Category Archives: Disease – Field and Forage

Plant Diseases to Watch For in 2021

Septoria Leaf Spot Septoria Leaf Spot and Early Blight
Host:  Tomato
Pathogens:   Septoria lycopersici and Alternaria solani
Signs/Symptoms:  Spotting and eventual total collapse of leaves working from the bottom of the plant up
For more information see:       UW Plant Disease Facts D0100/46
Late Blight Late Blight
Hosts:  Tomato, potato
Pathogen:   Phytophthora infestans
Signs/Symptoms:  Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death
For more information see:       UW Plant Disease Facts D0068
Septoria Leaf Spot of Lilac Septoria Leaf Spot of Lilac
Host:  Lilac
Pathogen:   Septoria sp.
Signs/Symptoms:  Dead spots on leaves, potentially leading to complete leaf browning
Rhizosphaera Needle Cast Rhizosphaera Needle Cast
Hosts:  Colorado blue spruce, other spruces
Pathogen:   Rhizosphaera kalkhoffii
Signs/Symptoms:  Browning/purpling of interior needles of lower branches, followed by needle drop
For more information see:       UW Plant Disease Facts D0093
Gymnosporangium Rusts
Hosts:  Juniper, apple, crabapple, hawthorn, quince
Pathogen:   Gymnosporangium spp.
Signs/Symptoms:  Brown blobs with orange gelatinous masses (juniper); yellow/orange leaf spots (other hosts)
For more information see:       UW Plant Disease Facts D0058
Elderberry Rust
Hosts:  Elderberry
Pathogen:   Puccinia sambuci
Signs/Symptoms:  Light yellow, powdery growths on branches
For more information see:       UW Plant Disease Facts D0049
Scab Scab (Apple and Pear)
Hosts:   Apple, crabapple, pear, mountain-ash
Pathogens:   Venturia inaequalis, Venturia pirina
Signs/Symptoms:  Feathery-edged spots on leaves and fruits often leading to leaf loss and tree defoliation
For more information see:       UW Plant Disease Facts D0004
Bur Oak Blight
Host:   Bur oak
Pathogen:   Tubakia iowensis
Signs/Symptoms:  Wedge-shaped dead areas on leaves leading to dead leaves that stay attached to trees
Powdery Mildew
Hosts:   Herbaceous and woody ornamentals, fruits, vegetables, turf
Pathogens:   Miscellaneous powdery mildew fungi
Signs/Symptoms:  Powdery white growth on leaves
For more information see:       UW Plant Disease Facts D0084/86/87
Diplodia Shoot Blight and Canker
Hosts:  Austrian pine, other pines
Pathogen:   Diplodia spp.
Signs/Symptoms:  Dieback of branch tips with dead needles showing uneven lengths
For more information see:       UW Plant Disease Facts D0042
Boxwood Blight
Host:  Boxwood
Pathogen:   Calonectria pseudonaviculata
Signs/Symptoms:  Circular, brown leaf spots followed by leaf drop and shrub death
For more information see:       UW Plant Disease Facts D0023

For more information on plant diseases to watch for:  See https://pddc.wisc.edu/ or contact your county Extension agent.

Ten Common Plant Diseases/Disorders You Can Diagnose by Eye

Powdery Mildew Powdery Mildew
Hosts:   Herbaceous and woody ornamentals, fruits, vegetables, turf
Pathogens:   Miscellaneous powdery mildew fungi
Signs/Symptoms:  Powdery white growth on leaves
For more information see:       UW Plant Disease Facts D0084/86/87
Tar Spot - Ten Common Plant Diseases Tar Spot
Hosts:  Maples
Pathogen:   Rhytisma spp.
Signs/Symptoms:  Tarry areas (either solid spots or clusters of small spots) on leaves
For more information see:       UW Plant Disease Facts D0110
Peach Leaf Curl Peach Leaf Curl
Hosts:  Peach
Pathogen:   Taphrina deformans
Signs/Symptoms:  Light-green, yellow or purplish-red puckered areas on leaves
For more information see:       UW Plant Disease Facts D0076
Sooty Mold Sooty Mold
Hosts:  Any plant
Pathogen:   Miscellaneous sooty mold fungi
Signs/Symptoms:  Powdery black growth on leaves or needles
For more information see:       UW Bulletin A2637
Chlorosis Chlorosis
Hosts:  Oak, red maple
Cause:   Iron or manganese deficiency, often induced by high soil pH
Signs/Symptoms:  Yellow leaves with dark green veins
For more information see:       UW Plant Disease Facts D0030
Gymnosporangium Rusts Gymnosporangium Rusts
Hosts:  Juniper, apple, crabapple, hawthorn, quince
Pathogen:   Gymnosporangium spp.
Signs/Symptoms:  Brown blobs with orange gelatinous masses (juniper); yellow/orange leaf spots (other hosts)
For more information see:       UW Plant Disease Facts D0058
Black Knot Black Knot
Hosts:  Prunus spp. (plum and cherry)
Pathogen:   Apiosporina morbosa
Signs/Symptoms:  Black poop-like growths on branches
For more information see:       UW Plant Disease Facts D0018
Elderberry Rust Elderberry Rust
Hosts:  Elderberry
Pathogen:   Puccinia sambuci
Signs/Symptoms:  Light yellow, powdery growths on branches
For more information see:       UW Plant Disease Facts D0049
Golden Canker Golden Canker
Hosts:  Pagoda dogwood
Pathogen:   Cryptodiaporthe corni
Signs/Symptoms:  Gold-colored branches with orange spots
For more information see:       UW Plant Disease Facts D0055
Dog Vomit Slime Mold Dog Vomit Slime Mold
Hosts:  Any plant and on mulch
Cause:   Fuligo septica
Signs/Symptoms:  Scrambled egg-like masses on mulch or at the base of plants
For more information see:       UW Plant Disease Facts D0102

For more information on common plant diseases:  See https://pddc.wisc.edu/ or contact your county Extension agent.

Tobacco Mosaic

Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.
Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.

What is tobacco mosaic?  Tobacco mosaic is a common viral disease of worldwide distribution that affects over 200 species of herbaceous and, to a lesser extent, woody plants.  Common hosts include tobacco, solanaceous vegetables (e.g., pepper, tomato) and vining vegetables (e.g., cucumber, melon, squash), as well as a wide range of ornamentals (e.g., begonia, coleus, geranium, impatiens, million bells, petunia).  The disease has its biggest impact on vegetables, where it can reduce yield and affect quality to the point that commercial crops cannot be marketed.

What does tobacco mosaic look like?  Symptoms of tobacco mosaic vary in type and severity depending on the plant infected, plant age, the variant of the virus involved, and environmental conditions.  On leaves, typical symptoms include blotchy light and dark areas (called mosaic); cupping, curling, elongation (strapping), roughening, wrinkling and other growth distortions; and smaller than normal size.  Fruits may have a blotchy color, ripen unevenly, be malformed or have an off flavor.  Entire infected plants are often stunted.  Other viral diseases like cucumber mosaic (see UW Plant Disease Facts D0036, Cucumber Mosaic) can cause symptoms similar to tobacco mosaic.  Often, multiple viral diseases can simultaneously affect a single plant.  Certain herbicide exposures (see UW Plant Disease Facts D0060, Herbicide Damage), nutrient deficiencies or toxicities, high temperature and even insect feeding can also cause similar symptoms.  Proper diagnosis of tobacco mosaic requires lab testing.

Where does tobacco mosaic come from?  Tobacco mosaic is caused by Tobacco mosaic virus (TMV), the first virus ever identified.  Numerous variants (strains) of the virus have subsequently been described.  TMV survives in infected plants (including viable seeds), as well as in debris from these plants.  Plant-based products (most notoriously tobacco products) can harbor the virus.  TMV is very stable and can survive for long periods of time; there are reports of TMV surviving and remaining infectious after 50 years in storage at 40°F.  Because of its stability, TMV can survive on and be picked up from hands, clothing, gardening tools, work surfaces and any other object (e.g., door knobs) that gardeners may handle.

TMV is highly transmissible and is commonly spread by handling infected plants, then healthy plants.  Spread via gardening tools is also very common.  No specific insects spread TMV (the way that aphids spread Cucumber mosaic virus).  However, bees and chewing insects (e.g., grasshoppers) can transmit TMV through casual contact or their feeding as they move from plant to plant.

Leaf growth distortions caused by tobacco mosaic.
Leaf growth distortions caused by tobacco mosaic.

How do I save a plant with tobacco mosaic?  There is no cure for tobacco mosaic.  Once infected, plants remain infected for life, and typically the virus spreads throughout the plant from the point of infection.  Infected plants and any associated debris should be burned (where allowed by local ordinance) or double-bagged and disposed of in a landfill.  DO NOT compost plants with this disease.  Thoroughly decontaminate any items that have come into contact with infected plants or their debris by treating them for a minimum of one minute with:

  • 2.75 tablespoons Alconox® (a lab detergent) plus 2.5 tablespoons sodium lauryl sulfate (SLS), also known as sodium dodecyl sulfate (SDS), in one gallon of water, or
  • 14 dry ounces of trisodium phosphate in one gallon of water.

These ingredients can be ordered on the internet.  If you decide to use SLS (SDS), be sure to wear gloves, safety goggles and a dust mask, and mix the solution in a well-ventilated area as SLS (SDS) is a known skin and eye irritant.  Once treated, rinse items with sufficient water to remove any residues.  Also, thoroughly wash your hands with soap and water, and launder any clothing that you wore while disposing of infected plants and debris.

How do I avoid problems with tobacco mosaic virus in the future?  Inspect plants prior to purchase for any symptoms of tobacco mosaic, and DO NOT buy symptomatic plants.  Purchase seed from a reputable supplier that routinely inspects their seed-producing plants for symptoms of viral (and other) diseases.  If you use tobacco products, DO NOT use them around plants.  Also, wash your hands thoroughly with soap and water prior to handling plants, and consider wearing freshly laundered clothing when gardening.  Finally, decontaminate (as described above) any items that might harbor TMV to help prevent spread.  Even if you do not use tobacco products, routine handwashing and decontamination of gardening tools and other items can help prevent tobacco mosaic from being a problem.

For more information on tobacco mosaic:  Contact your county Extension agent.

Sudden Death Syndrome of Soybean

What is sudden death syndrome?  Sudden death syndrome (SDS) is one of the most important diseases of soybean in the Midwest.  The disease was first observed in Arkansas in 1971, and has subsequently been reported throughout most soybean growing areas of the United States.  SDS was first documented in Wisconsin in 2005, and has become more common and severe since that time.  The disease is most severe when soybeans are planted into cool, wet soils, and when midsummer rains saturate the soil.  SDS often occurs in fields where soybean cyst nematode (SCN) is present.

Early symptoms of sudden death syndrome include yellow blotches between veins. (Photo courtesy of Craig Grau)
Early symptoms of sudden death syndrome include yellow blotches between veins. (Photo courtesy of Craig Grau)

What does sudden death syndrome look like?  The first noticeable symptoms of SDS are chlorotic (i.e., yellow) blotches that form between the veins of soybean leaflets.  These blotches expand into large, irregular, chlorotic patches (also between the veins), and this chlorotic tissue later dies and turns brown.  Soon thereafter, entire leaflets will die and shrivel.  In severe cases, leaflets will drop off leaving the petioles attached.  Taproots and below-ground portions of the stems of plants suffering from SDS, when split open, will exhibit a slightly tan to light brown discoloration of the vascular (i.e., water-conducting) tissue.  The pith will remain white or cream-colored.  In plants with advanced foliar symptoms of SDS, small, light blue patches will form on taproots and stems below the soil line.  These patches are spore masses of the fungus that causes the disease.

Foliar symptoms of SDS can be confused with those of brown stem rot (see UW Plant Disease Facts D0026, Brown Stem Rot of Soybean).  However, in the case of brown stem rot (BSR), the pith of affected soybean plants will be brown.  In addition, roots and lower stems of plants suffering from BSR will not have light blue spore masses.

Once symptoms of SDS are evident, yield losses are inevitable.  Yield losses can range from slight to 100%, depending on the soybean variety being grown, the plant growth stage at the time of infection, and whether or not SCN is present in a field.  If SDS occurs after reproductive stages R5 or R6, impact on yield is usually minimal.  If SDS occurs at flowering however, yield losses can be substantial.  When SCN is present, the combined damage from both diseases can be substantially more than the sum of the damage expected from the individual diseases.

Late-stage symptoms of sudden death syndrome include extensive death of tissue between veins and shriveling of leaflets. (Photo courtesy of Craig Grau)
Late-stage symptoms of sudden death syndrome include extensive death of tissue between veins and shriveling of leaflets. (Photo courtesy of Craig Grau)

Where does sudden death syndrome come from?  SDS is caused by the soilborne fungus, Neocosmospora phaseoli (synoym:  Fusarium virguliforme, Fusarium solani f. sp. glycines).  N. phaseoli can overwinter freely in the soil, in crop residue, and in the cysts of SCN.  The fungus infects soybean roots (by some reports as early as one week after crop emergence) and is generally restricted to roots as well as stems near the soil line.  N. phaseoli does not invade leaves, flowers, pods or seeds, but does produce toxins in the roots that move to the leaves, causing SDS’s characteristic foliar symptoms.

How can I save a soybean crop with sudden death syndrome?  SDS cannot be controlled once plants have become infected.  Foliar fungicides and fungicide seed treatments have no effect on the disease.

How can I avoid problems with sudden death syndrome in the future?  Use SDS-resistant varieties whenever possible in fields with a history of the disease.  If SDS and SCN are both problems in the same field, planting an SCN-resistant soybean variety may also be beneficial in managing SDS.  Avoid planting too early.  Wisconsin growers typically prefer to plant soybeans before May 10 to extend the length of the growing season and maximize yields.  However, planting when soils are cool and wet makes plants more vulnerable to infection by N. phaseoli.  Improve soil drainage by using tillage practices that reduce compaction problems.  Rotation, while useful in managing other soybean diseases, does not appear to significantly reduce the severity of SDS.  Even after several years of continuous production of corn, N. phaseoli populations typically are not reduced substantially.  Research from Iowa State University has shown that corn (especially corn kernels) can harbor the SDS pathogen.

For more information on sudden death syndrome of soybean:  Contact your county Extension agent.

 

 

Soybean Vein Necrosis Disease

Yellowing and death of leaf veins, as well as mosaic patterns, are typical symptoms of soybean vein necrosis disease.
Yellowing and death of leaf veins, as well as mosaic patterns, are typical symptoms of soybean vein necrosis disease.

What is soybean vein necrosis disease?  Soybean vein necrosis disease (SVND) is a relatively recent discovery in soybean.  SVND was first described in 2008 in Tennessee, but has since been confirmed in several other states including Arkansas, Delaware, Illinois, Iowa, Kentucky, Maryland, Michigan, Mississippi, Missouri, New York, Pennsylvania and Virginia.  SVND was confirmed in Wisconsin in 2012.  Researchers do not know if SVND can lead to significant yield reductions.

What does soybean vein necrosis disease look like?  Soybean plants with SVND exhibit vein clearing (i.e., lightening of vein color) and chlorosis (i.e., yellowing), as well as mosaic patterns (i.e., blotchy light and dark areas) on affected leaves.  Initially, symptoms develop around the veins of leaves and eventually expand outward.  As the disease progresses, vein and leaf browning and necrosis (i.e., death) occur.

Where does soybean vein necrosis disease come from?  SVND is caused by Soybean vein necrosis virus (SVNV).  SVNV is in the viral genus Tospovirus.  This group of viruses includes common vegetable viruses [e.g., Tomato spotted wilt virus (see UW Plant Disease Facts D0117, Tomato Spotted Wilt of Potato) and Iris yellow spot virus] and ornamental viruses [e.g., Impatiens necrotic spot virus (see UW Plant Disease Facts D0067, Impatiens Necrotic Spot)] that can cause severe damage and substantial loss of yield and crop quality.  Tospoviruses tend to have wide host ranges and are transmitted by several species of thrips.  SVNV is transmitted primarily by soybean thrips (Neohydatothrips variabilis), and to a lesser extent by tobacco thrips (Frankliniella fusca) and Eastern flower thrips (Frankliniella tritici).  SVNV may have been introduced to Wisconsin as thrips moved north on wind currents from the southern United States.

How can I save a soybean crop with soybean vein necrosis disease?  Currently very little is known about SVND.  Thus there are no specific management practices recommended for SVND at this time.

How can I avoid problems with soybean vein necrosis disease in the future?  Currently no specific control recommendations are in place.  Researchers at universities across the country are attempting to determine what impact SVNV will have.  Additional research is needed to determine how SVNV affects soybeans, how it is transmitted, how it overwinters, and what can be done to slow its spread.

For more information on soybean vein necrosis disease:  Contact your county Extension agent.

Soybean Rust

What is soybean rust?  Soybean rust is an extremely serious fungal disease of soybean that was first reported in the continental United States in November of 2004.  The disease has never been reported in Wisconsin.  Soybean rust had previously been reported in Asia, Australia, Africa and South America, where yield losses due to the disease ranged from 10 to 80%.  In addition to soybean (Glycine max), soybean rust affects approximately 90 other plant species in the legume family.  In Wisconsin, other potential hosts include snap and kidney bean (Phaseolus vulgaris), American bird’s-foot trefoil (Lotus unifoliolatus), crimson clover (Trifolium incarnatum), Korean clover (Kummerowia stipulacea), white clover (Trifolium repens), purple crownvetch (Coronilla varia), Chinese lespedeza (Lespedeza cuneata), lupine (Lupinus spp.), pea (Pisum sativum), rattlebox (Crotalaria spp.), yellow sweetclover (Melilotus officinalis), ticktrefoil (Desmodium spp.), and winter vetch (Vicia villosa).

Soybean rust causes small tan to reddish brown leaf spots (left) that first appear on lower leaves of soybean plants. Pimple-like pustules that are filled with tan spores form on the lower surface of infected leaves (right).
Soybean rust causes small tan to reddish brown leaf spots (left) that first appear on lower leaves of soybean plants. Pimple-like pustules that are filled with tan spores form on the lower surface of infected leaves (right).

What does soybean rust look like?  Initial symptoms of soybean rust include formation of small, gray spots on soybean leaves, particularly on the undersides of leaves.  Spots are most likely to occur first on lower leaves where conditions are more favorable for spores to germinate and infect.  Infections can also occur on petioles, stems and pods.  Spots increase in size over time and change color from gray, to tan or reddish-brown.  Tan lesions mature to form small pimple-like structures (called pustules) on the lower leaf surface.  Pustules contain powdery, tan spores that give the leaves the appearance that they have dandruff.  Reddish-brown lesions are composed of primarily necrotic (i.e., dead) tissue and typically have only a limited number of pustules.  As plant canopies close and pods begin to set, the soybean rust fungus can rapidly spread from lower to upper foliage of plants.  Other diseases of soybean including brown spot [see UW Plant Disease Facts, Brown Spot (Septoria Leaf Spot)], bacterial pustule and particularly downy mildew could potentially be confused with soybean rust.

Where does soybean rust come from?  Soybean rust is caused by the fungi Phakopsora pachyrhizi and Phakopsora meibomiaeP. pachyrhizi is the more aggressive of the two species, and the fungus that was introduced into the continental United States in 2004.  P. pachyrhizi is thought to have been brought to the U.S. through hurricane activity in the late summer of 2004.  Soybean rust fungi must overwinter on living plant tissue.  Therefore, if soybean rust fungi ever reach Wisconsin, they are not likely to survive Wisconsin winters.  In the South, however, plants such as kudzu (Pueraria montana var. lobata) can serve as overwintering hosts.  Soybean rust spores produced on these plants could be moved north each year by prevailing winds, as is known to occur with other rust fungi (e.g., the corn rust pathogen).  Soybean rust fungi may eventually reach Wisconsin via this route.  This movement of spores via prevailing winds could occur each year, thus making soybean rust a recurring problem.

How do I save a soybean plants infected with soybean rust?  If you suspect that your soybeans are suffering from soybean rust, proper diagnosis is crucial to document the presence of the disease in Wisconsin.  Contact the UW-Madison Plant Disease Diagnostics Clinic (https://pddc.wisc.edu/) about submitting a sample for diagnosis.  Keep in mind however that once soybean plants are infected and the soybean rust fungus has begun to produce spores, control of the disease is difficult and significant yield losses are likely.  Fungicides with “curative” properties are registered for use against soybean rust in Wisconsin.  However, curative fungicides have a very limited ability to eliminate existing disease and by the time soybean rust is observed, these products will likely not provide adequate control.  Therefore, every attempt should be made to prevent infections (see below), rather than to attempt to control soybean rust after infections have occurred.

How do I prevent problems with soybean rust?  Plant soybeans as early as possible, so that if soybean rust does occur, plants are as mature as possible when infection occurs, and yield loss can be minimized.  Researchers throughout the soybean-producing regions in the United States monitor for soybean rust each growing season (see https://soybean.ipmpipe.org/soybeanrust/).  Watch for reports of the disease to the south of Wisconsin and consider preventative fungicide treatments as the rust fungus approaches the state.  Products containing chlorothalonil, strobilurins and triazoles (the latter two types of active ingredients often combined into a single product) are labeled for preventative control of soybean rust.  Combining strobilurins with triazoles helps reduce the risk of selecting for variants of the soybean rust pathogen that will no longer be controlled by these active ingredients.  If you decide to use fungicides for control, be sure to select a formulation that is labeled for use on soybeans, and be sure to 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 soybean rust or help in diagnosing this disease:  Contact Damon Smith, Department of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI  53706-1598, [phone: (608) 286-9706, fax  (608) 263-3322, email:  damon.smith@wisc.edu]; or contact Brian Hudelson, Plant Disease Diagnostics Clinic, University of Wisconsin-Madison/Extension, 1630 Linden Drive, Madison, WI  53706-1598 [phone: (608) 262-2863, fax: (608) 263-3322, email: pddc@wisc.edu]; or contact your county Extension agent.

Sclerotinia Stem Rot of Soybean

What is Sclerotinia stem rot?  Sclerotinia stem rot (SSR), also known as white mold, is a serious and often lethal fungal disease that affects a wide range of agricultural crops in the United States including many broadleaf vegetable crops (e.g., carrots, cruciferous plants, peas, potatoes, snap beans) and field crops, especially soybean.  SSR is most severe on soybeans in high-yielding environments that have dense, fast-growing canopies.

Cottony white growth of the Sclerotinia stem rot fungus on a soybean plant.
Cottony white growth of the Sclerotinia stem rot fungus on a soybean plant.

What does Sclerotinia stem rot look like?  SSR causes sudden wilting of soybean leaves and rapid plant death.  Lower stems of affected plants become bleached and under moist conditions (e.g., high humidity, frequent rain), become covered with a cottony white fungal growth.  Small, black structures that look like rat or mouse droppings (called sclerotia) form on and inside the stems and pods of affected plants.

Where does Sclerotinia stem rot come from?  Sclerotinia stem rot is caused by the fungus Sclerotinia sclerotiorum, which survives as sclerotia in dead plant tissue or soil.  Sclerotia can survive for five years or more in soil.  A cool, moist environment favors Sclerotinia stem rot development.  Under these conditions, sclerotia germinate to produce small, mushroom-like structures (called apothecia) that produce spores.  These spores can be spread by wind, insects, or rain splash.  In soybeans, most infections occur via open or senescing (i.e., withering) flowers.  Occasionally, the fungus will spread from plant-to-plant via direct contact of roots or other plant parts.

How can I save plants with Sclerotinia stem rot?  SSR is difficult to control once the disease has occurred.  If affected plants are limited to a small area in a field, removal and destruction of plants may help to limit production of sclerotia that can further contaminate and cause long-term problems in the field.  This strategy usually is not feasible on a large scale, however.  If affected plants are removed, they should be burned.  DO NOT compost plants or till them into the soil.

How can I avoid problems with Sclerotinia stem rot in the future?  To prevent introduction of the SSR fungus into soybean fields, be sure to plant sclerotia-free soybean seed.  Also, harvest fields with SSR last to avoid spreading sclerotia of the SSR fungus from field to field on combines.

Sclerotinia stem rot can cause widespread plant death and substantial yield loss.
Sclerotinia stem rot can cause widespread plant death and substantial yield loss.

In fields with a history of SSR, grow soybean cultivars that have been bred for SSR resistance.  This is the most economical and successful long-term strategy for SSR control.  In addition, consider using no-till production for three to four years as this will reduce the number of viable sclerotia near the soil surface.  Rotate soybeans with small grain crops that are not susceptible to SSR (e.g., wheat, barley, oats) to further reduce the number of viable sclerotia in the soil.  Increase row spacing and reduce soybean seeding rates to promote a more open canopy that will have better air circulation and thus dry more rapidly.  Also, make sure fields are well drained and avoid excessive irrigation especially during flowering.  Remember that the SSR fungus prefers wetter conditions; under drier conditions the fungus is less likely to infect.  Maintain good broadleaf weed control.  Weeds not only decrease air circulation and promote wetter conditions, but can also be hosts for the SSR fungus.

Finally, there are fungicides and biological control products available for SSR management.  Fungicides containing an active ingredient that is a succinate dehydrogenase inhibitor (SDHI), such as boscalid, are often effective in SSR control.  The active ingredient picoxystrobin (a type of strobilurin fungicide) has also been shown to be effective in SSR control in university research trials.  Timing of fungicide applications is critical.  Fungicides should be applied during early flowering (R1) to early pod development (R3) growth stages.  Fungicide applications made at the full pod (R4) growth stage or later will NOT be effective.  In addition, applying fungicide treatments after symptoms are visible will not be effective.  Several biocontrol agents (the most effective being one that contains a fungus called Coniothyrium minitans) have been shown to be effective in controlling SSR.  Be sure to read and follow all label instructions of the fungicide/biological control product(s) that you select to ensure that you use the materials in the safest and most effective manner possible.

For more information on Sclerotinia stem rot:  Contact your county Extension agent.

Powdery Mildew of Wheat

What is powdery mildew?  Powdery mildew is a common fungal disease of wheat in Wisconsin.  The disease interferes with photosynthesis, thereby reducing plant growth, heading, and grain fill.  In extreme cases, powdery mildew can result in leaf, and even plant, death.  When weather is favorable and the disease occurs at flag leaf emergence or during heading, yield losses of up to 40% can occur.

Cottony, white growth on wheat leaves is characteristic of powdery mildew. (Photo courtesy of Craig Grau)
Cottony, white growth on wheat leaves is characteristic of powdery mildew. (Photo courtesy of Craig Grau)

What does powdery mildew look like?  Powdery mildew typically appears as white, cottony patches (masses of fungal threads and spores of the causal fungus) on the upper surfaces of leaves.  Patches also can occur on lower leaf surfaces, as well as on stems, seed heads and awns.  Fungal growth is confined primarily to the plant surface, with only limited penetration of the fungus into plant tissue.  As the fungal growth ages, it turns from white to dull gray or light brown.  When fully mature, the fungus forms reproductive structures called chasmothecia, which resemble small black dots or tiny seeds, among the fungal threads.

Where does powdery mildew come from?  Powdery mildew is caused by the fungus Blumeria graminis, which most commonly overwinters as ascospores (a type of spore) inside chasmothecia on wheat residue.  During mild winters or when sufficient snow cover is present to provide good insulation, the fungus also survives on wheat residue as fungal threads or as conidia (a second type of spore).  In the spring, both ascospores and conidia are blown onto actively growing wheat plants where infection occurs followed by development of typical cottony fungal threads.  New conidia that form on infected plants can lead to additional infections throughout the wheat growing season.  Once a wheat crop is harvested, volunteer wheat plants serve as a reservoir for the fungus until the next wheat crop is planted and begins to grow in the fall.  The wheat powdery mildew fungus does not infect other small grains or weed grasses and these plants do not serve as a reservoir for the fungus.  Similarly, fungi that cause powdery mildew on small grains other than wheat and weed grasses are unlikely to infect wheat.  Moist, humid weather with widely fluctuating temperatures favors the development of powdery mildew.  Long periods of excessive rain inhibit powdery mildew development by washing spores from plants before infection can occur.

How can I save wheat plants with powdery mildew?  Careful, routine scouting of a wheat crop throughout the growing season is important to detect powdery mildew as early as possible.  Frequent scouting allows for assessment of the likely impact of the disease on a wheat crop and helps to determine if and when fungicide applications are warranted.  If you scout only once for powdery mildew, be sure to scout just prior to flag leaf emergence.  Yield losses due to powdery mildew are greatest when the disease occurs prior to and at flag leaf emergence.  Therefore, protecting the flag leaf is critical in preserving proper head development and grain fill.  If powdery mildew is present at flag leaf emergence and weather is favorable for further disease development, consider applying a fungicide for control.  While there are a wide variety of fungicides available labeled for control of wheat powdery mildew, products or premixes containing demethylation inhibitor group active ingredients (FRAC 3) have performed particularly well in university research trials.  When using fungicides, be sure to read and follow all label instructions of the product that you select to ensure that you use it in the safest and most effective manner possible.

How can I avoid problems with powdery mildew in the future?  Consider using wheat cultivars with powdery mildew resistance, but keep in mind that the level of powdery mildew resistance can vary widely from cultivar to cultivar.  To reduce the amount of powdery mildew fungus in a field, use tillage practices (where feasible) to bury infested wheat residue, remove volunteer wheat plants, and routinely rotate wheat with other crops (e.g., corn, soybeans).  Powdery mildew tends to be more severe in fields that have excess nutrients (particularly nitrogen).  Therefore, fertilize (especially with nitrogen, potassium, and phosphorus) for optimal plant growth, but DO NOT overfertilize.

For more information on powdery mildew of wheat:  Contact your county Extension agent.

Phytophthora Root and Stem Rot of Soybean

What is Phytophthora root and stem rot?  Phytophthora root and stem rot (PRSR) is a common disease of soybean that can ultimately cause death of soybeans at any stage of development.  The disease can cause stand losses and severe yield reductions in susceptible soybean varieties.  In Wisconsin, PRSR of soybean is becoming increasingly important due to expansion of soybean acreage, increased frequency of planting of soybeans in given fields, and substantial variability in the organism that causes the disease.

Post-emergence damping-off of soybean seedlings due to Phytophthora root and stem rot. (photo courtesy of Craig Grau)
Post-emergence damping-off of soybean seedlings due to Phytophthora root and stem rot. (photo courtesy of Craig Grau)

What does What does Phytophthora root and stem rot look like?  Watch for symptoms of PRSR in fields or areas of fields with poor drainage (e.g., low-lying areas or areas with soil compaction problems).  In addition, watch for the disease in well-drained fields when soils are saturated due to heavy rain or excessive irrigation.  Symptomatic plants often occur in patches.

Symptoms of PRSR can vary depending on the age of affected plants.  Early stages of PRSR can lead to seed rot or death of seedlings prior to emergence (called pre-emergence damping-off).  Once plants emerge, PRSR can lead to yellowing, wilting, and death of seedlings (called post-emergence damping-off).  Infected seedlings can be pulled easily from the ground because of damage to developing roots.  Symptoms of PRSR in older plants (particularly those infected before flowering) include root decay, browning and water-soaking of stems extending six to 12 inches above the soil line, yellowing of leaves, wilting, and eventual death, with leaves on dead plants remaining attached.  Stem lesions of PRSR are typically brown, long, narrow, and sunken.  When infections remain confined primarily in roots, above-ground symptoms may be more subtle, and can include a lighter green color, stunting and uneven growth.  Death due to PRSR tends to occur more rapidly in younger plants than older plants.

Symptoms of PRSR can be similar to symptoms of other soybean diseases, particularly Pythium root rot (PRR) and stem canker.  PRR causes root symptoms similar to those caused by PRSR but typically not the expansive stem lesions seen with PRSR.  Stem canker, like PRSR, causes stem lesions, but stem canker lesions tend to be larger (eventually girdling stems), and become darker brown with age than those caused by PRSR.  Also, older stem canker lesions will have numerous black, pimple-like spots (actually reproductive structures of the fungus that causes the disease).  Such spots will not be present in lesions of plants suffering from PRSR.

Where does Phytophthora root and stem rot come from?  PRSR is caused by the water mold Phytophthora sojae, a soilborne organism that survives via specialized, thick-walled spores called oospores.  Oospores are produced in infected soybean plants, and can survive for many years in the soil after soybean residues decompose.  Oospores germinate when soil moisture is high.  P. sojae tends to be most active when temperatures are between 58 and 77°F, in contrast to Pythium species (the causes of Pythium root rot), which tend to be active over a wider temperature range (50 to 95°F).

How can I save a soybean crop with Phytophthora root and stem rot?  Once soybean plants become infected by P. sojae, there is no cure.  Therefore, management of PRSR relies on preventing infections from occurring.

Brown stem discoloration and plant death (with leaves remaining attached) is typical of Phytophthora root and stem rot. (photo courtesy of Craig Grau)
Brown stem discoloration and plant death (with leaves remaining attached) is typical of Phytophthora root and stem rot. (photo courtesy of Craig Grau)

How can I avoid problems with Phytophthora root and stem rot in the future?  Use PRSR-resistant varieties as a primary means of disease management.  Both race-specific resistance [complete resistance to a specific variant of the pathogen (called a race)] and field resistance (partial resistance to many races) are available in soybean varieties marketed in Wisconsin.  When choosing a race-specific variety, be sure to know which race(s) of the pathogen is/are prevalent in your area and match race-specific resistance genes with the predominant race(s).  The performance of race-specific resistant varieties can change over time.  Therefore, monitor the performance of race-specific resistant varieties very closely, and base future selection of race-specific resistant varieties on the performance (or lack thereof) of recently planted varieties.  Field (partial) resistance to PRSR is present at differing levels in most soybean varieties marketed in Wisconsin.  While field resistance can be useful in managing PRSR, this type of resistance is not particularly effective during early growth stages or under high disease pressure (e.g., when P. sojae levels are high in soil or when soil conditions are excessively wet).  In addition to using resistant varieties, consider using seed treatments containing metalaxyl or mefenoxam.  These active ingredients have been shown to be effective in providing early protection of soybean seeds and seedlings against P. sojae.  Also, improve soil drainage to promote drier soils that are less favorable for P. sojae growth and reproduction.  Crop rotation will not eliminate PRSR or eradicate P. sojae but should be used to prevent the rapid build-up of high levels of the pathogen that can reduce the effectiveness of field resistance.

For more information on Phytophthora root and stem rot of soybean:  Contact your county Extension agent.

Leaf and Glume Blotch of Small Grains

What is leaf and glume blotch?  Leaf and glume blotch is a common disease of wheat, and to a lesser extent barley and rye.  While the impact of the disease is typically relatively minor (usually 5% or less of a wheat crop is affected), under favorable environmental conditions, leaf and glume blotch can destroy upwards of 20% of a wheat crop.

Blotchy brown to purple discolorations on grain heads are characteristic of leaf and glume blotch of wheat. Photo courtesy of Craig Grau.
Blotchy brown to purple discolorations on grain heads are characteristic of leaf and glume blotch of wheat. Photo courtesy of Craig Grau.

What does leaf and glume blotch look like?  Leaf and glume blotch on leaves appears initially as small yellow flecks that enlarge to form brown, lens-shaped lesions, often surrounded by yellow halos.  These regions can merge, leading to large necrotic (i.e., dead) areas on leaves.  The disease can also affect stems and grain heads resulting in smaller brown to purple lesions on tillers and botchy brown to purple areas on glumes (i.e., the leafy, husk tissue that surrounds developing seeds).  As the disease develops, small, dark, pimple-like dots (reproductive structures of the fungus that causes the disease) form in the discolored tissue.  When the humidity is high, gelatinous masses of fungal spores are exuded from these tiny dots giving leaves, stems and grain heads a shiny, wet appearance.

Where does leaf and glume blotch come from?  Leaf and glume blotch is caused by the fungus Parastagonospora nodorum which can survive in wheat debris, as well as in wheat seed.  Spores of the fungus are produced on wheat debris and on infected plants during periods of high humidity and moderate temperatures (optimally around 68°F) and are easily moved within a wheat planting by splashing due to heavy rains.

How can I save plants with leaf and glume blotch?  Once plants are infected with the leaf and glume blotch fungus, curative treatments are not available.  Luckily, under weather conditions typical for small grain production in Wisconsin, damage due to leaf and glume blotch is not severe (at most perhaps 5% of an overall wheat crop in a typical growing season).  However, under weather conditions favorable for leaf and glume blotch to develop, early detection of the disease is important to provide greater flexibility in applying fungicide treatments that can limit disease development (see details below).  In particular, fungicide treatments that protect the flag leaves of wheat plants (i.e., the leaves just under the grain heads) can be important in preventing significant losses due to leaf and glume blotch.

On wheat leaves, leaf and glume blotch leads to brown, lens-shaped lesions with yellow halos. Photo courtesy of Craig Grau.
On wheat leaves, leaf and glume blotch leads to brown, lens-shaped lesions with yellow halos. Photo courtesy of Craig Grau.

How can I avoid problems with leaf and glume blotch in the future?  The best method for managing leaf and glume blotch is through the use of appropriate crop rotation.  Rotate wheat and other small grains with nonsusceptible crops (e.g., soybeans, corn, or vegetable crops) for at least one year.  Where feasible, also consider using tillage practices that partially or fully bury wheat debris, and avoid planting wheat excessively early.  Rotation, proper tillage and delayed planting all provide time for wheat debris to decay, which in turn eliminates the primary source of spores of the leaf and glume blotch fungus.  Also, use wheat seed that was produced in leaf and glume blotch-free fields to avoid introducing the fungus on contaminated seed.

Where leaf and glume blotch has been a chronic problem, use leaf and glume blotch-resistant wheat cultivars and reduce seeding rates to allow better air penetration and more rapid drying of plants.  Avoid overuse of nitrogen fertilizers as this will promote excessive leaf growth that will slow leaf drying.  Finally, scout wheat fields routinely, and consider using preventative fungicide treatments at the emerging flag leaf stage of development (Feekes 8).  Many strobilurin fungicides (FRAC group 11) and demethylation inhibitor fungicides (FRAC group 3), as well as mixes of active ingredients with these modes of action, provide very good control of leaf and glume blotch.  Use fungicides containing a strobilurin only prior to heading; avoid using these products after wheat has flowered.  Demythylation inhibitor fungicides can be applied both before and after wheat has flowered.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the material(s) in the safest and most effective manner possible.

For more information on leaf and glume blotch:  Contact your county Extension agent.