Diagnosing Leaf Diseases in Wheat

While many parts of the wheat growing area last year struggled with drought, other areas were fraught with conditions ideal for leaf diseases. Incidence and severity varied, as some disease symptoms were found across sites, whereas others were restricted to one or two sites. 

Many small grain diseases are favored by wet weather, including tan spot, Septoria leaf and glume blotch, scab and net blotch. However, some small grain diseases—such as common root rot, wheat streak mosaic virus, and barley yellow dwarf virus—can even result in losses in yield and quality when both soil moisture and humidity are low.  More information about these diseases provided by Dr. Marcia McMullen at NDSU, can be found online at www.ag.ndsu.nodak.edu/drought/ds-01-02.htm .

Following are diseases that generally prefer more humid conditions to develop, and were identified consistently from several locations in the Red River Valley area last year.

Leaf Rust
Rust spores produced in southern states are carried by northerly winds to our area. 

The fungus that causes leaf rust of wheat (Puccinia triticina) needs warm days and cool nights (60-80oF) with six to eight hours of leaf moisture. Plant leaves in the lower canopy are generally the first to become infected since they stay wet longer.  Under the right environmental conditions, masses of spores that are capable of spreading disease can be produced in about one week after plants become infected.

Plant symptoms differ depending on their disease resistance levels.  Bright orange to reddish spots, called pustules, tear leaf tissues as spores multiply within the leaves of plants and push outward.  Moderately resistant plants have smaller erupting pustules, than those that are susceptible, and have a narrow band of yellow plant tissues, called a halo, surrounding the pustule. Resistant plants can be identified by small off-white to yellow spots, called flecks, on the leaf where infections have been stopped.

Yield losses from susceptible varieties can occur if infection takes place before flowering.  Disease development is greatest when spores are plentiful and weather conditions are optimum for the fungus. Losses from leaf rust of wheat in North Dakota during 2002 were estimated at 8 to 9 million bushels. Leaf rust pressure seemed to be higher last season on later planted, susceptible wheat varieties such as Gunner, Ingot, Russ and Walworth.

Timely disease management is important since spore populations can build quickly. Fields should be scouted at regular intervals so timely application of fungicides can be made to protect the crop. The USDA Cereal Disease Lab in St. Paul monitors and reports rust development in the southern and central Plains, which in turn can help us keep on top of rust potential in our region.  You can find the CDL’s rust bulletins online at www.cdl.umn.edu .  Click on the link “Cereal Rust Bulletins.”

We will also be working with the Minnesota Department of Agriculture this year to conduct and disseminate information from surveys of leaf rust disease incidence and severity on wheat and barley, and FHB on wheat. 

Tan Spot
Tan spot is a common disease of wheat in the Red River Valley. The fungus that causes tan spot (Pyrenophora tritici-repentis) overwinters on aboveground crop stubble, and infects plants and causes disease over a wide range of temperatures as long as adequate moisture is present on leaves for an extended period (12-24 hours).

Tan spot and Septoria/Stagonospora leaf spotting diseases produce similar leaf symptoms. Tan spot produces oval-shaped, tan lesions that enlarge with age.  Lesions are surrounded by a yellow halo, while dark areas eventually form on tissues that were infected initially. Severe disease development causes spots to expand until most, if not all, plant leaf tissues are killed.  Yield and kernel weights are reduced if the seed-filling ability of diseased plants is limited early enough.

Yearly disease surveys in North Dakota list tan spot as the most common leaf disease since 1998. Wet years tend to favor tan spot epidemics, with the result often being yield loss. In fact, planting wheat into fields with infected small grain stubble is especially risky, since it increases the likelihood for seedling infection early in the growing season.

Management options are similar to those with Fusarium head blight, since both pathogens survive on aboveground crop stubble.  Crop rotation and residue management can prevent most early-season disease development.  However, as the growing season progresses, spores are produced locally on diseased, non-cropped plants, and spores produced on infected stubble can travel long distances with the help of gusty winds.

Even if growers routinely practice all known management options, it’s likely they could still find tan spot lesions on wheat plants.  If the disease is established prior to flag leaf emergence and extended wet weather is forecast, fungicide application provides a viable control alternative. Some spring wheat varieties have differing levels of resistance to the disease.

Septoria/Stagonospora Leaf Spot Complex
Three fungi are involved in this leaf spot complex (Stagonospora nodorum, Stagonospora avenae f. sp. triticea, and Septoria tritici). The disease can be spread by (1) fungi overwintering on crop stubble, (2) infected seed, and (3) volunteer wheat plants.  The pathogens can survive on crop stubble above the ground for three years and in, or on, wheat seed for at least one year.

Septoria/Stagonospora leaf spot can occur across a range of temperatures (50-81oF). Like leaf rust and tan spot, a period of leaf wetness is essential for plant infection, disease development and spread. Plant leaves must remain moist for at least six hours for infection to occur.  If newly infected plants remain wet, more spores will be produced in 10 to 20 days.  Wind and rain disperse spores to other leaves and plants. If it turns dry, not only do infections end, disease development is also stopped dead in its tracks.

Septoria/Stagonospora leaf spot diseases look much like tan spot early on. Young lesions appear oval-shaped and have yellow halos.  As lesions mature, either tiny black fungal fruiting bodies (pycnidia) form or lesions develop grey centers as light-colored spores are produced. Specific plant symptoms are dependent on which fungi are responsible for causing the disease.

Rotating out of small grains for two years reduces residue-borne inoculum of Septoria/Stagonospora leaf spot complex. In addition, volunteer wheat, rye, barley and wild grasses should be destroyed before rotating the field back into wheat. Spring wheat varieties have different resistance levels for the disease. Varieties with moderate resistance to one or more of the fungi should be used if there is a history of the problem in the field.  Fungicides may be necessary if wet weather is forecast and the disease is present.

Bacterial Stripe
Bacterial stripe, also known as Black chaff, is a periodic disease of wheat. Like many diseases, it’s extremely dependent on weather conditions for disease development. A primary means of spread is through infected wheat seed (bacteria have been found in the seed coat).  Bacteria are also suspected to overwinter on above-ground crop stubble in the field.

The bacterium that causes bacterial stripe (Xanthomonas t. pv. translucens) needs wet field conditions for plant infection and disease development. It’s less dependent on specific temperatures than on extended leaf wetness and high humidity. Even so, bacteria have been found to multiply much faster at warm rather than cool temperatures. The disease is spread by raindrop splash.  Plant-to-plant contact during windy periods or during field cultivation activities also spreads the disease.

All aboveground portions of a plant can be attacked, from the first leaf all the way up to the head.  Plant leaf symptoms appear initially as thin, light brown to golden stripes that appear water-soaked.  The bacteria follow plant veins, eventually expanding lengthwise from sheath to tip.

If the weather remains wet, leaf stripes multiply and entire leaves can be killed.  Sometimes, before dew dries in the morning, you can see slimy, thick, creamy-yellow colored droplets at wounds. Droplets contain masses of bacteria that can be spread to healthy plants via rain, wind, insects, tractors, etc. Infection of heads produces black streaks on glumes (Black chaff), and can result in a range of symptoms from complete head sterility to kernel contamination with bacteria.  Symptom severity depends on when the infection takes place.

Bacterial stripe was commonly noted across wheat fields in Minnesota last year.  Plants remained wet for long periods (especially in flooded areas), so there were ample opportunities for the disease to develop and spread.  Last year’s yield losses are unknown, but entire flag leaves were often killed during grain fill. Generally, if 50% of the flag leaf is killed, it results in a 20% average reduction in yield.  Yield losses have been documented up to 40% in other states following bacterial stripe epidemics.  But, yield reductions are more commonly 10% or less.

Management strategies have been difficult to evaluate since bacterial stripe is a periodic problem.  Hard, cold data haven’t been collected to determine the effectiveness of rotation, but it’s safe to say that it wouldn’t hurt. Tillage management would also be beneficial, since bacterial populations are known to crash when infected stubble is incorporated into the soil.  Infected seed provide the primary means of inoculum. If the disease was a problem in your field last year and you saved wheat for seed, it’ll be contaminated.  Spraying the crop with fungicides won’t help; the chemistry isn’t effective against bacteria. 

Given a field of susceptible host plants and optimum environmental conditions, plant diseases have the potential to wreak havoc.  On the other hand, if conditions don’t support disease development, the pathogen can be considered as merely another pest problem that’s lurking out there for the perfect opportunity to cause problems. A good tool to help scout for the formation and outlook of leaf diseases this season is the NDSU disease forecasting system, online at www.ag. ndsu.nodak.edu/cropdisease/cropdisease.htm.

—Charla Hollingsworth, extension plant pathologist, University of Minnesota, Crookston

Small Grain Fungicide Use Decision Guide Online
SDSU extension plant pathologist Marty Draper offers a Small Grain Fungicide Decision Guide designed to help the producer or applicator determine if a foliar fungicide applied to the crop will return an economic benefit.

The decision guide is in the form of a Microsoft Excel ver. 4.0 spreadsheet, which is used by inserting the values appropriate for your operation. The spreadsheet will make all the necessary calculations. Examples are presented and a guide to possible values is included.

Generally, says Draper, a yield increase of 10-15% will be attained from a fungicide application. The cost of the fungicide product, application costs, and the market price for the commodity are the greatest variables that will influence profitability of fungicide application.

The fungicide decision guide can be downloaded from the web site plantsci.sdstate.edu/planthealth/Scab/fungguid.htm or contact Draper, ph 605. 688.5157, email draper.marty@CES.sdstate.edu

White Wheat Heads May  Stem From Root Rot
White heads in wheat at the milk to soft dough stage were apparent last year in some wheat growing areas. Some instances were due to wheat stem maggot injury, others to root rot infections. When root rot causes white heads, the whole plant generally turns whitish to pale grey-green and the plant easily pulls from the soil. Roots and crowns often are discolored brown on these symptomatic plants, and the crown area is brown and “cheesy,” instead of healthy crowns which are cream to white colored with the cell structure intact. Incidence of white heads due to root rot or stem maggot in most fields was under 1% and generally was worse on the headlands. An NDSU bulletin online has more information on common root rot: Go to www.ext.nodak.edu/extpubs/plntdise.htm then click on the link, “Root and Crown Rots of Small Grains.”