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.

Susceptible Species

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)

• Ceanothus

• Oaks (Quercus species)
• Willows (Salix species)
• Honey Locust (Gleditsia triacanthos)
• Conifers
• 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.

Control of Powdery Mildew

Powdery Mildew is a white fungus found growing on the leaves of various trees, shrubs and annuals. It afflicts many commonly found trees and shrubs: Red-tipped Photinia (Photinia x fraseri), Sycamore Trees (Platanus acerifolia), Crape Myrtles (Lagerstroemia indica varieties), some Cultivars of Ninebark (Physocarpus species), some varieties of Euonymus shrubs, Rose bushes and some Stone Fruit Tree species (Rosa & Prunus ), Apple Trees (Malus), on both the leaves and fruit, and some species of squash and cucumber plants. Powdery Mildew can also be a big problem when growing grape vines.
It can grow on the tops or undersides of leaves, stems and fruits.

How to Identify

Powdery Mildew is white or pale gray in color, and tends to grow in circular patterns on leaves. The spores are spread by the wind. They do not need water to germinate.
Powdery Mildew can distort and stunt new growth on plants, and cause premature leaf drop, as well as being unsightly. Powdery mildew is ugly, but will not kill plants.

Cultural Control of Powdery Mildew

Despite being called Mildew, Powdery Mildew, does prefer and thrive in dry conditions, which is why it takes off in our drier seasons. The spores will survive on plant tissue only, on leaves and buds, but will not live in soil. So it is important to dispose of all diseased plant material, don’t put it in your compost pile, as spores can over-winter and infect plants in the Spring.

Favorable conditions for spores growing, are areas with warm days and cool nights. Ideal temperatures for the fungus spreading are 70-85 Deg. F. Direct Sunlight and rain will prevent powdery mildew from spreading.

Improve Cultural Conditions of Plants

A good first step of cultural control, is to pay attention to the cultural conditions of afflicted plants. If plants are stressed, they will be more susceptible to disease:

• Pruning to increase air circulation while dormant, or earlier in the season, will help, particularly with branches or plants that are in the shade.
• Adequate Water Make sure that the plants are healthy, pay attention to their water requirements
• Drainage make sure that drainage is good, dig in some soil amendment
• Fertilize plants to make sure that they’re as healthy as they can be.
• Remove Dead Leaves, or buds that drop to prevent mildew from spreading

Treatment

• Wash off leaves regularly with a jet of water
• Spray with Neem Oil or Horticultural Oils
• Spray with Organic Fungicides, such as sulfur

Choose Mildew Resistant Varieties:

• Platanus acerifolia ‘Columbia’, ‘Liberty’ and ‘Yarwood’ have some resistance.
• Popular Roses such as ‘Just Joey’ and ‘Olympiad’ are resistant. Here’s a list of Roses resistant to different diseases
• Crape Myrtles with excellent resistance are: Tonto, Muskogee, Acoma, Souix, Tuskegee

As always, it is worth seeking out the advice of Tree and Landscape professionals to ensure the very best care for your plants.

Good references for more information on Powdery Mildew: UC Davis and University of Washington