Category Archives: Disease – Field and Forage

Fusarium Head Blight (Scab)

What is Fusarium head blight?  Fusarium head blight (FHB) or scab is a fungal disease that affects wheat, barley, oats, and many grasses.  FHB is important, not only because it reduces yield, but because it reduces the quality and feeding value of grain.

Bleached spikelets on a wheat grain head due to Fusarium head blight.
Bleached spikelets on a wheat grain head due to Fusarium head blight.

In addition, the FHB fungus may produce mycotoxins, including deoxynivalenol (also known as DON or vomitoxin), that when ingested, can adversely affect livestock and human health.  Maximum allowable levels of DON in feed for various animals (as set by the U.S. Food and Drug Administration) are ≤ 10 parts per million (ppm) for beef and feedlot cattle, ≤ 10 ppm for poultry, and ≤ 5 ppm for swine and all other animals.

What does Fusarium head blight look like?  Diseased spikelets on an infected grain head die and bleach prematurely.  Healthy spikelets on the same head retain their normal green color.  Over time, premature bleaching of spikelets may progress throughout the entire grain head.  If infections occur on the stem immediately below the head, the entire head may die.  As symptoms progress, developing grains are colonized by the FHB fungus causing them to shrink and wrinkle.  Often, infected kernels have a rough, sunken appearance, and range in color from pink or soft gray, to light brown.

Where does Fusarium head blight come from?  FHB is caused by the fungus Gibberella zeae (also known as Fusarium graminearum), which is not only a pathogen of wheat, but also of corn.  The fungus can overwinter in infested stubble and straw of cereals and weed grasses, and on stalks and rotted ears of corn.  The severity of FHB varies greatly from year to year.  Infection is favored by extended periods of high moisture or high (>90%) relative humidity, and moderately warm temperatures (59 to 86°F), particularly when these conditions occur just before or during wheat flowering (Feekes 10.5).  If favorable weather conditions persist, infections can continue to occur through the early dough stage (Feekes 11.2).  Extended periods of infection can be especially problematic in wheat stands where plants have varying levels of maturity.

How can I save a small grain crop with Fusarium head blight?  Fungicides are available for FHB control.  Triazole fungicides [Fungicide Resistance Action Committee (FRAC) class 3] are recommended.  In particular, fungicides containing prothioconazole and tebuconazole, or a mix of these two compounds, have provided the best control of FHB in university research trials.  Avoid using strobilurin fungicides (FRAC class 11) as research indicates that use of these products can result in an increase in DON levels in harvested grain.  A web-based FHB risk assessment tool (http://www.wheatscab.psu.edu) is available to help make decisions about fungicide applications.  The tool also provides real-time, local commentary by extension personnel about the status of diseases in wheat.  When using fungicides for FHB control, be sure to read and follow all label instructions to ensure that you use the product in the safest and most effective manner possible.

Fusarium head blight can cause grain heads to become completely bleached.
Fusarium head blight can cause grain heads to become completely bleached.

How can I avoid problems with Fusarium head blight in the future?  DO NOT plant small grains into small grain or corn residue.  Also, avoid planting grain crops near areas where there are large amounts of small grain or corn residue on the soil surface.  When possible, plant small grains following a legume crop (e.g., soybeans) and maintain a rotation with two to three years between small grain crops.  Deep plow all infested plant debris, where feasible.  DO NOT apply manure containing infested straw or corn stalks onto fields planted to small grains.  Certain grain varieties have moderate levels of resistance to FHB.  Consider using these varieties as a means to reduce disease severity and increase grain quality.  Finally, plant several varieties of a small grain that vary in flowering date.  This will decrease the risk that an entire crop will be vulnerable to FHB when weather conditions favor the disease.

 For more information on Fusarium head blight:  Contact your county Extension agent.

Ergot

What is ergot?  Ergot is a fungal disease of worldwide distribution that is common in the northern two-thirds of North America.  Ergot affects wild and cultivated grasses, as well as small grain crops such as wheat, oats, barley and especially rye.

Production of honeydew (red arrow) and sclerotia (white arrows) are typical of ergot.
Production of honeydew (red arrow) and sclerotia (white arrows) are typical of ergot.

The ergot pathogen produces alkaloid toxins many of which reduce blood flow in animals (e.g., cattle, sheep, swine, horses and even humans) that eat ergot-contaminated grain.  Ergot poisoning is cumulative.  Symptoms can develop rapidly if animals eat large quantities of ergot or more slowly if they eat small quantities of ergot on a regular basis.  In many animals (e.g., cattle), the first symptom of ergot poisoning is lameness that occurs two to four weeks after ergot is first eaten.  Gangrene in extremities (e.g., hooves and ears) follows.  Dairy cows that eat ergot-contaminated grain typically have a marked reduction in milk yield.  Other symptoms of ergot poisoning can include convulsions, hallucinations and death.  Symptom development is often more severe in very hot or very cold weather.  Interestingly, some ergot toxins, when purified and used at low dosages, have pharmaceutical applications (e.g., inducing labor and treating migraine headaches).

What does ergot look like?  Signs of ergot first appear as droplets of a sticky exudate (called honeydew) on immature grain heads.  Honeydew contains asexual spores of the ergot fungus.  Over 40 species of insects are attracted to honeydew and can carry spores from infected to healthy plants.  After approximately two weeks, infected grains are replaced by dark (often purplish), compact fungal structures called sclerotia.  Sclerotia range in size from 1/16 to ¾ inches in length, and often look like seeds, rodent droppings, or insect parts.

Where does ergot come from?  Ergot is caused by several species of the fungus Claviceps, most commonly Claviceps purpurea.  Sclerotia of these fungi survive in soil and harvested grain.  Sclerotia require a one to two month period of cold temperatures (32 to 50°F) after which they germinate to form small, mushroom-like structures that produce sexual spores (different from those produced in honeydew).  Germination occurs most commonly in cool (57 to 84°F), damp weather and is inhibited at higher temperatures.  Sexual spores are blown to developing grain heads where infection occurs.  Humid weather (> 90% relative humidity) contributes to honeydew production.  Ergot is also often more severe if frosts occur at the time of spore production.

Ergot sclerotia germinate to form mushroom-like, spore-producing structures.
Ergot sclerotia germinate to form mushroom-like, spore-producing structures.

How can I save a small grain crop with ergot?  Fungicide treatments are not recommended to control ergot.  When ergot is a problem, you can attempt to remove sclerotia from grain using commercial seed-cleaning equipment.  However, if sclerotia are broken or are the same size as the grain itself, this may be difficult and costly, and may still result in grain that has a contamination level that is above marketable thresholds (tolerances for ergot sclerotia in harvested grain can be as low as 0.05% by weight).  Ultimately destroying the contaminated grain may be the best course of action.  Be sure to also destroy the hay from the affected field.  DO NOT use the hay as feed or for animal bedding.

How can I avoid problems with ergot in the future?  Maintain a rotation with at least one year between small grain crops.  Use crops that are not susceptible to ergot (e.g., soybeans, alfalfa, corn) in years when small grains are not grown.  Plant grain seed that is free of ergot sclerotia.  Ergot-resistant varieties are not available, but avoid longer-flowering varieties as they tend to be more susceptible to infection.  Keep weed grasses under control.  Also, mow areas adjacent to small grain fields to prevent grasses from flowering and developing ergot.  In fields where ergot becomes a problem, consider clean, deep plowing that will bury ergot sclerotia to at least three to four inches, thus preventing sclerotia from germinating.  Mow, remove and destroy ergot-infected grasses from pastures and hayfields.  DO NOT allow animals to graze in these areas and DO NOT use the harvested material for feed or as bedding material.

For more information on ergot:  Contact your county Extension agent.

Dodder

What is dodder?  Dodder is the name of several species of parasitic plants that are widely distributed in North America and Europe.  Plants parasitized by dodder include alfalfa, carrots, onions, potatoes, cranberries, a variety of herbaceous and woody ornamentals, and many weed species.  Parasitized plants become weakened, have reduced yields (in the case of agronomic crops), and can potentially die.

Spaghetti-like dodder plants parasitizing carrots.
Spaghetti-like dodder plants parasitizing carrots.

What does dodder look like?  Dodders lack roots and leaves, and also lack chlorophyll, the green pigment found in most plants.  Dodders have slender, yellow-orange stems that cover infected plants in a spreading, tangled, spaghetti-like mass.  From May through July, dodders produce white, pink, or yellowish flowers. 

Where does dodder come from?  Dodders produce large numbers of seeds that germinate in the spring to produce shoots that attach to suitable host plants.  Dodders penetrate host tissue, and absorb nutrients via specialized structures called haustoria.  Once established on a host, the bottom of a dodder plant dies (thereby severing its connection with the soil), and the dodder plant becomes dependent on the host plant for water and nutrients.

How do I save plants parasitized by dodder?  On woody ornamentals, simply prune out dodder-parasitized branches.  When small patches of dodder occur among herbaceous plants, apply contact herbicides such as 2,4-D early in the season, preferably before dodder seedlings have parasitized host plants.  Keep in mind that use of contact herbicides will likely also kill host plants.  Alternatively, cut or burn dodder and parasitized plants to keep dodder from spreading, and to prevent seed production.  For widespread dodder infestations, a combination of frequent tilling, burning and herbicide applications may be needed to achieve control.  Be sure to read and follow all label instructions of the herbicide that you select to ensure that you use the product in the safest and most effective manner possible. 

How do I avoid problems with dodder in the future?  Dodder’s wide host range and ability to survive as dormant seeds in soil make eradication difficult.  Preventing introduction of dodder is the best method of control.  Use dodder-free seed, and be sure to clean equipment after working in a dodder-infested area.  Try to restrict animal movement between infested and non-infested areas as well.  Depending upon the specific crop or location, use of pre-emergent herbicides containing DCPA, dichlobenil, propyzamide, or trifluralin may be possible to prevent germination of dodder seeds.  Destroy actively growing dodder and any parasitized plants before the dodder produces seeds.  In agricultural settings where dodder has been a problem, rotate away from susceptible crops and grow non-host crops (e.g., corn, soybeans, or small grain cereals).  In conjunction with rotation, adequate control of weed hosts is critical to achieve control.

For more information on dodder:  Contact your county Extension agent.

Corky Ringspot

What is corky ringspot? Corky ringspot (also known as spraing) is a potentially serious viral disease of potato that has recently been detected in Wisconsin. The disease can cause severe losses due to the fact that it reduces potato tuber quality, making tubers unsuitable for use in potato chip production and undesirable to consumers as table stock. Variants of this disease [usually referred to as tobacco rattle (see University of Wisconsin Garden Facts XHT1180)] affect a variety of other plants including vegetable crops (e.g., beans, beets, peppers, and spinach), many herbaceous ornamentals (e.g., astilbe, bleeding heart, coral bells, daffodil, epimedium, gladiolus, hyacinth, marigold, tulip, and vinca) and many weed species (e.g., chickweed, cocklebur, henbit, nightshade, pigweed, purslane, prickly lettuce, shepherd’s-purse and sowthistle).

Internal necrosis of tubers, often in fleck or arc patterns is typical of corky ringspot.
Internal necrosis of tubers, often in fleck or arc patterns is typical of corky ringspot.

What does corky ringspot look like? Symptoms of corky ringspot vary depending on the variety/cultivar of potato affected, and depending on environmental conditions. Foliar symptoms are rare, but on occasion can include reduced leaf size, puckering and mottling (i.e., blotchy light and dark coloring). More commonly, corky ringspot manifests itself underground as corky arcs, rings or flecks that form on or within tubers. Thinner-skinned and lighter-colored potato varieties are more likely to exhibit obvious ring symptoms on the surfaces of tubers. Symptoms similar to those caused by corky ringspot can be caused by other potato viruses such as alfalfa mosaic virus, potato mop-top virus, and certain strains of potato virus Y.

Where does corky ringspot come from? Corky ringspot is caused by the tobacco rattle virus (TRV) which is spread primarily by stubby-root nematodes, a group of microscopic, worm-like organisms in the genera Trichodorus and Paratrichdorus. These nematodes feed on the roots of infected plants (vegetables, ornamentals or weeds), acquiring TRV, then move to non-infected plants where their subsequent feeding spreads the virus. TRV also can be spread mechanically when knives or other tools that are used to cut tubers for seed pieces, or that are used to divide ornamental plants become contaminated. In addition, on ornamentals, TRV can be spread by pruning and grafting, and via movement of seed from infected plants.

How do I save potatoes with corky ringspot? Once potatoes have become infected with TRV, they remain infected indefinitely. Infected plants cannot be treated in any way to eliminate the virus and should be removed and disposed of by burning, burying or composting. You may want to have symptomatic plants tested to verify the presence of TRV. Note that ELISA (a technique commonly used to test for other potato viruses) is not a reliable test for TRV; polymerase chain reaction (PCR) should be used to test for this virus. Once TRV is introduced into a field, it is likely to remain there indefinitely. Stubby-root nematodes can carry the virus for extended periods and weed species can serve as reservoirs of the virus indefinitely.

How do I avoid problems with corky ringspot in the

Tubers with corky ringspot may, but do not always have target-like ring patterns on their surfaces.
Tubers with corky ringspot may, but do not always have target-like ring patterns on their surfaces.

future? The best way to prevent problems with corky ringspot is to avoid introducing TRV onto your property. Be sure to grow potatoes from seed that is certified as being free of TRV. Currently, seed potatoes produced in Wisconsin are considered TRV-free. Also avoid introducing the virus on infected ornamental plants. Carefully inspect ornamentals (see above for a partial list of susceptible species) prior to purchase for symptoms caused by TRV and DO NOT buy symptomatic plants. Alternatively (and preferably), avoid growing susceptible species altogether, and grow plants that are not susceptible to TRV. Non-susceptible plants include, but are not limited to, annual phlox, carnation, carrot, devil’s trumpet (downy thorn-apple), Scotch spearmint, sorrel, sweet William, zinnia and zombie cucumber. To limit potential spread of TRV, routinely decontaminate tools (e.g., knifes or other cutting tools) that come into contact with potentially infected plant material (e.g., whole tubers that are cut into seed pieces, or ornamentals that are being divided). Also decontaminate tires, tools (e.g., spades or hoes) and any other object (e.g., shoes or boots) that might transport stubby-root nematode-infested (and thus TRV-infested) soil from field to field. First rinse any excess plant tissue or soil from these items, then treat them for at least 30 seconds in a solution that is a combination of 1% sodium lauryl sulfate (use 10% shampoo as a source of this chemical) and 1% Alconox (an industrial detergent). Trisodium phosphate (available at most local hardware stores) can also be used. Also consider routinely testing soils for the presence of stubby root nematodes. Knowing the level of these nematodes in a field can provide information on the likelihood that TRV will spread should the virus be introduced. Finally, DO NOT ever produce seed potatoes in fields with a history of corky ringspot or other TRV diseases. Also avoid using infested fields for potato or other vegetable production. If you decide to use a TRV-infested field for non-seed potato production, be sure to grow a TRV-resistant potato variety. The potato varieties ‘Castile’, ‘Millennium Russet’, ‘Red Pearl’, ‘Symfonia’, and ‘St. Johns’ have all been reported to have at least moderate levels of resistance to TRV.

For more information on corky ringspot: Contact your county Extension agent.

Charcoal Rot of Soybean

What is charcoal rot? Charcoal rot, also known as summer wilt or dry weather wilt, is a fungal disease of soybean that most commonly occurs in plants that are under heat and water stress. Charcoal rot is most prevalent in the southern United States, but can occur in the North Central region when weather is hot and dry. In Wisconsin, charcoal rot is observed most often in fields with sandy soils.

A dusty, gray discoloration of stems and roots is characteristic of charcoal rot of soybean.  (photo courtesy of Theresa Hughes)
A dusty, gray discoloration of stems and roots is characteristic of charcoal rot of soybean. (photo courtesy of Theresa Hughes)

What does charcoal rot look like? Plants suffering from charcoal rot may display premature yellowing of their top leaves, as well as premature leaf drop that may be mistaken for normal plant maturity. Plants with charcoal rot often initially wilt in the midday heat and then recover at night. Eventually permanent wilting will occur. In some cases, the upper third of a plant may have unfilled, flat seedpods. At flowering, a light gray discoloration develops in the surface tissues of both tap and secondary roots, as well as lowers stems. These tissues will appear as if they have been dipped in charcoal dust, hence the name of the disease. The dusty appearance is due to the presence of tiny survival structures (called microsclerotia) of the fungus that causes the disease.

Where does charcoal rot come from? Charcoal rot is caused by the fungus Macrophomina phaseolina which has an extensive distribution and is known to infect over 500 plant species including corn (where it causes charcoal stalk rot), alfalfa, and many ornamental and weed species. M. phaseolina can survive for two or more years in dry soils as microsclerotia embedded in plant residue. In wet soils however, sclerotia do not survive for more than seven to eight weeks. Hyphae (i.e., fungal threads) of the fungus typically do not survive in soil for more than seven days. Infections of M. phaseolina primarily occur in the spring when soil moisture is high. The fungus enters plants via roots and then grows very slowly until plants reach their reproductive stage (usually coinciding with the hottest, driest part of the growing season). Then more extensive colonization of plant tissue occurs. M. phaseolina is most active when soil temperatures are high (80 to 95°F), unlike many soilborne, disease-causing fungi which have reduced activity when soil temperatures are high.

How can I save a soybean crop with charcoal rot? By the time that typical symptoms of charcoal rot are evident, control of charcoal rot is difficult, and losses in yield are likely inevitable. Foliar fungicides and fungicide seed treatments have no effect on charcoal rot.

How can I avoid problems with charcoal rot in the future? Plant high quality, pathogen-free seed to prevent introduction of the charcoal rot pathogen into fields that are not currently infested. In fields where M. phaseolina is already present, any cultural practices that minimize plant stress will reduce the risk of charcoal rot. Use tillage practices (e.g., no-till) that maintain soil moisture, and irrigate where possible during dry periods to reduce drought stress. Lower plant populations and maintain good weed control to minimize stress from competition for soil nutrients. In addition, optimize soil fertility levels, particularly phosphorus. Rotations with wheat may provide some control of charcoal rot. However, because M. phaseolina has a wide host range (including corn), crop rotation may not provide sufficient control of charcoal rot. A moderate level of partial resistance is known in soybean varieties in maturity groups IV and higher. Unfortunately these varieties are not suitable for production in Wisconsin. Whether partial resistance is present in commercial varieties suitable to be grown in Wisconsin (maturity groups I and II) is not known.

For more information on charcoal rot of soybean: See UW-Extension Bulletin A4037 “Charcoal Rot Management in the North Central Region” (available at http://learningstore.uwex.edu) Contact your county Extension agent.

Brown Stem Rot of Soybean

What is brown stem rot? Brown stem rot (BSR) is a disease of soybean that was first observed in central Illinois in 1944 and is now prevalent throughout the North Central States of the US, and Canada. BSR has been identified as the third most important disease of soybeans in Wisconsin, attributable to the expansion of soybean acreage and shorter crop rotations used in the state. The agronomic impact of BSR is greatest in high yield potential environments. BSR negates the benefits of management practices intended to increase yield potential.

Brown pith discoloration of soybeans suffering from brown stem rot (top) compared with the white pith of a healthy soybean plant (bottom).  (photo courtesy of Craig Grau)
Brown pith discoloration of soybeans suffering from brown stem rot (top) compared with the white pith of a healthy soybean plant (bottom). (photo courtesy of Craig Grau)

What does brown stem rot look like? Symptoms of BSR are usually not evident until late in the growing season and may be confused with signs of crop maturity or the effect of dry soils. The most characteristic symptom of BSR is the brown discoloration of the pith especially at and between nodes near the soil line. This symptom is best scouted for at full pod stage. Foliar symptoms, although not always present, typically occur after air temperatures have been at to below normal during growth stages R3-R4, and often first appear at stage R5, peaking at stage R7. Foliar symptoms include interveinal chlorosis and necrosis (i.e., yellowing and browning of tissue between leaf veins), followed by leaf wilting and curling. Yield loss as a result of BSR is generally greatest when foliar symptoms develop. The severity of BSR symptoms increases when soil moisture is near field capacity (i.e., when conditions are optimal for crop development). Foliar symptoms of BSR can be confused with those of sudden death syndrome (see University of Wisconsin Farm Facts XGT1015, “Sudden Death Syndrome of Soybean”). However, in the case of sudden death syndrome (SDS), the pith of affected soybean plants will remain white or cream-colored. In addition, roots and lower stems of plants suffering from SDS (but not those suffering from BSR) often have light blue patches indicative of spore masses of the fungus that causes SDS.

Where does brown stem rot come from? BSR is caused by the soilborne fungus Phialophora gregata. There are two distinct types (or genotypes) of the fungus, denoted Type A and Type B. Type A is the more aggressive strain and causes more internal damage and plant defoliation than Type B. P. gregata Type A also is associated with higher yield loss. P. gregata survives in soybean residue, with survival time directly related to the length of time that it takes for soybean residue to decay. Thus, P. gregata survives longer when soybean residue is left on the soil surface (e.g., in no till settings) where the rate of residue decay is slow. P. gregata infects soybean roots early in the growing season. It then moves up into the stems, invading the vascular system (i.e., the water-conducting tissue) and interfering with the movement of water and nutrients. Several factors can influence BSR severity. Research from the University of Wisconsin has shown that the incidence and severity of BSR is greatest in soils with low levels of phosphorus and potassium, and a soil pH below 6.3. In addition, P. gregata and soybean cyst nematode (Heterodera glycines) frequently occur in fields together, and there is evidence that BSR is more severe in the presence of this nematode.

Interveinal chlorosis and necrosis typical of brown stem rot.  (photo courtesy of Craig Grau)
Interveinal chlorosis and necrosis typical of brown stem rot. (photo courtesy of Craig Grau)

How can I save a soybean crop with brown stem rot? BSR cannot be controlled once plants have been infected. Foliar fungicides and fungicide seed treatments have no effect on the disease.

How can I avoid problems with brown stem rot in the future? Use crop rotations of two to three years away from soybean with a non-host crop (e.g., small grains, corn, or vegetable crops), as well as tillage methods that incorporate plant residue into the soil. Both of these techniques will help reduce pathogen populations by promoting decomposition of soybean residue. Also, make sure that soil fertility and pH are optimized for soybean production to avoid overly low phosphorus and potassium levels, as well as overly low soil pH. Finally, grow soybean varieties with resistance to BSR. Complete resistance to BSR is not available in commercial varieties. However several sources of partial resistance that provide moderate to excellent BSR control are available. Also, some, but not all, varieties of soybean cyst nematode (SCN) resistant soybeans also are resistant to BSR. Most soybean varieties with SCN resistance derived from PI 88788 express resistance to BSR. However, the same is not true of varieties with SCN resistance derived from Peking. Therefore growers should consult seed company representatives about BSR resistance when selecting a variety with SCN resistance derived from this source.

For more information on brown stem rot of soybean: Contact your county Extension agent.

Brown Spot (Septoria Leaf Spot)

What is brown spot? Brown spot of soybean, also referred to as Septoria leaf spot or Septoria brown spot, is a common and usually relatively minor foliar disease of soybean in Wisconsin. Brown spot typically does not lead to significant yield loss in soybeans produced in the state, although yield losses of up to 15% have been reported from other areas of the US. In Wisconsin, brown spot tends to be more prevalent on soybeans that are under stress [e.g., stress due to drought, low fertility (particularly low potassium), high insect feeding, or other diseases such as soybean cyst nematode].

Angular, reddish-brown leaf spots are typical of brown spot.  (photo courtesy of Craig Grau)
Angular, reddish-brown leaf spots are typical of brown spot. (photo courtesy of Craig Grau)

What does brown spot look like? The most typical symptom of brown spot is the formation of angular, reddish-brown spots (pinpoint to 3mm in diameter) on both primary and trifoliolate soybean leaves. Using a hand lens, one can often also see small, roughly spherical pycnidia (reproductive structures) of the causal fungus in the brown areas. The pycnidia often ooze strands or masses of tannish fungal spores. When brown spot is severe, plants may begin to defoliate from the ground up. The disease is often more prevalent where drainage is poor.

Where does brown spot come from? Brown spot is caused by the fungus Septoria glycines, which survives in residue from previously diseased soybean crops. The fungus can also survive on diseased seeds. Brown spot tends to be more common during warm, wet weather, and when relative humidity is high.

How can I save a soybean crop with brown spot? Brown spot is not a lethal disease and in Wisconsin, it rarely leads to economic loss. However brown spot is more prevalent, and can be yield limiting, in late planted soybeans and in early maturing soybean varieties. Fungicide treatments for brown spot are typically neither warranted nor economical.

How can I avoid problems with brown spot in the future? Brown spot is best managed through proper rotation. DO NOT grow soybeans continuously in the same field, but rotate soybeans with other crops for at least one year to allow time for soybean residues to naturally decay. Tillage techniques that bury crop residue and promote more rapid decay of residues that harbor the brown spot pathogen may also help provide control. Also, avoid using seed that has been produced in fields with high levels of the disease. Finally, reduce other stresses on your soybeans that may predispose plants to brown spot. Plants that are properly fertilized, have sufficient water and are insect- and pathogen-free are less likely to develop the disease. For more information on brown spot of soybean: Contact your county Extension agent.

Aphanomyces Root Rot of Alfalfa

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.

Stunting and yellowing of alfalfa plants.
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 the 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.  Oftentimes 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, can 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 as the southwest region), 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.