Foliar Diseases of Wheat

Powdery Mildew
Powdery mildew is favored by cool temperatures and humid, but not wet conditions.  In fact, spore germination is inhibited by water on foliage.  This season, the cool, humid conditions likely have flared up this disease in some areas where susceptible wheat is planted. In recent years, population shifts in the pathogen that have resulted in it being able to overcome some commonly deployed powdery mildew resistance genes in wheat, resulting in flareups in some areas.  

The disease typically occurs in focal points within a field, often in areas that favor cooler temperatures and greater humidity.  These areas include along fence lines, low lying areas of the field, or shaded forest edges.  Signs of the pathogen are observed as white fuzz on foliage that will turn grey and potentially contain black fungal growths on these grey or white masses over time.  This disease is often more problematic in situations where we are really pushing our N rates early or using manure in our production systems.   

 
Figure 2. Powdery mildew 

Stagonospora Leaf and Glume Blotch 
Stagonospora leaf and glume blotch is residue borne and occasionally seed borne. This is a disease that is favored by warm, wet conditions, and typically doesn’t start to develop until canopies have closed and air temperatures remain above ≈ 80°F.  This disease is rain splashed up the plant and can be observed on the foliage and heads.  When the pathogen infects the glumes early it can cause the grain to shrivel, or completely abort.  Resistance to SNB differs between foliage and heads, so you may observe it on foliage but not heads, and vice versa.  Look for lens-shaped lesions on foliage with a dark center.  Sometimes you may see small, light brown bumps within lesions, which are the fruiting bodies of the fungus.   

 
Figure 3.  Stagonospora leaf blotch 

Tan Spot (Yellow Leaf Spot)
Tan spot is another residue borne disease that infects wheat canopies and can be difficult to distinguish from SNB, as both diseases have similar environmental requirements.  In fact, many times both diseases occur simultaneously, and when combined with SLB, results in something called leaf blotch complex.  Lesions are typically oval with dark centers.  Lesions are bordered by yellow halos.  Centers of lesions are often dark, and if you have a good hand lens, you may be able to see that these spots contain black “hairs” with hot dog shaped spores on them.  The tan spot pathogen does not cause disease on heads like SNB. 

Stripe Rust
Stripe rust (NOT strip rust) is an obligate pathogen and requires a living host to grow and reproduce.  Under some circumstances, it can overwinter in parts of the Midwest if conditions favor fall infection of plants and heavy snows insulate seedlings.  However, in most cases the pathogen needs to blow into the state from warmer climates in the South.  Stripe rust is a cool season beast and can be a major issue when it arrives early, and cool (≈60-65°F) wet conditions persist.  Like powdery mildew, the disease starts in fields as foci, which expand over time.  Pustules of orange to yellow spores develop in stripes on foliage, and glumes may also be affected.  Keep an eye on local and regional reports of this disease, as if it arrives before heading and cool conditions persist, serious damage can occur on susceptible cultivars.   

 
Figure 4.  Stripe rust 

Scouting and Management
It is important to make sure that you are getting into your wheat fields weekly to scout for diseases and pests every 7-10 days from flag leaf emergence through soft dough.  Make sure you are checking about 10 locations across the field for every 100 acres, and at each location assess at least five plants.  There are no thresholds for managing foliar diseases, but in general, if you are seeing elevated foliar disease severity in the mid canopy of at least 20-25% of your plants near FGS 8, you may need to consider a fungicide application in the near future.  The situation is slightly different with stripe rust due to the rapid rate at which the fungus can reproduce and spread under cool wet conditions.  In most instances, fungicide applications to the head that are targeting Fusarium head blight also will be efficacious for managing foliar diseases.  However, if a disease such as stripe rust comes in early, waiting until flowering to make an application may not be advisable.  Do not apply products containing strobilurins (FRAC group 11) after flag leaf emergence as research has shown that these fungicides may increase vomitoxin levels if Fusarium head blight occurs.  Always read and follow all label instructions.