By Emmanuel Byamukama

Fungicides applied with herbicides at tillering in wheat may not always result in a profitable investment, partly due to low disease pressure at this point in time. With the aid of a disease-forecasting tool now available, producers can now combine disease forecasting and scouting to determine the need for fungicide treatment in wheat. 

There are two reasons why applying fungicide with herbicides at the tillering stage is common. First, fungicides are relatively inexpensive and are mostly applied at half the rate at this time. Second, applying a fungicide and herbicide at the same time further reduces cost because the two pesticides share one application cost. 

However low the cost is, there is still an expense involved, so the question of whether this expense is justified still needs to be answered. In most cases, the answer to this question is no. Only in certain circumstances (wheat following wheat, an excessively wet spring, or susceptible cultivars) does including a fungicide with an herbicide application at the tillering stage result in increased yield. 

One of the reasons why this timing does not result in increased yield is that wheat at the tillering growth stage usually has very little disease developing. Diseases such as tan spot, powdery mildew and Stagonospora/septoria leaf blotch pathogens survive on crop residue (Figure 1), and develop and become severe when the canopy starts to close in. This is when more moisture is trapped under the canopy and there is poor air circulation. These conditions promote the development of leaf spot and powdery mildew. Usually at tillering, the undergrowth is not yet too dense to promote disease development except under the above listed risk factors.

Above: Figure 1. Wheat straw with fruiting bodies of tan spot fungus. Photo by: E. Byamukama

Weather conditions do influence the level of disease development and therefore influence yield gain resulting from fungicide application. Research done at SDSU shows a strong relationship (R2 = 78%) between rainfall in the month of May and the yield gain between early strobilurin treated plots and non-treated winter wheat plots (Figure 2). More rainfall in May was associated with higher yield gain from a strobilurin fungicide application. Although this is not an extensive data set (two sites, three years), it shows the likelihood of seeing profit when there is higher spring precipitation. 

The SDSU Plant Pathology Extension program has partnered with the North Dakota State University small grains program to develop a small grains disease forecasting system based on weather conditions. This tool will provide information on the likelihood of development for tan spot, stagonospora nodorum blotch and Fusarium head blight or scab. 

The system indicates whether weather conditions are conducive for these diseases and records a “Yes” or “No” for each day. Accumulation of six or more days with “Yes” when the crop is at the late jointing stage would indicate that applying fungicide is warranted. The Fusarium head blight forecast is provided when heading or a later growth stage is selected. The producer should select a weather station closest to the field being assessed. For details and to use the tool, please visit the SDSU South Dakota Climate and Weather Web site. 

The decision to apply a fungicide should hinge on the weather conditions, type of crop residue, and any indication of diseases beginning to develop. Producers are encouraged to scout and consider disease in the forecast using the above tool and decide on the need for fungicide application. 

Above: Figure 2. Relationship between yield increase due to a strobilurin fungicide application at tillering and rainfall in the month of May. The more rain in the month of May, the higher the yield from treated plots. Data is from Volga, Brookings County and South Shore, Codington County from 2010 through 2013.