The genus Verbascum includes multiple species of biennial plants commonly known as mullein. Verbascum thapsus is a common species that grows in all 50 states, Canada and Greenland. It is native to some regions, but considered invasive in others, according to the USDA Plants Profile database. You can grow Verbascum as an ornamental and for its medicinal uses. Because it is a wildflower, it is relatively carefree, but it can develop some plant diseases.
The website "A Modern Herbal" reports that Verbascum has been known and used in Europe since the 1500s. One of the names given to Verbascum is candlewick plant--it is said that witches of olden times used the plant in their lamps and candles for incantations. The ancient Anglo Saxons called mullein "moleyn," and the French called it "malen." Both names derive from the Latin word malandrium. It was believed to have the power to drive away evil spirits in Europe and Asia.
In addition to North America, Verbascum grows in many places around the world. From Western Europe to East Asia, it is a common roadside and pasture plant often considered a weed. Scientists believe that the plant originated in Europe and Asia and that early settlers introduced it to the colony of Virginia in the United States in 1770, where it was used to poison fish. It has spread to many other areas since that time, through both natural seed dispersal methods and purposeful introduction.
Verbascum requires only a small amount of annual rainfall. In environments that receive as little as 3 inches per year, Verbascum can survive, although it prefers a slightly wetter climate. Verbascum grows naturally in meadows and openings in forests that receive full sun and it performs well in dry or sandy soil. It can thrive in rocky areas as well. If you decide to grow it, plant Verbascum in a sunny area that has well-drained soil. Each plant can produce up to 180,000 seeds, according to the National Park Service website, so the chances of it naturalizing in your yard are high.
Powdery mildew is a common plant disease (fungus) that affects many plants. If not treated, powdery mildew can kill a plant. Root rot is another plant disease that can strike Verbascum. Known as Phymatotrichum omnivorum, this disease is also called cotton root rot and Texas root rot. A soil-borne fungus causes it, and it is difficult to control, according to Texas A&M University. Other microorganisms can sometimes affect Verbascum--one of them is known as Phoma thapsi. This fungal disease occurs on many types of plants and can cause Verbascum plants to die.
Treating Common Fungal Diseases
You can sometimes control fungal plant diseases by spraying your plant with a commercial chemical fungicide or organic sulfur. If you allow some airflow around your Verbascum plants and other plants, you can help to prevent most fungal diseases. It's also important to keep the ground around your plant clean by picking up leaf litter where microorganisms can grow. As soon as you notice a fuzzy white to gray coating beginning to form on your Verbascum's leaves, snip off the affected foliage and spray the entire plant with a fungicide or sulfur. You can also control the other fungal diseases that affect Verbascum by applying fungicide or sulfur at the first sign of plant weakness.