In case you have never seen the image above, it is called the disease triangle. Like any triangle if one of the points does not exist, the triangle does not exist. Typically, in Eastern Montana, we may have the host and pathogen present in our crop fields but because we do not normally receive much in the amount of rainfall and therefore, our crops do not remain constantly wet, we do not have the correct environment for most diseases to get established.
Well, this year we have had a wetter than normal spring and, because of this, I have received a few calls regarding patches of diseased winter wheat stands. Here is an article from the recent issue of the NDSU Crop and Pest Report.
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 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.
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.
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
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
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