Continue Checking For Wheat Diseases

May 29, 2014

It is difficult to summarize the current disease picture for winter wheat in Missouri unless to say that it is as variable as the weather has been.  There is also a range in stage of growth although across the state wheat development is behind normal.  The May 12 “Crop Progress and Condition” report from the Missouri Field Office of the National Agricultural Statistics Service reported 34 percent of the wheat crop headed which is about the same as last year but significantly lower than the 5-year average of 56% headed by May 12.  With the majority of the crop still to head and flower, it is important to continue to scout wheat for foliage diseases and to consider the risk of the development of Fusarium head blight.

So what is the current disease situation on winter wheat in Missouri?

Foliage diseases:  Have not been a significant issue yet this season.  Much of the state has been cooler than normal with short periods of warm weather and the early part of the season was fairly dry.  States to the south, in particular Texas, Oklahoma and Kansas, have been extremely dry so foliage diseases on wheat have not been a problem.  No or low levels of diseases such as leaf rust and stripe rust in those states means little inoculum to be blown into Missouri thus lowering the potential for those diseases to develop here.

Powdery mildew has been reported in one or two isolated fields but has not been as prevalent as is was in 2013.  Septoria is beginning to show up but is coming in late enough in the season that impact on yield should be minimal.  Neither leaf rust nor stripe rust have been reported on wheat in Missouri thus far.  Although the current period of wet weather could favor the development of foliage diseases, the cool temperatures associated with this current rainy period slow down their development.  The later in the season foliage diseases become established and the later they begin to increase in severity, the less yield loss is likely to occur.  Descriptions of the common wheat foliage diseases are given below.

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.

Septoria leaf blotch

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.

Leaf Rust of Wheat

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.

Stripe Rust of Wheat

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.

Leaf rust and Septoria leaf blotch on same wheat leaf

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. 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.  Table 1 accompanies this alert.

Table 1.

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. But also scout fields for stage of growth.  The unusually cool temperatures during March and April have resulted in a late wheat crop.  Most parts of the state are reporting wheat 14-17 days behind 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 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 cool temperatures, wheat may just be approaching these growth stages.  If yield potential is good and weather conditions continue to be conducive for disease development, foliar fungicide applications may be warranted.

Fusarium head blight or Scab:  It is later in the year than normal to be thinking about Fusarium head blight or scab but with the unusually late development of the wheat crop, the wheat in the southern part of the state may be in or just past 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.  If fungicide applications for Fusarium head blight management are being considered the stage of growth needs to be monitored carefully. If the frequent rains continue throughout the state conditions could be conducive for the development of Fusarium head blight in fields in which the crop is beginning to flower or is flowering.

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. 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 alert 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.

Other Head Diseases of Winter Wheat

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.

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