Verticillium wilt, also known as blackheart, is caused by the fungus Verticillium dahliae. The disease is found worldwide but is more serious in temperate areas.
The disease is usually observed in early summer as a progressive loss of leaves from infected limbs, starting at the base of each branch (Figure 1).
Leaves may become yellow and dull in appearance before dropping prematurely. By late summer, only a tuft of leaves may remain at the tip of a severely affected branch (Figure 2).
Occasionally leaves may show a true wilt, and when the death of these leaves is very rapid they may remain attached to the plant for several weeks. An entire tree can show these symptoms, or infection may be confined to one side, or even one branch, of the tree.
The most important diagnostic symptom of this disease is found when the trunk, branches or twigs are cut open to reveal the internal wood (Figure 3). Diseased wood shows in the cross section as a series of light to dark brown, irregularly shaped spots that sometimes merge into a ring of stained tissue. In mild cases of infection, wood staining may occur in the absence of any leaf symptoms.
The disease is serious for commercial growers because, though affected trees rarely die quickly, they may remain stunted and unproductive for many years. There is also a strong probability that replacement trees will be attacked in later seasons.
The fungus is widespread and particularly prevalent on land that has been repeatedly cropped with susceptible crops (alternative hosts for the fungus) such as:
The fungus survives in the soil for many years as resting-bodies (micro-sclerotia) and as a root-parasite of many crop plants and weeds. Mycelium enters a healthy plant by penetrating the root hairs, or through any wound. The fungus then grows into the water-conducting wood, in which it forms spores.
These spores are carried upwards, with the flow of water, to the stem and leaves, giving rise to a continuous strand of infected wood from the invaded root to the branches and leaves vertically above it.
Verticillium wilt is difficult to control because the source of infection occurs in the soil and the fungus spreads internally throughout the tree. No method is available for treating infected orchard trees.
Verticillium wilt is a notoriously destructive fungal disease that infects the vascular systems of hundreds of woody and herbaceous plants, impeding water transport and eventually killing the plant. However, some tree species have natural genetic resistance or immunity to verticillium wilt, making them ideal choices for landscapes where this soil-borne pathogen is present.
Understanding Verticillium Wilt
Verticillium wilt is caused by two related soil-borne fungal pathogens – Verticillium dahliae and Verticillium albo-atrum. The microscopic fungi enter plants through the roots and then spread through the water-conducting xylem tissues, forming blockages This prevents water and nutrients from reaching the stems and leaves Affected plants display wilting, yellowing, defoliation, and dieback symptoms, usually starting on one side.
The fungi can persist in soil for years. Once a plant is infected, there is no cure. Verticillium mainly affects herbaceous annuals and vegetables, but also numerous trees and shrubs.
Gymnosperms – Naturally Resistant Conifers
An entire division of woody plants the gymnosperms, are broadly resistant to verticillium wilt. Gymnosperms have naked seeds and include conifers like pines firs, spruces, redwoods, junipers, cedars, and more. Their resinous sap, thick bark, and other defenses make them poor hosts for the Verticillium fungi.
Any species of pine, fir, spruce, cedar, redwood, cypress, or other conifer can be planted in areas affected by verticillium wilt. They’ll grow unaffected by the disease.
Dicots – Select Deciduous Trees With Resistance
Among deciduous woody plants, there are certain species with proven natural immunity to verticillium wilt. These resistant species mainly come from the rose, legume, birch, and oak families.
Oaks
All species of oak trees have strong resistance, including white oaks, red oaks, live oaks, willow oaks, etc.
Birches
River birch, paper birch, yellow birch and European white birch are unaffected
Black Locust
A legume tree resistant to the disease. Fixes nitrogen in soil.
Mulberries
Both red and white mulberry trees are immune. The berries are edible.
Buckeyes
Ohio buckeye and yellow buckeye are resistant trees.
Willows
Weeping willows, black willows, pussy willows resist verticillium.
Dogwoods
Kousa dogwood and Cornelian cherry dogwood are resistant varieties.
Smoke Tree
An ornamental tree with airy puffy blooms.
Catalpa
Southern and northern catalpa trees are unaffected.
Black Gum
Also known as tupelo or pepperidge tree.
Monocots – Palms, Yuccas, Agaves
Monocots like palms, yuccas, and agaves are immune to verticillium wilt. These make striking vertical accents in hot, arid climates where verticillium wilt is active.
Palms
Queen palms, date palms, fan palms, etc.
Yuccas
Stiff, spikey yucca plants resist the disease.
Agaves
Large sculptural shapes and succulent leaves.
Key Facts on Resistant Trees
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Conifers, oaks, birches, willows, dogwoods and other trees have natural genetic immunity to verticillium wilt.
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Once infected, there are no fungicide sprays or cures for susceptible trees like maples, ashes, elms, etc.
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Remove and destroy infected plants to limit the spread of fungal inoculum in soil.
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Verticillium can persist in soil for over 10 years after susceptible plants are removed.
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Rotate herbaceous crops, but resistant trees can be continually planted in affected soil.
Planting Advice for Landscapes With Verticillium
Here are some tips for incorporating resistant trees in garden beds or landscapes where verticillium wilt is a problem:
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Test soil to confirm verticillium is present before planting.
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Remove dead plants and till soil deeply to disrupt fungal resting structures.
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Solarize soil by moistening and covering with plastic sheeting in summer.
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Add resistant varieties of both woody and herbaceous ornamentals.
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Incorporate organic matter to encourage beneficial microbes that suppress fungal pathogens.
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Use raised beds for herbs and vegetables if drainage is poor.
By selecting tree varieties genetically unfazed by verticillium wilt, you can create beautiful, thriving gardens even where this destructive pathogen lurks in the soil. Focus on inherently resistant conifers, oaks, birches, dogwoods, willows, and palmettos. With knowledge of plant immunity, verticillium won’t limit your landscape design dreams.
Frequently Asked Questions
What crops does verticillium wilt not affect?
The best protection against verticillium wilt is growing plants with resistance or immunity to the disease. Verticillium-resistant tomato varieties carry a “V” on the plant tags or labels. Birch trees naturally resist the disease, and verticillium wilt doesn’t affect conifers such as pine and spruce trees.
What plants are resistant to verticillium wilt rhs?
The Royal Horticultural Society produces a list of resistant plants: Alnus (alder), Gleditsia (honey locust), Betula (birch), Ilex (holly), Carpinus (hornbeam), Liquidamber (sweet gum), Cercidiphyllum (katsura), Morus (mulberry), Crataegus (hawthorn), Platanus (plane), Eucalyptus (gum tree), Populus (poplar), Fagus (beech), and more.
Are any maples resistant to verticillium wilt?
Although maples are generally thought to be highly susceptible to Verticillium wilt, red and sugar maples appear to be much more resistant than Norway maples. ‘Jade Glen’ and ‘Parkway’ are reported to be more resistant than other cultivars of Norway maples.
Are dogwoods resistant to verticillium wilt?
A partial list of resistant shrubs includes dogwood, firethorn, flowering quince, holly, and rhododendron.
Reduce fungus in soil
To prevent Verticillium from attacking fruit trees, try to reduce the amount of fungus in the soil. You can do this in the following ways:
- Only plant new blocks of stone fruit on land that has been under grass for several years and avoid planting stone fruit into sites that have had a history of crops such as potatoes, tomatoes or strawberries.
- If fruit trees must be planted in soil known to be infested with Verticillium, then plant apples or pears, as these species show marked resistance to the disease. Apricots grown on plum rootstocks also show some resistance to the disease, and can be planted where other options are not available.
- Use only high-quality, disease-free planting material, because transmission of V. dahliae in vegetative planting stock is significant.
- Avoid intercropping blocks of young stone fruit with tomatoes, potatoes, peppers, strawberries or melons, as these susceptible crops can increase the population of Verticillium in the soil.
- Suppress weeds after planting or sow the block down to grass and clover to prevent a rapid increase of Verticillium in the soil. Important weed hosts of Verticillium belong to the families Chenopodiaceae (fat hen), Solanaceae (nightshade), and Amaranthaceae (red-root amaranthus).
- Remove seriously affected, unproductive trees and as much of their roots as possible. Fumigate or otherwise treat (for example, by soil solarisation) the area to kill any remaining fungus before replanting.
- Avoid subjecting young plants to water stress, root damage and excessive quantities of fertiliser.