Insect Pests of Hosta

Leaf Nematodes

Leaf nematodes are microscopic round worm that enter stomatal openings on hosta leaves to feed on the inside of the leaves. They are unable to cross the veins from inside the leaf and must exit the leaf to cross the barrier or to move to another section of the leaf. They require a layer of moisture in order to move about on the outer surface. If it is raining, they can even be splashed onto other leaves or close by plants. Hosta have parallel venation and thus damage appears as dead stripes on the leaves.

Root Nematodes

Root nematodes live in the soil and feed on the roots of Hosta, rather than the leaves. Their feeding damage causes small galls to form on the roots and root distortion. These microscopic round worms feed inside of these galls on the plants fluids which weakens the plant. They also vector several diseases of Hosta such as Tobacco Rattle Virus, Arabis mosaic (ArMV), and Tobacco ringspot virus (TRSV). For more information on root nematodes.

Cutworms

Are the caterpillar larvae of moths. They live in the soil by day and come out at night to feed on the above ground parts of their hosts. Damage to hostas appears as large irregular holes in stems and leaves. Damage may look similar to slug and snail damage but there will be no slimy trail left behind. For more information on cut worms.

Slugs and Snails

Slugs and snails live in the soil and come out to feed on above ground plant parts on overcast or rainy days and at night. They usually require a layer of moisture in order to move about. Unlike the nematodes however they are able to produce a layer of mucus under themselves to help them move about. Damage to hostas appears as irregular holes in stems and leaves usually with a slimy trail left behind. This trail appears shiny when dry but can be washed off during periods of rain. For more information on slugs and snails.

Black Vine Weevils

Black vine weevil adults take cover in leaf litter or in the soil by day and come out at night to feed on leaves. They cut small half-moon like notches out of the outer edges of leaves. Their larvae grubs remain in the soil feeding on roots and stems and can cause extensive damage to the plant.

Leaf Cutter Bees

Leaf cutter bees cut rounded holes from the edges of leaves. These holes are larger than those of black vine weevils. Feeding damage is considered cosmetic. They are most active in late summer.

Aphids

Aphids can sometimes be a pest to Hosta. They particularly like the flowers but may also be found on the underside of leaves. Damage to Hosta appears as tiny brown spots on leaves with possibly some twisting or contorting, along with some sticky honeydew, that may attract wasps and bees. Damage to the flowers can be quite unsightly. As the aphids suck out the plant juices plants become weakened. Aphids are also notorious for spreading disease in the garden, especially viral diseases. In Hosta they vector Cucumber mosaic virus (CMV). For more information on aphids.

Thrips

Thrips are a very tiny insect that are difficult to see without the aid of a magnifying glass. To the naked eye they look like pieces of brownish thread. When they are disturbed then tend to jump or fly up. Damage to Hosta appears as faint spots (stippling) on leaves. If their populations are high leaves may appear silvery or papery. Black fecal spots may also be present. Like aphids they are notorious for spreading serious viral diseases such as Hosta Virus X (HVX), Impatiens Necrotic Spot (INSV), Tobacco streak virus (TSV). They can also damage flowers causing distortion. Thrips are particularly troublesome in green houses.

Grasshoppers

Grasshoppers have voracious appetites chewing large, irregular holes in all above ground parts, even to the point of consuming entire plants.

Sowbugs/pillbugs

Are considered only a minor pest of Hosta. They primarily feed on decaying or damaged plant material but may also feed on young new shoots, roots and flowers especially those touching the soil.  Damage to Hosta appears as small irregular holes in leaves and reduced root systems. They often move in to complete the decomposition of leaves that have been damaged by slugs and snails. They mainly feed at night and are attracted to moist areas of the garden.

Blister Beetles

Feed on all parts of Hosta. Adults chew leaves, usually beginning at the outer edges working in. Grub-like larvae feed on roots. If blister beetles are lightly or firmly squeezed, they fluid they emit can create blisters on human skin and be dangerous if ingested by pets or live stalk. Wear gloves when handling this insect and never smack them if they land on you.

Earwigs

Earwigs prefer to feed on decaying plant material close to the soil level, but they also commonly feed on leaves and flowers chewing irregular holes that looks much like slug and snail damage but without the slimy trail. The feed at night and hide out in the day in cool, dark moist places in and around the garden.

Hosta Diseases

Fungal Diseases

Note: Fungal diseases account for approximately 85% of all plant diseases. A tell-tale sign that you are dealing with a fungus is the presence of fungal fruiting bodies that can be seen growing on the diseased tissue. Look for these to help determine if you are dealing with a fungus or other disease-causing pathogen.

Cercospora Leaf Spot

If your Hosta is infected with Cercospora Leaf Spot the first thing you will notice is randomly spaced brown spots; more specifically small reddish-brown to tan coloured spots, with a darker rusty brown border, that may have a yellow hallo. The spots enlarge and may coalesce. Eventually entire leaves may die. This is a common disease of Hosta especially during hot weather. It usually is not fatal and managing the disease begins with removing infected leaves and practicing good garden sanitation. The fungus spreads by splashing water, wind and mechanical transfer. Appling a copper spray can help manage the disease but can also cause phototoxicity especially on hot days. Read and follow the manufacturers recommendations for use.

Phytophthora Foliar Blight

Phytophthora is a soil borne, fungal-like genus of water molds consisting of about 170 named species. Many species are fairly host specific while others can infect a broad range of plants. Some species attack only above ground parts, while other species attack roots, stems and collars. This is a serious disease that can kill the entire plant. Phytophthora Foliar Blight symptoms appear as water-soaked spots on leaves that continue to enlarge and coalesce eventually covering the entire leaf. These spots usually first occur in areas where water tends to collect. The centers of these spots die out, turning brown and dry then fall out of the leaf. Plants look like they are falling apart. To avoid this disease, improve drainage, reduce humidity, avoid overhead watering and only plant disease free plants. Once infected you may be able to save the Hosta if you catch the disease early. Remove affected leaves and spray with a copper-based fungicide and apply a soil drench fungicide.

Phytophthora Root, Stem, and Collar Rot

Phytophthora is a Greek work that means “plant destroyer.” This soil borne fungus-like disease causes the roots to rot destroying first the feeder roots then moving into the main roots and finally spreading to the crown and stems. Roots turn a black or brown colour. Above ground the leaves begin to yellow beginning at the edges, working its way inward. Yellow areas turn brown and dry and drop out of the leaf. Overly wet soil is the primary cause of this disease as the zoo spores produced by this pathogen require wet soil in order to swim around in the soil to reach and infect roots. Remove infected plant and soil. Improve site drainage and site conditions before replanting. If the disease is minor wash the soil off the roots and cut away diseased areas. Pore hydrogen peroxide over roots and let dry, then spray with a copper-based fungicide and replant in uninfected soil.

Fusarium Root and Crown Rot (FRCR)

Fusarium root and crown rot is a serious soil borne fungal disease that can affect the whole plant. There are several species of the genus Fusarium, but the problematic specie for Hosta is Fusarium hostae. This pathogen invades through root wounds and hostas being propagated or divided are most at risk. Other conditions that favour this pathogen are dry soil conditions and warm weather. Symptoms of infection appear as reduced size and number of roots, which may exhibit decay on the outer layers. Crowns that are split open appear brown with dry decay (instead of white) and white root areas near the crown will be discoloured on the inside. Above ground, hostas may be later out of dormancy, have stunted growth, leaves turn yellow then tan and wilt. If caught early remove infected part of plant, soak root in a fungicide, then cover root with wood ash then repot plant in peat-based soil. For more serious infections remove plant and destroy.

Phyllosticta Leaf Spot

Phyllosticta is a fungal genus with many species that infect mainly the leaves (fruits can also be affected), of a broad range of hosts. While it is a common leaf spot disease of hosta it is rarely a serious pathogen. Symptoms of infection appear as small irregular brown to tan spots with darker borders on leaves. The dead tissue drops out leaving holes with a brown edge. If the spots begin to coalesce damage can be more extensive. Remove affected leaves and spray plant with a copper- based fungicide. Make sure plants are not overcrowded and avoid overhead watering.

Anthracnose

Anthracnose is a fungal leaf spot disease caused by various species of the pathogen genus Colletotrichum. This is one of the most common diseases of hosta and commonly occurs along with other fungal pathogens, like Cercospora. It is usually not life threatening but it can certainly cause some damage. Symptoms appears as randomly spaced round or irregular spots that can be whitish, tan, grey or brown in colour with a dark border. Spots enlarge and may coalesce; Center of spots dries out and may drop out of the leaf. Reduce moisture to leaves and manage humidity by spacing plants, eliminating weeds and organic material around the plant.

Gray Rot aka Botrytis Blight

Gray Rot caused by the fungus Botrytis cinera can be a serious disease able to infect over 200 plant species, mainly ornamental plants and food crops. It becomes most problematic during cool damp weather. Hosta infected with grey rot will develop small water-soaked spots that form a dark halo ring around the outside. As the spot continues to expand new rings are formed. In the center of these lesions a fuzzy gray mold grows when humidity is high. Flowers and crown can also be affected. This fungus is difficult to treat due to its spores being released on the air currents where they can infect a wide range of plants in your garden. The fungus can also live in the soil for periods of time. Help prevent this disease by reducing humidity to the area by ensuring good air circulation, not overcrowding, and not overhead watering. Remove dead plant material and apply a protective copper-based fungicide spray before symptoms begin. Once infected remove diseased leaves and disinfect all tools and surfaces that come in contact with the disease. Spray plant with a fungicide and used a soil fungicide drench. If the plant is seriously infected remove it and the surrounding soil and destroy them.

Rust

Rust disease of Hosta is a fungal disease caused by the pathogen Puccinia funkiae. Hosta infected with this fungus develop tiny rust coloured spots on the leaves. At first the spots on the upper leaf surface will appear yellow or tan in colour with rust coloured wart like blisters forming usually on the underside of the leaves. Eventually the rust colour becomes apparent on the upper surface. Infected leaves begin yellowing, may become distorted and die completely. Some rust species, like this one, have a unique life cycle where by 2 plant hosts are required in order to complete its life cycle. The alternate host in this case is C. The fungal spores on the Hosta can be blown on the wind or splashed onto Patrinia villosa (Pennycress) where the fungus spends part of its life then the fungus will sporulate again and blow the spores back to the Hosta. To manage this disease do not grow the 2 host species anywhere near each other. Reduce humidity, ovoid overcrowding and clean up weeds and garden debris. Treat with a fungicide.

Alternaria Leaf Spot

Alternaria leaf spot begins as small irregular, grey coloured spots on leaves that have a reddish-brown border. The spots enlarge and leaves can become blighted. To avoid this disease, avoid overhead watering, overcrowding, and ensure good air circulation. Protective copper based fungicides can be sprayed prior to infection if the disease is known to be in your area. Once infected remove diseased leaves and spray with a fungicide.

Bacterial Diseases

Bacterial Soft Rot

Bacterial Soft Rot is caused by the bacterial pathogen Pectobacterium sp. formerly Erwinia sp. This is a serious fast acting disease that can kill the entire plant and be spread to others nearby on infected hands and tools. The disease is most problematic during hot humid weather. The bacteria primarily entire the plant through wounds and once inside they release an enzyme that breaks down cell walls creating a watery mushy plant tissue that begins collapsing. First symptoms may be water-soaked areas on the leaves that may ooze, quickly followed by leaves yellowing, wilting, then collapsing on the ground as the stems and crowns decay. There is an unpleasant odour that accompanies this disease that smells like rotted fish. Remove plant and soil and destroy. If infection is mild remove plant, wash roots and soak in a fungicide and replant in new soil.

Viruses

Note: There is no cure for viral diseases, plants should be removed and burned or black bagged to prevent their spread. Plants can be infected with more than one virus at a time making identification all the more challenging. A laboratory is usually required to accurately identify specific viral diseases.

Hosta Virus X (HVX)

Potexvirus HVX symptoms can appear somewhat variable depending on the Hosta species infected. Hosta with lots of natural venation can be quite tricky to diagnose. Leaves may develop green spots or blotches, mosaic patterns, chlorosis or variegation-like mottling beginning at leaf margins. Leaves may twist, appear deformed and may be smaller in size and in severe cases whole leaves may die. White flower breaks may appear in purple flowering hostas. Plants may be symptomless for a year or more after infection. There is no cure, remove and destroy plant. Control thrips that vector the disease and avoid spreading to other plants by practicing good sanitation practices.

Impatiens Necrotic Spot (INSV)

Impatiens Necrotic Spot disease is caused by the Tospovirus INSV. Leaves develop subtle circular spots that often are darker green with lighter coloured rings within the spots. The spot may be quite small and appear more like mottling or they can become quite large and begin to coalesce. The plants tissue at the outer ring of the spot may begin to die and turn brown. Leaves may yellow and pucker. To prevent this disease control thrips that vector this disease. Once infected remove plants and burn or black bag them.

Tomato Spotted Wilt Virus (TSWV)

Tomato Spotted Wilt Virus is caused by the Tospovirus TSWV. Leaves develop chlorotic spots that have fuzzy margins. The spots may have a brown ring inside of them. Control root feeding nematodes that vector this disease. Remove infected plant and burn or black bag it.

Tobacco Rattle Virus (Tobravirus)

Tobravirus TRV Steaking, mottling, spots in ring pattern. Transmitted by soil nematodes. Remove and destroy plant. Remove soil or grow annuals instead for a while.

Tobacco streak virus (TSV)

Ilarvirus TSV Streaking and mottling. Symptomatic plants may become asymptomatic in time. Control the thrips that vector this disease.

Arabis mosaic (ArMV)

NepovirusArMV Flecking between the veins. May also experience stunting and leaf deformation. Transmitted by soil borne Dagger nematodes. Remove and destroy plant and soil.

Tomato ringspot virus (ToRSV)

Nepovirus ToRSV Chlorotic ringspots, mottling, mosaic and line patterns. Flowers and leaves may appear distorted. Remove and destroy plant and soil.

Cucumber Mosaic Virus (CMV)

 (Cucumovirus CMV) Transmitted by aphids. Variegation-like steaking. Leaves may have yellow spots, become deformed and stunted. Remove and destroy plant and soil. Manage aphid populations.

Animal Pests of Hosta

Rabbits

In some yards rabbits will completely leave hosta alone but in other yards they can be a problem for Hosta growers. Damage appears as nice clean cuts made at a 45-degree angle. There are often unconsumed leaves left lying on the ground. A sure sign that there are bunnies in your yard are the presence of small round droppings (poo). Nesting activity can also damage roots as they dig a well in the soil, fill it with grass and hair, then lay their babies in this.

Deer

Hungry deer can sometimes be a problem for Hosta growers. This damage is distinguishable from rabbit damage by the ragged edge they leave behind. Deer pull entire leaves off to consume, leaving a stalk with a ragged edge, whereas rabbits make a nice clean cut.

Voles

Rather than feed on the leaves voles tunnel around under the ground and may feed on the roots of Hosta, especially during the winter months. In the spring plants may stunted in size, may wilt, or fail to come up at all.

Mice, Gophers, Squirrels, Chipmunks and Moles

Usually not a huge problem for Hosta but they can damage the crown of Hosta in late fall, winter and early spring when food is scarce. Moles do not feed on roots or leaves of Hosta but their tunneling activity may disrupt roots.

Environmental and Other Damage to Hostas

Frost

When tender new growth is pushing forth it is particularly vulnerable to frost damage. Leaves may appear puckered and cupped or may have a shredded look that later browns.

Drought

When Hosta is experiencing drought, it will begin browning on the tips and margins of the leaves. This may be accompanied by wilting and a stunting of the growth.

Sunburn

Sun burned Hosta leaves at first appear bleached. Then these bleached areas turn brown and fall out of the leaf. The damage is most prominent on sunny side of plants.

Sun Scald

Margins brown first followed by discolouration of tissue between the veins and browning.

Herbicide and Insecticide Damage

Can look like disease symptoms or other conditions. If there has been any spraying in your yard our neighbouring yards the damage may be due to this and not be an indication of disease.

References:

Driesche R.V., Western Flower Thrips. UC Riverside. https://biocontrol.ucr.edu/western-flower-thrips

Fisher John R. and Mortko Robert A., (n.d.). Tobacco Streak Virus a New Virus in the Genus Hosta. http://www.americanhostasociety.org/AHSMembers/OnlineJournals/44/Virus.html

Kirk W., Dudek T. and Byrne J., (2007). Managing botrytis blight on Hosta. https://www.canr.msu.edu/news/managing_botrytis_blight_on_hosta

Lockhart B. E. L. (2006). Occurrence of Arabis mosaic virus in Hostas in the United States. Plant disease, 90(6), 834. https://doi.org/10.1094/PD-90-0834C

Michigan State University, (2015). Botrytis blight. MSU Extension Integrated Pest Management. https://www.canr.msu.edu/resources/botrytis_blight

Missouri Botanical Garden., (n.d.). HOSTA LEAF NEMATODES. https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/nematodes/hosta-leaf-nematodes

Mueller D., Gleason M., and Lewis D., (Iowa State University), Jeffers S., (Clemson University), Edmund B., and Warfield C., (North Carolina University), and Williams-Woodward J., (University of Georgia). Hosta Diseases and Pests. Sustainable Urban Landscapes. https://ronpaulgardencentre.com/wp-content/uploads/hosta-diseases.pdf

Oregon State University, (n.d.). WHAT IS PHYTOPHTHORA? OSU College of Agricultural Sciences. https://horticulture.oregonstate.edu/nursery/nursery/what-phytophthora

Ono, Y., (2005). Plantainlily rust fungus is distinct from daylily rust fungus. J Gen Plant Pathol 71, 99–106 (2005). https://doi.org/10.1007/s10327-004-0169-9

Plants Galore. com, (n.d.). Virus Diseases of Hosta. http://www.plantsgalore.com/care/hostas/Hosta-pests-virus.htm

Pscheidt J.W. and DeAngelis J., (n.d.). Impatiens Necrotic Spot: New Name, Same Threat. A Pacific Northwest Pest Management Handbook. https://pnwhandbooks.org/plantdisease/pathogen-articles/common/plant-viruses/impatiens-necrotic-spot-new-name-same-threat

The Connecticut Agricultural Experiment Station, (n.d.). Hosta (Hosta) Plant Health Problems. https://portal.ct.gov/CAES/Plant-Pest-Handbook/pphH/Hosta-Hosta

University of Massachusetts Amherst, (n.d.).  Hosta – Leaf spot (Botrytis cinera). Center for Agriculture, Food, and the Environment. UMass Extension Greenhouse Crops and Floriculture Program. https://ag.umass.edu/greenhouse-floriculture/photos/hosta-leaf-spot-botrytis-cinera

University of Massachusetts Amhers, (n.d.). Hosta – Alternaria Leaf Spot. UMass Extension Greenhouse Crops and Floriculture Program. https://ag.umass.edu/greenhouse-floriculture/photos/hosta-alternaria-leaf-spot

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