ID-125
In a 1992 survey, Kentucky wheat producers ranked disease as one of the top three factors limiting wheat yields on their farms. Disease management, in fact, is a key component of high-yielding wheat. Some diseases, such as take-all disease, must be managed proactively and cannot be affected once they are established. Other diseases, such as foliar diseases caused by fungi, can be managed by the timely application of foliar fungicides. Generally, Kentucky producers place too much emphasis on disease control using foliar fungicides only. Most diseases are best managed through the use of multiple tactics, both proactive (e.g., crop rotation, delayed planting, resistant varieties, proper fertility, seed treatment fungicides) and reactive (application of foliar fungicides).
Scouting for diseases is important for two reasons. Yearly scouting helps you build an on-farm database that can be used to select appropriate disease management tactics for future crops. Scouting also helps you determine if and when to spray fungicides.
Once you have properly scouted your fields, use the data to determine your disease control options. For help with this, contact your local Extension office or crop consultant for the most recent recommendations. Begin a course of action only when you are fully armed with up-to-date, accurate information.
Most Kentucky wheat producers have their total disease management program in place once the seed is in the ground. By that time, decisions have been made relative to crop rotation, tillage/seedbed preparation, variety selection, seed quality, seed treatment, planting date, seeding method, seeding rate, and fall fertility. Individually and collectively, these decisions can play an important role in influencing which diseases develop, their severity, and their effect on crop yield and test weight. Because preplant and planting decisions are so important in the management of wheat diseases, you need to understand how they affect disease.
Decisions relating to variety selection are, perhaps, the most important decisions you can make in managing diseases. Every commercially available wheat variety has a unique range of reactions to diseases common in Kentucky. Which and how many varieties are planted determines the potential for certain diseases. Failure to consider the ramifications of variety selection in managing diseases is a costly mistake made by many wheat producers. Select two or three varieties with the greatest amount of available resistance to the diseases most common on your farm and in your community. To do this, you must have some idea about the disease history on your farm. If you don't have this information, talk with your county Extension agent, farm supply dealer, and/or neighbors. This information won't be as good as actual data from your farm, but it is far better than basing decisions on no information. It is important to plant more than one variety for this key reason: it is common for a single disease to severely damage a single variety. When multiple varieties are planted, your risk level is reduced. Planting more than one variety, especially when different maturities are represented, also can help with the logistics of harvesting and soybean planting.
Crop rotation helps in the management of wheat pathogens that survive between wheat crops in wheat residue. When a crop other than wheat is grown in a field, levels of wheat pathogens decline. This occurs simultaneously as the residue of previous wheat crops deteriorates. Reduced levels of pathogens can translate into reduced disease pressure the next time wheat is produced. Rotation is helpful in the management of hidden diseases, such as Pythium root rot, and destructive diseases, such as take-all. In fact, rotation of fields out of wheat or barley is the only practical means of controlling take-all. Rotation also can reduce infections by the fungi Stagonospora and Septoria. However, the effect of rotation on these diseases can be negated by spores blowing into fields from neighboring fields. Most wheat in Kentucky is planted following corn. Corn is generally a good non-host crop to grow in non-wheat years. There has been some talk that planting conventionally-tilled wheat behind corn increases the chances for a head scab problem in wheat because the head scab fungus also can attack corn and result in stalk and ear rots. Most producers find, however, that planting wheat behind corn does not increase the amount of head scab in wheat. So much head scab inoculum blows around anyway that as long as conditions are favorable for head scab development, the disease will be a problem regardless of the rotation.
Tilling wheat stubble hastens the breakdown of residue that harbors certain disease organisms. This can help reduce levels of take-all and foliar diseases, such as Septoria leaf blotch and tan spot. "Help" is the operative word here since it is unlikely that tillage will be of much good in the absence of other control methods. For fields in a wheat/double-crop soybean/corn rotation, tillage prior to planting corn should cause a significant decline in surviving wheat stubble. The year between wheat crops in this rotation also helps, except where high levels of the take-all fungus exist. In those cases, two or more years between wheat crops may be required.
Planting no-till wheat following corn is not a problem except that this practice can increase the risk for head scab in borderline situations. However, there are no data to suggest that planting wheat directly into corn residue will be the difference between a serious head scab problem and a light one.
All of these can affect stand establishment and development. To achieve the highest possible yields, you must have sufficient stands. To achieve the desired stands, you must have excellent seed germination and development of seedlings. Using high-quality seed treated with a broad-spectrum fungicide and good planting techniques foster good stand establishment. Excess stands, however, encourage foliar and head diseases by reducing air circulation and light penetration into the canopy later in the season. Use your experience, and calibrate your equipment to achieve sufficient but not excessive stands (see Section 3—Cultural Practices for more information).
Producers tend to plant wheat earlier and earlier each year. Early-planted wheat, defined as wheat planted prior to the Hessian fly-free planting date, is at greater risk of damage caused by barley yellow dwarf, take-all disease, and Hessian fly than is later-planted wheat. If logistic considerations cause you to plant some of your wheat acres prior to the fly-free date for your area, make sure those acres have been well rotated and plant a variety that can tolerate some barley yellow dwarf. Planting all your wheat acreage prior to the fly-free date is extremely risky.
Too much nitrogen in the fall can encourage excessive fall growth that can increase your problems with barley yellow dwarf and most foliar diseases caused by fungi. Increased problems with barley yellow dwarf have to do with an extended period of aphid activity (aphids transmit barley yellow dwarf) when stands are dense in the fall. The same situation encourages infection and overwintering of foliar diseases, such as leaf rust, powdery mildew, and leaf blotch complex. Excessive spring nitrogen results in lush stands that promote disease in a manner similar to that associated with excessive seeding rates.
Getting and keeping a good stand of wheat is a key component of high yields. One management strategy many wheat producers use to attain excellent stands is to treat seed with a fungicide. (See Table 7-1 for activity of seed treatment fungicides against diseases.) However, attaining good stands is just one reason to consider seed fungicides. Wheat seed treatment fungicides:
Most producers who plant wheat according to recommended guidelines, however, have little difficulty achieving dense, vigorous stands of wheat seedlings. The fact that a good percentage of the wheat seed planted in Kentucky is treated with a fungicide probably has something to do with this situation. Routine use of high-quality, high-germ seed, however, is probably the key contributing factor. In fact, most small-plot seed treatment research done over the last five years has shown that treating high-quality seed with a fungicide only rarely results in stand or yield increases. It is still advisable to treat seed with a good general-use fungicide to protect seedlings from adverse soil conditions should they develop after planting. In this regard, look at seed treatment fungicides as a form of low-cost crop insurance.
Carboxin is still considered by many to be the standard for loose smut control. It is inexpensive and highly effective under most situations. However, sporadic, reduced activity of carboxin, due to poor application procedures or soil conditions that caused the active ingredient to be washed off the seed, supported the development of a new generation of seed treatment materials that are highly effective against loose smut. All of these new-generation fungicides are extremely active at low use rates. Because of this, application of product to seed can only be done by seed conditioners with the experience and equipment to do the job properly. This eliminates the option of on-farm seed treatment by producers and increases the cost in many cases.
In general, new-generation sterol-inhibiting seed treatments, because of their specific mode of action, are not highly effective against many common soil-borne pathogens. For this reason, most are marketed as a mixture with either thiram or captan, both of which provide at least moderate activity against a wide range of soil-borne fungi.
| Table 7-1. Activity of Common Seed Treatment Fungicides Against Diseases Common in Kentucky | |
| Fungicide | Activity |
| Carboxin | Slight control of general seed- and soil-borne pathogens and excellent control of loose smut. |
| Difenoconazole | Moderate control of general seed- and soil-borne pathogens, very good control of Fusarium seed rot and seedling blight, and excellent control of loose smut. Minor control of early powdery mildew and rust and good control of Stagonospora and Septoria seedling blights. |
| Imazalil and Thiabendazole | Similar to thiram and captan except for much improved control of Fusarium seed rot and seedling blight. |
| Metalaxyl | Provides protection from Pythium for a limited time following seeding. |
| Pentachloronitrobenzene | Provides protection from Rhizoctonia for a limited time following seeding. |
| Tebuconazole | Similar to difenoconazole, except provides no control of fall powdery mildew or leaf rust. |
| Thiram, Captan | Moderate activity against many common seed- and soil-borne fungi. |
| Triadimenol | Similar to difenoconazole, but provides excellent control of fall leaf rust and powdery mildew. In high mildew areas, can often be used as a replacement for foliar fungicide sprays for mildew in early spring (up to head emergence). Limited control of Fusarium seed rots and seedling blights. |
When contemplating the use of wheat seed treatment fungicides, consider the following factors. Costs of materials and disease control vary widely, so it is critical to assess cost-benefit ratios of the various fungicides. The main consideration is to determine why you want to use a seed treatment fungicide. Specifically, what diseases are you trying to control, and what do you want to accomplish? Check fungicide labels to determine which diseases the products control. In this regard, labels are pretty specific. Once you have these determinations, select the most appropriate material (i.e., best disease control for the money). For example, triadimenol seed treatment is relatively expensive. However, if powdery mildew is a major problem on your farm, triadimenol may negate the need for an early foliar application of a fungicide on a mildew-susceptible variety. Taken in this context, the economics of triadimenol seed treatment become more favorable. If loose smut or general soil-borne pathogens are your main concern, and the risk of powdery mildew is minimal, triadimenol is not the most economical choice since less expensive materials are as good as triadimenol at controlling these diseases.
Although fewer options are open to producers regarding on-farm treatment of seed, some hopper-box treatments are still available. If you decide to try a hopper-box treatment, remember that coverage is everything. Poor coverage equals poor results and, perhaps, a waste of money. Do everything in your power to assure that the seed treatment is evenly distributed on all of the seed. This can only be accomplished, on-farm, with considerable effort and planning. Having seed treated by a professional eliminates poor fungicide distribution and variable rates on seed as potential problems.
Deciding whether or not to apply foliar fungicides to wheat is one of the most difficult decisions a producer has to make. This is because of the many variables that influence the need for and effectiveness of foliar fungicides. First, fungicides must be applied in the early stages of a disease epidemic to be of much use. Applying fungicides too far in advance of an epidemic or waiting too long to spray results in poor disease control and little or no economic benefit. Similarly, there is no economic gain from using foliar fungicides if yield-reducing levels of disease fail to develop or if crop yield potential is low. Finally, foliar fungicides only control certain foliar and head diseases caused by fungi. They do not control bacterial or viral diseases, and they have no effect on some fungal diseases, such as loose smut, take-all, and head scab.
Below are the steps you should take when making foliar fungicide use decisions:
Step 1. Commit to scouting fields. When considering the use of foliar fungicides, you first need to ask yourself some questions. The first question is, "Am I willing to scout my fields to determine the need to spray?" (If you're not sure how to answer this question, see steps 3 through 6, below, for specific field scouting requirements.) If you are not willing to commit to field scouting (you do it or pay to have it done), question your decision to even consider fungicide use. In the end, you will need to decide how important wheat is to your total farm operation. If wheat is important to the profitability of your farm, it is advisable for you to make both time and monetary commitments to get the best crop possible. If wheat is of only secondary importance relative to your other farm operations, perhaps you should back off from using fungicides and focus on controlling diseases using resistance and cultural practices.
Step 2. Determine the number of potential sprays. Once you have made the commitment to scout your wheat fields, the next significant question is, "How many fungicide applications am I willing to make?" If answering truthfully, nearly every producer says only one. Few say two. The answer to this question is important because it determines your approach to fungicide use.
If you are going to make only one fungicide application, you had better make it count. Research and experience show that a single application made during heading performs at least as well, and usually better than, a single application made at flag leaf emergence.
The problem with single applications at flag leaf emergence (regardless of the fungicide used) is that they typically allow late-season disease pressure to build to excessive levels. As a result, the crop is damaged even though early diseases may have been kept in check. Heading applications, on the other hand, limit disease buildup on the flag leaf (F), the second leaf down (F-1), and the head even though disease is allowed to develop unchecked early in the season. Protection of the F and F-1 leaves and the head is much more important to yield and grain quality than is protection of lower leaves. The risk in making a single late application is that you will wait too long to apply the heading treatment. Fungicides are of little or no value once the flag leaf and head are severely diseased. The best way to limit this risk is to start scouting operations during flag leaf extension. It is unlikely that significant disease will have developed on the F and F-1 leaves by this time.
Two fungicide applications (i.e., an early application followed by a late application) often outyield even the best single application. The question, however, is whether or not the economic benefit that results from the additional treatment is greater than its cost. As a general rule, the extra treatment at least pays for itself if early disease pressure is moderate to heavy. If early disease pressure is minimal or nonexistent, it probably would not be economical.
Step 3. Know the disease reaction of the wheat variety planted. Typically, foliar fungicides are not necessary on wheat varieties rated as resistant or moderately resistant to a particular fungal disease. Vigilance is still the watchword. Leaf rust and powdery mildew can adapt to and attack a formerly resistant variety. This can happen in a single season.
Step 4. Estimate crop yield potential. Does the field have sufficient yield potential to justify spraying? Spraying with fungicides only protects yield already built into a crop; fungicides do not "make" additional yield. Although various techniques can be used to estimate crop yield potential, most producers can look at a crop after greenup and know, intuitively, if the crop is worth protecting. In most cases, you will need to harvest an additional 3 to 5 bushels per acre (depending on grain price) to offset the cost of fungicide and application. The higher the yield potential of a crop, the more likely you are to benefit economically from applying a foliar fungicide should disease become a problem.
Step 5. Know the disease(s) with which you are dealing. As indicated earlier, fungicides control a relatively small number of fungal diseases. Fortunately, the diseases they control are those that commonly reduce yields in Kentuckyleaf rust, powdery mildew, leaf blotch complex, and glume blotch. Other diseases (except tan spot, which is rarely a problem) are not controlled with foliar fungicides, so proper identification of the disease is critical.
Step 6. Scout your fields. Scouting your fields to determine crop growth stage and current disease situation is critical to making good fungicide-use decisions. When scouting fields, walk over the entire field. Do not base decisions on what you find along the edges or what you think you see from the seat of your pickup truck. The key is to make a decision based on the average disease situation in a field, which requires assessing disease levels in eight to ten randomly selected sites in the field.
Once in a field, it is important to determine the growth stage of the crop for two reasons:
| Table 7-2. Thresholds for applying foliar fungicides at various growth stages. | |||
| Growth stage | Disease | Indicator leaf | Threshold |
| Early stem erection to first node | Powdery mildew | Uppermost fully expanded green leaf | Average of 10 pustules per indicator leaf examined |
| Leaf rust | Do not spray at this time | ||
| Leaf blotch complex | Do not spray at this time | ||
| Second node; flag leaf not emerged | Powdery mildew | Uppermost fully expanded green leaf | Average of 5 pustules per leaf |
| Leaf rust | Any green leaf | Average of 1 pustule per leaf | |
| Leaf blotch complex | Uppermost fully expanded green leaf | 25% of leaves sampled with expanding blotches | |
| Flag leaf extension | Powdery mildew | F-2 leaf | Average of 5 pustules per leaf |
| Leaf rust | F-3 leaf and above | Average of 1 pustule per leaf | |
| Leaf blotch complex | F-2 leaf | 25% of leaves with blotches | |
| Flag leaf fully expanded to head emergence | Powdery mildew | F-1 leaf | Average of 5 pustules per leaf |
| Leaf rust | F-2 leaf and above | Average of 1 pustule per leaf | |
| Leaf blotch complex | F-2 leaf and above | 25% of leaves with blotches | |
| Heads completely emerged through flowering | Powdery mildew | F-1 leaf | Average of 5 pustules per leaf |
| Leaf rust | Flag or F-1 leaf | Average of 1 pustule per leaf | |
| Leaf blotch complex | Flag or F-1 leaf | 25% of leaves with blotches | |
| Early milk to late milk | Powdery mildew | Do not spray | |
| Leaf rust | Flag or F-1 leaf | Average of 5 or more pustules per leaf | |
| Leaf blotch complexa | Do not spray | ||
| a Glume blotch cannot be controlled once disease symptoms appear. | |||
Thresholds must be used, along with some good old common sense. For example, if a threshold is reached for powdery mildew, you may want to hold off spraying if an extended period of hot, dry weather is predicted. The thresholds indicate that yield loss due to one or more of the above diseases is likely; however, they do not mean losses will definitely occur. Weather can always intervene and short-circuit a disease epidemic. There also may be some situations in which you believe the disease thresholds over- or underestimate the disease situation you are faced with. In these situations, by all means, go with your gut instinct. There is no way to develop disease thresholds appropriate for all situations.
Step 8. Select a fungicide. Propiconazole (Tilt), triadimefon (Bayleton), mancozeb (e.g., Dithane DF), and benlate (Benomyl) are all labeled for use on wheat in Kentucky. Table 7-3 shows the levels of disease control each material is capable of providing. Product labels provide detailed use instructions and product limitations. Apply all pesticides according to label specifications!
| Table 7-3. Levels of disease control provided by fungicides. | |||
| Material | Powdery mildew | Leaf rust | Leaf/glume blotch |
| Tilt | ++a | +++ | +++ |
| Bayleton | +++ | +(+++) | -(+) |
| Mancozebb | - | ++ | +++ |
| Benlate | -(+) | - | +(++) |
| a Key: - = no control or not labeled; + = slight control; ++ = very good control; +++ = excellent control;
parenthetical ratings are those at the highest labeled rate.
b Combination sprays of mancozeb and Bayleton or mancozeb and Benlate are labeled and provide a greater spectrum of disease control than any product applied by itself. | |||
Step 9. Understand the risks. One problem facing fungicide users is the inability to determine if disease-favorable conditions will persist after a fungicide is applied. Fungicides are valuable only if yields and test weights are threatened by disease. Similarly, fungicides are of limited value if other diseases develop that are not controlled by those fungicides. Examples are all virus and bacterial diseases and fungal diseases such as take-all, loose smut, and head scab. Lastly, fungicides may be of limited value if yields and test weights are reduced by non-disease factors such as spring freeze, lodging, delayed harvest, poor grain fill period, etc. Unfortunately, the above situations are always a risk to the fungicide user. You can reduce some of this risk by monitoring crop development throughout the season. This might allow you to pick up on yield-limiting factors indicating that applying fungicides might not be a good idea. Of course, this is a moot point once a fungicide is applied. In all instances where fungicides are used, check the response of your crop to the treatment by leaving a non-treated strip in the field for comparison.
The following are general descriptions of the wheat diseases most common in Kentucky. Diseases are listed seasonally. More specific information on each disease is available through your county Extension office. If you are using picture sheets to help identify a disease, be aware that many diseases look similar and can be confused with one other. The University of Kentucky staffs two plant disease diagnostic laboratories to assist you, at no charge, in identifying plant diseases.
Plate 7-1.
Patch of stunted and yellowed plants caused by barley yellow dwarf virus.
Plate 7-2.
Foliar symptoms of barley yellow dwarf. Note yellowing and purpling of leaf tips.
Occurrence: Greenup through late milk.
Symptoms: Primary symptoms include plant stunting, reduced tillering, and yellow to red-purple discoloration of leaf tips and margins. Affected plants may have a spiked appearance. Symptoms can occur in the fall or spring but are most common in the spring on the top two leaves of the plants. Foliar symptoms are frequently accompanied by secondary bacterial infections. These infections are visible as brown spots and streaks on BYD-symptomatic plants. Infected plants frequently occur in random, small groups. Large portions of fields or entire fields can be affected in severe cases.
Damage: BYD reduces grain yield and test weight.
Key features of disease cycle: Barley yellow dwarf virus (BYDV) is transmitted from infected grasses into wheat and barley by several species of aphids. In Kentucky, the bird-cherry oat aphid and, to a much lesser extent, the corn leaf aphid are the most important vectors in the fall. In the spring, overwintered bird-cherry oat aphids and migrating English grain aphids are the most important vectors. Regardless of the aphid species, winged adults migrate into wheat fields from neighboring and distant sites, feed, and deposit live young on plants. Typically, they develop into wingless adults that produce more offspring over several generations. Wingless aphids spread gradually in fields by crawling from plant to plant leaving behind their young.
BYDV is transmitted into wheat through the feeding activities of both winged and wingless aphids. Aphids acquire the virus by feeding on diseased plants for as little as 30 minutes. For this reason, the percentage of winged aphids originally carrying the virus into a field is an important piece of the picture. This percentage can vary greatly from field to field and from season to season. Although you can never tell which aphids are carrying BYDV and which are not, having knowledge of seasonal aphid activities can help you assess the potential for BYDV to occur.
Crops that emerge long before a hard freeze have a greater potential for aphid infestation (and exposure to BYDV) than those emerging after a hard freeze. The fly-free date, which is used to control Hessian fly infestation, is based on that principle and works well as long as the freeze occurs when expected.
Management: Plant after the Hessian fly-free date. Plant wheat varieties tolerant to BYDV. Limiting BYDV infection by controlling aphids with insecticides can be successful, but the results are erratic. The greatest probability for the successful use of insecticides exists when the following criteria are met: the crop is planted prior to the fly-free date or first killing frost; crops were not drought-stressed during the previous summer; there is an extended period of mild weather in the fall; there is a mild winter or good snow cover during cold periods; there is an early, mild spring; at least ten aphids per row foot are observed in the crop; the crop is at the stage prior to flag leaf emergence; and there is high crop yield potential.
If the ten-aphids-per-row-foot level is reached in the fall or spring, it is an indication that at least some of the above criteria have been met. If this aphid level is reached in the fall especially within 30 days of seedling emergence, it may be advisable to make an insecticide application. If it turns cold after the application, wait and scout again in the spring. If the fall is mild and winged aphids continue to arrive in the field, a second fall application might be needed to achieve acceptable BYD control. Regardless of what was done in the fall, a spring application may be needed if greenup is early and the aphid treatment guideline of ten aphids per row foot is reached prior to flag leaf emergence. Failure to make the necessary spring applications may negate any gains associated with fall applications.
Keep in mind that the above aphid treatment guideline is not chiseled in stone. In some years, ten aphids may be too low and in other years too high. Herein lies the difficulty when attempting to control BYD indirectly using insecticides; the system is not perfect. However, until our understanding of BYD epidemiology and aphid biology is enhanced by new research, the ten-aphids-per-row-foot treatment guideline is the only one available with any experimental basis.
Plate 7-3.
Foliar symptoms of wheat soil-borne mosaic. Note mosaic pattern with darker green islands on a light green background.
Occurrence: Symptoms are most prominent from greenup through stem erection, but plants may remain permanently stunted.
Symptoms: Leaves of infected plants exhibit a mild green to prominent yellow mosaic. Small green islands and short streaks may be evident on an otherwise yellowed leaf. Infected leaves may be somewhat elongated and have rolled edges, and tillering of plants can be reduced. Wheat soil-borne mosaic can occur throughout fields, but may be most severe in poorly drained areas. Symptoms are most prominent early to mid season when day temperatures are between 55°F and 70°F. Symptoms tend to fade as the weather warms up, but in severe cases, plants can remain permanently stunted.
Damage: Yield is reduced.
Key features of disease cycle: Virus is transmitted by a soil fungus, Polymyxa graminis, that is common throughout Kentucky. Infection can occur in the fall, winter, or spring, but autumn infections lead to the most serious problems. High soil moisture favors infection.
Management: Plant resistant wheat varieties. Delay fall planting operations past the Hessian fly-free date to limit fall infections. Improve internal and surface drainage of fields where problems exist. Avoid crop production practices that encourage soil compaction.
Plate 7-4.
Foliar symptoms of wheat spindle streak mosaic. Note mosaic pattern with short, yellow dashes or "spindles" on an otherwise dark green leaf.
Occurrence: Greenup through flowering.
Symptoms: Symptoms are highly variable, depending on the wheat variety and growing conditions. Foliar symptoms appear as random, yellow to light green dashes running parallel with the leaf veins. Early in the spring, the dashes may have a nondescript appearance. With age, however, some dashes are pointed at one or both ends, hence the name spindle streak. Spindles may have an island of green tissue in their centers. Plant stunting and reduced tillering can be associated with severe infection by the virus. Symptoms usually appear during the period the crop should be greening up in early spring. Symptoms are frequently uniformly distributed across fields and usually fade as temperatures warm in mid spring. During cool springs, symptoms may be evident throughout the season.
Damage: Yield is reduced.
Key features of disease cycle: The virus is transmitted to wheat in the fall, winter, or early spring by the soil fungus Polymyxa graminis. The onset and degree of symptom expression can be highly variable in a field from one year to the next, even though P. graminis and the virus are present at relatively constant levels. This is related to the time of year wheat becomes infected and the range and consistency of winter and early spring temperatures. Disease is favored in wet soils, although excessive moisture is not required for severe disease to occur.
Management: Same as wheat soil-borne mosaic.
Plate 7-5.
Powdery mildew lesions. Note black structures which are often embedded within the lesions. With age, lesions turn a dull gray-brown.
Occurrence: Season-long.
Symptoms:White, powdery patches form on upper leaf surfaces of lower leaves and eventually can spread to all aboveground portions of plants. Patches turn dull gray-brown with age.
Damage: Yield and test weight are reduced, directly due to infection and indirectly due to harvest losses associated with lodging.
Key features of disease cycle: Fungus persists between seasons in infested wheat stubble and in overwintering wheat. Spores infect plants during periods of high moisture (not necessarily rain) and cool to moderate temperatures.
Management: Plant resistant or moderately resistant varieties, and avoid farming practices that favor excessively dense, lush stands. Protect upper leaves and heads of susceptible varieties by using foliar fungicides.
Plate 7-6.
Spore-bearing structures (pycnidia) visible with a 10X hand lens in foliar lesions caused by Stagonospora nodorum, the cause of leaf and glume blotch. Note the brown color of the pycnidia. Compare with black pycnidia associated with Septoria tritici lesions.
Plate 7-7.
Typical foliar lesions caused by Septoria tritici (top) and Stagonospora nodorum (bottom).
Occurrence: Stem erection through late dough.
Symptoms: Foliar symptoms caused by Septoria tritici infection include brown, elongated rectangular lesions with irregular margins. Lesions have numerous pinpoint, black specks (pycnidia) throughout. Pycnidia are most evident in the morning following a heavy dew or after rain. Symptoms usually start in the lower leaves and move upward. Lesions are often first found at the tips of leaves.
Foliar symptoms caused by Stagonospora nodorum infection are lens-shaped, tan-brown lesions of varying sizes with regular borders that are frequently surrounded by a yellow halo. Lesions contain light brown pycnidia, but these are difficult to see without the aid of a hand lens. Infections can occur very early in the season but are most evident just prior to and after heading. Infections start in the lower leaves and move to the upper leaves and heads (see glume blotch). Symptoms become evident seven to ten days following infection.
Damage: Yield and test weight are reduced.
Key features of disease cycle: Septoria and Stagonospora overwinter in wheat stubble of previously diseased crops or on infested seed. Spores are produced during wet weather and are either splashed or wind-blown onto leaf surfaces. Infection of plants by S. tritici is greatest during cool to moderate temperatures. Infection by S. nodorum can occur over a wide range of temperatures but is favored in the mid to late stages of crop development. The fungi that cause leaf blotch complex can occur individually in a crop or at the same time, even on the same leaves.
Management: Plant moderately resistant varieties and high-quality, well-cleaned, disease-free seed that is treated with a fungicide (e.g., certified seed). Avoid excessive seeding rates as well as nitrogen deficiencies and excesses. Protect the upper two leaves and heads of susceptible varieties with fungicides. Crop rotation and tillage of infested wheat stubble may help in leaf blotch management, but neither provides a high degree of control.
Plate 7-8.
Orange pustules of leaf rust.
Occurrence: Seedling emergence through late dough, although typically is most severe after head emergence.
Symptoms: Infections are evident as pinpoint, yellow flecks on upper leaf surfaces. After about one week, flecks develop into orange pustules, each containing thousands of spores. Many things can cause wheat leaves to fleck, so flecks are a good indicator of leaf rust only when at least some mature pustules are also visible. Leaf rust pustules usually form in random patterns, primarily on the upper surfaces of leaves.
Damage: Yield and test weight are reduced. Indirect losses associated with crop lodging can occur when leaf rust is severe.
Key features of disease cycle: The leaf rust fungus can overwinter in Kentucky, but more commonly it is blown into Kentucky from more southern states. Spores blow to and infect foliage during moderate to warm temperatures and six or more hours of continuous leaf wetness. Leaf rust is a potentially explosive disease and requires just a short time to go from low to epidemic levels on a susceptible variety.
Management: Plant resistant or moderately resistant wheat varieties. Avoid excessively dense stands that decrease air circulation and light penetration into the crop canopy. Protect the upper two leaves of susceptible varieties with foliar fungicides.
Plate 7-9.
Heads are showing various degrees of glume blotch. Glume blotch is evident as brown to gray blotches on glumes and awns and
is often first visible on the tips of individual glumes. Once glume blotch appears, fungicides are of no value.
Occurrence: Early milk through maturity.
Symptoms: Infected glumes and awns develop gray-brown blotches, usually starting at the tips of glumes.
Damage: Infected heads develop low-test-weight, shriveled grain. Seed quality can be affected when glume blotch is severe.
Key features of disease cycle:Spores of Stagonospora nodorum blow or are splashed onto wheat heads. Spores originate from diseased foliage (see leaf blotch complex) or infested wheat stubble. Infections occur during periods of extended wetness.
Management: Plant moderately resistant varieties and high-quality, well-cleaned, disease-free seed (e.g., certified seed). Control foliar and head infections on susceptible varieties with fungicides applied prior to infections. Avoid nitrogen excesses and deficiencies, which encourage glume blotch. Crop rotation and tillage of infested wheat residue may help to moderate glume blotch severity, but neither provides a high degree of control.
Plate 7-10.
Bleached area on head is the result of head scab, also known as Fusarium head blight.
Plate 7-11.
Grain affected by scab. Note pink to white discoloration and shriveled appearance of diseased grain.
Occurrence: Early milk through maturity.
Symptoms: Individual spikelets or groups of spikelets turn cream to white on otherwise green heads. Entire heads may become diseased when extended periods of warm, wet weather occur during flowering and early grain fill. Salmon-colored patches of fungal growth frequently can be seen at the base of infected spikelets. Infected spikelets often fail to develop grain, or grain is extremely shriveled and of low test weight. Shriveled grain may have a pinkish discoloration.
Damage: Low-test-weight, shriveled grain is produced in diseased heads. Germination and viability of seed and milling qualities of grain are reduced. Scabby grain often is contaminated with mycotoxins, especially vomitoxin, which affects feed and food uses.
Key features of disease cycle: Scab fungi overwinter in cereal, grain sorghum, and corn stubble. Spores are produced and heads infected when warm, wet weather occurs just prior to and during the flowering period for wheat. Most fields, in most years, escape serious infection because flowering does not occur during warm, wet weather. Scab epidemics occur when extended periods of disease-favorable weather occur while much of the Kentucky wheat crop is in flower.
Management: Nature provides the best management by limiting disease-favorable conditions during crop flowering. No chemical controls exist, and all wheat varieties are susceptible. Crop rotation and tillage have little effect on head scab because of the widespread occurrence of the causal fungi throughout Kentucky. Planting different varieties that flower at different times may reduce the incidence of head scab in a moderate to light disease year.
Plate 7-12.
Loose smut has caused this head to turn into a mass of fungal spores.
Occurrence: Head emergence through maturity.
Symptoms: Floral parts of infected plants are transformed into a mass of black, powdery spores. Diseased tillers usually head out in advance of healthy tillers.
Damage: 100 percent grain loss occurs in diseased heads.
Key features of disease cycle: Spores produced by diseased heads blow to and infect the flowers of healthy heads during rainy weather. Infected flowers give rise to infected grain. Infected grain develops normally, but harbors the loose smut fungus. The fungus remains dormant until the seed is planted and germinates. Infected plants appear to be normal, but develop smutted heads.
Management: Plant certified or otherwise high-quality, disease-free seed. Infections in seed can be eradicated by treating seed with various systemic fungicides.
Plate 7-13.
Darkening of roots and basal region of stem typical of take-all disease.
Occurrence: Stem erection through maturity.
Symptoms: Infected plants appear normal through crop greenup but eventually become stunted and uneven in height and experience some premature death of tillers. Tillers that head out are sterile and turn white or buff colored. Affected plants easily can be pulled out of the soil because of extensive root rotting. A shiny black discoloration is evident under the leaf sheaths at the bases of diseased plants. Infected plants can occur individually but more typically occur in small to large groups. Entire fields or large portions of fields can be diseased in severe situations.
Damage: Diseased plants yield little or no grain.
Key features of disease cycle: The take-all fungus survives from season to season in infested wheat and barley stubble and residue of grassy weeds. Infections are favored in neutral to alkaline, infertile, poorly drained soils.
Management: Allow at least one year (preferably two years) between wheat (or barley) crops. Soybeans, corn, grain sorghum, and oats are acceptable alternative crops. Control grassy weeds, especially in years between small grain crops. Fertilize fields and lime fields according to soil test recommendations. Do not allow fall or spring nitrogen deficiencies in the small grain crops. Improve surface and internal drainage of fields.
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*Adapted from material written by Lee Townsend and Douglas Johnson, Extension
entomologists.
