Should You Spray for Early Season Tan Spot?

By Charla Hollingsworth
Extension Plant Pathologist, Northwest Research & Outreach Center, holli030@umn.edu

Tan spot, shown in the photo at right, is a common leaf disease of wheat in the Red River Valley.  The fungus that causes tan spot (Pyrenophora tritici-repentis) overwinters on aboveground crop stubble, infects plants in the spring and causes disease over a wide range of temperatures (68o-82oF) as long as adequate moisture is present for an extended period (12-24 hours).

Tan spot and Septoria/Stagonospora leaf spotting diseases produce similar leaf symptoms. Tan spot produces oval-shaped, tan lesions that enlarge with age.  Lesions are generally surrounded by a yellow halo, while dark areas eventually form on tissues that were infected initially. Severe disease development causes spots to expand until most, if not all, plant leaf tissues are killed.  Yield and kernel weights are reduced if infection occurs early enough.

Spring wheat varieties have differing levels of foliar leaf disease resistance. Rotation with broadleaf crops and residue management can help prevent early-season disease development.  However, as the growing season progresses, spores are produced locally on diseased, non-cropped plants, as well as wheat fields in other locations, and these spores can travel long distances with the help of gusty winds.

Even if growers routinely practice all known management options, they may still find tan spot lesions on wheat plants. If the disease is established prior to flag leaf emergence and extended wet weather is forecast, a fungicide application provides a viable control alternative.

Many growers interested in applying a fungicide for early-season disease control prefer to do so at the 4-5 wheat leaf stage, when they apply herbicides and can tank mix both for a one-pass application. However, it’s advised to scout for disease potential before considering a tank-mixed herbicide/fungicide application. The fungicide application is recommended when the following conditions apply:

1. A susceptible variety is grown (especially if planted into wheat stubble)

2. Plant tissues remain wet for an extended period

3. Field scouting has identified tan spot lesions on lower plant leaves

Several fungicide products are labeled for early-season tan spot control on wheat (Table 1). Of the fungicides listed, only one is a protectant (Dithane) and must be on leaf surfaces before the fungal spores are present to infect the plant. If the fungicide is used after a spore lands on the leaf, there is no protective barrier against infection.  All other fungicides are systemic and can be applied before or after plants become diseased.

There were some reports of crop injury last year with certain herbicide/fungicide tank mixes, which might have been weather related. Read labels of both herbicide and fungicide products carefully for allowable tank mix partners, and consult with an agronomist or product retailer for more details.

Producers who practice good integrated pest management by using crop rotation and tolerant wheat varieties can still have tan spot develop if the weather stays wet for several days.  Tan spot can worsen rapidly, so regular field scouting is recommended if plant tissues remain wet. Current wheat disease survey information will be posted online by the Minnesota Dept. of Agriculture at www.mda.state.mn.us/pestsurvey/pestreports/pestreport.html .

Another good tool to help scout for the formation and outlook of leaf diseases this season is the NDSU disease forecasting system, online at www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm

Small Grain Fungicide Use Decision Guide Online
SDSU extension plant pathologist Marty Draper offers a Small Grain Fungicide Decision Guide designed to help the producer or applicator determine if a foliar fungicide applied to the crop will return an economic benefit.

The decision guide is in the form of a Microsoft Excel ver. 4.0 spreadsheet, which is used by inserting the values appropriate for your operation. The spreadsheet will make all the necessary calculations. Examples are presented and a guide to possible values is included.

Generally, says Draper, a yield increase of 10-15% will be attained from a fungicide application. The cost of the fungicide product, application costs, and the market price for the commodity are the greatest variables that will influence profitability of fungicide application.

The fungicide decision guide can be downloaded from the web site http://plantsci.sdstate.edu/planthealth/Scab/fungguid.htm or contact Draper, ph 605. 688.5157, email draper.marty@CES.sdstate.edu .