Wheat Foliar Fungicides

Transcription

1 Wheat Foliar s Robert L. Bowden, Extension Plant Pathologist Leaf rust, tan spot, Septoria leaf blotch (also called speckled leaf blotch) and powdery mildew are important fungal foliar diseases of wheat in Kansas. Average annual losses are approximately 7 percent and range up to 16 percent on a statewide basis. Of course, individual fields occasionally have much higher losses. Crop rotation and crop residue destruction are the traditional means of decreasing disease severity. However, many producers find their choices of rotational crops limited. Also, the need for soil conservation practices often prevents destruction of the wheat crop residue, which provides refuge for disease organisms between crops. Genetic resistance is probably the most effective and economical method of control. Unfortunately, most common wheat varieties are susceptible to at least one foliar disease. Because of this vulnerability to disease, foliar fungicides are sometimes a good investment for Kansas wheat growers. Nevertheless, experience has shown that fungicide benefits are variable, depending on many factors. s are more likely to Department of Plant Pathology pay in the following cases: high yield fields (greater than 45 bu/a), seed production fields, disease-susceptible varieties, minimum tillage continuous wheat, STAGE Feekes Growth Stages Wheat Growth Stages irrigated fields, wet spring seasons, years when rust overwinters extensively in Kansas, Ž later-maturing varieties, Ž high market prices for wheat. In order to use fungicides profitably, producers must carefully choose the circumstances where fungicides are used.

2 2 Disease Severity Diagram Wheat leaves show different percentages of disease severity. The Decision Process The first step in the decision-making process is to determine the disease potential. Crop scouting is critical to good decision-making. It needs to begin around the early jointing stage and continue on a regular basis until a final decision is made. Obtaining an accurate assessment of disease is important. Randomly select 25 tillers in each of several different areas of the field. Determine which diseases are present, and which leaves are being attacked. Use the diagram on page 1 to determine the crop growth stage and the diagram above to determine disease severity. Wheat Diseases, Cooperative Extension publication S-23, has color pictures of major diseases and is very helpful for wheat disease identification. Diagnosis is also available through your local county Extension office. Producers also need information about the disease susceptibility of the variety. Wheat Variety Disease and Insect Ratings, Cooperative Extension publication MF-991, gives the most recent results on disease reactions of common varieties. For example, Chisholm, Hawk, Lamed, Newton, and TAM 107 are some varieties that are highly susceptible to most foliar diseases. Caldwell, Karl and 2163 are moderately resistant. The last bit of information needed is the weather during the past two weeks and the weather forecast for the next week. Fungal diseases are favored by high humidity and frequent rains during the early jointing stage through the hard dough stage of crop development. Moderate temperatures in the 60s or 70s are ideal for rapid disease progress. If the weather is hot and dry, then foliar diseases will be suppressed. Use the worksheets on pages 3 and 4 to estimate disease potential for the crop. Disease potential can be used to estimate yield increases from fungicide applications. If disease potential is high, estimate a 15 percent increase. If disease potential is moderate, estimate a 10 percent increase, and if low, then estimate a 5 percent increase in yield. Of course, these figures are not exact, but they are realistic estimates based on many years of fungicide trials. Calculate expected bushels of increase by multiplying the expected percentage by the realistic yield goal for the field. For example, if the field typically yields 50 bu/a, and the disease potential is moderate, then the expected increase is 10% X 50 bu/a = 5 bu/a. Calculate the value of the increase in dollars by multiplying by the expected selling price of the wheat. For example, if the expected price is $3.50, then 5 bu/a x $3.50/bu = $17.50/A. In order to make the fungicide decision, you need to know the cost of application. Approximate chemical costs are presented in the table. Aerial application costs are approximately $5/A. So total costs are usually around $15/A. Use the worksheet on the last page to calculate expected returns. Obviously, since there is some risk involved, you don t want to just break even. Therefore, expected net returns need to be significantly more than the break-even point. The exact level will depend on the type of risk the producer wants to assume. When in doubt, it s probably wisest not to spray. If the field is for seed wheat, then expected price and resulting return are much more favorable for fungicide application. In addition, test weight is frequently increased by fungicides, so clean-out may be reduced. Seed producers should seriously consider fungicide applications each year, especially for disease susceptible varieties. Before you spray, be sure there are no other serious yield constraints in the field. s control only foliar fungal diseases, so do not spray fields with serious levels of insect pests, weeds, or diseases like take-all root rot, wheat streak mosaic, soilborne mosaic, or barley yellow dwarf. Application Available fungicides are listed in the table. Mancozeb is a protectant fungicide with no systemic action. Benlate is a systemic with good activity against Septoria leaf blotch and powdery mildew. Bayleton and Tilt have similar chemistries and both are partially systemic. They translocate within the leaf on which they are applied, but they do not move to newly emerging leaves. That is why it is continued on page 5

3 Septoria Leaf Blotch or Tan Spot Disease Prediction Worksheet* I. Growth Stage and Disease Development RECOMMENDED POINTS ASSESSED POINTS Feekes Stage Number of leaves from the top 6, 7 or 8 of the plant free of disease (early joint to flag Do Not Spray leaf just visible) or Do Not Spray (base of flagleaf showing to boot stage) (flowering) Do Not Spray II. Cultivar Susceptibility Resistant (Rating of 1-3) Do Not Spray Intermediate (Rating of 4-6) 1 Moderately Susceptible (Rating of 7-8) 3 Highly Susceptible (Rating of 9) 4 III. IV. Environment. (Based on the past 2 weeks and a one week forecast, select the point value most closely associated with the appropriate average temperature/moisture condition. Temperature selected should be average daytime temperature.) Moisture Temperature ( F) Dry Ave Wet < > Prediction of Disease. Add the score for sections I, II, and III above. If any of the previous sections resulted in Do Not Spray then do not apply fungicide. Total Score Disease Potential Return (%) 1-5 Low Moderate High 15 TOTAL SCORE *This worksheet should be used as a guide only. Disease epidemics are very difficult to predict with certainty. Parts of this were taken from the Kentucky Decision Guide for wheat fungicides.

4 4 Leaf Rust Disease Prediction Worksheet* RECOMMENDED ASSESSED I. Growth Stage and Disease Development POINTS POINTS Feekes Stage % of Flag Leaf and Flag-1 7, 8, 9 or 10 Covered by Rust Pustules (two joint through Do Not Spray boot) <1% % % >15% Do Not Spray (too late) (head emergence o do Not Spray and flowering) <1% % % >15% Do Not Spray (too late) II. Cultivar Susceptibility Resistant (Rating of 1-3) Do Not Spray Intermediate (Rating of 4-6) 2 Moderately Susceptible (Rating of 7-8) 4 Highly Susceptible (Rating of 9) 5 III. Environment. (Based on the past 2 weeks and a one week forecast, select the point value most closely associated with the appropriate average temperature. Temperature selected should be average daytime temperature. Leaf rust requires 6 hr of wetness for infection, If there is no rain or dew then enter 0) Temperature ( F) < >75 2 IV. Prediction of Disease. Add the score for sections I, II, and III above. If any of the previous sections resulted in Do Not Spray then do not apply fungicide. Total Score Disease Potential Return (%) 1-6 Low Moderate 10 TOTAL SCORE High 15 * This worksheet should be used as a guide only. Disease epidemics are very difficult to predict with certainty. Parts of this were taken from the Kentucky Decision Guide for wheat fungicides.

5 5 Wheat Foliar s Effectiveness Common Name Trade Name Company Rate Leaf Septoria Tan Powdery Approx. Cost* per Acre Rust Leaf spot Mildew per Acre ($) Blotch benomyl Benlate 50 WP DuPont 8-16oz N** F N F-G mancozeb Dithane M-45 Rohm & Haas 2 lb G G F P 5.50 Manzate 200 DF DuPont Penncozeb Atochem propiconazole Tilt Ciba- Geigy 4 fl oz VG G G E triadimefon Bayleton 50 WP Miles Laboratories 2 oz F P P E oz G F P E oz VG F P E oz VG F P E tank mix mancozeb 2 lb VG G F-G E Bayleton 50 WP 2 oz * Add $5 per acre for aerial application. ** N=not recommended, P=poor disease control, F= fair, G=good, VG=very good, E=excellent READ THE LABEL Mention of specific products does not constitute a guarantee or warranty of the product by Kansas State University. Read the label and select only fungicides that may be used legally. Consult the label for directions and restrictions on fungicide usage. continued from page 2 critical to wait for the flag leaf to begin emerging before applying any of these fungicides. Bayleton remains effective for 2 to 3 weeks and Tilt lasts slightly longer. Mancozeb is subject to weathering and has a shorter residual than the other fungicides. If mancozeb is used alone, multiple applications are often required to achieve good control. The effectiveness of the fungicides against the various diseases differs. It is important to select a fungicide that is effective against the disease(s) present in the field. Tank mixes of these fungicides can often be used to increase the spectrum of disease control. However, unless the tank mix is recommended on the label, the user assumes responsibility for compatibility of the mixture. The most commonly recommended tank mix is mancozeb plus Bayleton. Proper application timing is critical for success. s must be applied after flag leaf emergence, but before significant disease develops on the flag leaf. Applications after the flowering stage are rarely economical because significant damage will have already occurred. The Tilt label prohibits application after Feekes stage 8 (flag leaf emergence). In some cases this may be too early, but later application is illegal. The tank mix of mancozeb and Bayleton is usually applied at about the boot stage (Feekes 10). Bayleton and Benlate require a minimum 21-day waiting period between application and harvest. Mancozeb requires a 26-day interval after application. selection will depend primarily on when application can be made. If the wheat is at Feekes stage 8, then Tilt is usually preferred. If the wheat is past Feekes 8, then the tank mix of Bayleton and mancozeb is probably the most effective choice. If neither tan spot nor Septoria leaf blotch are present, then Bayleton alone could be used. It is important to apply these fungicides in enough water to get adequate coverage. For aerial application, 5 gallons of water per acre are required to get good coverage. For ground rigs, a minimum of 15 gallons of water per acre should be used. A spreader/sticker is recommended for the dry flowable or wettable powder formulations.

6 6 Field 1 E X A M P L E Profit Worksheet 10% X 50 Bu/A X $ 3.50 = $ Percent Increase Realistic Yield Goal Selling Price $ _ $ = $ 2.50 Application Cost Net Return per acre Field 2 X Bu/A X $ = $ Realistic Percent Increase Yield Goal Selling Price $ - $ = $ Application Cost Net Return Field 3 X Bu/A X $ = $ Realistic Percent Increase Yield Goal Selling Price $ $ = $ Application Cost Net Return Field 4 X Bu/A X $ = $ Realistic Percent Increase Yield Goal Selling Price $ - $ = $ Application Cost Net Return Cooperative Extension Service, Manhattan, Kansas Issued in furtherance of Cooperative Extension Work, acts of May 8 and June 30, 1914, as amended. Kansas State University, County Extension Councils, Extension Districts, and U.S. Department of Agriculture Cooperating, Richard D. Wootton, Associate Director. All educational programs and materials available without discrimination on the basis of race, color, national origin, sex, age, or disability. March 1995 (Revised) File Code: Plant Pathology, 2-A M; M; M