THE EFFECT OF DOWNY MILDEW ON SUGARCANE YIELD IN THE VARIETY B72177 AT RAMU SUGAR, GUSAP, PAPUA NEW GUINEA.

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1 THE EFFECT OF DOWNY MILDEW ON SUGARCANE YIELD IN THE VARIETY B72177 AT RAMU SUGAR, GUSAP, PAPUA NEW GUINEA By 1 G.B. RAUKA, 2 S. SUMA, 3 R.C. MAGAREY and 1 L.S. KUNIATA 1 Ramu Sugar Limited 2 Secretariat of the Pacific Communities 3 BSES Limited grauka@ramusugar.com.pg KEYWORDS: Peronosclerospora Sacchari, Downy Mildew, Yield Loss. Abstract B72177 is a major commercial cultivar for Ramu Sugar Limited, grown on 20% of the total area. Studies were carried out on the effect of downy mildew on the yield of this variety, through the planting of varying proportions of healthy and diseased cane in a range of field plots. Higher infestations of downy mildew reduced cane yield parameters. Thirty-three per cent loss in cane yields and 45% loss in tonnes sugar were observed for 60% observed downy mildew disease levels. Further reductions in yields of up to 58% tonnes cane and 75% tonnes sugar were observed for 100% observed disease. These results show that downy mildew is a disease of serious economic importance. Introduction Downy mildew of sugarcane is caused by the fungal pathogen Peronosclerospora sacchari (T. Miyake) Shirai and K. Hara. The main symptoms of the disease are leaf streaks that vary in colour as the symptoms age, and the production of down (with a white powdery appearance) on the underside of infected leaves (Suma and Magarey, 2000). Another feature is the abnormal elongated growth of stalks referred to as jump ups. Oospore production in diseased leaves can lead to leaf shredding under certain conditions (Suma and Magarey, 2000) and is sometimes associated with jump ups. Downy mildew transmission occurs via the planting of systemically-infected planting material (stalk pieces) or through conidia spread by air currents. Conidia may travel up to 400 m (Egan and Leu, 1989). Conidia production occurs at night with optimum conditions including temperatures of C and relative humidity close to 100%. Rain (leaf surface water) and low light hinder sporulation. The disease is restricted to the western Pacific and Southeast Asian regions. Lenon in 1938 first reported downy mildew in Papua New Guinea on Saccharum spontaneum growing along the headwaters of the Purari River (Hughes and Robinson,1961). The disease was later 353

2 reported to be widespread in PNG. It is likely that Papua New Guinea is the centre of origin of this disease (Egan and Leu, 1989). Significant yield losses caused by the disease have been reported in Australia, Fiji, Philippines and Taiwan (Egan and Leu, 1989). Losses of 21% were observed in Taiwan during the epidemic in moderately susceptible varieties and 46% in susceptible varieties (Chu, 1968). Losses in Fiji of up to 73% were recorded in individual fields in 1912 (Daniels et al., 1971). Suma and Pais (1996) reported losses from downy mildew of up to 15% at Ramu Sugar estate in Papua New Guinea. There is no doubt the disease can significantly reduce sugarcane yields under favourable conditions. In Papua New Guinea, downy mildew is controlled through the planting of resistant varieties, and selection for disease resistance is an important component of the Ramu breeding program. There remains a need to quantify the effect of downy mildew on crop production in order to determine the level of disease resistance needed in commercial varieties to minimise losses. This paper reports on the losses attributed to downy mildew in a variety grown extensively at Ramu Sugar, B Materials and methods The experiment was conducted at Ramu Sugar between 1998 and The aim was to create differing disease intensities by planting different ratios of healthy and diseased planting material, and to relate disease incidence with yield through regression analyses (Husmillo, 1982). The trial was planted on a deep loam soil with 20 disease levels. Plot size was four rows of 10 m length, with a 1.5 m row spacing. There was no replication of each individual disease level. Three successive crops were grown from the single planting (plant, first ratoon and second ratoon crops) with each crop lasting approximately 12 months. The no-disease control was treated with metalaxyl at 1.25 ai/l to ensure the crop remained free from downy mildew. SuSCon blue (granular insecticide) was applied in all plots to minimise white grub infestations. A basal dressing of 200 kg urea/ha was applied in the planting furrow. Downy mildew incidence was assessed in each crop at monthly intervals for the first 4 6 months. Disease incidence was expressed as a percentage of diseased stalks. Disease incidence was correlated with the proportion of diseased setts planted in each plot. At the end of each crop cycle (12 months), yield measurements were made to determine total harvested product (expressed as tonnes cane per ha), sugar content (rendement) and total sugar production (expressed as tonnes sugar per ha, a derived figure). Prior to harvesting, sugar content was estimated by selecting 10 stalks at random for juice and sucrose analyses. The 10 stalks were weighed and then ground using a Jeffco cutter grinder. 500 g of fibre was pressed to remove the juice, dried in an oven at 80 O C (to constant weight), and the data used to generate percentage fibre in the harvested product figures. The pressed juice was assayed to determine both Brix (total dissolved solids) and polarity (sucrose content) using the modified formulae for CCS which were derived using the standard formulae used by BSES (Anon., 1984). Total harvested product per plot was quantified using a tractor-hauled weigh bin in association with a commercial harvester. 354

3 Results Positive correlations were observed between initially planted downy mildew (DM) and observed disease in all crops. The DM levels used for correlation with yields were derived from averaging plant-cane and first ratoon data only. These averaged data showed significant correlation with initially planted disease (Figure 1(b)). For unknown reasons, the observed disease levels were low in the second ratoon crop, and the data were therefore discarded (Figure 1(a)). Fig. 1 Relationship between initially planted downy mildew (DM) and observed DM in (a) all crop classes and (b) average of plant-cane and 1 st ratoon crops in B Fig. 2 Relationship between observed DM and cane and sugar yields (tc/ha and ts/ha) and rendement. Data averaged over 2 crop classes, plant-cane and 1 st ratoon. 355

4 Yield parameters were also averaged as plant-cane and 1R crop data. A negative correlation was observed between yields of cane and sugar (t/ha and ts/ha) and observed disease levels (Figure 2(a), (b)). Although a negative correlation was observed between observed DM and sugar content (rendement), the relationship was not strong (Figure 2c). Discussion The results showed that planting differing proportions of healthy and diseased sugarcane setts provided a reliable way of creating differing downy mildew disease intensities in the resultant crops, as found by Husmillo (1982). The proportion of diseased setts significantly influenced observed downy mildew incidence. The weather during the years of the experiment appears to have had no influence on disease incidence. Higher infestations of downy mildew reduced cane yield parameters, as well as sugar content (rendement) in all crop classes. Our data confirm the potential for downy mildew to very significantly reduce sugarcane yield, as found in Taiwan, Fiji, Philippines and Australia (Egan and Leu, 1989; Chu, 1968; Daniels et al., 1971). In many countries, the disease is one of the more important to be considered in disease control programs and these data confirm why this is the case. The results from this work were used in conjunction with Ramu Estate disease survey data to estimate economic losses from downy mildew during the study period ( ). In these years ( ), the average downy mildew disease level on B was 1.3% diseased stalks. Our estimates of cane yield losses are 0.8% tonnes of cane and 0.9% tonnes of sugar. At an average current selling price of K3400 per tonne of sugar, and estimated sugar production of tonnes, losses are estimated at 400 tonnes sugar, valued at K1.5 million. B72177 is planted to 20% of the Estate and these very significant losses emphasise the importance of the downy mildew control program at Ramu. Other trials have been initiated investigating the influence of varietal resistance on losses and the results will be used to further refine the disease control program.. The presence of the disease at Ramu prevents the exploitation of susceptible but otherwise highly desirable varieties, with higher yielding (tonnes cane, increased rendement) or preferred agronomic features. The need to select lower yielding, but more resistant varieties, leads to very large hidden losses for Ramu Sugar Limited. This is exacerbated by additional costs associated with direct disease control measures, such as fungicide applications and the plough out of heavily diseased fields. In conclusion, this experiment confirms that downy mildew has a significant effect on sugarcane yields in PNG, specifically in the variety B The estimated losses are very significant and this emphasises the need for purposeful disease management. The relationship between planted and observed disease highlights the importance of using disease-free seedcane, obtained from well-managed nurseries. This has not always been the case at Ramu; these data will be used to ensure nurseries are managed more effectively. With hot-water-treatment of nursery material becoming routine with the recent finding of ratoon stunting disease (RSD) at Ramu Sugar, a positive outcome should be reduced levels of downy mildew, as the disease is also eliminated by this treatment. This treatment, together with effective roguing practices, should lead to better disease control, and reduced losses. 356

5 Acknowledgements Funding for this work was provided by Ramu Sugar Ltd, Papua New Guinea. The pathology technicians, namely Zure Maingo and Lucas Kuniata, are acknowledged for most of the field work. REFERENCES Anon. (1984). The Standard Laboratory Manual for Australian Sugarcane Mills: Volume 1, Principles and Practices. BSES Publications, Brisbane. Chu, Hsio-Tseng (1968). Studies of downy mildew disease of sugarcane in Taiwan. Proc. Int. Soc. Sugar Cane Technol., 12: Daniels, J., Husain, A.A. and Hutchinson, P.B. (1971). The control of sugarcane diseases in Fiji. Proc. Int. Soc. Sugar Cane Technol., 14: Egan, B.T. and Leu, L.S. (1989). Downy Mildew. In: Ricaud, C., Egan, B.T., Gillaspie Jr, A.G. and Hughes, C.G. ed. Diseases of Sugarcane. Amsterdam, Elsevier, Hughes, C.G. and Robinson, P.E. (1961). Downy mildew disease. In: Martin, J.P., Abbot, E.V. and Hughes, C.G. ed. Sugarcane Diseases of the World. Vol. 1. Elsevier Publ. Co., Amsterdam, Husmillo, R. Florante (1982). Assessment of yield loss due to downy mildew of sugarcane caused by Peronosclerospora philippinensis (Weston) C.G. Shaw. Sugarcane Pathologists Newsletter. Suma, S. and Magarey, R.C. (2000). Downy mildew. In: Rott, P., Bailey, R.A., Comstock, J.C., Croft, B.J. and Saumtally, A.S. ed A Guide to Sugarcane Diseases, CIRAD and ISSCT, Suma, S. and Pais, E. (1996). Major diseases affecting sugarcane production on Ramu Sugar Estate, Papua New Guinea. In: Croft, B.J., Piggin, C.M., Wallis, E.S. and Hogarth, D.M. ed. Sugarcane Germplasm Conversation and Exchange. Report of an International workshop held in Brisbane, Queensland, Australia, June, ACIAR Proceedings No. 67,