1. Fungicides as a first line of defense for effective control of stem rust in hard red spring wheat
A. Adhikari1, R. Dill-Macky 2, J. J. Wiersma2, S. Haguen2 and M. J. Smith2 University of Minnesota, St. Paul, MN, U.S.A; 2University of Minnesota, Northwest Research and Outreach Center, Crookston, MN, USA.
Introduction:
Stem rust caused by Puccinia graminis f.sp. tritici is a major constraint of wheat production worldwide. Stem rust infection manifests as pustules on stems (Fig.1) and leaves (Fig.2) that drains nutrients from the plant and cause water loss from plant tissues. Stem rust may lead to severe lodging and this is the only disease of wheat capable of eliminating all yield. Development of resistant cultivars and elimination of the alternate host, the common barberry (Berberis vulgaris) served as a very effective control of Stem Rust for three decades, until 1998, when a new race (TTKSK, a.k.a Ug99) was found in otherwise resistant CIMMYT germplasm in Africa.
The race TTKSK has unique combination of alleles that makes it virulent on 84% of all US spring wheat cultivars in the Northern Great Plains and 84% of the CIMMYT cultivars tested. New races within the Ug99 lineage have already spread from Africa into the Middle East (Fig. 3). Some resistance genes effective against Ug99 have been identified; including Sr2, Sr36 and SrTmp. The process of introgressing resistance genes in to commercial cultivars may take years. Barberry bushes in close proximity to wheat fields in the US opens the possibilities of new virulent races of stem rust developing locally.
In this context, fungicide treatments may provide the only effective means to control of stem rust if races of the Ug99 linage arrive in the US before the release of resistant cultivars. This study was undertaken with the aim of finding the most effective fungicide against stem rust and establishing effective timings of fungicide applications. Twelve commercial fungicides were tested against a local race of stem rust (QCFS) at two Minnesota locations (Crookston and St Paul) in 2013.
Fig. 4: Syringe inoculation into leaf whorl
Fig. 5: Fungicide treatments, growth stage and application rates (ml/ha)
Results:
Needle and spray inoculations resulted in successful infection of wheat and pustule development on stems and leaves in both locations. Severities in the untreated control averaged 15.5% and 15.0% in St Paul and Crookston, respectively.
All fungicides tested reduced stem rust incidence and severity (P =  and 0.04, respectively) at Crookston except treatment 2(Fig. 6a). All fungicides reduced stem rust incidence (P = ) and reduced yield losses compared to the untreated control (P=0.001) in St. Paul (Fig. 6b).
Linear contrasts between fungicide application timings were significant for Headline® , Priaxor® and Tilt® applied at Feekes 8 (flag leaf) and Feekes (anthesis) for incidence (P = ), suggesting that the later application timing provided effective disease control of stem rust longer into the growing season.
The fungicides classed in the demethylation inhibitor (DMI) mode of action group gave, on average, better control at Feekes (linear contrast significant at p=0.03).
Fig.1: Stem rust pustules
(source: aps.net.org )
Fig 3: Spread of Ug99 lineage
(source:
Fig.2: Stem rust pustules on leaf
Fig. 6 a: Treatments 1 (Control),
2,3,4 (Feekes 8), 5-12 (Feekes )
Fig. 6 b: Fig. 6 a: Treatments 1 (Control),
2,3,4 (Feekes 8), 5-9 (Feekes )
Methods:
Cultivar = Max, a hard red spring wheat
Stem rust race: QCFS (a North American race of P. graminis f. sp. tritici)
Experimental Design: randomized complete block design with 4 replicates, plot size = 1.5 x 4.5 m2
Inoculation:
Inoculum = uredeniospores (2mg/ml aqueous suspension with one drop of polysorbate (Tween 20) added per 100ml inoculum)
Thirty plants per plot were needle inoculated at flag leaf (growth stage = Feekes 8) with 0.5 ml of inoculum delivered into the leaf whorl (Fig. 4)
Plots were also sprayed with a urediniospore suspension at flag leaf stage at the concentration of mg spores?/ml isoparrafin (Soltrol 170) / application rate?) applied using an Ulva+ sprayer
Disease Assessment: Incidence and severity were assessed visually on 20 stems per plot
Post-harvest data collected included: yield, thousand kernel weight and grain protein
Fungicides were sprayed with CO 2 powered sprayer at the rate of ?. (See Fig. 5. for treatments)
Conclusion:
The data showed that fungicides Caramba® and Prosaro® provide the best reduction in both severity and incidence of stem rust when applied at anthesis (growth stage =Feekes ). Data from multiple seasons will enable us to reach more robust conclusions – this we plan to repeat these experiments in 2014.
Acknowledgements:
Dr. Yue Jin, Galen Thompson, Jim Cameron, Justin Stanton
Source (Background Picture):