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Wheat Stripe rust update, April 11, 2013
Author: Arkansas Row Crops

There have been numerous reports of wheat stripe rust from eastern Arkansas, and the levels are much higher that at this time last year. Varieties most frequently reported to have stripe rust include Ricochet and Beretta that account for a significant portion of the wheat acres. In past years, these varieties were susceptible early and then changed to moderately susceptible and moderately resistant as the season progressed and adult-plant resistance was expressed more strongly. Other varieties with stripe rust include Arcadia and Progeny 117 that were susceptible early and moderately susceptible to susceptible as the season progressed.

Why is stripe rust so much worse this year compared to last year? Let’s assume that there were equal amounts of fall infection each year. Last year was warm and dry. Stripe rust came out of the winter fairly strong, but moisture was lacking for infection which slowed development even on susceptible varieties. Wheat grew quickly, and adult-plant resistance was expressed strongly early in the season. This year, temperatures were cool and moisture was plentiful. Stripe rust had frequent periods for reinfection, and cool temperatures were ideal for stripe rust but slowed wheat growth and development of adult-plant resistance.

Apparently some growers did not take the stripe rust threat seriously this year and did not make an early fungicide to fields that had stripe rust during the late winter and early spring. These fields are now being diagnosed with stripe while driving down the road. Other fields that received an early fungicide are still developing stripe rust. A 4 fl oz application of propiconazole or tebuconazole with the spring broadleaf herbicide likely would give 4 weeks of control because plants were small (less dilution of the fungicide) and temperatures were cool (slower degradation of the fungicide). Once the fungicide plays out, stripe rust starts developing again. The situation can be worse if portions of fields were not sprayed or other fields upwind are contributing a lot of inoculum. If a fungicide application is to be made to control stripe rust, an earlier application is almost always better than a later application of the same product!

Why should a fungicide be applied if the variety with stripe rust has adult-plant resistance? 1) If stripe rust develops early (like this year), it can reduce yield potential significantly before adult-plant resistance is expressed. 2) The adult-plant resistance in our varieties likely is race specific. If these varieties are grown over large areas for a long time, a new race of the stripe rust fungus likely will evolve to overcome the resistance. New races arise by mutation which is a random event. The larger the population size (number of spores) of the fungus, the greater the probability of a new race arising by mutation. If the mutation occurs in the field of the variety with adult-plant resistance, it will quickly multiply and spread. Controlling stripe rust early reduces the risk of new races. 3) Spores blow to other fields near and far causing more stripe rust. Controlling stripe rust early slows its spread across the region and country. Everyone farms downwind of others – be a good neighbor!

Given the early onset of stripe rust and the cool rainy weather in recent weeks, some wheat growers are considering two or three fungicide applications. This is a new phenomenon for Arkansas growers and requires some planning to stay within the legal limits for total amounts of particular fungicides that can be applied to wheat fields. The fungicide label lists to total amount of each active ingredient that can be applied per acre per year. These amounts usually are given in pounds of active ingredient (lb ai) which require some math to translate into fluid ounces (fl oz) of particular products. The total amounts and usual application rates below are for fungicides most likely to be used in Arkansas.

Tebuconazole: total amount = 0.11 lb ai/A = 4 fl oz/A.
Products containing only tebuconazole include Folicur (no longer being sold), Orius, Tebucon, Tebustar, Tebuzol, Tegrol, and Toledo.

Prosaro: total amount = 8.2 fl oz /A = 0.11 lb ai each of prothioconazole and tebuconazole.
(Note that no Prosaro can be applied if 4 fl oz of a tebuconazole product was applied earlier because Prosaro is half tebuconazole.)

Propiconazole: total amount = 0.22 lb ai/A = 8 fl oz/A.
4 fl oz/A = 0.11 lb propiconazole.
Products containing propiconazole include Tilt, Bumper, Fitness, Propiconazole E-AG, and PropiMax

Quilt Excel: total amount = 28 fl oz/A = 0.22 lb propiconazole + 0.26 lb azoxystrobin
14 fl oz Quilt Excel = 0.11 lb propiconazole + 0.13 lb azoxystrobin.
(Therefore 4 fl oz of a propiconazole product + 14 fl oz of Quilt Excel can be legally applied.)

Pyraclostrobin (Headline): total amount = 18 fl oz/A = 0.29 lb ai/A
9 fl oz = 0.147 lb Pyraclostrobin (Note the slight discrepancy between the total amounts expressed as fl oz/A and lb ai/A.)

Metconazole (Caramba): total amount = 34 fl oz/A = 0.20 lb ai/A
17 fl oz = 0.10 lb Metconazole

Twinline: total amount = 18 fl oz/A = 0.10 lb metconazole + 0.15 lb pyraclostrobin
9 fl oz/A = 0.05 lb metconazole + 0.076 lb pyraclostrobin
(Note the discrepancies for total amounts metconazole and pyraclostrobin depending on which products are used.)

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