The 2012 growing season has already been an "interesting" one for winter wheat in Missouri. The record mild winter allowed for early infestations of the aphids that may transmit barley yellow dwarf virus and the foliage disease Septoria leaf blotch. Sporulating lesions of Septoria were found on wheat leaves in southwest Missouri as early as February 6, 2012. Powdery mildew was also an early season issue on winter wheat in southwest Missouri. Eastern Arkansas has been reporting stripe rust outbreaks and stripe rust has been confirmed in southeast Missouri fields and fields in St. Charles, County, MO. Most recently there have been calls and questions from many areas of Missouri about purpling of flag leaves possibly from the barley yellow dwarf virus.
So what is the current disease situation on winter wheat in Missouri?
Foliage Diseases: Although Septoria was actively sporulating on wheat foliage very early in the season, the disease does not appear to have moved up the plants. Most lesions which are present are on the lowest leaves and the flag leaf and 2-3 leaves below the flag leaf are clean. Powdery mildew seems to have been controlled by fungicide application or slowed down by the change in weather conditions from March to April. Stripe rust hasn't been reported outside of the areas in which it was initially reported. There haven't been any confirmed reports of leaf rust yet this season.
If foliar fungicide applications are being considered it is important to scout fields first. Look for the presence of foliage diseases which might be impacting yield and could be controlled with a fungicide application. Descriptions of the common foliage diseases on winter wheat in Missouri are given below. But also scout fields for stage of growth. The unusually warm temperatures during January, February and March have resulted in a very early wheat crop. Most parts of the state are reporting wheat 25-28 days ahead of average as far as stage of growth. Many of the wheat foliar fungicides are applied at flag leaf emergence, heading or until the beginning of flowering. Most of the these fungicides have harvest restrictions of Feekes growth stage 10.5 (head completely emerged) or 30, 35 or 40 days prior to harvest. Due to the warm temperatures, wheat may be passed the time when it can be sprayed.
Lesions of Septoria leaf blotch begin as light yellow flecks or streaks. These flecks expand into yellow to reddish-brown, irregularly shaped blotches. Dark brown specks (fruiting bodies or pycnida of the causal fungus, Septoria tritici) may be scattered within the centers of mature lesions. Lesions may coalesce killing larger areas of leaf tissue.
Stagonospora glume blotch (formerly called Septoria glume blotch) may also begin as light yellow flecks or streaks on leaves. The lesions also turn yellow to reddish-brown but usually have a more oval to lens shaped appearance than those of Septoria leaf blotch. Again, the dark brown specks or fungal fruiting bodies of the causal fungus Stagonospora nodorum may be evident within the lesions. Symptoms of Stagonospora glume blotch are more common on heads than foliage of wheat. Infected heads will have dark blotches on the glumes.
The initial symptoms of tan spot are small tan to brown flecks on the leaves. These expand into tan to light brown, elliptical lesions which often have yellow borders. The centers of mature tan spot lesions may have a dark brown region caused by outgrowth of the fungus. But the fungus which causes tan spot, Pyrenophora tritici-repentis, does not produce pycnidia or fruiting bodies as the Septoria fungus does. So mature tan spot lesions do not have the distinct dark brown specks scattered throughout the centers of the lesions as do Septoria leaf blotch lesions. Although tan spot can occur in Missouri, it is not usually a problem in the state.
Leaf rust lesions appear primarily on the upper leaf surfaces and leaf sheaths. Initially, lesions are small, yellow to light-green flecks. Eventually, leaf rust appears as small, circular to oval shaped, orange-red pustules. These pustules break open to release masses of orange-red spores of Puccinia recondita. The edges of the open pustules tend to be smooth without the tattered appearance of stem rust pustules. Heavily rusted leaves may yellow and die prematurely.
Stripe rust, caused by the fungus Puccinia striiformis, has become more prevalent in Missouri over the last few years. Stripe rust may develop earlier in the season than leaf rust or stem rust. The pustules of stripe rust are yellow or yellowish-red and occur in obvious stripes or streaks running lengthwise on the wheat leaves. This disease is more commonly associated with cooler temperatures, especially cooler night temperatures.
Stem rust, caused by the fungus Puccinia graminis f. sp. tritici, is most common on stems and leaf sheaths of wheat plants but may develop on any of the above ground portions of the plant including both upper and lower leaf surfaces and glumes and awns. Stem rust pustules are small, oval, and reddish-brown. The ruptured pustules tend to have more ragged edges than leaf rust pustules. Frequently both leaf rust and stem rust occur on the same plant and both types of pustules may develop on an individual leaf.
Powdery mildew infections begin as light-green to yellow flecks on the leaf surface. As powdery mildew develops the leaf surfaces become covered with patches of cottony white mold growth of Erysiphe graminis f. sp. tritici, the causal fungus. These patches eventually turn a grayish-white to grayish-brown in color and small black fungal fruiting bodies may be visible within the patches of mildew growth.
The fungi which cause most of these wheat foliage diseases survive in infested wheat residues left on the soil surface. The next growing season spores are produced during moist periods and are carried by wind currents to susceptible wheat leaves where infection may begin. Disease problems tend to be more severe when wheat is planted in fields with infested wheat residue left on the soil surface. Eventually spores that are produced in the initial lesions on plants are wind blown to other leaves or other plants causing secondary infection.
Leaf rust, stem rust and stripe rust are exceptions to this simplified explanation of disease development. The rust fungi do not survive in infested residue left in a field. Rather, the rust fungi are reintroduced into this area each season when spores are carried up on air currents from the southern United States.
Most of the foliage diseases of wheat are favored by warm, wet or humid weather. Frequently infection begins on the lower portion of the plant. If weather conditions are favorable for disease development, the disease may move up through the plant. Severely infected leaves may yellow and die prematurely. Yield losses tend to be highest when the flag leaves are heavily infected.
There are several fungicides that are labeled for use on wheat to control fungal foliage diseases. It is important to scout wheat fields and determine which leaf diseases are occurring as well as the level of their severity before making a decision to apply a foliar fungicide. In particular be on the lookout for Septoria leaf blotch, Stagonospora glume blotch, leaf rust and stripe rust. When scouting fields, try to identify the disease or diseases which are present, determine the average percent of infection on a leaf and the number of leaves showing infection and determine the stage of growth of the crop. Generally, the profitable use of foliar fungicides on wheat depends on a number of factors including varietal resistance, disease severity, effectiveness of the specific fungicides and timing of fungicide application. The greatest increases in yield are usually obtained when fungicides are applied to disease susceptible varieties with high yield potential at the early boot to head emergence growth stage when the flag leaf is in danger of severe infection. Fungicide applications are seldom beneficial if applied after flowering or after the flag leaf is already severely infected. It is also important to read the fungicide label for specific information on rates, recommended timing of application, frequency of applications, preharvest intervals and grazing restrictions.
A management program for foliage diseases of wheat should include the following steps.
The North Central Regional Committee on Management of Small Grain Diseases (NCERA-184) developed a table containing information on fungicide efficacy for control of certain foliar diseases of wheat. These efficacy ratings were determined by field testing the materials over multiple years and locations by members of the committee. This table is included in this issue of the cropPest newsletter.
Fusarium head blight or Scab: It seems early in the year to be thinking about Fusarium head blight or scab but with the unusually early development of the wheat crop, the wheat in the southern part of the state may be in a susceptible stage of growth and wheat in the remainder of the state could be in the susceptible stage of growth, i.e. flowering, in the next week or two. Again this would be about 24-28 days ahead of average. This may be delayed some by the cooler temperatures in April but flowering is likely to be ahead of normal and if fungicide applications for Fusarium head blight management are being considered the stage of growth needs to be monitored carefully. The rain throughout most of the state this past weekend could be conducive to the development of Fusarium head blight in fields in which the crop is beginning to flower or is flowering. The forecast for the coming week doesn't show significant chances for precipitation so fields flowering this week might escape infection. Information on Fusarium head blight symptoms are given below.
Fusarium head blight or scab of wheat develops on plants in the flowering to early grain fill stages of growth. Infection is very dependent on environmental conditions while wheat is in susceptible stages of growth, i.e. flowering. Moderate temperatures in the range of 77-86°F, frequent rain, overcast days, high humidity and prolonged dews favor infection and development of scab. Weather conditions over the next several weeks will determine the extent and severity of scab in this year's wheat crop. Fusarium head blight or scab problems will be more severe if rains coincide with flowering of wheat fields. After a warm, dry March, many parts of the state have been cooler and wetter in April. If the rain continues as the crop moves through the flowering stages, the risk for scab will increase.
The characteristic symptom of scab on wheat is a premature bleaching of a portion of the head or the entire head. Superficial mold growth, usually pink or orange in color, may be evident at the base of the diseased spikelets. Bleached spikelets are usually sterile or contain shriveled and or discolored seed.
Scab is caused by the fungus Fusarium graminearum. This fungus overwinters on host residues such as wheat stubble, corn stalks and grass residues. Spores are carried by wind currents from the residues on which they have survived to wheat heads. If environmental conditions are favorable, i.e. warm and moist, the spores germinate and invade flower parts, glumes and other portions of the spike. Scab infection occurs when favorable environmental conditions occur as the wheat crop is in the flowering to early grain fill stages.
Unfortunately, the detrimental effects of scab are not limited to its adverse effects on yield. The fungi which cause scab may also produce mycotoxins. Vomitoxin (deoxynivalenol or DON) and zearalenone may occur in wheat grain infected by scab fungi. This is a primary concern where grain is fed to non-ruminant animals. Ruminants are fairly tolerant of these two mycotoxins. Also, the fungi which cause scab may survive on the seed and can cause seedling blight and root rot problems when scabby grain is used for seed.
Crop rotation, variety selection and residue management are preventative measures for managing scab in wheat. At this point in the season the only remaining management option would the application of a fungicide to try to reduce scab levels. The fungicide table in this issue of the Integrated Pest & Crop Management Newsletter lists the fungicides labeled for the suppression of Fusarium head blight or scab. Growers should be scouting fields to get a feel for incidence and severity of scab in this year's wheat crop. Because of possible mycotoxin concerns and seed quality concerns, grain from fields with scab may require special handling. Wheat planted on corn, sorghum or wheat residue (even wheat double cropped with soybeans) has a greater risk for scab.
From flowering through the early stages of grain fill is also the time to scout for other head diseases of wheat such as loose smut, Septoria and Stagnospora infections on heads, bacterial stripe and black chaff on heads and take-all.
Loose smut is obvious as heads emerge from the boot and for several weeks after that. The kernels on infected heads are replaced with masses of powdery black spores. So the heads have a very distinct black, powdery appearance. These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left. Spores produced on smutted heads are wind carried to adjacent plants in the field and infect through the flowers. The fungus that causes loose smut survives within the embryo of wheat seeds. If infected seed is planted, the plants growing from those seeds will be infected and develop smutted heads the next season. If seed from a field that has a "small" amount of smut in one season is used for seed, the field planted with that seed may have a substantially higher level of smut. Loose smut is best controlled by planting either disease-free seed or using a systemic fungicide seed treatment.
Septoria leaf blotch is present in the lower canopy of many fields this year. It hasn't seemed to move up in the canopy to the flag leaf or head but with increased precipitaiton and high humidity it could still develop on flag leaves and heads. On the heads dark brown to black blotches may develop. Stagnospora nodorum may also cause leaf lesions but is usually more common on heads- again causing dark blotches on glumes of part or all of the head.
Bacterial stripe or black chaff is a bacterial disease that produces symptoms on both leaves and heads. Water-soaked lesions may develop on young leaves. These expand into reddish-brown to brownish-black streaks on the leaves. Glumes and awns show brown-black blotches or streaks. Fungicides are not effective against bacterial stripe or black chaff so the use of resistant or tolerant varieties and crop rotation are the main management options.
Take-all is one of the more common root and crown rot diseases of wheat in Missouri. The fungus which causes this disease may infect seedlings in the fall. Symptoms are usually most evident after heading as white heads on the wheat plants. Entire heads on infected plants may be bleached (white heads) and sterile. Infected plants are also stunted and slightly yellow, have few tillers and ripen prematurely. Plants with take-all typically have poorly developed root systems and roots are sparse, blackened and brittle. With sufficient soil moisture, a black-brown dry rot may extend into the crown and up the lower stem. This shiny, black discoloration of the lower stem and crown may be seen if the lowest leaf sheath is scraped off with a knife or fingernail. A management program for take-all should including planting good quality seed of adapted, disease resistant varieties, planting in well-drained sites under good seed bed conditions, rotating with nonhost crops for one to three years, controlling weed-grass hosts and volunteer wheat, using seed treatment fungicides and maintaining good plant vigor with adequate fertility.
Virus Diseases of Wheat: Although aphid numbers were extremely high in some fields in southwest Missouri earlier this spring, the symptoms of barley yellow dwarf have been slow to develop. Over the last week to ten days, calls related to purpling of the flag leaf have been received. Several samples from eastern and central Missouri have been tested for the four most common virus diseases of winter wheat in Missouri (descriptions follow) and, thus far, the only positives have been for barley yellow dwarf. At this point in the season and with much wheat in the boot stage or beyond, there are no management recommendations for barley yellow dwarf or other wheat virus diseases for this year's crop.
Descriptions of the wheat virus diseases most likely to occur on winter wheat in Missouri are given in the following paragraphs.
Symptoms of wheat spindle streak mosaic appear in early spring as yellow-green streaks or dashes on the dark green background of the leaves. These lesions usually run parallel to the leaf veins and tend to be tapered at the ends giving the lesions a spindle shaped appearance. Foliage symptoms are most obvious when air temperatures are about 50°F. As temperatures warm-up, foliage symptoms of wheat spindle streak mosaic tend to fade. Plants may be slightly stunted and have fewer tillers than normal. Wheat spindle streak mosaic tends to be more prevalent in lower, wetter areas of a field. The virus which causes this disease is soilborne and is spread by the soil fungus Polymyxa graminis. Wet falls tend to favor outbreaks of wheat spindle streak mosaic the following spring.
Wheat soilborne mosaic causes light green to yellow green to bright yellow mosaic patterns in leaf tissues. Symptoms are most evident on early spring growth, and warmer temperatures later in the season slow disease development. Symptoms of wheat soilborne mosaic are not always particularly distinctive and might occur as a more general yellowing similar to that caused by nitrogen deficiency. Infected plants may be stunted. This disease may be more severe in low lying, wet areas of a field. The soilborne wheat mosaic virus survives in the soil and is spread by the soil fungus Polymyxa graminis. Again, wet falls tend to favor outbreaks of wheat soilborne mosaic the following spring.
Barley yellow dwarf is an extremely widespread virus disease of cereals. Symptoms include leaf discoloration ranging from a light green or yellowing of leaf tissue to a red or purple discoloration of leaf tissue. Discoloration tends to be from the leaf tip down and the leaf margin in towards the center of the leaf. Plants may be stunted or may have a rigid, upright growth form. Symptoms are most pronounced when temperatures are in the range of 50-65°F. The barley yellow dwarf virus persists in small grains, corn and perennial and annual weed grasses. More than twenty species of aphids can transmit the barley yellow dwarf virus. Symptoms may be more severe and yield losses higher if plants are infected in the fall or early in the spring. Infections developing in late spring or summer may cause discoloration of upper leaves but little stunting of plants or yield loss.
The other virus disease likely to occur on winter wheat in Missouri is wheat streak mosaic, but symptoms of this disease are not usually evident until later in the season when air temperatures increase. Wheat streak mosaic causes a light green to yellow green mottling and streaking of leaves. Symptoms may vary with variety, virus strain, stage of wheat growth when plants are infected and environmental conditions. Plants may be stunted. As temperatures increase later in the spring, yellowing of leaf tissue and stunting of plants may become more obvious. The wheat streak mosaic virus is spread by the wheat curl mite. Symptoms are frequently found along the edges of fields where the mite vector first entered the field. Both the wheat streak mosaic virus and the wheat curl mite survive in susceptible crop and weed hosts. Thus, the destruction of volunteer wheat and weed control are important management options for wheat streak mosaic.
A management program for virus diseases of wheat should include the following steps.
|Efficacy of fungicides for wheat disease control based on appropriate application timing|
|Septoria leaf blotch||Tan spot||Stripe rust||Leaf rust||Stem rust||Head scab||Harvest Restriction|
|Strobilurin||Fluoxastrobin 40.3%||Evito 480 SC||2.0 – 4.0||G||--³||--³||VG||--³||VG||--³||NL||40 days|
|Pyraclostrobin 23.6%||Headline SC||6.0 - 9.0||G||VG||VG||E||E2||E||G||NL||Feekes 10.5|
|Triazole||Metconazole 8.6%||Caramba 0.75 SL||10.0 - 17.0||VG||VG||--³||VG||E||E||E||G||30 days|
|Propiconazole 41.8%||Tilt 3.6 EC4||4.0||VG||VG||VG||VG||VG||VG||VG||P||Feekes 10.5|
|Prothioconazole 41%||Proline 480 SC||5.0 - 5.7||--³||VG||VG||VG||--³||VG||VG||G||30 days|
|Tebuconazole 38.7%||Folicur 3.6 F4||4.0||G||VG||VG||VG||E||E||E||F||30 days|
|Prosaro 421 SC||6.5 - 8.2||G||VG||VG||VG||E||E||E||G||30 days|
|Mixed mode of action||Metconazole 7.4%
|TwinLine 1.75 EC||7.0 – 9.0||G||VG||VG||E||E||E||VG||NL||Feekes 10.5 &
|Quilt 200 SC||14.0||VG||VG||VG||VG||E||E||VG||NL||Feekes 10.5|
|Quilt Xcel 2.2 SE||10.5 - 14.0||VG||VG||VG||VG||E||E||VG||NL||Feekes 10.5|
|Stratego 250 EC||10.0||G||VG||VG||VG||VG||VG||VG||NL||35 days|
|Stratego YLD||4.0||G||VG||VG||VG||VG||E||VG||NL||35 days|
|Absolute 500 SC||5.0||G||VG||VG||VG||VG||E||VG||NL||35 days|
¹ Efficacy categories: NL=Not Labeled and Not Recommended; P=Poor; F=Fair; G=Good; VG=Very Good; E=Excellent.
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REVISED: April 19, 2012