Physiological leaf spots (PLS) on winter wheat variety ‘Bobcat’ are once again observed. PLS are associated with chloride deficiency in combination with unfavorable environmental conditions. Fungicide applications are not effective, but chloride fertilizer can help. The issue is more prevalent on coarse soils in wet areas (because chloride easily leaches), especially when no potash has been applied, but has been observed in a range of environments.
Fungal leaf spots (FLS) are most likely to occur in wheat-on-wheat, no-till, and reduced-till systems and are favored by rain events and high moisture. Fungicide applications are effective but may not be necessary depending on the crop growth stage.
Take the time to diagnose/get diagnosed the cause for the leaf spot before you make treatment decisions.
It’s the time of year again where leaf spots are on the rise. If you are growing ‘Bobcat’ winter wheat, chances are you see Physiological leaf spots (PLS). PLS are not caused by a (fungal) pathogen but are instead related to environmental conditions, including chloride deficiency and fluctuating weather conditions. This means that fungicide applications will not be effective in managing PLS.
If your winter wheat is growing in a wetter part of the state, if it is a variety other than ‘Bobcat’, and/or if the field has a cereal-intensive rotation with high residue build-up, Fungal leaf spots (FLS) may be more likely. FLS, such as tan spot, can look very similar to PLS but can be managed with fungicides. Distinguishing between fungal leaf spots and PLS is difficult but important.
This AgAlert discusses risk factors and treatment options for PLS and FLS on wheat and includes resources on fungicide applications.
1. Risk Factors for Physiological and Fungal Leaf Spots
Factors Favoring Physiological Leaf Spot (PLS):
- Growing ‘Bobcat’ winter wheat.
- Low soil and tissue chloride levels. Soils may have adequate Cl levels but if they are dry the plants may not be able to take up the water-soluble Cl. On the other end of the spectrum, Cl may have leached from the root zone in areas that received a lot of precipitation.
- Fluctuating weather conditions where overcast, cloudy days are followed by sunshine-intensive days.
Factors Favoring Fungal Leaf Spot (FLS):
- Wet, rainy weather and moderately warm temperatures between 59-80 degrees F.
- Cereal-intensive rotations, reduced- and no-till systems promoting residue buildup.
- Black pycnidia on wheat residue.
- Growing a FLS-susceptible wheat variety. Moderately susceptible winter wheat varieties include for example Yellowstone, CDC Falcon, Warhorse, Genou, Decade, Northern, Loma.
- A history of FLS in the field/area.
In addition, FLS usually shows a severity gradient on the plant where symptoms develop first on the lower leaves and are more severe. This is because the disease-causing pathogens spread from the residue via rain-splash, establishing first in the lower canopy. If rainy weather persists, FLS will spread to the younger leaves higher up on the plants over time.
Discerning between PLS and FLS is difficult. Consult your local Extension agent or submit a sample to the Schutter Diagnostic Lab for support with the diagnosis.
2. Physiological Leaf Spots (PLS)
Certain winter wheat varieties are more prone to PLS. Over the past years, Bobcat has repeatedly developed PLS. Please note that Bobcat has been performing very well throughout its development process at MSU, even in the presence of PLS.
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 more discrete margins compared to FLS, 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. .
Figure 1. Physiological leaf spot symptoms on winter wheat variety Bobcat on an individual leaf (left) and across the canopy (right). Photo credits: Uta McKelvy, Montana State University (left); Brandee Johnston, Montana State Foundation Seed Program (right). |
PLS is more likely when there are low soil and tissue Cl levels, but we have seen no to minimal PLS in Bobcat at low tissue Cl levels (<0.10%) and low to moderate soil Cl levels (10-22 lb Cl/ac in top 2′), so available Cl levels and PLS symptoms are not consistently correlated, suggesting that weather plays a role.
Based on two years of research near Bozeman, potash (0-0-60-45Cl) does prevent PLS, but doesn’t always result in a yield benefit. Because chloride is highly soluble and mobile, chloride fertilizers applied now or soon, should still reach your crop’s roots. Potash is 45% chloride and is likely the cheapest option. Twenty lb potash per acre should be sufficient (10 lb Cl/acre) to mitigate chloride deficiency, although the actual amount will depend on yield potential, PLS severity, and Chloride levels in the soil. 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, though it’s likely that most foliar Cl ends up on the soil and will be taken up by roots, not leaves.
Physiological leaf spots can be caused by other environmental stressors such as weather extremes. The recent back and forth of cold and warmer days, sunshine, heavy rains, and snow has stressed wheat plants which may respond with spotting and discoloration. 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 fertilizer recommendations.
3. Fungal Leaf Spots (FLS)
FLS on Montana wheat include Tan spot, Septoria tritici blotch, and Stagonospora leaf spot. Tan spot tends to be most prevalent in the state. See Figure 2 for characteristic FLS symptoms.
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. |
Fungicide applications are effective at managing FLS. 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 to 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.
Fungicide applications to manage FLS are most economical when they are applied between flag leaf emergence and early flowering. The goal is to protect the flag leaf and penultimate leaf from foliar disease, as these leaves contribute significantly (75% and more) to grain yield. In most years in Montana, environmental conditions are not favorable for FLS by the time the flag leaf has emerged and fungicide applications wouldn’t be necessary. You can find resources on fungicide use, application decisions, and suitable modes of actions below. Areas with high wheat residue cover are likely disease hot spots and can be managed by practices of residue decomposition and removal. Crop rotation is also effective.
4. Where Can You Find More Information?
Physiological Leaf Spots:
Fungal Leaf Spots and other Wheat Diseases:
- MSU Extension Plant Pathology Resources Website (Plant Diseases and Management)
- Crop Protection Network
Resources on Fungicide use (decision) and suitable modes of actions:
Contacts: Uta McKelvy, Assistant Professor Extension Plant Pathology
uta.mckelvy@montana.edu
Clain Jones, Professor and Extension Soil Fertility Specialist
clainj@montana.edu
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MSU Extension
