Among the three fungicides tested in vitro against Sclerospora graminicola, metalaxyl and oxadixyl were effective in completely inhibiting sporangial production and germination at 250 ppm concentration. With mancozeb, complete inhibition of sporangial production was observed only at 1000 ppm concentration while there was no complete inhibition of sporangial germination even at 1000 ppm.



In a field trial conducted during the kharif season of 1985, metalaxyl 25 WP seed treatment protected the pearl millet crop from downy mildew upto 30 days. Seed treatment followed by one foliar spray with metalaxyl or mancozeb was more effective than seed treatment alone. Seed treatment with metalaxyl followed by a single combined foliar spray of metalaxyl + mancozeb was superior to seed treatment with metalaxyl followed by a single foliar spray of either of these fungicides

Rao, K C S

Rao, M V B

Reddy, M S

Shankara Rao, R

Singh, S D

Venkateswarlu, B

ICRISAT Open Access Repository

Indian J. Plant Prot. 15: 146-15], 1987 FUNGICIDAL CONTROL OF DOWNY MILDEW OF PEARL MILLET R. SHANKARA RAO, KC.S. RAO, S.D. SINGH l , M.S. REDDY, M.V.B. RAo2 AND K VENKATESWARLU3 Department of Plant Pathology, College of Agriculture, Hyderabad-500 030, Andhra Pradesh. ABSTRACT Among the three fungicides tested in vitro against Sc1erospora graminicola, metalaxyl and oxadixyl were effective in completely inhibiting sporangial production and germination at 250 ppm concentration. With mancozeb, complete inhibition of sporangial production was observed only at 1000 ppm concentration while there was no complete inhibition of sporangial germination even at 1000 ppm. In a field trial conducted during the kharif season of 1985, metalaxyl 25 WP seed treatment protected the pearl millet crop from downy mildew upto 30 days. Seed treatment followed by one foliar spray with metalaxyl or mancozeb was more effective than seed treatment alone. Seed treatment with metalaxyl followed by a single combin-ed foliar spray of metalaxyl + mancozeb was superior to seed treatment with metalaxyl followed by a single foliar spray of either of these fungicides. INTRODUCTION Downy mildew of pearl millet caused by Sc!erospora graminicola (Sacc.) Schoret, is the most widespread and destructive disease. The development of metalaxyl (N-(2 methoxyacetyl)-N-(2, 6-xylyl)-DL-alani-nate) in the seventies has resulted in excellent control of downy mildew at many locations (Venugopal and Safeulla, 1978, Rajagopal, 1981, Williams and Singh, 1981) even under high inoculum pressure. There are reports of certain Oomycetes, such as Peronospora hyoscyami, Phytophthora infestans and Pseudoperonospora cubensis, developing resistance to metalaxyl (Bruck et al., 1982,. Davidse et al., Reuveni et al., 1980). Thus it is possible that the cereal downy mildew may also have the capacity to develop resis-tance to this fungicide. One of the ways of coping with acquired fungicide resistance in plant pathogens is by using companion fungicide with different mode of action (Delp, 1980). In the present studies, an attempt was made to study the effect of metalaxyl, oxadixyl and mancozeb in vari-ous combinations and the results are re-ported in this paper. MATERIALS AND :METHODS In vitro : Three fungicides viz., metalaxyl 25 WP, oxadixyl 50 WP and mancozeb 75 WP were tested at 2000, 1000, 500, 250, 100 and 50 ppm on sporangial germination (test tube dilution method, Anonymous, 1947) and production. Downy mildew infected leaves of the cultivar 7042 were collected from unirrigated plot and their old downy growth was removed with moist cotton. The leaves were cut into one sq. cm. bits and were floated on fungicidal solution in petridishes with abaxial side up. For control, leaves were floated on distilled water in petri plate. These leaf bits were incubated for 6 hours at 21°C. After 6 hours, sporangia were harvested with cold distilled water. Sporangial concentration was determined using haemocytometer (6 counts/treatment) and the average was 1,2,3 - Scientists of ICRISAT, CPPTI and NAARM, respectively. 147 Shankara Rao et ai. calculated, Leaf area involved in sporangial production was measured by an automatic area meter and the number of sporangia produced per cm2 leaf was determined. In vivo : Seeds of three pearl millet cultivars viz., WC-C-75 (resistant) and 7042 and NHB-3 (susceptible) were treated with metalaxyl 25 WP at 2 g a.i. per kg seed and sown in the downy mildew sick plot of Andhra Pradesh Agricultural University, Rajendra-nagar in a Factorial Randomized Block Design replicated thrice during the kharif season of 1985. The plot size was 4.5 X 2.7m with a spacing of 45 cm X 15 cm. Sporangial inoculum was continually provided throughout the experimental period from the infector rows planted on two sides of each plot 21 days before planting the test rows. Thirty days after planting the test rows, one foliar spray of metalaxyl at 0.1 % a.i. and mancozeb at 0.2 % a.i. was given . separetely and in combination. Per cent downy mildew incidence was recorded at 25, 45 and 70 days after sowing. Infection index (%) was calculated according to the scale given by Williams and Singh (1981). Yield measurements were recorded by taking the weights of sun-dried grain. The data were subjected to statistical analysis. RESULTS AND DISCUSSION In Vitro: It is evident from Table 1 that sporangial production was effectively inhi-TABLE 1. In Vitro evaluation of the effect of fungicides on sporangiaI production and germination of Sclerospora graminicola. Fungicide Concentration Sporangial Percent sporangial production germination Metalaxyl 25 WP 25 ppm 27.30 (32.21) Metalaxyl 25 WP 50 ppm 56854 (10.94) 21. 00 (27.23) Metalaxyl 25 WP 100 ppm 14540 ( 9.58) Metaiaxyl 25 WP 125 ppm 9 . 25 (17.77) MetalaxyI 25 WP 250 ppm o (0) 0.00 ( 0.00) Metalaxyl 25 WP 500 ppm o (0) 0.00 ( 0.00) Metalaxyl 25 WP 1000 ppm o (0) 0.00 ( 0.00) Oxadixyl 50 WP 25 ppm 30.00 (33.03) Oxadixyl 50 WP 50 ppm 58550 (10.98) 22.60 (28.35) Oxadixyl 50 WP 100 ppm 16695 ( 9.72) Oxadixyl 50 WP 125 ppm - 10.90 (19.50) Oxadixyl 50 WP 250 ppm o ( 0) 0.00 ( 000) Oxadixyl 50 WP 500 ppm o (0) 0.00 ( 0.00) OxadixyI 50 WP 1000 ppm o ( 0) 0.00 ( 0.00) Mancozeb 75 WP 25 ppm 46.50 (42.99) Mancozeb 75 WP 50 ppm 238325 (12.38) 36.30 (37.00) Mancozeb 75 WP 100 ppm 195720 (12.18) Mancozeb 75 WP 125 ppm 29.60 (32.76) Mancozeb 75 WP 250 ppm 123492 (11.72) 25.30 (28.22) Mancozeb 75 WP 500 ppm 31563 (10.28) 14.10 (20.67) Mancozeb 75 WP 1000 ppm o (0) 5.30 (13.31) Check (water) 41239 (12.93) 72.30 (58.13) 'F'Test Sig. Sig S.Em 0.08 0.72 C.D. at 5% 0.214 2.058 Mean of 9 replications Figures in parenthesis are transformed values and log values. Shankara Rao et al. 148 bited by metalaxyl and oxadixyl at· 250 ppm whereas mancozeb inhibited sporangial production only at 1000 ppm. Further, metalaxyl and oxadixyl completely inhibited sporangial germination at 250 ppm while mancozeb could not completely inhibit even at 1000 ppm. This clearly indicates the systemic nature of metalaxyl and oxadixyl which were able to penetrate the host tissue and prevent the sporangial formation. Similar observations were made by Muthusamy and Narayanasamy (1985) who reported effective inhibition of sporan-gial germination at 0.02 per cent concentra-tion and sporangial production at 0.01 per cent concentration. In Vivo: It is evident from Table-2 that meta-laxy! seed treatment reduced the disease in the early stages of plant growth (30-45 days) but the effect did not persist long enough. This was probably due to air-borne infection coming into the plots from infector rows of from neighbouring infected crops, the loss of fungicide with time, or its dilution within the plants as they grow, or a combination of these factors (Dang et. aI, 1983). At harvest time, the plants raised from treated seeds have less downy mildew than plants raised from the un-treated seeds and had significantly better grain yield than checks. This is because initial disease development and build up of inoculum was checked due to metalaxy1 seed treatment. One foliar spray of metalaxyl 0.1 % in combination with seed treatment resulted in excellent disease control and increased grain yield. The ratio of healthy tillers to diseased ones was very high, the disease intensities were only 7.13, 15.66 and 0.79 per cent as against 74.04, 81.42 and 3.21 per cent in the corresponding checks of NHB-3, 7042 and WC-C75 cultivars res-pectively. The grain yields were 12.45 and 9.77 qjha as against 5.99 and 3.29 qjha in the corresponding checks of NHB-3 and 7042, respectively. Dang et. al. (1983) recorded a less disease intensity of 5.4 per cent in plots given with one foliar spray of metalaxyl 0.25 per cent a.i. in combination with seed treatment as against 78.8 per cent in the untreated plots of NHB-3. It has to be noted that in the present study, meta-laxyl was sprayed only at 0.1 per cent a.i. even then, it was quite effective in reducing the disease intensity and increasing grain yield. . One foliar spray of mancozeb 0.2 per cent in combination with metalaxyl seed treatment, though significantly superior to the untreated check, was found to be less effective when compared with metalaxy1 seed treatment plus one foliar spray with metalaxyl. Obviously, because mancozeb is a protectant fungicide, it was not as effective as metalaxyl which is curative III action. The results obviously show that a combined foliar spray of metalaxyl plus mancozeb mixture with metalaxyl seed treatment provided a much better control of downy mildew of pearl millet than either metalaxyl or mancozeb alone with meta-laxyl seed treatment. These findings stand in accordance with those of Samoucha and Cohen (1984). In their study on downy mildew of cucumbers incited by Pseudo-peronospora cubensis, the relative increase in control of metalaxyl-mancozeb mixtures was found to be 1.06 - 24.41 times greater than that of the individual fungicides com-bined. Their results also showed that a fungal SUb-population resistant to metalaxyl could still be controlled by metalaxyl if appropriate mixtures with mancozeb are applied. The mechanism governing syner-..j:>. \D TABLE 2. Effect of seed treatment with metalaxyl plus foliar sprays of metalaxyl and mancozeb on downy mildew incidence, senrity, and grain yield in three pearl millet cultivars. Downy mildew (%) at Infection index Grain yield , Cultivar Treatment % 25 days 45 days 70 days at 70 days (Q/ha) NHB-3 Metalaxy1 seed treatment 0.6 ( 3.7 ) 9.8 (18.2) 22.3 (28.2) 16.3 (23.8 ) 10.09 (Susceptible) Metalaxyl seed treatment 0.4 ( 3.0 ) 6.2 (14.7 ) 11.9 (20.2 ) 7.1 (15.4) 12.45 + metalaxyl spray Metalaxyl seed treatment+ O.S ( 4.1 ) 9.1 (17.6 ) 18.4 (25.4 ) 11.5 (19.9 ) 11.37 mancozeb spray Metalaxy1 seed treatment + metalaxyI + mancozeb spray 0.2 (1 .5 ) 3.0 ( 9.9 ) 8.8 (17.2 ) 6.0 (14.0 ) 13.53 Check 56.4 (48.7) 79.8 (63.3 ) 83 .. S (66.2 ) 74.0 (59.5 ) 5.99 i:./:J 7042 Metalaxyl seed treatment 0.2 (1 .5) 31.0 (33.7 ) 45.7 (42.5 ) 40.2 (39.3 ) 7.90 ;:r P> (Susceptible) MetalaxyI seed treatment 0.4 (2 .0 ) 11.4 (19.7 ) 20.4 (26.9 ) 15.7 (23.3 ) 10.11 ]::I + metalaxyI spray ::>;" P> Metalaxyl seed treatment 0.6 (2 .6 ) 19.8 (29.7 ) 29.8 (33.1) 27.3 (29.2 ) 9.03 I-j P> + mancozeb spray :;c MetalaxyI seed treatment + 0.2 (1 .5 ) 6.9 (14.8 ) 12.3 (20.5 ) 11.5 (19.6 ) II. 64 P> metalaxyl + mancozeb spray 0 Check 49.7 (4 . S ) 89.0 (70.8 ) 91.9 (73.5 ) 81.4 (64.5 ) 3.29 '" WC-C75 Metalaxyl seed treatment 0.2 (l .5 ) 1.8 (7 .0 ) 3.3 (10.4 ) 1.0 (5 .7 ) 15.25 .... \::> (Resistant) Metalaxyl seed treatment + 0.2 (1. 5 ) 0.6 (4 .4 ) 1.6 (6 .9 ) 0.8 (4 .4 ) 15.68 :-metalaxyl spray Metalaxyl seed treatment 0.2(1 .5) 1.0 (5 .6 ) 2.9 (9 .8 ) 1 .2 (6 .3 ) 15.55 + mancozeb spray Metalaxyl seed treatment + 0.2 (1 .5 ) 0.2 (1 .5 ) 0.6 (3 .6) 0.2 (1 .7) 15.76 metalaxy1 + mancozeb spray Check 2.0 (8 .03) 5.2 (13.0 ) 6.5 (14.7 ) 3.2 (lO.2 ) 15.66 'F'test Sig Sig Sig Sig Sig S.Em± 1. 75 1.18 1.42 1.49 0.29 C.D. at 5% 5.0S 3.41 4.12 4.33 0.8296 Figures in parenthesis are transformed values. Shankara Rao et al. 150 gism between the fungicides is not known. A reasonable speculation would be that since metalaxyl and mancozeb have diff-rent modes of action (the former mostly inhibits fungal RNA synthesis while the later impairs energy production), exposure of sporangia to sub-lethal concentrations of one fungicide weakens them to an extent that sublethal doses of the second fungicide will be detrimental. Fisher and Hayes (1982) showed that the primary effect of metalaxyl on Phytophthora paimivora proba-bly involves impaired biosynthesis of RNA so that mitosis is inhibited. The present study clearly indicates that out of the three cultivars tested, the disease development was more in plots sown with CV 7042 treated seed followed by NHB-3. Variations among cultivars in the effectiveness of fungicide treatments in controlling downy mildew indicated that the lower the resistance of a cultivar to the pathogen, the more fungicide or more effective treatment is needed to provide acceptable level of downy mildew control in all but the most highly susceptible cul-tivar 7042. For cultivars WC-C75, NHB-3 and 7042 the variations in grain yield are readily explained by the differences in the level of downy mildew. In the resistant cultivar WC-C75, the downy mildew inci-dence increased from 1.97 per cent to only 6.49 per cent between 25 and 70 days after planting. Hence, no significant difference in grain yield could be observed between plots sown with treated seed and untreated plots. In cultivars NHB-3 and 7042, the early infection levels in the untreated plots were considerably higher than in cultivar WC-C75 and there was a considerably greater increase in downy mildew incidence at final scoring, particularly in the highly susceptible cultivar 7042. The later development of symptoms would have resulted in plants competing for light and nutrients throughout crop development and yet producing heads without grain (Singh, 1983). Hence, keeping in view the develop-ment of resistance by fungi belonging to Peronosporales against metalaxyl, a com-bined foliar spray of metalaxyl and manco-zeb will be very much useful to minimise the risk of crop losses. ACKNOWLEDGEMENTS The facilities provided by the authori-ties of Andhra Pradesh Agricultural Univer-sity and ICRISAT are greatfully acknow-ledged. REFERENCES Anonymous, 1947. Test tube dilution technique for use with the slide germination method of evaluating protectant fungicides. Phytopathology, 37:354-356. Bruck, R.I., Gooding, G.V., and C.E. Main, 1982. Evidence for resistance to metalaxyl in isolates of Peronospora hyoscyami. Plant Disease. 66: 44-45. Dang, J.K., Thakur, D.P., and R.K. Grover, 1983. Control of pearl millet downy mildew caused by Sc/erospora graminicoia with systemic fungicides in an artifically contaminated plot. Ann. apl. Bioi., 102:99-100. Davidse, L.c., Looijan,· D., Turkenstun, L.J. and D. Vanderwal, 1981. Occurrence of metaIaxyl resistant strains of Phytophthora infestans in Dutch Potato fields. Neth. J. Pl. Path., 87:65-68. Delp, c.J., 1980. Coping with resistance to plant disease control agents. Plant Disease. 64:652-657. Fisher, D.J., and A.L. Hayes, 1982. Mode of action of the systemic fungicides furalaxyl, meta-laxyl and ofurace. Pest. Sci. 13:330-339. Muthusamy, P .and P. Narayanasamy, 1985. Evaluation of fungicides for the control of pearl millet downy· mildew disease. Pesticides. 19: 20-21. Rajagopal, K. 1981. Newsletter, IntI. working group on Graminaceous downy mildews: 3(2),5. Samoucha, Y. and Y. Cohen, 1984. Synergism between metalaxyl and mancozeb in controll-ing downy mildew in cucumbers. Phytopatho-logy, 74:1434-1437. 151 Shankara Rao et al. Singh, S.D., 1983. Variable cultivar response to metalaxyl treatment in pearl millet. Plant Disease. 67 :1013-1015. Venugop31, M.N., and K.M. Safeeulla, 1978. Chemical control of the downy mildews of pearl millet, sorghum and maize. Ind. J. Agric. Sci. 48:537-539. Williams, R.J. and S.D. Singh, 1981. Control of pearl millet downy mildew by seed treatment with metalaxyl. Ann. appl. Bio. 97:263-268. Received Accepted 29-3-1987 1-8-1987 

Fungicidal Control of Downy Mildew of Pearl Millet

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Fungicidal Control of Downy Mildew of Pearl Millet

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