As a gardener, you have probably heard about the benefits of soil fungi or even have added mycorrhizae into your soil – but not all fungi are beneficial. Verticillium wilt is a fungal disease that impacts over 400 plant species including trees and shrubs, vines, flowers, and vegetables. Some of our most prized garden plants such as roses and tomatoes are susceptible.
There are two types of this soil-borne fungi: Verticillium dahliae and Verticillium albo-atrum. These fungi live in the xylem of plants which are tiny tubes that help to transport water and nutrients throughout the plant. They essentially create a blockage in the plant vascular system and results in them not being able to get enough water. Some infected plants may wilt as a result. Verticillium also produces toxins that harm host plants.
There are currently no treatments available for verticillium, but there are many ways to prevent or manage the disease. If you know that verticillium is an issue in your soil, start by selecting plant cultivars that are bred to be resistant or immune to verticillium wilt. In addition, maintaining healthy plants and improving plant vigor can give them a leg up.
What Is Verticillium Wilt?
Verticillium is a common soil fungus that thrives in temperate climates across the world and can be present in the soil for decades. Verticillium overwinters in the soil as dormant mycelium or tiny black resting structures called microsclerotia, waiting for favorable conditions to return. They enter damaged plant tissue through the roots and multiply. Many common weeds like dandelions and pigweeds can be host species for verticillium.
Types Of Verticillium
Verticillium dahliae is found in temperate parts of the world and affects many common garden favorites such as members of the nightshade family (tomatoes, peppers, eggplants), mints, berries, roses, and maples. However, all monocots (tulip, corn, ginger, grass, etc.) and all gymnosperms (conifers, ginkgos, cycads) are resistant to this disease. Compared to Verticillium dahliae, Verticillium albo-atrum has a more limited host range and distribution. It is most commonly associated with agricultural crops like cotton, alfalfa, and hops.
Life Cycle Of Verticillium
Verticillium belongs to the fungal class Deuteromycetes, which are not known to have a sexual reproduction system. Instead, the verticillium fungi have asexual reproductive spores called conidia. The name verticillium derives from the Latin word “verticillus” meaning “whorled” because these single-celled conidia are born from specialized hyphae that are arranged in a whorled pattern.
The fungal spores infect plant tissue and occupy the xylem vessels where more conidia are produced. Conidia move throughout the plant’s vascular tissue system as the plant transports water and eventually leads to the narrowing of the xylem. This issue is akin to humans having blocked arteries. As plants’ xylem become more blocked, the xylem will not be able to efficiently transport water and nutrients and the plant will wilt. As the plant declines and dies, the verticillium fungi will form structures called microsclerotia or masses of melanized hyphae on the dying plant tissue. These little black dots are typically visible using a handheld magnifying glass.
Microsclerotia are released into the soil through the decomposition of the original host plant and contain conidia that can infect other plants through the plant’s root system and can even exist on the surface of resilient plant roots, poised to infect neighboring non-resilient plants. The pathogen can spread through contaminated soil or infected roots. This disease can also be transmitted through pruning tools, so it’s very important to sanitize your tools between plants. Plants that already suffer root damage, such as from nematodes, are more susceptible to verticillium wilt.
Symptoms Of Verticillium Wilt
Plants often show very similar symptoms of stress so it can be confusing to properly identify the root cause of the stressor. Verticillium wilt symptoms can also manifest in different ways despite the name of this plant pathogen.
In many trees, verticillium wilt can be an acute or chronic issue. In acute infection which occurs in new wood or sapwood, branches might suddenly show yellow leaves, drop leaves and die back. When the fungus lives in older wood, the tree might show chronic illness by developing smaller leaves, leaves that yellow at the edges, and decreased plant vigor overall. Trees that are infected with verticillium may not die immediately but might suffer a slow decline.
In fruiting plants like strawberries and tomatoes, you may see leaves with reddish-brown edges that curl and fall off prematurely. For strawberry plants, the entire crown might die off. This disease travels from the bottom of the plant upwards. New leaves may be yellow and stunted. In tomatoes, you may first notice yellow blotches on the lower leaves. Leaves may turn brown but the stems typically will not, unlike fusarium wilt.
Controlling Verticillium Wilt
Although there is no treatment for verticillium wilt, there are several effective mitigation practices which include soil sanitation, crop rotation, and selecting resilient cultivars. Proper pruning, watering, and fertilization can also help to keep up the plant’s overall vigor and health and better prepare it to fight against diseases.
Avoid planting susceptible plants in areas that have known inoculations of verticillium. If you suspect that your plants have verticillium wilt, cut off the infected plant tissue and make sure not to put them in your regular compost to reduce the spread of future fungal disease.
Practicing crop rotation with resistant plants may also help to disrupt the life cycle of the fungi. A study published in the Journal of General Plant Pathology showed a promising trial of rotating broccoli after eggplants to suppress the presence of V. dahliae in the soil since eggplant is one of the susceptible species. Similarly, the University of California Davis published a guide on soil solarization. It describes how solarizing soil in warm climates can help control fungi and weeds, especially in combination with using plant residues from mustards, broccoli, and other cruciferous crops.
Commercial farms may use soil fumigation with high concentrations of metham-sodium or methyl-bromide to eradicate the fungus but this is not a method used for home gardeners and is an expensive and toxic control.
Plant resilient cultivars and disease-free plants when possible. For example, some readily available types of resistant tomatoes include Better Boy, Big Girl, Rutgers 39, and more. They are indicated by the letter “V” to show resistance to verticillium wilt. For strawberries, Camino Real, Petaluma, Albion, and San Andreas are cultivars that are more tolerant to verticillium. Check with your garden center or seed supplier to see which cultivars they carry that are resistant to the fungus verticillium.
Frequently Asked Questions
Q: How do I know if I have verticillium wilt?
A: It is very difficult to tell if a plant is affected by verticillium wilt specifically because the symptoms are very similar to many other plant diseases and deficiencies.
Q: How do you remove verticillium wilt from soil?
A: Verticillium wilt can live in the soil for a long time. Commercial farms may use chemicals to fumigate soil. Home gardeners can try to manage the verticillium population through crop rotation, soil solarization and general sanitation, and removal of infected plants.
Q: What plants are affected by verticillium wilt?
A: Over 400 plant species are affected by verticillium wilt. The Missouri Botanical Garden has a comprehensive list of plant species that are susceptible and those that are tolerant. Many susceptible crops are garden favorites such as rose, watermelon, tomato, or strawberry, or commercial crops such as cotton or maple.