Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison
The wheat crop in southern Wisconsin is progressing through growth stages quickly now. About 10 days ago we were at jointing and now we have fully expanded flag leaves (growth stages ranging from Feekes 8 to Feekes 9 depending on variety in Arlington, WI). Now is the time to be scouting diligently for diseases of wheat. Fungicide applications to protect the flag leaf may be necessary if active disease epidemics are found in your crop.
In general the wheat I have scouted has been relatively free of disease. I have not seen any rust or powdery mildew. The only disease that has been active and continues to be active is Septoria leaf blotch. Severity has been low in most plots; however, this disease should be monitored carefully. Under rainy conditions, spores can be splashed to leaves in the upper plant canopy, which can cause yield-limiting damage if severity is high. For more information on this disease consult my previous article at https://ipcm.webhosting.cals.wisc.edu/blog/2013/04/wisconsin-winter-wheat-disease-update-april-24-2013/ and our fact sheet that can be found at http://fyi.uwex.edu/fieldcroppathology/files/2013/04/Leaf-Blotch-Diseases-of-Wheat-1.pdf.
Other diseases to be on the lookout for during the “flag leaf stages” include powdery mildew (Fig. 1), leaf rust (Fig. 2), and stripe rust (Fig. 3). I haven’t talked much about powdery mildew on wheat yet this season. Levels have been low to non-existent, which is unusual for Wisconsin.
Powdery Mildew is a frequent pathogen of wheat in Wisconsin. The powdery mildew fungus damages wheat plants by utilizing nutrients, reducing photosynthesis, and increasing plant respiration and transpiration. High incidence and severity of powdery mildew on wheat can cause reduced vigor, heading, and seed fill. In some severe cases, powdery mildew can result in leaf and whole plant death. Yield losses as high as 40% have been recorded as a result of powdery mildew. Greatest yield reduction will occur when infection occurs around the time of wheat heading.
Initial infections by the powdery mildew fungus typically result in white cottony growth (mycelium) of the fungus. This growth can occur on all above ground parts of the plant, but is typically most prevalent on the upper surface of the leaves. The fungal colonies will turn dull gray to light brown as they mature. Once the colonies are fully mature, fruiting structures (cleistothecia) form, which appear as small black dots that resemble small seeds nestled in the fungal mat. The fruiting structures are visible with the naked eye. The fungus exists entirely on the surface of the plant except for microscopic appendages, which penetrate the host epidermis.
Powdery mildew is caused by the fungus Blumeria graminis. B. graminis overwinters on infested wheat residue as spores in fruiting structures called cleistothecia. If the winter is mild or there is snow cover the fungus can also survive on infested wheat debris as mycelium or another type of spore called conidia). In the spring spores are windblown to wheat plants and subsequent spore cycles can occur throughout the wheat-growing season. Volunteer wheat can serve as a “green bridge” between wheat crops and act as a source for spores that can infect wheat planted in the fall. If plants are infected in the fall, the fungus can survive as long as the wheat leaf survives.
Currently there are a wide variety of fungicides available to control powdery mildew. Careful scouting of the wheat crop should be performed on a regular basis throughout the growing season. Fungicides should be applied when powdery mildew is first observed. Focus should be made on protecting the flag leaf from powdery mildew. Yield losses are greatest when powdery mildew infections occur prior to, and at, flag leaf emergence.
Management of powdery mildew is through the use of resistant varieties, foliar fungicides, tillage, crop rotation and nutrient management. Cultivars of wheat with varying levels of resistance to the powdery mildew fungus are available. Regular scouting should be done to determine disease progress and crop yield potential prior to making decisions on fungicide applications. Fungicide applications are based on the risk of disease on or before flag leaf emergence. Tillage to bury infested residue and to manage volunteer wheat can reduce the level of inoculum in wheat fields. Crop rotation with a non-wheat host can help reduce initial inoculum levels. Powdery mildew can be more severe on plants with high nitrogen. Nutrients, especially nitrogen, potassium, and phosphorus should be optimized for plant growth but not applied in excess.
If you have been following my articles lately, you know that Dr. Carl Bradley has found stripe rust to the south of us in Illinois. However, spread has been slow and severity low enough that yield has likely not been impacted except in very susceptible varieties. All rusts (stripe rust, leaf rust, and stem rust) are favored by wet weather and periods of extended leaf wetness. However, temperature optimums vary among the different rusts. Stripe rust is favored during periods when temperatures are between 55 and 65F, while leaf rust tends to occur at a higher temperature range of 60 to 72F. Stem rust tends to occur at temperatures between 60 and 104F. In most years, spores of the rust pathogens will not readily overwinter in Wisconsin. Cold temperatures (<32 F) will reduce the ability of rust to overwinter. Spores are typically carried into Wisconsin on wind currents from southern states. In years like 2012 where the winter was mild, it is likely that rust pathogens were able to overwinter on wheat debris in Wisconsin. This could be one reason for the unusually high incidence and severity of stripe rust observed in 2012 in Wisconsin. For More about rust, see my previous article at https://ipcm.webhosting.cals.wisc.edu/blog/2013/04/wheat-scouting-and-little-more-about-rusts/.
Fusarium Head Blight
Finally, Fusarium head blight (FHB; a.k.a Fusarium head scab) is a disease of concern during the heading growth stage of wheat (Feekes 10 and later). Severe FHB can result in high levels of the mycotoxin deoxynivalenol (DON or vomitoxin). Dockage at the elevator can occur if a load of wheat grain tests positive for DON at levels above 2ppm. Therefore, managing FHB can be important in areas with a history of the disease and where susceptible varieties are grown.
Diseased spikelets on an infected grain head die and bleach prematurely. Healthy spikelets on the same head retain their normal green color (Fig. 4). Over time, premature bleaching of spikelets may progress throughout the entire grain head (Fig. 5). If infections occur on the stem immediately below the head, the entire head may die. As symptoms progress, developing grains are colonized causing them to shrink and wrinkle. Often, infected kernels have a rough, sunken appearance, and range in color from pink or soft gray, to light brown.
FHB is caused by the fungus Gibberella zeae (also known as Fusarium graminearum). In addition to wheat, the fungus is a pathogen of corn, barley, and other grasses. This fungus overwinters on infested stubble and straw of cereals and weed grasses, and on stalks and rotted ears of corn. The severity of FHB varies greatly from year to year. Infection is favored by extended periods of high moisture or high (>90%) relative humidity, and moderately warm temperatures (59 to 86°F). These conditions have to occur just before or during wheat flowering (Feekes 10.5) and wheat can remain susceptible to infection through the early dough stage (Feekes 11.2). If favorable weather conditions persist, flowering tillers can continue to be infected. This is especially problematic in wheat stands that are at varying levels of maturity.
Fungicides are available for control of Fusarium head blight. Triazole fungicides (Fungicide Resistance Action Committee; FRAC class 3) are recommended for control of FHB. Fungicides containing prothioconazole and tebuconazole, or a pre-mix of these two compounds, have given the best control of FHB in University research trials. Strobilurin fungicides (FRAC class 11) should be AVOIDED for controlling FHB. Research has demonstrated that the use of strobilurin fungicides to control FHB can result in an increase in DON levels in harvested grain. A web-based FHB risk assessment tool is available at http://www.wheatscab.psu.edu. This tool can be used by crop management personnel when making decisions about applying a fungicide just before or at flowering. The tool also provides real-time, local commentary by extension personnel about the status of diseases in wheat.
DO NOT plant small grains into small grain or corn residue as this increases the chance of FHB. Also, avoid planting grain crops near areas where there are large amounts of small grain or corn residue on the soil surface. When possible, plant small grains following a legume crop (e.g., soybeans) and maintain a rotation with two to three years between small grain crops. In addition, deep plowing of all infested plant debris is recommended. DO NOT apply manure containing infested straw or corn stalks onto fields planted to small grains. Certain grain varieties have moderate levels of partial resistance to FHB, and use of these varieties can lead to a reduction in disease severity and an increase in grain quality. Finally, plant several varieties of a small grain that vary in flowering date. This will decrease the risk that an entire crop will be vulnerable to FHB when weather conditions favor the disease.
For more information about fungicide options in wheat and tips on using fungicides, see my previous article at https://ipcm.webhosting.cals.wisc.edu/blog/2013/03/using-fungicides-on-wheat/.