symptoms, showing that there is some other pathogen at work in the garden.

Rather than going into a full profile of verticillium wilt here, I've provided links to useful information, with an emphasis on lists of plants that are resistant to the disease:

• Verticillium Wilt in the Pacific Northwest, from the Pacific Northwest Pest Management Handbook
• Plants Resistant or Susceptible to Verticillium Wilt, again from the PNW Pest Management Handbook
• Plants Resistant or Susceptible to Verticillium Wilt, from UC Davis
• Missouri Botanical Garden list of resistant and susceptible plants
• Verticillium wilt plant list, from the Royal Horticultural Society

You may notice, looking through those lists, that some list a specific plant as resistant while another might list the same plant as susceptible. Plants that are healthy, growing in their preferred environmental conditions with little or no stressing factors, are generally better able to resist diseases, including verticillium. So, one region may have the perfect growing environment for a plant, and that plant is considered resistant because it's so healthy that it can successfully fend off the disease. In a region where the climate is less amenable but still allows that plant to survive, if only barely, it will be stressed and more susceptible to pests and diseases. It's just another of the many reasons to grow plants adapted to your climate and your specific garden conditions.

A summer-dry bed on the south end of the house, full of plants adapted to hot, dry conditions. Along the dry creek bed, the plants are both tolerant of dry summer soils and heavily saturated winter conditions.

Keep in mind that "resistance" is not the same as "immunity." It only means a plant is less likely to contract the disease, and keeping it as vigorous and healthy as possible will ensure the strongest resistance. For that reason, I rather like the list from the RHS, which shows the number of reported cases of verticillium for each plant listed since 1996. It lists plants, to some extent, by degree of resistance. Of course, the climate of the Pacific Northwest is quite distinct from that of the United Kingdom, despite being able to grow many similar plants. All of the lists above are guidelines, not guarantees. One thing those lists don't do, is distinguish between plants that are resistant and those that are immune to verticillium. Conifers, true ferns (sorry, Selaginella lovers, I don't know about fern allies), and monocots (grasses, bamboo, palms, bromeliads, lilies, iris, etc.) are immune, and a beautiful garden can be created from those groups alone. Dry gardens, such as those where you would grow Arctostaphylos, Cistus, and other plants from summer-dry climates, with minimal to no irrigation, are less likely to have problems with verticillium as the conditions aren't conducive to the growth of the fungus. As if gardeners in the Pacific Northwest needed more reason to plant such gardens!

Stipa gigantea capturing the morning light in part of my summer-dry garden. I did lose a Cotinus 'Grace' to what I believe was verticillium in one bed in this area, before it was turned into a dry garden (in fact, before most of this area was garden). That bed is now planted with resistant plants and the rest of this area is dry enough that it shouldn't be a problem.

So now you have some basic knowledge, if you didn't before, of verticillium wilt and a list of resources to guide you in selecting resistant or immune replacements if you've lost plants to wilt as I have. Now it's time to show you how I've been dealing with this unfortunate situation in my garden.

The first tree to be removed was an Acer metcalfii, which I detailed in this post. The Fargesia robusta 'Wenchuan' with which I replaced it has quickly become a favorite. Next up were two magnolias, M. globosa and M. 'Silk Road' x insignis. I wanted to replace them with resistant trees, rather than shorter plants.

Photo from the end of last summer showing the Acer metcalfii on the far right and the Magnolia 'Silk Road' x insignis at the center.

A similar angle, if a bit closer, from this spring. You can just see the Fargesia robusta 'Wenchuan' at the far right, and you may be able to see the little twig between the two stakes left of center (with the bright blue-green tape). It hurt to lose that nice vigorous magnolia, one of the larger non-Douglas fir trees in this young garden.

Below, you can see the Magnolia 'Silk Road' x insignis last summer (left) and this spring (right), after it had fully leafed out. It went through a wilting period before seeming to rally and hold its leaves turgid, but note how much smaller the leaves are, less than half the size, compared to last year.

 I was digging through anything I could find to figure out what I could replace the two magnolias with, not really finding anything that satisfied me, when Paul Bonine of Xera Plants suggested Nothofagus, or southern beech, saying it was resistant to verticillium. The lists of verticillium-resistant plants I linked to above are nice guides, but woefully inadequate beyond the most common garden plants, so of course Nothofagus wasn't listed. Research into verticillium-resistant plants in general is rather sparse beyond major agricultural and nursery crops because there isn't enough money to justify it.

So I focused my research on Nothofagus, to determine the best species for my garden. Hardiness and suitability to the conditions in my garden were obvious factors, but so was availability. What use figuring out the perfect plant if you can't find it? I looked at photos and read descriptions and fell in love with Nothofagus pumilio, but I was concerned that it would need better drainage, growing primarily up on mountain slopes in their native habitat. Seeing a photo of them growing around a high-elevation bog in saturated soil settled my concerns in that regard. Nonetheless, I was actually going to use the more common N. antarctica because it was easier to find. I had also been looking at the evergreen Nothofagus dombeyi, having read reports of it being hardy to USDA zone 6. Once again, Paul chimed in to guide me, having grown many species of Nothofagus himself years ago. Nothofagus dombeyi and even antarctica had been damaged by bad winters in his garden. N. pumilio was the hardiest and his favorite of the bunch. Luckily, another friend on Facebook pointed me toward Windcliff as a source for N. pumilio, and I remembered seeing one at their table at Hortlandia. So two friends and I ventured north to Windcliff and Far Reaches Farm (I'll cover that trip eventually...) so that I could acquire my trees.

Not much to look at, are they? Spindly, thin, awkward little things, but the best trees often are in their youth. The one that replaced the larger Magnolia 'Silk Road' x insignis, especially, fails to fill the hole left by that large-leaved tree. But they will grow, their intricate branch structure and tiny leaves creating a relatively light shade beneath their architectural canopies. The fall color and branch structure will both be more interesting than the magnnolias they replaced, and the tiny leaves will filter into the lawn and beds instead of being a bit of a nuisance as the large magnolia leaves were. I hope I don't lose any more trees or large plants to verticillium, but if I do, I won't mind replacing them with more Nothofagus, bamboo, and other resistant or immune plants!