Hostas are popular shade garden perennials. Their attractive foliage, endless diversity of shape and size, tolerance of shady areas, minimal maintenance needs, and relatively few pest and disease problems are what make them so popular. While they are tough, easy-to-grow plants, they are not free of problems. Several diseases and pests can reduce plant vigor and aesthetic value.
Learn about some of the most common issues gardeners face when growing hostas and what to do to manage them. Even more information can be found in this publication: Hosta Diseases and Pests.
Slugs | Deer & Rabbits | Dead Center | Leaf Spot | Petiole & Crown Rot | Viruses | More Information
Slugs
Slugs are one of the most common issues hosta growers face. Slugs are close relatives of snails, clams, and oysters. Slugs have soft, unsegmented bodies and look very similar to snails, but without the external shell. When stretched out, the average Iowa garden slug will be up to 1 inch in length.
Feeding damage appears as large, irregular holes. When abundant, they may cause considerable damage.
Slugs require a damp environment to survive. Periodic drying that occurs in Iowa gardens may be one of the factors that limits this animal to relative obscurity except in wet years. Slugs are protected from drying by hiding during the day and feeding at night. They spend the daytime hiding in soil crevices or under boards, rocks, mulch, debris or the foliage of low, dense plants.
Slug treatment may be occasionally necessary, but not always. Cleaning up the garden and eliminating slug hiding places may help. Heavy leaf litter, boards, bricks, and other piles of damp debris in contact with the ground should be removed. Dense ground covers that are harboring slugs can be thinned to promote sunshine, air circulation, and drying.
Minor slug problems can be controlled by handpicking. Check carefully around the base of damaged plants and favored hiding places. Night checking with a flashlight may improve your efficiency, or you can leave "trap sites" to be checked on a regular basis. Good slug "traps" or convenient hiding places under which the slugs will retreat include overturned pots and a piece of board or asphalt shingle.
Beer is a well-known trap attractant for slugs, though any fermenting or yeast-containing liquid appears to work. The traditional trap design is to bury a shallow pan in the soil with the top edge level with the soil surface. Renew the beer or attractant regularly and empty the pan of trapped slugs frequently.
Learn more in this article: Slugs.
Deer & Rabbits
The damage deer and rabbits can do to a hosta garden is extensive. Hostas are a favorite for deer and a plant can be reduced to "celery stalks" in just an evening or two. When browsing, deer grab and tear leaf blades from the plant. This leaves behind a characteristic ragged edge on the petiole. When rabbits feed (another common hosta connoisseur), they snip off leaves, leaving behind a petiole that looks as if it was cut by scissors.
Management of deer browsing can be difficult. Ideally, fencing is used to exclude deer from the area. If the garden area cannot be fenced off, then fencing can be placed around individual plants. A dome or cylinder with a "cap" can be used to keep a prized plant safe. Other methods can be used to prevent deer damage including repellents, scare devices, and savvy placement of plants in areas where deer are less likely to visit (such as by the house). Learn more in this article: How to Protect Gardens from Deer.
Management for rabbit browsing is best done with fencing. Placing a chicken wire enclosure around the plant to protect will keep gnawing bunnies away. Entire gardens can be fenced off if required. Be sure to check fencing frequently for damage or areas where rabbits may slip under. Other tactics, such as repellents and scare devices, can also be used, but are not typically as effective as fencing. Learn more in this article: How to Protect Gardens from Rabbits.
Dead Centers
It is common for the center of large, established hosta clumps to die out. While a little alarming at first, a dead center in an otherwise healthy plant is not an indication of disease or insect issues. The hosta puts on most of its new growth on the outside of the clump each year, and over several years, the old, dead growth from past years accumulates in the center.
Hostas showing this problem can be dug up roots and all, and the dead center can be cut out. The remaining "ring" of live material can be divided into several new plants, which can then be replanted.
Learn more in this article: How to Divide and Transplant Perennials.
Leaf Spot
Several fungi can cause leaf spotting and severely impact the appearance of hostas. Anthracnose is one of the more common ones. Infection can degrade appearance, but plants are seldom killed. Disease development is favored by extended periods of leaf wetness (rain, dew, or overhead
irrigation) and warm temperatures. The anthracnose fungi cause large, irregular spots with dark borders. The centers of spots often fall out, and leaves
become tattered. Under certain conditions, these spots can grow together, causing entire leaves to die back.
Other fungi cause similar symptoms and, like anthracnose, reduce the aesthetic value of the hosta but seldom kill plants. These other foliage diseases are caused by species of Cercospora, Alternaria, and Phyllosticta fungi. These fungi cause leaf spots that are often distributed randomly across the leaf during warm, rainy weather.
Management of all these leaf spot diseases looks similar. Integrating multiple management strategies is most effective. Cultural practices that help manage anthracnose include regular irrigation early in the day, keeping plants cool, and removing infected leaves. Fungicides can be used when issues are significant and have been observed in the past. To be effective, however, fungicides need to be applied before symptoms appear. Among the fungicides registered for use on anthracnose include chlorothalonil, iprodione, mancozeb, and thiophanate-methyl. Fungicides such as azoxystrobin, chlorothalonil, copper hydroxide, or myclobutanil may be used to treat other leaf spot diseases. Always read and follow label directions and select a product that will control the fungi you have in your hosta garden.
Hosta Petiole and Crown Rot
Petiole rot is a serious disease of hosta. It is caused by the fungus Sclerotium rolfsii var. delphinii. Disease symptoms start as marginal yellowing and browning of the lower leaves. A soft, brown, mushy decay may be seen at the base of the petiole, sometimes accompanied by white fungal threads. As the disease progresses, the leaves discolor and wilt. In the final stages of the disease, the bases of the petioles rot away and the leaves collapse and lay flat on the ground. Collapsed leaves pull easily from the crown of the plant.
The disease fungus produces small, round fungal fruiting structures about the size of mustard seeds that appear at the base of the infected petioles. These structures, called sclerotia, are a cream color when young, but gradually turn to a dark, brick red color. The sclerotia are long-term survival structures of the fungus. Sclerotia germinate during warm, humid weather and fungal mycelium grows in the ground until it finds a susceptible nearby plant.
Effective management requires a combination of strategies. Before planting hostas in the garden, carefully inspect each plant. Do not plant any suspect or symptomatic hostas. Remove and destroy infected plants. Also, carefully remove the soil in the area to a depth of 6 to 8 inches and replace it with non-contaminated soil. Avoid moving plants from contaminated beds to other parts of the garden. A mulch-free zone of several inches at the base of susceptible plants might also help. Common garden fungicides available at garden centers and other retailers are not effective.
Learn more in this article: Hosta Petiole and Crown Rot.
Viruses
Several viruses can cause issues in hostas. Some common ones include Hosta virus X (HVX), Impatiens necrotic spot virus (INSV), Tomato ringspot virus (ToRSV), Tobacco rattle virus (TRV), and Arabis mosaic virus (ArMV). These viruses can be transmitted during vegetative propagation if an infected plant is used as the source of propagation material. They can also be carried in seeds and pollen and by pests that feed on plants and move infected plant sap from plant to plant, like nematodes, insects, and mites.
Symptoms for all viral diseases look similar but can vary depending on the cultivar and environmental conditions. In most cases, light or dark green discoloration is present along leaf veins. Leaves can become puckered, distorted, wilted, and eventually, tissue death can be seen. Holding an infected leaf and a healthy leaf up to some light can make seeing symptoms a little easier.
Viruses often shorten the lifespan of plants or reduce growth, but they rarely are lethal. Plants are weakened, which may make them more likely to experience issues with other pests and disease, or less tolerant of extreme environmental conditions like drought.
The best method to manage virus diseases is to avoid introducing these pathogens into the nursery or landscape. Only use virus-free plants for propagation to avoid transmitting viruses to new plants. If symptomatic plants are found in a nursery or landscape, they should be removed and destroyed so that the virus cannot be transmitted to healthy plants. Weeds are known to harbor many different types of viruses, so keeping weeds out of hosta beds will help, as well. There are no pesticides available to control virus diseases in plants.
Learn more in this article: Hosta-infecting Virii.
More Information
Photo credits: 1: Iowa State University Plant and Insect Diagnostic Clinic; 2: Laura Jesse; 3: Aaron Steil; 4: Aaron Steil; 5: Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University, Bugwood-.org 6: Iowa State University Plant and Insect Diagnostic Clinic; 7: John Fisher, Ohio Department of Agriculture, Bugwood.org; 8: Alan Windham, University of Tennessee, Bugwood.org; 9: Anette Phibbs, WI Department of Agriculture, Trade & Consumer Protection, Bugwood.org