EPIDEMIOLOGY AND CONTROL OF PEARL MILLET DOWNY MILDEW, SCLEROSPORA GRAMINICOLA, IN SOUTHWEST NIGER

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1 EPIDEMIOLOGY AND CONTROL OF PEARL MILLET DOWNY MILDEW, SCLEROSPORA GRAMINICOLA, IN SOUTHWEST NIGER E. GILIJAMSE 1 and M.J. JEGER 2 1 Rijk Zwaan Zaadteelt en Zaadhandel b.v., P.O. Box 40, 2678 ZG De Lier, the Netherlands 2 Imperial College at Wye, University of London, Wye, Ashford, Kent, TN25 5AH, United Kingdom 1. Introduction Pearl millet, Pennisetum glaucum, is one of the most important staple crops, both for human consumption and animal fodder, in the drier regions of the Sahel area. Due to droughts, low soil fertility, pests and diseases, yields are low. One of the main pearl millet diseases is downy mildew, caused by the oomycete Sclerospora graminicola. The pathogen causes chlorosis, starting at the base of a leaf ( half-leaf or partial leaf symptom), but gradually covering a greater proportion of leaves until the entire leaf is chlorotic (Williams, 1984). Severely infected plants are barren and produce malformed heads, so-called green ears or crazy tops. Crop losses of up to 50% have been reported (Werder and Manzo, 1992), although in general more conservative estimates are between 0 and 20% (Mbaye, 1992; Gilijamse et al., 1997; Jeger et al. 1998). Initial infection of plants is caused by oospores which are thick-walled resting spores, produced at the end of the growing season when conditions are unfavourable for crop growth. Oospores are incorporated in the soil together with crop debris providing a continuous inoculum source in areas where no crop rotation is practised. During the growing season, secondary infection is possible through asexually produced sporangia when relative humidity exceeds 95%. In the drier regions of the Sahel, oospores are probably not only the initial but also the main source of infection. The role of oospores in the epidemiology of pearl millet downy mildew was studied in the sahelian parts of southwest Niger. Possibilities for downy mildew control, that may be implemented in low-input agriculture, were also studied. 2. Materials and Methods 2.1. EPIDEMIOLOGY Oospore Production Production of oospores was studied in different parts of pearl millet plants at two periods in time (Gilijamse, 1997). One partially resistant (HKP) and two susceptible (NHB3 and P.T.N. Spencer-Phillips et al. (eds.), Advances in Downy Mildew Research, Kluwer Academic Publishers. Printed in the Netherlands. 189

2 190 E. GILIJAMSE AND M.J. JEGER 7042) cultivars were studied. Inoculation took place by mixing the seed with ground infected pearl millet leaves containing approximately oospores per gram dry weight. At 60 and 90 days after sowing (d.a.s.) plants were harvested, dried and different plant parts were ground. The number of oospores was determined in the roots, stems, leaves and ears of the three cultivars Role of Oospores in Downy Mildew Infection In two regions in the southwest of Niger around the villages Dosso and Gaya downy mildew incidence and severity were monitored during the growing season following the method of Williams (1984) modified by Mbaye (1994). In each region 10 fields were visited at 30 and 60 d.a.s. and at crop maturity (90 d.a.s.). At the beginning of the rainy season soil samples were taken from the upper layer of each field and analysed for several soil characteristics. Oospore density was determined in each soil sample through extraction by centrifugation in a sucrose suspension (Van der Gaag and Frinking, 1997) CONTROL Several control methods have been investigated for low-input pearl millet cultivation. Crop rotation, deep tillage, changing planting date and fertilization, however, are all of limited use in the drier parts of the Sahel. No or limited resources are available for deep tillage and fertilization whereas crop rotation and changing the planting date are not feasible because of the risk of harvest failure. Traditional control methods using herbal extracts are practised sometimes (Gilijamse et al., 1997). The most common control method is roguing of diseased plants (Gilijamse et al., 1997). According to Mbaye (1994) this method results in the reduction of asexual spores of S. graminicola when it is done within the first month after sowing. No reports exist on the long-term effects. Roguing is also rather time-consuming and therefore not practised sufficiently often to control the disease adequately. The use of chemical fungicides as a seed treatment might be an option for control of pearl millet downy mildew. The amount of fungicide applied is low and a limited input of labour is necessary, making it a rather inexpensive control method. In the southwest of Niger, downy mildew control by seed treatment was investigated using Apron Plus 50 DS (Ciba Geigy) which consists of two fungicides (metalaxyl and carboxine) and one insecticide (furathiocarb). Apron Plus was applied at the recommended dose of 10g per kg of seed. A local pearl millet variety was used in an experiment in five replications. Seedling emergence, disease incidence, severity and yield were assessed. 3. Results and Discussion 3.1. EPIDEMIOLOGY Oospore Production At 60 d.a.s. the number of oospores ranged from about oospores per gram dry weight in roots and stems of the two susceptible cultivars to about oospores in ears and leaves. No oospores were found in any part of the partially resistant cultivar. At

3 EPIDEMIOLOGY AND CONTROL OF MILLET DOWNY MILDEW d.a.s. oospores were found in all plant parts. There were no differences between the three cultivars. The number of oospores in roots and stems differed significantly from that in leaves and ears, ranging from oospores per gram in roots and stems to 2.3 in leaves and ears. In ears collected from farmers fields, up to oospores per gram were found. These data largely correspond with those of others although Michelmore et al. (1982) found oospores in seedlings as early as 13 days after sowing. Since S. graminicola is heterothallic, the co-existence of two mating types in the same zone of host tissue is the primary determinant for oospore production. This is one explanation for why oospores are not always or only at later times present in diseased host tissue. Williams (1984) suggested that oospore production is related to environmental factors and physiological changes in the maturing plant. In the sahelian area with a dry, crop-free period, the formation of thick-walled oospores at the end of the growing season is the only strategy for the pathogen to survive. In pearl millet cultivars with a certain level of (partial) resistance, oospore production might be restricted. It was found that fungal structures are encased by thin membranous structures which deprive the fungus of essential nutrients (Sharada et al., 1995). However in our experiment there were no differences in oospore production between the three cultivars at the end of the growing season Role of Oospores in Downy Mildew Infection In the Dosso region the oospore density ranged from 26 to 133 oospores per gram soil whereas around Gaya it ranged from 74 to 768 oospores per gram soil. Downy mildew incidence (percentage of diseased plants) ranged from 0 to 23.2% in Dosso and from 7.6 to 42.0% in Gaya. Disease severity (degree of infection of individual plants systemically infected) showed a similar pattern ranging from 0 to 12.9% in Dosso and from 4.3 to 24.8% in Gaya. Oospore density in the soil was highly correlated (r = 0.83, 18 df) with disease incidence at 90 d.a.s. During the crop-free period no alternative hosts are present for S. graminicola to survive on and therefore no sporangia are present to infect pearl millet at the beginning of the cropping season, thus strenghtening the view that oospores are responsible for initial infection by S. graminicola. Since S. graminicola is the only downy mildew pathogen occurring on pearl millet in the Sahel, it was assumed that the oospores found were from S. graminicola. Soil characteristics were found to play a role in disease development. Organic C content, and fraction of loam were correlated significantly with disease incidence at 90 d.a.s. (r = 0.70, 0.47 and 0.46 respectively, 18 df). No significant correlation was found between disease incidence at 90 d.a.s. and the fraction clay and sand (r = 0.39 and 0.13 respectively, 18 df). This is the first time a relationship has been shown between oospore density in the soil and pearl millet downy mildew incidence. For sorghum downy mildew, Schuh et al. (1988) found that the spatial pattern of oospores in soil was comparable to the pattern of infected sorghum plants. However, Pratt and Janke (1978) did not find a significant correlation between oospore density of this pathogen in the soil and disease incidence (r = 0.33, 16 df). In both studies the number of oospores per gram soil was low ( and 8-95 respectively) compared to this survey (26-768). Possibly with such low numbers,

4 192 E. GILIJAMSE AND M.J. JEGER disease incidence is much more influenced by secondary infection due to sporangia and the percentage of viable oospores in the soil (Shetty, 1987). It is probable that soil characteristics determine to a large extent the percentage viability of oospores and hence indirectly disease incidence. Other studies indicated the relation between soil characteristics and disease incidence although results are sometimes contradictory. Disease incidence of sorghum downy mildew on sorghum was higher on sandy soils than on clay soils according to studies by Pratt and Janke (1978) and Schuh et al. (1987). However, Van der Westhuizen (1977) found the opposite for sorghum downy mildew on maize. None of the authors mentioned other soil characteristics such as ph and organic C content. The relationship between soil characteristics and epidemiology of downy mildew of pearl millet and sorghum certainly needs further investigation in the development of new control strategies CONTROL Disease incidence and severity ranged from 11.1 to 25% and from 6.9 to 14.6% respectively for treated seeds, and from 5.6 to 27.8% and from 4.2 to 17.4% respectively for non-treated control seeds. Statistical analysis showed no significant differences between the treatment and the control. No differences were found for seedling emergence and yield. These results largely correspond with preliminary data collected during the two previous years of the study. Under the conditions of the experiment it seems that seed treatment using the chemical fungicide Apron Plus 50 DS does not result in an efficient control of pearl millet downy mildew. Metalaxyl however, has been shown to be effective for seed treatment followed by foliar application with metalaxyl (Shishupala et al., 1990). Combined applications for control of downy mildew is probably more important for pearl millet compared to sorghum and maize, since pearl millet produces tillers that keep the crop susceptible for up to 60 days. However, foliar application of metalaxyl is again too expensive in lowinput agriculture. Additionally, the fungicide might easily be washed off the leaves during heavy showers which occur regularly in the rainy season in the Sahel. The most effective strategy to control or at least reduce pearl millet downy mildew in the Sahel is probably through the production of resistant cultivars that meet the characteristics (i.e. taste, drought resistance and low fertiliser need) of the traditional varieties, but these are not currently available. 4. References Gilijamse, E. (1997) Downy mildew on pearl millet in south Niger, Joint report of WAU Department of Phytopathology, IAH Larenstein and DFPV Niamey/Niger. Gilijamse, E., Frinking, H.D. and Jeger, M.J. (1997) Occurrence and epidemiology of pearl millet downy mildew, Sclerospora graminicola, in southwest Niger, International Journal of Pest Management 43, Jeger, M.J., Gilijamse, E., Bock, C.H. and Frinking, H.D. (1998) The epidemiology, variability and control of the downy mildews of pearl millet and sorghum, with particular reference to Africa, Plant Pathology 47,

5 EPIDEMIOLOGY AND CONTROL OF MILLET DOWNY MILDEW 193 Mbaye, D.F. (1992) Les maladies du mil au Sahel: Etat des connaissances et propositions de lutte, in Institut du Sahel (ed), La lutte intégrée contre les ennemis des cultures vivrières dans le Sahel, CILLS/INSAH, Bamako, Mali, pp Mbaye, D.F. (1994) Une étude du pathosysteme Pennisetum glaucum-sclerospora graminicola, Application à la gestion du mildiou du mil au Senegal, Montpellier, France: Ecole nationale supérieure agronomique de Montpellier, PhD thesis. Michelmore, R.W., Pawar, M.N. and Williams, R.J. (1982) Heterothallism in Sclerospora graminicola. Phytopathology 72, Pratt, R.G. and Janke, G.D. (1978) Oospores of Sclerospora sorghi in soils of south Texas and their relationships to the incidence of downy mildew in grain sorghum, Phytopathology 68, Schuh, W., Jeger, M.J. and Frederiksen, R.A. (1987) The influence of soil temperature, soil moisture, soil texture and inoculum density on the incidence of sorghum downy mildew, Phytopathology 77, Schuh, W., Jeger, M.J. and Frederiksen, R.A. (1988) Comparisons of spatial patterns of oospores of Peronosclerospora sorghi in the soil and of sorghum plants with systemic downy mildew, Phytopathology 78, Sharada, M.S., Shetty, S.A. and Shetty, H.S. (1995) Infection processes of Sclerospora graminicola on Pennisetum glaucum lines resistant and susceptible to downy mildew, Mycological Research 99, Shetty, H.S. (1987) Biology and epidemiology of downy mildew of pearl millet, in ICRISAT (ed), Proceedings of the International Pearl Millet Workshop, Patancheru, India, pp Shishupala, S., Lokesha, S. and Shetty, H.S. (1990) Efficacy of metalaxyl and oxadixyl formulations against pearl millet downy mildew, Advances in Plant Sciences 3, Van der Gaag, D.J. and Frinking, H.D. (1997) Extraction of oospores of Peronospora viciae from soil, Plant Pathology 46, Van der Westhuizen, G.C.A. (1977) Downy mildew fungi of maize and sorghum in South Africa, Phytophylactica 9, Werder, J. and Manzo, S.K. (1992) Pearl millet diseases in Western Africa, in W.A.J. de Milliano, R.A. Frederiksen and G.D. Bengston (eds), Sorghum and Millets Diseases:A second World Review, ICRISAT, Patancheru, India, pp Williams, R. J. (1984) Downy mildew of tropical cereals, Advances in Plant Pathology 3,