Verticillium wilt (Verticillium dahliae) is a fungal disease that affects over 300 species of woody trees and shrubs. It also affects perennials and annuals in temperate climates. It’s worth finding out more about this disease since it affects such a wide range of plants.
Common woody hosts to this disease include Japanese maples (Acer species), redbud trees (Cercis species), magnolia trees, lilacs (Syringa species), viburnums, Sumacs (Rhus species), smoke trees (Cotinus species), ash trees (Fraxinus species), and cherry trees (Prunus species).
This fungal disease can also affect perennial and annual plants. This includes many crops like strawberries, tomatoes, peppers, pumpkins, watermelons, cruciferous vegetables such as broccoli, and berries (Rubus species) such as currants and raspberries. Here is a good list of species that are susceptible to verticillium wilt and some that are resistant too.
How Verticillium Wilt Affects Plants
The mycelium (strands of fungi) of Verticillium infect soil. Nursery stock can be infected through the growing medium. Unfortunately, fungicides have proven to be ineffective. Once the fungus is in the soil, it is very difficult to remove, although it can go dormant in high temperatures.
Susceptible species will be infected through their roots from contaminated soil. Verticillium affects the vascular system of plants – the vessels (xylem) that carry water through the plants from the roots to the shoots in the canopy. Once the spores reach the twigs of trees and shrubs, they block the vascular system. This prevents water from being transported, therefore causing wilting in select twigs and branches.
Often only a few parts of the tree will be affected or just one side of a tree or shrub. This is a characteristic of the disease that helps diagnose verticillium wilt.
Symptoms of Verticillium Wilt
The sapwood of the affected plants will turn either dark green, black, or reddish brown. The outsides of the twigs and branches can eventually blacken too and die. Other common symptoms include:
- Chlorosis (yellowing) of the leaves
- Dieback, which is intensified when the tree is stressed during heat, drought, or when transplanting trees and shrubs
- Wilting at the ends of twigs, leaves, branches, or whole branches
- Premature fall color
- Slow growth/stunting
- Leaf curl
- Plant death
Plants Resistant to Verticillium Wilt
Since this fungal disease has no treatment, if you know that your soil is infected with the fungus, opt for planting a resistant species. For example, if you remove a dead Japanese maple tree that is infected with the disease, it would be wise to not plant another maple or susceptible tree species in its place. Here is a list of resistant species you can consider:
- Dogwoods (Cornus species)
- Crabapples (Malus species)
- Oaks (Quercus species)
- Willows (Salix species)
- Honey Locust (Gleditsia triacanthos)
- Grasses, including bamboo
It is always wise to seek professional advice and expertise when trying to diagnose a tree or shrub disease. Other diseases can have similar symptoms, such as yellowing of the leaves and dieback. Some disease could also be due to nutritional deficiencies, drought stress, or trees planted in conditions that don’t align with their needs.
Contact Frontier Landscape for help with plant disease diagnosis. Whatever symptoms your tree is exhibiting, you want to make sure that it is structurally sound and safe to keep on your property.