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Tan Spot (06/06/13)

Tan spot is beginning to be found in several areas of the state. Growing degree accumulations across the state indicate that wheat fields planted from May 1 to May 10 are in the 3 to 5 leaf stage now.

Tan Spot

Tan spot is beginning to be found in several areas of the state.  Growing degree accumulations across the state indicate that wheat fields planted from May 1 to May 10 are in the 3 to 5 leaf stage now.tan_spot_symptoms.markell.png

Tan spot is the common fungal disease caused by Pyrenophora tritici-repentis, which infects winter wheat and spring wheat regularly in all the counties of North Dakota. The greatest likelihood of a yield loss to tan spot occurs when spring rains favor disease development with temperatures above 10°C (50°F), a susceptible or moderately susceptible variety is planted, and when the crop is planted into wheat residue.

Tan spot showed up in winter wheat fields in regular weekly scouting in some counties of ND (McHenry and Ward) already.

The early infections are small tan to brown spots surrounded by yellow halos often referred to as lens shaped lesions (Figure 1). These lesions coalesce into large blotches as they age, resulting in a premature senescence. The tan spot pathogen commonly infects leaves (Figure 1) and stems (Figure 2) contributing to severe yield losses.

There is an excellent disease forecasting system available in North Dakota. NDSU coordinates with North Dakota Agricultural Weather Network (NDAWN) sites across the state to make fungicide spray decisions.fruiting_bodies_on_wheat_pseudothecia.markell.png

The continued rain showers have resulted in long dew periods at many NDAWN sites, which are favorable for infection and disease development.    Disease models are confirming a risk of tan spot infection in many areas in the state. 

The NDSU small-grain disease forecasting system also information for the risk of late season tan spot and other Leaf spot infections in the web: www.ag.ndsu.nodak.edu/cropdisease/

On appearance of tan spot, an early season fungicide application may be warranted for disease management. Quite a few fungicides from triazole and strobilurin groups are available for early season sprays to manage tan spot (refer to 2013 North Dakota Field Crop Fungicide Guide for the latest options). Generally for early season tan spot control, half the full label rates can be used. Sprays can also be tank mixed with early herbicide application, generally at the 4-5 leaf stage.  For more information about tan spot refer to NDSU Extension bulletin PP-1249

Venkat R Chapara 
Area Extension Specialist/ Crop Protection 
NCREC, Minot, ND-58701

Marcia McMullen 
Professor Emeritus, Cereal Crops

Sam Markel
Extension Plant Pathologist

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