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Physiological Leaf Spots on winter wheat widespread – don’t confuse them with fungal leaf spots !

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Quick Summary for Busy People

  • Physiological leaf spots are caused by chloride deficiency. Fungicide applications are not effective, but chloride fertilizer can help.
  • Fungal leaf spots may occur where wet and cool weather is observed. Fungicide applications are effective but may not be necessary, depending on the crop growth stage.
  • Take the time to diagnose/get diagnosed the leaf spot cause before you make treatment decisions.

 

I’ve seen several winter wheat samples, mainly variety Bobcat, with tan spot-like symptoms in the Schutter Diagnostic Lab. These leaf spots turned out to be Physiological Leaf Spots (PLS). PLS are not caused by a (fungal) pathogen, which also means that fungicide applications will not be effective. Distinguishing between fungal leaf spots and PLS is difficult but important. This AgAlert discusses differentiation, characteristic symptoms, risk factors, and treatment options for PLS and fungal leaf spots on wheat and includes resources on fungicide applications.

 

Differentiation between Physiological and Fungal Leaf Spots – a “quick and dirty” key (by Mary Burrows)

  1. Physiological spots occur on all leaves, not just in the thicker, lower canopy.
  2. Physiological spots are more uniform than fungal spots (there aren’t more spots on lower leaves than upper leaves).
  3. Physiological spots are not highly associated with patches of residue in the field, it is more uniform across the field.
  4. Physiological spots have a very sharp edge to the lesion, whereas fungal spots are more fuzzy and continue to expand over time.
  5. Fungal spots, when left in a moist chamber 24-48 h, develop small black or brown spots that are fungal structures called Pycnidia. Physiological spots do not form pycnidia. You can see these under high humidity in the field with a 10-20x lens.
  6. Look on residue for black pycnidia – these are the source of fungal leaf spots. If there are none, it might be physiological.

 Read on below for more details on Physiological and Fungal leaf spots and treatment options.

 

Physiological Leaf Spots (PLS)

Certain winter wheat varieties are more prone to PLS. Bobcat develops PLS, and other varieties are known to develop PLS as well, for example CDC Falcon, Raptor, Peregrin, Promontory, and others. Please note that Bobcat has been performing very well throughout its development process at MSU, even in the presence of PLS.

PLS symptoms typically appear from flag leaf emergence to boot stage, but I have seen plants at earlier growth stages developing PLS this spring. PLS appear as circular to oblong, dark brown or chlorotic lesions and the tissue in the center looks bleached, white to gray in color (Figure 1). The lesions are often surrounded by narrow chlorotic (yellow) halos and tend to be more numerous toward the tip of the leaf blade. PLS lesions have a more discrete margins compared to fungal leaf spots, which are more diffuse and continue to expand over time. Most importantly, PLS do not develop small black spots in the center of the lesion when kept in a humid environment for 24-48 hours.

PLS are often caused by chloride deficiency. Low chloride uptake by the plants could occur in sandy soil with high rainfall because the chloride has leached, but also in droughty soils where root growth is reduced. If you want to rule out or rule in chloride deficiency, I suggest you do a tissue test for chloride concentration. The suggested critical level in plant material (entire plant) is between 0.1 and 0.4%, with large yield increases and leaf spot decreases when chloride was applied at tissue chloride concentrations below 0.1%. Applying 10-20 lb potash (0-0-60) per acre should be sufficient (5-10 lb Cl/acre and 12 lb K2O/acre) to mitigate chloride deficiency. Alternatively, a similar amount of chloride can be applied as a liquid (for example calcium chloride or ammonium chloride). Dry fertilizer applications can be applied at any time through the late jointing stage, depending on sufficient rain (or irrigation). Liquid formulations can be applied to the foliage up to the time of flag leaf emergence.

Please note that the Schutter Diagnostic Lab does not offer tissue analysis or soil testing services but most independent labs test for chloride.

Please contact Clain Jones (Phone: 406-994-6076; Email: clainj@montana.edu) for questions on tissue testing and fertility recommendations.

 

Fungal Leaf spots: Tan spot, Septoria tritici blotch, Stagonospora leaf blotch

Fungal leaf spots on Montana wheat include Tan spot, Septoria tritici blotch, and Stagonospora leaf spot. These diseases are residue borne. Tight cereal rotations and high residue cover are factors that promote fungal leaf spots. Fungal leaf spots are more concentrated in the lower canopy, which is closer to the residue (source of inoculum) and where it is more humid. Wet and humid weather favors fungal leaf spot diseases. Septoria tritici blotch develops at cool temperatures of 59 - 68°F; Tan spot and Stagonospora leaf blotch develop at moderate temperatures of 68 - 75°F. In other words, if you live in a part of Montana that has experienced a lot of precipitation lately while temperatures are still cool, fungal leaf spot diseases are to be expected.

Fungal Leaf Spot Symptoms (Figure 2)

Tan spot is caused by Pyrenophora tritici-repentis. Initial symptoms will be small, brown leaf spots that enlarge and develop tan necrotic spots surrounded by yellow halos. The lesions are initially oval and take on a diamond shape as they enlarge. When kept in a humid environment, a pinhead-sized black spot will develop in the center of the lesion.

Septoria tritici bloch is caused by Zymoseptoria tritici. The disease starts out as tan to brown lesions on the lower leaves, which have an elliptical to rectangular shape and form between leaf veins. The lesions enlarge and their centers become necrotic. Dark fruiting bodies will develop in the lesions, giving them a speckled appearance (as if sprinkled with cracked peppercorns).

Stagonospora leaf blotch is caused by Stagonospora nodorum. The disease may first become evident on the lower leaves as small yellow spots. The spots will enlarge into elliptical, lens-shaped lesion of gray or brown color and will be surrounded by yellow margins. In the center of the lesions, you may observe small, brown dot-like structures, which are called pycnidia. The lesions can coalesce to form large areas of brown, necrotic tissue on the leaves.

Fungicide applications are effective at managing fungal leaf spots. I strongly recommend you bring a sample to your local Extension agent or submit one to the Schutter Diagnostic Lab for diagnosis before you plan a fungicide application. If you send a sample into the clinic for confirmation, please let us know if you’ve already applied a fungicide – if you have, it’s unlikely we will be able to recover spores.

With a fungicide application, protecting the flag leaf from disease is key. You can find resources on fungicide use, application decisions, and suitable modes of actions below. Varieties that are less susceptible to leaf spot pathogens are available. Areas with high cereal residue cover are likely disease hot spots and can be managed by practices of residue decomposition and removal. Crop rotation is also effective.

 

Powdery Mildew

I have also heard reports of powdery mildew on the winter wheat crop, so I thought I include some info on that while we’re at it.

Powdery mildew lesions are sporulating on lower leaves and may have an orangish cast. The lesions are defined and powdery, not in stripes (Figure 3). This is NOT stripe rust. Powdery mildew starts out grey or white and forms hard black pinprick sized spores called cleistothecia further along in disease development. Disease development is optimal between 59 - 72°F and is significantly reduced above 77°F. The disease prefers high humidity but does not prefer active rainsplash. If symptoms are on lower leaves only, no treatment is recommended. If the temperatures warm and it becomes dry, it’s likely the disease will dry up too. If symptoms start moving up the crop and the weather remains favorable, a fungicide may be recommended depending on the yield expectation of your crop and whether it’s irrigated or not. All varieties should be considered susceptible in Montana. Please see the resources below for fungicide options.

 

Where you can find more information

Physiological Leaf Spots:

Fungal Leaf Spots and other wheat diseases:

Resources on Fungicide use (decision) and suitable modes of actions:

 

Figure Captions

Figure 1. Physiological leaf spot symptoms on winter wheat variety Bobcat at early jointing (left) and boot stage (right). Photo credit: Uta McKelvy, Montana State University.

Figure 2. Leaf spot symptoms associated with tan spot (left), Septoria tritici blotch (middle), and Stagonospora leaf blotch (right). Photo credit: (left) Mary Burrows, Montana State University; (middle) Jack Kelly Clark, UC IPM Program; (right) Paul Bachi, University of Kentucky.

Figure 3. Islands of fuzzy, powdery growth on a wheat leaf associated with powdery mildew. Photo credit: Mary Burrows, Montana State University.

 

Please don’t hesitate to email (uta.mckelvy@montana.edu) or call (406-994-557) if you have any questions. I’m here to help.

Best,

Uta McKelvy, Extension Field Crop Pathologist