All posts by hudelson

Bird’s Nest Fungi

What are bird’s nest fungi? 

Bird’s nest fungi are a group of organisms named for their resemblance to miniature bird’s nests.  These fungi are found all over the world, growing and reproducing on decomposing organic matter.  In temperate regions, bird’s nest fungi can be found virtually anytime there are damp, shady conditions, but they are most commonly seen in the autumn.

Fruiting bodies of Crucibulum laeve found on decaying wood. (Photo courtesy of Mark Steinmetz)
Fruiting bodies of Crucibulum laeve found on decaying wood. (Photo courtesy of Mark Steinmetz)

What do bird’s nest fungi look like? 

Bird’s nest fungi can be identified by the appearance of a brown, gray, or white outer “nest”, with brown or white “eggs” inside.  The “eggs” are actually spore-containing structures called peridioles that rest inside the cup-shaped fruiting body (reproductive structure).  Individual fruiting bodies are usually only ¼ to ½ inch in diameter and vary in shape, size, and color depending upon species.

Where do bird’s nest fungi come from? 

Bird’s nest fungi are classified in the fungal group gasteromycetes.  Cyathus spp., Nidula spp., Crucibulum spp., Nidularia spp., and Mycocalia spp. are the most common genera of bird’s nest fungi.  These fungi are often found in moist, shaded areas, and typically survive on soil, plant remains, decaying wood, or horse and cow excrement.  The characteristic “cup and egg” structure of the bird’s nest fungi provides a unique method of dispersing spores.  In a storm, the “eggs” are splashed out of the “cups” by raindrops.  “Eggs” can travel a three feet or more before sticking to another object.  When the “egg” dries, it splits open releasing fungal spores.

What do I do with bird’s nest fungi in my yard? 

Bird’s nest fungi are not harmful to living plants, and control of these fungi is typically not necessary.  In fact, bird’s nest fungi can be fascinating organisms to observe when you find them in your yard.  On occasion, bird’s nest fungi can be a nuisance, particularly when the “eggs” stick to objects such as houses or cars where they are difficult to remove.  If bird’s nest fungi become a nuisance, the number of fruiting bodies can be reduced by decreasing irrigation and raking the affected area.  The use of non-mulch groundcovers, such as ivy, can also reduce the occurrence of these fungi.  Fungicides are not recommended for treatment of bird’s nest fungi.

For more information on bird’s nest fungi: 

Contact your county Extension agent.

Bacterial Wilt of Cucurbits

What is bacterial wilt? 

Bacterial wilt is a common and destructive disease that affects cucurbits (i.e., plants in the cucumber family), including economically important crops such as melon (Cucumis melo), cucumber (Cucumis sativus) and, to a lesser extent, squash and pumpkin (Cucurbita spp.).  This disease is distributed throughout the United States; and can be found anywhere that cucurbits are grown.

Sudden wilting and eventual death of melon, cucumber and squash plants can be due to bacterial wilt. (Photo courtesy of ISU-PIDC.)
Sudden wilting and eventual death of melon, cucumber and squash plants can be due to bacterial wilt. (Photo courtesy of ISU-PIDC.)

What does bacterial wilt look like? 

The most distinctive symptom exhibited by a plant with bacterial wilt is wilting and ultimately death.  These symptoms are a consequence of the blockage of water movement inside of the plant.  Symptoms appear first on leaves of a single runner (vine).  Leaves may develop chlorotic (i.e., yellow) and necrotic (i.e., dead) areas as the disease progresses.  Symptoms typically develop rapidly along individual runners, and eventually, the plant’s crown is affected, resulting in the entire plant dying.  To determine if a symptomatic plant has bacterial wilt, cut a wilted vine near the base of the plant.  Next cut a section from this vine and look for sticky threads to form between the two vine sections as you slowly pull them apart.  The presence of these sticky threads is diagnostic.  This technique works best for cucumbers and melon, but less well for squash and pumpkins.

Where does bacterial wilt come from? 

Bacterial wilt of cucurbits is caused by the bacterium Erwinia tracheiphila.  This bacterium is moved from plant to plant by two insects: the striped cucumber beetle (Acalymma vittatum) and the spotted cucumber beetle (Diabrotica undecimpunctata).  See University of Wisconsin Garden Facts XHT1092, Cucumber Beetles, for details on these insects  The bacterium primarily overwinters in the guts of adult beetles, and is released when beetles feed on healthy cucurbit plants and excrete contaminated frass (i.e., feces) onto fresh feeding wounds.  E. tracheiphila has been found in association with wild cucurbits and other plants such as goldenrods (Solidago nemoralis and S. altissima), Johnson grass (Sorghum halepense) and even corn (Zea mays).  However, most of these plants never show wilt symptoms, and none are considered an important reservoir for the bacterium.

How do I save a plant with bacterial wilt? 

Bacterial wilt cannot be controlled once a plant is infected.  In particular, chemical sprays are not effective for control once plants show symptoms.  If you find bacterial wilt in your garden, immediately remove infected plants, and dispose of them by burning (where allowed by law) or burying them.  DO NOT compost infected plants.  Prompt removal and disposal of infected plants is important not only because they serve as a source of E. tracheiphila, but because they attract more cucumber beetles, thus increasing the risk of spread of the bacterium to other, healthy plants.

How do I avoid problems with bacterial wilt in the future?  

Management of bacterial wilt relies on control of cucumber beetles to prevent infection.  Target non-chemical and chemical control methods to protect plants at the beginning of the growing season when plants are more attractive to cucumber beetles.  Use mechanical barriers, such as row covers, to protect plants from cucumber beetle feeding.  Also, inspect cucurbits on a regular basis for cucumber beetles and their damage (two to three times per week early in the season, and weekly thereafter).  When cucumber beetle numbers are high (more than 20 per plant), spray plants with an appropriate insecticide (see UW Garden Facts XHT1092 for recommended insecticides).  Be sure to read and follow all label instructions of the insecticide(s) that you select to ensure that you use the insecticide(s) in the safest and most effective manner possible.  Cucurbit varieties resistant to bacterial wilt are not currently available.  However, some cucurbits such as watermelons and pickling cucumbers tend to be less attractive to cucumber beetles and thus tend to have fewer problems with bacterial wilt.  These crops can be used as alternatives to more susceptible cucurbit species.

For more information on bacterial wilt of cucurbits: 

Contact your county Extension agent.

Bacterial Wetwood

What is bacterial wetwood? 

Bacterial wetwood, also known as “slime flux”, is a visually frightening-looking, but typically non-lethal, disorder of many types of deciduous trees.  This disorder can reduce the aesthetic appeal of landscape trees, and more seriously, can substantially reduce the value of forest trees used for lumber.  Bacterial wetwood most commonly affects elm and poplar, but can also be a serious problem on aspen, maple, and mulberry.

Bacterial wetwood leads to discolored, rancid-smelling areas on tree trunks.
Bacterial wetwood leads to discolored, rancid-smelling areas on tree trunks.

What does bacterial wetwood look like? 

Trees suffering from bacterial wetwood have areas where liquid oozes from their trunks.  This ooze may flow freely at certain times of the growing season, but then may stop flowing at others.  The ooze leads to streaked, discolored, water-soaked areas on tree trunks.  The ooze is often colonized by bacteria, as well as yeasts and other fungi.  These organisms can give the ooze a slimy, sometimes brightly-colored (i.e., pink or orange) appearance as well as a highly disagreeable, rancid smell.  Internally, bacterial wetwood can be associated with localized areas of wood decay.

Where does bacterial wetwood come from? 

Bacterial wetwood arises when localized wet areas develop in the heartwood or sapwood of tree trunks.  These areas are colonized by a diverse assortment of bacteria (e.g., Enterobacterium, Klebsiella, Pseudomonas and many others) that can enter trees through root, branch or trunk wounds.  As these bacteria feed and grow, often under anaerobic conditions (i.e., conditions without oxygen), they can produce gases such as methane, carbon dioxide, or nitrogen gas.  These gases build up pressure, causing movement of interior liquids to the exterior of the trunk where they escape through wounds and cracks.

How do I save a tree with bacterial wetwood? 

Bacterial wetwood is a chronic disorder and affected trees cannot be cured.  To limit the unsightly staining of bark caused by bacterial wetwood, try to identify where the ooze is exiting from the trunk and insert a long, plastic tube at this location to direct the ooze away from the trunk and to the ground at the base of the tree.  There has been speculation that the build-up of gases due to bacterial wetwood might cause a tree to explode.  However, there have been no reliable reports of this ever happening.

How do I avoid problems with bacterial wetwood in the future? 

There is little you can do to prevent problems with bacterial wetwood.  Many affected trees were likely invaded by wetwood-associated bacteria in the seedling stage.  Developing a healthy tolerance for bacterial wetwood, when it occurs, is perhaps the best method for coping with this disorder.

For more information on bacterial wetwood: 

Contact your county Extension agent.

Bacterial Soft Rot

What is bacterial soft rot? 

Bacterial soft rot describes a group of diseases that cause more crop loss worldwide than any other bacterial disease.  Bacterial soft rots damage succulent plant parts such as fruits, tubers, stems and bulbs of plants in nearly every plant family.  Soft rots commonly affect vegetables such as potato, carrot, tomato, cucurbits (e.g., cucumbers, melons, squash, pumpkins), and cruciferous crops (e.g., cabbage, cauliflower, bok choy).  These diseases can occur on crops in the field as well as on harvested crops in storage.  Rot can occur over a wide temperature range (with the worst decay between 70 and 80°F) and is particularly severe when oxygen is limited.

Bacterial soft rots cause the collapse of plant parts such as potato tubers. (Photo courtesy of the UW-Madison/Extension Plant Disease Diagnostics Clinic)
Bacterial soft rots cause the collapse of plant parts such as potato tubers. (Photo courtesy of the UW-Madison/Extension Plant Disease Diagnostics Clinic)

What does bacterial soft rot look like? 

Soft rot bacteria degrade pectate molecules that bind plant cells together, thus causing plant structure to fall apart.  Woody tissues are not susceptible.  Initially, bacterial soft rots cause water-soaked spots.  These spots enlarge over time and become sunken and soft.  Interior tissues beneath the spots become mushy and discolored, with the discoloration ranging anywhere from cream to black.  Seepage from affected areas is common.  Soft rots are known for a strong, disagreeable odor that accompanies the breakdown of plant tissue.

Where does bacterial soft rot come from? 

Soft rots are caused by several bacteria, most commonly species of Pectobacterium [particularly Pectobacterium carotovorum (previously called Erwinia carotovora)], Dickeya species [particularly Dickeya dadantii (previously called Erwinia chrysanthemi)], and certain species of Pseudomonas, Bacillus and Clostridium.  These bacteria can enter plants through wounds caused by tools, insects, and severe weather such as hail, as well as through natural openings.  The bacteria can be spread from plant to plant by insects, on contaminated tools, or by movement of infested plant debris, soil, or contaminated water.  Bacterial soft rots tend to be more of a problem during wet weather and can be more severe when plants lack sufficient calcium.

How do I save a plant with bacterial soft rot? 

Once soft rot bacteria have infected plant tissue, there are no treatments.  Immediately remove and discard infected plants or plant parts.  DO NOT bury or compost this material.

How do I avoid problems with bacterial soft rot in the future?  

Avoiding wet conditions is key for managing soft rot.  Plant vegetables in well- drained soils, and control watering times and amounts, making sure plants are watered adequately (but not excessively) and uniformly.  DO NOT crowd plants; wider spacing will promote more rapid drying of plants and soil.  Make sure that soil fertility (particularly soil calcium) is optimal for the vegetables that you are growing based on a soil nutrient test.  Add calcium (e.g., bone meal) at planting as needed.

Use soft rot-resistant vegetables in rotation with susceptible vegetables.  Corn, snap beans and beets are vegetables that are not considered susceptible to soft rot.  When growing broccoli, avoid varieties with flat/concave heads that trap moisture and promote soft rot.  Instead, select varieties with domed heads where water readily drains away.

Avoid damaging vegetables when weeding and during harvest.  Minimize any handling of soft-rotted plants, but if you must handle such plants (e.g., to remove them from the garden), wash your hands afterwards with soap and water.  Decontaminate garden tools before and after use by treating them for at least 30 seconds with 10% bleach or preferably (because of its less corrosive properties), 70% alcohol.  Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol.  Also, keep insects that can wound vegetables such as cabbage maggot under control (see University of Wisconsin Garden Facts XHT1030, Cabbage Maggot, for details).

Harvest only during dry conditions.  Closely inspect vegetables from infected gardens that will go into long-term storage, and be sure not to store any diseased vegetables.  Cure vegetables where appropriate prior to storage.  Store vegetables in a cool, dry, well-aerated place to suppress bacterial growth.

At the end of the growing season, remove any infested plant debris remaining in your garden, and destroy the material by burning (where allowed by local ordinance) or landfilling it.  If soft rot is a serious, recurring problem in an area in your garden, DO NOT grow susceptible crops in that area for a minimum of three years.

For more information on bacterial soft rot: 

Contact your county Extension agent.

Bacterial Canker

What is bacterial canker? 

Bacterial canker is a common and sometimes lethal disease of trees in the genus Prunus including cherry, plum and peach.  Bacterial canker is sometimes also referred to as “gummosis”, “blossom blast”, “dieback”, “spur blight” and “twig blight”.

Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.
Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.

What does bacterial canker look like? 

Often branch dieback is the first symptom of bacterial canker that homeowners notice.  However, other more subtle symptoms of flowers, leaves, fruits and branches typically precede this dieback.  Initially, infected trees are symptomless.  Infected flowers often open but then collapse.  Infected leaves become spotted and yellowed.  The centers of leaf spots often shothole (i.e., fall out).  If spotting is severe, leaves may fall off.  Infected fruits develop dead spots surrounded by water-soaked tissue.  Spots can eventually develop into a fruit rot.  Branch or trunk infections often occur at pruning sites and lead to cankers (i.e., sunken, dead areas).  Cankers often produce a gummy, resinous ooze.  Wood in the cankered area is typically discolored.  Flower, fruit and branch infections can become systemic, leading to twig dieback, death of larger branches, or even death of an entire tree.

Where does bacterial canker come from? 

Bacterial canker is caused by the bacteria Pseudomonas syringae pv. syringae (Pss) and P. syringae pv. mors-prunorum (Psm).  These bacteria overwinter in cankers, in asymptomatic, systemically infected branches, and in buds of susceptible trees.  Pss is also known to occur naturally on the leaves of many plants including many weed species.  Both Pss and Psm can subsequently be spread by rain splash, wind, or insects.  The bacteria can also be spread to healthy branches when contaminated pruning tools are used.  Infections most often occur during cool, wet conditions.

How do I save a plant with bacterial canker? 

Prune infected branches at least 12 inches below cankers or other dead tissue, and dispose of branches by burning (where allowed by local ordinance) or burying them.  Prune branches only during the winter (e.g., Jan. and Feb.) or during dry periods in late summer (e.g., Aug.).  DO NOT prune during the cool, wet periods (e.g., spring and fall).  Disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

How do I avoid problems with bacterial canker in the future? 

Healthy trees are better able to slow the development of bacterial canker.  Therefore make sure that trees are watered and fertilized properly.  Keep weeds and other plants that may harbor bacterial canker pathogens from around the base of susceptible trees.  Copper-containing sprays have been advocated for bacterial canker management.  However, in many areas, copper-resistant strains of Pss and Psm are present and therefore copper sprays are ineffective.

For more information on bacterial canker: 

See UW Bulletin A3639, Growing Apricots, Cherries, Peaches, and Plums in Wisconsin (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

Bacterial Blight

Death of lilac branch tips and leaves due to bacterial blight.
Death of lilac branch tips and leaves due to bacterial blight.

What is bacterial blight? 

Bacterial blight, also known as blossom blight or shoot blight, is a common and often serious disease of Chinese, Japanese, Persian and common lilac, as well as walnut, apple, pear, plum and cherry.  White flowering varieties of common lilac are most susceptible to the disease.

What does bacterial blight look like? 

Initial symptoms of bacterial blight may include dark brown necrotic (dead) leaf spots with yellow halos.  If leaf spots develop before leaves are fully expanded, leaf curling and twisting may result.  More advanced symptoms include necrotic blotches starting at the leaf margins and advancing inward, as well as black streaking on twigs.  In its most severe form, bacterial blight can result in the death of branch tips, leaves and blossoms.

Where does bacterial blight come from? 

Bacterial blight is caused by the bacterium Pseudomonas syringae pv. syringae (Pss), which survives in diseased stem tissue (cankers), plant debris, and soil.  Pss can be spread by insects and on pruning tools but is more commonly spread by wind and rain.  Often Pss is found on the surface of healthy plants and does not cause disease.  Infections can occur when the bacterium enters tissue through natural openings or through wounds caused by insects, pruning, wind damage or hail.

How do I save a plant with bacterial blight? 

Prune diseased twigs 10 to 12 inches below the point of visible symptoms, and dispose of the branches by burning (where allowed by local ordinance) or burying them.  Always prune in dry weather, and disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

How do I avoid problems with bacterial blight in the future?  

When planting lilacs, provide adequate spacing between shrubs.  Thin individual shrubs each winter to promote good air circulation (see University of Wisconsin Garden Facts XHT1015, Pruning Deciduous Shrubs, for pruning tips).  Properly water, fertilize and mulch shrubs to avoid stress that may predispose them to disease.  Avoid overhead watering that may keep leaves wet.  If you have had chronic problems with bacterial blight, you may want to use a combination of copper and mancozeb-containing fungicides for control.  Apply fungicides two to three times at seven to 10 day intervals as leaves emerge, but before symptoms develop.  Read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.

For more information on bacterial blight: 

Contact your county Extension agent.

Aster Yellows

What is aster yellows? 

Aster yellows is a chronic, systemic disease that affects over 300 species of broad-leafed, herbaceous plants in at least 38 families.  Members of the aster family (Asteraceae), such as asters, marigolds, Coreopsis and purple coneflower are commonly affected by this disease.  Vegetable crops such as carrots and potatoes are also susceptible.  Aster yellows occurs throughout North America.

Coneflowers with aster yellows (right) often have deformed, discolored flowers.
Coneflowers with aster yellows (right) often have deformed, discolored flowers.

What does aster yellows look like? 

Infected plants are typically stunted and twisted, with foliage that is yellow, purple or red.  Infected plants are often sterile.  Floral parts that are normally brightly colored may be green, and petals and sepals may become puckered and distorted.  In purple coneflower, secondary flower heads (often in a cluster) may emerge from the primary flower head.  In marigolds, flowers are often leafy and a muddy green-orange color.  Infected carrots have purple/red leaves and form taproots with tufts of small, white “hairy” roots.  Tap roots from infected carrots often have a bitter taste.

Where does aster yellows come from? 

Aster yellows is caused by the aster yellows phytoplasma, a bacterium-like organism that lives in the food-conducting tissue (phloem) of plants.  Aster yellows is rarely lethal.  Thus, infected perennials can serve as a source of the aster yellows phytoplasma for many years.  The aster leafhopper (Macrosteles fascifrons), a common insect, moves the aster yellows phytoplasma from plant to plant.

How do I save a plant with aster yellows? 

There is no known cure for aster yellows.  Plants suspected of having aster yellows, including weeds such as dandelions, should be removed immediately so that the aster yellows phytoplasma cannot be spread from infected plants to other non-infected plants in the area.  Proper diagnosis of aster yellows is important because management of herbicide and eriophyid mite damaged plants does not require plant removal.

How do I avoid problems with aster yellows in the future? 

Some herbaceous plants (e.g., geraniums and impatiens), as well as most woody ornamentals, are not susceptible to aster yellows.  Therefore these plants should be used in areas where aster yellows is a problem.  In landscape settings, attempts to control aster leafhoppers as a means of controlling aster yellows are typically not effective and are not recommended.

For more information on aster yellows: 

See UW Bulletins A2595 and A3679 (available at https://learningstore.extension.wisc.edu/) or contact your county Extension agent.

Ash Yellows

What is ash yellows? 

Ash yellows is a chronic, systemic disease that affects ash trees of all ages.  White ash is particularly susceptible to ash yellows.  Ash yellows likely occurs wherever ash is grown and has been reported widely in the United States and southern Canada.  The organism that causes ash yellows also causes a disease called lilac witches’-broom.

Brooming symptoms in an ash tree caused by ash yellows.
Brooming symptoms in an ash tree caused by ash yellows.

What does ash yellows look like? 

Symptoms of ash yellows usually occur within three years of infection.  Infected trees typically grow at a much slower rate than non-infected trees, although this may be difficult to detect in an isolated, single tree.  The rate of growth of an infected tree may be as little as one half that of a healthy tree.  Leaves on infected trees are frequently smaller, thinner and lighter green than normal.  Often, but not always, affected trees will produce branches in tufts, a symptom that is called “brooming”.  Eventually, branches in the crown will die and this dieback can continue until the entire crown is dead.

Where does ash yellows come from? 

Ash yellows is caused by the phytoplasma, Candidatus Phytoplasma fraxini.  Phytoplasmas are bacteria-like organisms that live and survive in the phloem (the food-conducting tissue) of infected plants.  Leafhoppers are thought to be the primary means by which this pathogen is moved from tree to tree.

How do I save a tree with ash yellows? 

There is no known cure for ash yellows, but some infected trees may live and grow slowly with the disease for many years.  Ash trees suspected of having ash yellows should be tested for the disease, and those trees that test positive should be removed immediately to prevent spread of the ash yellows phytoplasma to other trees in the area.  Wood harvested from infected trees does not serve as a source of the phytoplasma and can be used for woodworking or firewood, or chipped for mulch.

How do I avoid problems with ash yellows in the future? 

Avoid growing ash trees in areas where ash yellows is prevalent.  When choosing a lilac, select a variety of common lilac as these varieties appear to have tolerance to the ash yellows phytoplasma.  Avoid using S. josikaea, S. reticulata and S. sweginzowii (or hybrids of these species with either S. komarowii or S. villosa), as these lilacs appear to be highly susceptible.  It is unclear if the use of insecticides (or other means) to control leafhoppers can help control the spread of this pathogen.

For more information on ash yellows and ash yellows testing: 

Contact your county Extension agent.

Armillaria Root Disease

What is Armillaria root disease? 

Armillaria root disease, also known as shoestring root rot, is an often lethal disease of tree and shrubs.  It can affect almost any conifer or hardwood species, from seedling to maturity.  Herbaceous plants can also be affected.  Trees and shrubs stressed due to drought or defoliation can be particularly susceptible to Armillaria root disease.

White mats of fungal tissue called mycelial fans (arrow) may be present within and beneath the bark of stems and roots affected by Armillaria root disease.
White mats of fungal tissue called mycelial fans (arrow) may be present within and beneath the bark of stems and roots affected by Armillaria root disease.

Where does Armillaria root disease come from? 

Armillaria root disease results from colonization of trees and shrubs by fungi in the genus Armillaria.  These fungi produce tough, cord-like strands called “rhizomorphs” that grow from decaying stumps and roots through the soil.  Infection of other trees or shrubs can result from penetration of intact roots by rhizomorphs.  In late summer or early fall, honey-colored mushrooms of Armillaria fungi develop near the bases of colonized plants and produce spores that are distributed by wind.  Infection also can occur after these spores germinate in wounds on stems or roots.

What does Armillaria root disease look like? 

Above-ground symptoms of Armillaria root disease may include slow growth, yellowing and dwarfing of foliage, and thin crowns.  Dieback of twigs and branches also may occur as the disease progresses.  These symptoms may develop slowly and intensify over many years.  However, trees and shrubs also may be rapidly killed, with leaves or needles suddenly wilting or browning on a plant that appeared healthy just days or weeks earlier.  Bark on lower stems or roots may be killed and crack, with flow of resin common on conifers.  Thin white mats of fungal tissue called “mycelial fans” may be present within and beneath killed bark.  Stem and root tissue decayed by Armillaria fungi is often water-soaked, creamy to yellow in color, and spongy or stringy in texture.  Rhizomorphs are commonly seen on or beneath the bark and growing from decayed stumps and roots.

How do I save a tree affected by Armillaria root disease? 

There is no practical way to eliminate Armillaria from trees that are already colonized by the fungus.  The useful life of an affected tree might be prolonged however, by supplemental watering during dry periods and appropriate fertilization to improve overall host condition.  In very vigorous trees, the Armillaria fungi may be “walled off” and confined to just a portion of the root system or root collar.  There are no chemical treatments that can effectively target Armillaria fungi within diseased trees.

How do I avoid Armillaria root disease in the future? 

Practices that maintain trees in vigorous condition are the best means of preventing Armillaria root disease.  Watering and fertilization to avoid stress will help trees resist infection.  Because Armillaria root disease often develops in response to defoliation, suppression of both insect and leaf pathogen defoliators will indirectly reduce the occurrence and severity of Armillaria root disease.  Because stumps and root systems of previously colonized trees can serve as “food bases” supporting rhizomorph growth for many years, thorough removal of stumps and root systems will reduce the risk of infection of other trees.

For more information on Armillaria root disease: 

Contact your county Extension agent.

Aphanomyces Root Rot of Alfalfa

Stunting and yellowing of alfalfa plants (leading to increased weed pressure) is typical of Aphanomyces seedling blight and root rot. (Photo courtesy of Craig Grau)
Stunting and yellowing of alfalfa plants (leading to increased weed pressure) is typical of Aphanomyces seedling blight and root rot. (Photo courtesy of Craig Grau)

What is Aphanomyces root rot of alfalfa? 

Aphanomyces root rot (ARR) is a serious disease of both recently seeded alfalfa and established alfalfa stands.  ARR can cause severe yield reductions in affected alfalfa fields.  Variations of the disease also occur on many other legumes (including soybean, snap bean, faba bean, red kidney bean, pea, red clover, and white clover) and can cause significant losses in these crops as well.

What does Aphanomyces root rot look like? 

Typically, alfalfa emergence is not dramatically affected by ARR, but symptoms appear shortly after seedlings emerge.  Young plants appear stunted and yellow and may eventually die.  The root systems of affected seedlings are smaller than normal, and what roots remain appear gray and water-soaked.  Older alfalfa plants suffering from ARR also tend to be stunted and yellow.  They may have a well-developed tap root but typically relatively few smaller, fine roots.  Often, growers realize they have a problem with ARR when they notice that weeds in their fields are growing more vigorously than their alfalfa crop.

Where does Aphanomyces root rot come from? 

ARR is caused by the soilborne water mold (i.e., fungus-like organism) Aphanomyces euteichesA. euteiches is commonly found in fields that are poorly drained, fields with heavier (i.e., clay) soils, fields with compaction, and fields that receive excessive water.  A. euteiches produces microscopic, long-lived resting spores (called oospores) in the roots of infected plants, and these spores can remain dormant in the soil for up to 10 years, even in the absence of a susceptible crop.  Once a susceptible crop is present, oospores can germinate and directly infect plants, or under wetter conditions produce numerous microscopic swimming spores (called zoospores) that can subsequently infect plants.

There are several variants of A. euteiches and these variants tend to have preferences for which plant hosts they will infect.  For example, some variants tend to infect alfalfa, others tend to infect peas and others tend to infect snap beans.  A. euteiches that infects alfalfa can be further divided into two races (race 1 and race 2), which can be distinguished based on the particular alfalfa varieties that they most readily infect.  Other races of A. euteiches that can infect alfalfa likely exist, but at this time have not been fully documented.

Alfalfa plants with Aphanomyces seedling blight/root rot have reduced numbers of small, fineroots. (Photo courtesy of Craig Grau)
Alfalfa plants with Aphanomyces seedling blight/root rot have reduced numbers of small, fineroots. (Photo courtesy of Craig Grau)

How can I save plants with Aphanomyces root rot? 

There is no way to save an alfalfa crop once ARR has occurred.  Fungicide seed treatments may provide short-term protection of alfalfa seedlings.  However, foliar fungicides do not provide any ARR control.

How can I avoid problems with Aphanomyces root rot in the future?  

The most important management strategy for ARR is to make sure fields are properly drained.  Reducing standing water is important to prevent development of zoospores, which can dramatically increase disease severity.  Reducing compaction, using sub-surface drainage tiles and/or re-routing surface water drainage pathways can help alleviate wet soil conditions.  If there is a past history of ARR in a field, use alfalfa varieties with resistance to the specific race(s) of A. euteiches present in the field.  Which race(s) are present can be determined using a soil bioassay.  Contact your local county Extension office for more information on how to collect a soil sample for A. euteiches testing, as well as for recommendations on appropriate alfalfa varieties to use once the results of the soil bioassay are available.  In some areas of Wisconsin (such the southwest), both race 1 and race 2 of A. euteiches are widespread.  Therefore, routine use of alfalfa varieties resistant to both races may be warranted.  Crop rotation is not an effective management strategy for ARR because oospores of A. euteiches survive for long periods in the soil.  Alfalfa seed treatments may provide protection to seedlings only up until shortly after emergence.  Foliar fungicides, fumigants and other biological control products are also not effective in managing ARR.

For more information on Aphanomyces root rot:  

Contact your county Extension agent.