Special Edition: Stripe Rust Alert!

Apr 20, 2016

By Erick DeWolf, Extension Wheat Pathologist

The recent rains across Kansas (Figure 1) will be beneficial to the wheat crop, but will also increase stripe rust activity on wheat. Stripe rust was already established at low levels in many areas of central Kansas and the recent rainfall greatly increases the risk that the disease will move the upper leaves soon (Figure 2). This movement to the upper leaves is important because they contribute most of the energy used by the plant to make grain. The disease has already reached the upper leaves in many fields in the southeast region of the state, which received more rain in previous weeks than other regions.

Figure 1. Precipitation summary for April 12-18, 2016. This rainfall may stimulate additional infections of the stripe rust fungus.

Figure 2. Distribution of stripe rust in Kansas as of April 18, based on observations from K-State Research and Extension plant pathologist, agronomists, and agents.

Where stripe rust was not yet present on wheat before the recent rains, producers have a little more time before making a decision about applying a fungicide. But it is still important to monitor for disease regularly. Fields planted to varieties susceptible to stripe rust should be scouted closely until the wheat is past the stage at which fungicides can be applied – which is flowering for most fungicides.

Where stripe rust was already present on lower leaves before the weekend of April 16-17, it may now quickly spread to the more critical upper leaves. A fungicide application now would likely be beneficial in those situations, depending on the yield potential of the crop.

In some areas of southeast and south central Kansas, stripe rust had already infected the flag leaf as of this past weekend. Where stripe rust is currently present on the flag leaves at low levels and most of the leaf’s green area is still intact, a fungicide application will still be beneficial in most cases. There is a point of diminishing return however. If the disease has already destroyed more than 25% of the upper leaves, the crop will be less likely to benefit fully from the fungicide application. In this case, the disease has already damaged a good portion of the leaf area and has likely already begun to infect much of the remaining green tissue. The remaining green tissue may still die even after the fungus has been suppressed by the fungicide.

In short, I think we are headed for trouble with stripe rust. Growers should be checking fields and ready to spray when the weather clears.

Wheat farmers have a lot of fungicide options to chose from although product availability may vary regionally in the state. Most of the products are rated very good to excellent on stripe rust (Table 1). In general, the largest reductions in disease severity and greatest increases in wheat yield or grain quality occur when fungicides are applied between full extension of the flag leaves and anthesis (when the male flower parts have just begin to emerge). Applications intended for the management of glume blotch or head scab should be made between the beginning of anthesis and 50 percent flowering. Always consult the product label for specific growth stage restrictions and preharvest intervals (PHI) before making fungicide application.

Table 1. Efficacy of many widely marketed fungicide products against stripe rust.

Fungicides
Class	Product	Active Ingredient	Rate (fl. oz./acre)	Stripe rust rating	Preharvest Interval
Strobilurin	Approach SC	Picoxystrobin 22.5%	6.0 - 12	Excellent**	Feekes 10.5
	Evito 480 SC	Fluoxastrobin	2.0 – 4.0	--	Feekes 10.5 and 40 days
	Headline 2.09 EC	Pyraclostrobin 3.6%	6.0 – 9.0	Excellent**	Feekes 10.5
Triazole	Caramba 0.75 SL	Metconazole 8.6%	10.0 – 17.0	Excellent	30 days
	Tilt 3.6 EC*	Propiconazole 41.8%	4.0	Very good	Feekes 10.5
	Proline 480 SC	Prothioconazole 41%	5.0 – 5.7	Very good	30 days
	Folicur 3.6 F*	Tebuconazole 38.7%	4.0	Excellent	30 days
	Prosaro 421 SC	Prothioconazole 19% Tebuconazole 19%	6.5 – 8.2	Excellent	30 days
Mixed modes of action	Absolute Maxx SC	Tebuconazole 22.6% Trifloxystrobin 22.6%	5.0	Very good	35 days
	Fortix	Fluoxastrobin 14.8% Flutriafol 19.3%	4.0 – 6.0	Excellent	Feekes 10.5 and 40 days
	Trivapro A EC + Trivapro B SE	Benzovindiflupyr 10.3% Propiconazole 11.7% Azoxystrobin 13.5%	4.0 + 10.5	Excellent	Feekes 10.5.4
	TwinLine 1.75 EC	Metconazole 7.4% Pyraclostrobin 12%	7.0 – 9.0	Excellent	Feekes 10.5
	Priaxor	Fluxapyroxad 14.3% Pyraclostrobin 28.6%	4.0 – 8.0	Excellent	Feekes 10.5
	Quilt Xcel 2.2 SE*	Propiconazole 11.7% Azoxystrobin 13.5%	10.5 – 14.0	Excellent	Feekes 10.5
	Stratego YLD	Prothioconazole 10.8% Trifloxystrobin 32.3%	4.0	Very good	Feekes 10.5 and 35 days
	Approach Prima SC	Cyproconazole 7.17% Picoxystrobin 17.94%	3.4 – 6.8	Excellent	45 days