1. Introduction

Stem rust caused by Puccinia graminis f.sp. tritici (Pgt) is a destructive disease of wheat which occurs frequently in warm and humid environments such as those found in Kenya. A new and significant threat has been posed in the Eastern Africa region, due to reemergence of a new virulent race, TTKS (Ug99) which broke down resistance genes that had been used to control the disease ^[14]. In the early 1990’s, wheat stem rust epidemics were experienced in Kenya following the breakdown of resistance (Sr36) in the popular cultivar Enkoy. A new pathotype of Pgt was detected in Eastern Africa in 1999 (Ug99) ^[9]. Since then, Ug99 has been detected in Uganda, Kenya and Ethiopia. The pathotype has caused considerable concern because of its broad virulence spectrum that include gene Sr31, one of the most widely deployed stem rust resistance gene that remained effective until the detection of Ug99. Growing wheat in diverse agro-ecological zones in Kenya throughout the year is a major contributing factor to the pool of stem rust inoculum ^[2]. It is therefore difficult to prevent or reduce infection of susceptible cultivars. Regular occurrence of disease outbreaks in Kenya are due to favourable environmental conditions hence the designation of the country as a “hotspot” for stem rust ^[15]. Genetic resistance has provided adequate protection against stem rust disease for over 30 years ^[6]. However, the susceptibility of most wheat cultivars and heavy yield losses exceeding 80% ^[16] incurred by farmers has entailed the adoption of an integrated management strategy, hence the necessity to evaluate the available fungicides for efficacy against stem rust pathogen. Widespread growing of resistant cultivars to control the disease effectively has been adopted ^[4]. It is impossible to grow a profitable crop of wheat without fungicide application as all the current commercial wheat cultivars are susceptible to the new race. A stem rust severity level greater than 5% in hot-spot regions should be controlled to reduce yield losses. In the past there was no need for fungicide since there was widespread deployment and cultivation of resistant cultivars providing adequate protection ^[6]. Fungicides can be used as a short term management of the disease until new cultivars with genetic resistance become available. Fungicide treatments, if applied under high and moderate disease pressure at critical growth stages, may reduce large yield losses by suppressing or eliminating stem rust pathogens ^[7].

Stem rust caused 32 to 57% of grain yield loss and 17 and 24% of thousand kernel weight reduction ^[15]. The use of fungicide to control wheat stem rust specifically related to stem rust race Ug99 and its variants, has very little published information. Epidemics of leaf rust were more intense in Uruguay in 2010 and in main wheat growing areas of Argentina, but caused moderate epidemics in all countries in 2008 and 2009. In areas favorable for disease development, leaf rust can cause grain yield losses higher than 50% in severe epidemics if fungicides are not applied. Majority of cultivars are either susceptible or moderately susceptible, the fungicide application cost of controlling leaf rust in affected regions totals 50million US dollars. Farmers without resources have the option of using fungicides against Ug99 as a last resort, however, this option is too expensive for many smallholder farmers in the developing world making them rely entirely on resistance to control wheat diseases ^[5].

2. Materials and Methods

During the rainy season of 2014 (June-November), and dry season of 2015 (February-May), an experiment was conducted at the Kenya Agricultural and Livestock Research Organization (KALRO) Njoro, (0 20′S, 35^o 56′E) at 2185 m above sea level with mean minimum and maximum temperatures of 9.7°C and 23.5°C, respectively and an average annual rainfall of 900 mm. The site was chosen because environmental conditions are conducive for stem rust infection and it is a representative of wheat and barley growing areas ^[12].

The cultivar Kwale, adapted cultivar, was used for fungicide evaluation. The cultivar shows high yield, medium maturity, but susceptibility to stem rust.

Experimental plots were disc ploughed and harrowed twice to fine tilth suitable for wheat growing. Seeds were sown in plots of four rows of 80cm × 2.0m. Sowing was made by a seed driller at the seeding rate of 108.3kg ha^-1. At sowing time, diammonium phosphate (DAP) fertilizer was applied at the rate of 125kg ha^-1 in order to supply an equivalent of 22.5kg N ha^-1 and 25kg P ha^-1. A randomized Complete Block Design (RCBD) of split plot arrangement with three replicates was used. The experimental plots and rows were separated by 0.5m wide paths within and between the blocks. The fungicide was sprayed at three stages of the plant growth at three rates: Fosphite 53 SL (Monopotassium + dipotassium 530 g a.i. L^-1) at 5L ha^-1, Fosphite 53 SL Fosphite 53 SL (Monopotassium + dipotassium 530 g a.i. L^-1) at 7.5L ha^-1 and Fosphite 53 SL Fosphite 53 SL (Monopotassium + dipotassium 530 g a.i. L^-1) at 10L ha^-1. One standard fungicide: Folicur 25 EW (tebuconazole 250 g a. i. L^-1) was applied at the rate of 2.5L ha^-1 and non-treated control was used for comparison. At tillering stage (Zadok’s growth stage 20-29) (Zadok’s et al., 1974) the plots were sprayed with Buctril MC (bromoxynil ectanoate 225g ha^-1 and MCPA Ethyl Hexyl Ester 225 g ha^-1) to control broad-leave weeds. The trial was top dressed with Calcium Ammonium Nitrate (CAN) at stem elongation (Zadok’s GS30) ^[19] at the rate of 100kg ha^-1 in order to supply additional 33 kg N ha^-1. Bulldock (beta-cyfluthrin) was sprayed to control pest infestation at the rate of 31 g ha^-1 and repeated at 14-day intervals when required.

Stem rust severity scoring begun when the plants had attained 50% susceptibility based on modified Cobbs scale where 0% = immune, and 100% = completely susceptible ^[8] for three readings with an interval of 10 days between heading (GS 50-69) and plant maturity (GS 70-89) ^[18]. Plant response to rust infection at the adult plant stage was termed “infection response”. Based on the size of pustules and the associated necrosis or chlorosis, infection responses were classified into four discrete categories: R = resistant, MR = moderately resistant, MS =moderately susceptible, S = susceptible, MRMS denoted an infection response that overlaps the MR and MS categories ^[10]. Other agronomic traits such as date of emergence, date of flowering and date of maturity were recorded; 5 plants in the two middle rows were measured when 50% of the plants per entry had headed for parameters such as, plant height, measured from the base at ground level to the tip of plant; number of tillers were counted manually; spike length was measured from the first node where the first spikelet emerges to the spike tip; number of spikelet/spike were counted; 100 scale, while the yield was determined after drying to 12.5% HR.

The data for the yield and agronomic traits, once collected and organized, was analyzed using Statistical Analysis System 9.4 portable version (SAS Institute, Inc 2012) using the following statistical model:

Where:

Y_{ijklm =} Observation of experimental units, µ = Overall mean, S_i = i^th effect due to season R_{j(i) =} effect due to j^th replicate, F_k = effect due to k^th fungicide rates, SF_ik = i^th effect due to season and k^theffect due to fungicide rate, V_l = effect due to l^th crop stage, SV_il = i^th effect due to season and l^th effect due to crop stage, FV_kl = k^th effect due to fungicide rate and l^th effect due to crop stage, SFV_ikl = i^th effect due to season, k^th effect due to fungicide rate and l^th effect due to crop stage, ε_ijklm = random error components.

Area under Disease Progress Curve (AUDPC) was used. It was computed using the formula by ^[18] and AUDPC CIMMYT programme

Where;

t_i is the time in days of each reading, y_iis the percentage of affected part of the plant at each reading, n is the number of readings, t (i+1) is the second assessment date of two consecutive assessment, y (i+1) is the disease severity on assessment date t (i+1).

3. Results

Stem rust and yellow rust infection was observed across the two seasons and was high in dry season (February to May, 2015) compared to rainy season (June to November, 2014). There was a variation in the time of disease set in in the two seasons, with the+ disease setting in earlier.

The effect of season were significant (P≤0.001) for AUDPC-SR, AUDPC-YR, plant height, tillers, spike length, biomass and yield. Significant (P≤0.01) effects (fungicide rate) were observed for AUDPC-SR and AUDPC-YR. Crop stage showed significant (P≤0.01) for AUDPC-YR. There was significant (P≤0.05) effect (season × crop stage) for AUDPC-YR and (P≤0.001) for plant height. In addition, season × fungicide rate × crop stage interaction were significant for plant height (Table 1).

Effect of fungicide rates on wheat.

Mean values for the parameters observed for the different fungicide rates are given in Table 2. The effect of fungicide rate was noted on AUDPC-SR, AUDPC-YR, tillers, spike length and TKW.

Table 1. Mean squares of season, replicate, variety and variety × season on Audpc-SR, Audpc-YR, plant height, number of tillers, spike length, spikelet per spike, thousand kernel weight, biomass and yield on wheat cultivar Kwale Njoro, 2014-2015

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Table 2. Effect of fungicide treatments on AUDPC-SR, AUDPC-YR, plant height, number of tillers, spike length, spikelet per spike, thousand kernel weight, biomass and yield on wheat cultivar Kwale Njoro, 2014-2015

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For Fosphite 53SL application at 0.5L ha^-1, disease decrease was 6.836% and 1.277% for AUDPC-SR and AUDPC-YR, respectively, while mean for TKW and yield was 12.567% and 31.609% higher than the control, respectively. The rate registered a higher percentage for mean decrease in AUDPC for SR.

Fosphite 53SL application at 0.75L ha^-1: showed no effect on control of stem rust and yellow rust. Also, there was a reduction in % mean increase in TKW and yield.

Fosphite 53SL application at 1.0L ha^-1: Disease management for stem rust was low at 5.05% over the control and -0.718% for yellow rust. The rate registered highest % in TKW at -14.792% over the control with the highest yield increase at -35.052% for all the product rates.

Folicur EW 250 application at 0.25L ha^-1: The mean % disease decrease for AUDPC for stem rust and yellow rust registered higher % at 40.580 and 32.930, respectively. It also had higher mean % increase in TKW and yield at -25.960 and -43.969, respectively. The mean performance was significantly different from the Fosphite 53SL rates.

Effect of crop stage on the effectiveness of fungicide.

Table 3, shows mean values for the parameters observed for the three stages (tillering, stem elongation and heading) while the reduction was observed for yellow rust at tillering and stem elongation stage. There was reduced stem rust severity at all the growth stages: tillering, stem elongation and heading. Heading stage showed higher values for AUDPC-SR, AUDPC-YR, plant height, tillers, spike length and TKW. All the other parameters were not significantly different from each other for the three crop stages. Tillering stage had lower values for AUDPC-SR and AUDPC-YR by 3.41% and 17.617%, while it had high yield weight as compared to stem elongation and heading stage.

Table 3. Effect of three growth stages on AUDPC-SR, AUDPC-YR, plant height, number of tillers, spike length, spikelet per spike, thousand kernel weight, biomass and yield on wheat cultivar Kwale Njoro, 2014-2015

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Effect of season on the effectiveness of fungicide on wheat

Mean values for the parameters observed in two different seasons are given in Table 4. There was high disease pressure in main-rain (June to November, 2014) compared to off-rain (February to May, 2015) season by 12.472%. The effect of season was noted on AUDPC-SR, AUDPC-YR, plant height, tillers, spike length, biomass and yield. Off-rain, 2014 season had higher values for AUDPC-SR and AUDPC-YR at 40.579% and36.777% and lower values for plant height, tillers, spike length, spikelet per spike, TKW, biomass and yield by 17.127%, 36.874%, 19.543%, 1.27%, 3.26%, 39.263%, and 40.148%, respectively as compared to main-rain season. There was no significant effect of season on spikelet per spike and TKW.

Table 4. Means of main-rain, 2014 and off-rain, 2015 for AUDPC-SR, AUDPC-YR, plant height, number of tillers, spike length, spikelet per spike, thousand kernel weight, biomass and yield on wheat cultivar Kwale Njoro

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4. Discussion

Fungicides can play a major role in the integrated management of the disease, until new cultivars with genetic resistance are available ^[6]. The use of fungicide to control stem rust disease has shown 50% higher yield in treated versus untreated plots in various wheat growing regions of Kenya ^[15]. Use of resistant cultivars remains the only alternative in reducing stem rust severity because of the high costs in using fungicides and the serious health problem it causes for the users and environment. Most of the fungicides tend to be effective at specific growth stages ^[1]. Sometimes, crops that receive three poorly timed sprays suffer as much disease as the untreated crops, suggesting that either the fungicide are applied too late, or do not effectively control the disease. There was no evident fungicide phytotoxicity in our treatments. Wheat cultivar, Kwale, used in the experiment did not show any visible injurious effects. There was no chlorosis of wheat in all the treated plots as well as in the plots that were not sprayed with fungicides. This showed that all the test rates of Fosphite 53SL and Folicur 250 EW were safe on Kwale.

The study was carried out at KALRO-Njoro which is known to have favorable weather conditions for stem rust of wheat. Plots that had the highest disease score as shown by the AUDPC, had low grain yield and TKW. This concurs with the study by ^[17] that reported plots with the highest AUDPC had low grain yield, thousand grain and test weights. Application of fungicide at critical growth stages under high and moderate disease pressure, may reduce large yield losses by suppressing or eliminating stem rust pathogen.

In the study, of all the tested rates of Fosphite 53 SL only the rate of 0.5L ha^-1 showed significant reduction in the disease pressure and positive influence in the yield and grain weight. Folicur 250 EW which was the positive control was more effective than Fosphite 53 SL in reducing disease and increasing yield. There was inconsistency in the effectiveness of Fosphite 53 SL. In a study by ^[6] it was reported that Folicur (tebuconazole) was more effective than Triad (triadimefon) or Impact (flutriafol) in disease reduction and yield increase hence the variation in the efficacy of different fungicides in controlling stem rust. Regions with favourable weather conditions for stem rust occurrence, with stem rust severity level greater than 5% under field conditions should be controlled to reduce yield losses ^[6]. The three growth stages (tillering, stem elongation and heading) did not show any variation in the reduction of stem rust severity, while there was a low score for yellow rust severity at tillering stage and a high yield. This is consistent with the study made by ^{[15, 17]} which showed that spraying at tillering and flowering growth stages of the crop, higher rates and more fungicide applications could achieve greater rust control and greater yield increases. Stem rust is very severe in susceptible varieties when it begins to develop in the crop before flowering. There is a clear relationship between grain yield and disease severity by demonstrating that prevention of a 1% increase in rust severity saved 2% loss in grain yield ^[7].

5. Conclusion

The study displayed that wheat grain yield of susceptible cultivars could be highly reduced by stem rust and yellow rust, therefore, the adoption of foliar fungicides to combat stem rust disease as a short term control strategy until resistant cultivars are developed is encouraged in Kenya. Growth stage was found to be an important factor in fungicide management of wheat rusts. Fungicidal activity of Folicur 25 EW and Fosphite 53 SL could have played a major role in their efficacy differences. The ability to suppress disease development and protect the crop canopy, which is vital for dry matter accumulation and yield significantly differ among fungicides ^[13]. Fosphite 53 SL foliar fungicide applied at tillering growth stage at the rate of 0.5L ha^-1 was the most appropriate in effectively reduced/controlled the rust infection and increased grain yield. This study showed that foliar fungicide can be used to manage wheat rust as a short term control strategy. However, more research should be done to identify the right timing of application and the dosage.

Acknowledgements

We acknowledge the technical and financial support of KALRO, Njoro through the Durable Rust Resistance in Wheat project (DRRW) and Juanco Limited for provision of the test fungicides.

Competing Interests

The Authors have no competing interest.

References

[1]	Cook, R.J., Hims, M.J. and Vaughan, “Effects of fungicide spray timing on winter Wheat Disease Control,” Plant Pathology, 48: 33-50, 1999.
	In article		View Article
[2]	Green, G.J., Martens, J.W. and Ribeiro, O, “Epidemiology and specialization of wheat and oat stem rust in Kenya in 1968,” Phytopathology, 60:309-314, 1970.
	In article		View Article
[3]	Gomez, K.A. and Gomez, A.A, “Statistical procedures for Agricultural Research,”1974.
	In article
[4]	Jin, Y., Pretorius, Z.A., Singh, R.P. and Fetch, T.Jr, “Detection of virulence to resistance gene Sr24 within race TTKS of Puccinia graminis f. sp. Tritici,” Plant Disease, 92:923-926, 2008.
	In article		View Article
[5]	Jin, Y., Szabo, L., Rouse, M., Fetch, T.Jr., Pretorius, Z.A., Wanyera, R. and Njau, P, “Detection of virulence to resistance gene Sr36 within race TTKS lineage of Puccinia graminis f. sp. Tritici,” Plant Disease, 93:367-370, 2009.
	In article		View Article
[6]	Loughman, R., Jayasena, K. and Majewski, J, “Yield loss and fungicide control of stem rust of wheat,” Australian Journal of Agricultural Research, 56:91-96, 2005.
	In article		View Article
[7]	Mayfield, A. H, “Efficacies of fungicides for the control of stem rust of wheat,” Australian Journal of Experimental Agriculture, 25: 440-443, 1985.
	In article		View Article
[8]	Peterson, R.F., Campbell, A.B. and Hannah, A.E, “A diagrammatic scale for estimating rust intensity of leaves and stems of cereals,” Canadian Journal of Research, 26:415-421, 1948.
	In article		View Article
[9]	Pretorius, Z.A., Singh, R.P., Wagoire, W.W. and Payne, T.S, “Detection of virulence to wheat stem rust resistance gene Sr31 in Puccinia graminis f. sp. tritici in Uganda,” Plant Disease, 84:203, 2000.
	In article		View Article
[10]	Roelfs, A.P., Singh, R.P. and Saari, E.E, “Rust Diseases of Wheat: Concepts and methods of disease management. Mexico, CIMMYT,” 1992.
	In article
[11]	SAS Institute, “SAS procedure for personal computers. Version 9.4 SAS Institute Inc., 22 Cary, NC, USA,” 2012.
	In article
[12]	Singh, R.P., Hodson, D.P., Jin, Y., Huerta-Espino, J., Kinyua, M.G. and Wanyera, R, “Current status, likely migration and strategies to mitigate the threat to wheat production from race Ug99 (TTKS) of stem rust pathogen,” CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 1, No. 054, 2006.
	In article		View Article
[13]	Viljanen-Rollinson, S.L.H., Marroni, M.V. and Butler, R.C, ”Wheat stripe rust control using fungicides in New Zealand. N. Z,” Plant Protection, 59:155-159, 2006.
	In article
[14]	Wanyera, R., Kinyua, M. G., Jin. Y. and Singh, R. P, 3”The spread of the stem rust caused by Puccinia graminis f. sp. tritici, with virulence on Sr31 in wheat in Eastern Africa,” Plant Disease, 90:113, 2006.
	In article		View Article
[15]	Wanyera, R., Macharia, J.K., Kilonzo, S.M. and Kamundia, J.W, “Foliar fungicides to control wheat stem rust, Race TTKS (Ug99), in Kenya,” Plant Disease, 93 929-932, 2009.
	In article		View Article
[16]	Wanyera, R., Macharia, J.K. and Kilonzo, S.M, ”Challenges of fungicide control on wheat rusts in Kenya, in: O. Carisse (Ed.), Fungicides, ISBN: 978-953-307-266-1, Publisher: in Tech,” Plant Pathology, 123-138, 2010.
	In article		View Article
[17]	Wanyera, R., Wamalwa, M., Odemba, M., Wanga. H., Kinyanjui, P., Onyango, V. and Owuoche, J. (2016). Management of Wheat Rusts at Different Growth Stages using Nativo 300 SC (trifloxystrobin 100g/L+tebuconazole 200g/L) fungicide. Australian Journal of Crop Science, 10(9): 1273-1280.
	In article		View Article
[18]	Wilcoxon, R.D, “Genetics of Slow Rusting in Cereals,” Phytopathology, 71: 898-992, 1975.
	In article
[19]	Zadoks, J.C., Chang, T.T. and Konazak, C.F, “A decimal code for growth stages of cereals,” Weed Research, 14:415-421, 1974.
	In article		View Article