Field Guide     Crop Diseases     Verticillium Wilt

Verticillium Wilt

CROPS IMPACTED: Trees, shrubs, cotton, potatoes, tomatoes, oilseed rape

Verticillium Wilt

Family: Incertae sedis

Verticillium Wilt Verticillium Wilt

About Verticillium Wilt

Life Cycle

Verticillium will enter a host plant through their root system. This can happen either directly, or through natural wounds that occurred on the roots from growth. As soon as verticillium infects a host, this fungus will create a toxin that enters the xylem of the plant, allowing it to move through the plant by way of spores. These spores will then get into the vascular tissue to spread the infection. This toxin can kill plant cells, even at a fair distance from where the host was invaded. This means that while some symptoms may appear to only affect a certain area of the plant, this is usually not the case. When the host plant notices an infection, it secretes tyloses, which is a substance that is meant to act as a barrier between the infection and other parts of the plant that are still healthy. Unfortunately, this substance also limits water flow from the roots to the rest of the plant. This, mixed with the effects of the toxin is when symptoms are easier to notice on the plant externally. Trees that display the symptoms of this disease, especially more than one year in a row, are unable to be saved and should be removed from the area. This fungus can live in the soil for long periods of time. If leaves are infected and fall to the ground, the fungus can endure as microsclerotia for more than 10 years. However, if soil is extremely wet and warm, this can effectively kill off resting microsclerotia.

Verticillium Wilt Identification and Habitat

Identification

Verticillium wilt can spread through the plant internally and in many cases can eventually lead to death. This disease can infect host plants throughout the entire growing season. Symptoms are chronic or acute and include leaf scorch, yellow foliage, slower than usual growth, branch dieback, sudden foliage death across entire branches, leaf curling, and drying. It often appears that only certain branches or a certain section of the plant is infected. Sometimes bark will even die off, on both the trunk and branches. With some tree species, green leaves will fall to the ground before they have been noticeably infected (yellowing or scorched). Streaking of the wood can also sometimes occur; the streaking will vary depending on the host plant, but it can be anywhere from gray to green to brown to black.

Habitat

As stated by its name, this is a wilt disease and is caused by V. dahliae and V. albo-atrum, which are soil-borne fungi. While they both cause Verticillium wilt, V. dahliae is more common to North American woody ornamentals. This disease is found in many different soil types and has the ability to infect hundreds of different wood plants and herbaceous plant types. Due to Verticillium’s ability to last in the soil for long periods of time, this disease can sometimes cause major problems to vulnerable host plants. It is also hard to know if the fungus is present in soil because some host plants do not have any visible symptoms. Sometimes Verticillium wilt is confused as other causes, such as herbicide or mechanical damage. This disease is unlikely to cause significant damage to forested areas or natural stands. Symptoms on host plants tend to be commonly seen from July to August. If cool weather conditions are present, symptoms may become more severe. Verticillium wilt can affect many different tree species such as ash, dogwood, elm, honeysuckle, maple, oak, pine, rose, coffee, and cork tree. In any land that has been used for Verticillium, vulnerable vegetable or fruit types are likely to become infected. It is important to know that this fungus can also remain on roots of plants that are resistant to the disease. While this plant shows no symptoms, as soon as a susceptible species comes in contact with their roots, it can become fatally affected. This also means that trying to track the disease can be a great challenge.

Verticillium Wilt Management and Control Methods

Cultural Control

There are plants that are fairly resistant to this fungus. Plants resistance levels can be increased by providing them with fertile soil, especially with low nitrogen and high potassium levels. Sufficient watering can also keep disease symptoms at bay. Practice effective equipment cleaning protocols to stop possible spread. If hosts are badly infected, replace them with a resistant species, some of which include apple, beech, birch, juniper, spruce, pear, poplar, fir, and mulberry trees. Fertilize and water trees continuously to promote healthy growth. Also, remove dead branches, reducing the chance of another infection by a different disease (removing the branches does not get rid of Verticillium though). If you attempt treatment methods instead of replacing the tree, make sure the infected plants are in fact a result of Verticillium wilt before any treatment methods are put in place. Vascular tissue streaking is one of the most obvious symptoms demonstrating this specific disease; however, a lab culture test could confirm this.

Chemical Control

No fungicide is available to treat this disease for trees. For small gardens or greenhouses, soil fumigants can be applied to the soil before planting.

Sources

http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/verticillium-wilt/

http://www.ipm.ucdavis.edu/PMG/r61100111.html