Annals of Sri Lanka Department of Agriculture. 2007.9:113-118.
MANAGEMENT OF TOMATO LEAF BLIGHT WITH FUNGICIDES R.G.A.S. RAJAPAKSE, K.J.P. KAHAWATTA, S. WIJESEKARA and R. RANATHUNGA Horticultural Crops Research and Development Institute, Gannoruwa, Peradeniya
ABSTRACT Yield of tomato is reduced significantly due to fungal leaf diseases. Farmers use different systemic and protective fungicides to control these diseases but seldom get better results. Fungicides were evaluated in the field under natural conditions of infection at two locations to identify effective fungicides to control the diseases during rainy period. In one location, three fungicides i.e. Difenoconozole 250 EC (12ml/10 l and 16ml/10 l), Pyraclostrobin 250 EC (8ml/10 l) and Mancozeb 75 WP (20g/10 l) were tested and in the other location two concentrations of Difenoconozole 250 EC (12ml/10 l and 16ml/10 l), Pyraclostrobin 250 EC (8ml/10 l), Dimethomorph/ Mancozeb 90/600 WP (45g/10 l) and Mancozeb 75 WP (20g/10 l) were tested. Spraying was started just after initiation of symptoms i.e. 30 days after planting and continued 3 times at 14 days interval. Disease severity on leaves was measured at regular intervals during crop growth and yield recorded. Late blight caused by Phytopthora infestans was identified as the major fungal disease of tomato in experimental sites. Early blight caused by Alternaria solani was also identified but with low frequency. Spraying of Difenoconazole 250 EC (12ml/10 l or 16ml/10 l) and Pyraclostrobin 250 EC (8ml/10 l) significantly reduced (p=0.05) blight and increased tomato fruit yield compared to the other fungicides tested. KEYWORDS: Blight diseases, Fungicides.
INTRODUCTION Leaf diseases caused by several fungi are one of the major reasons for yield loss of tomato in Sri Lanka and many other countries (Anon., 1990 and 1993). Coincidence of rainy period with cropping period of tomato has aggravated the disease problems. Among the fungal leaf diseases reported in Sri Lanka, late blight caused by Phytophthora infestans and early blight caused by Alternaria solani severely damage tomato cultivation (Anon., 1990 and 1997). It has been reported that late blight occurs in wet and humid climates while early blight occurs in any climate where frequent dew provides sufficient moisture to permit disease development (Anon., 1993). Both diseases cause yield reduction of varying levels and such losses are correlated with the presence of rainy and humid weather conditions during the season (Anon., 1993). Control of fungal diseases has been accomplished preliminarily by the cultivation of resistant varieties and application of chemical fungicides (Nene and Thapliyal, 2000). However, plant resistance to late blight does not play an important role in disease control (Anon., 1993). Also tomato varieties
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grown in Sri Lanka do not have considerable resistance to blight diseases. Currently, management of blight of tomato relies mainly on foliar application of fungicides (Anon., 1993). Several protective fungicides are currently recommended in Sri Lanka to control the disease (Anon., 1997). However, application of fungicides is not effective when symptoms have developed, during the rainy period, as wet weather conditions are conducive for rapid disease development of P. infestans and A. solani (Anon., 1993; Agrios, 1997). Also farmers claim that most of the fungicides do not effectively control fungal leaf diseases of tomato. Therefore, identification of more effective fungicides with systemic, curative properties is necessary to control blight diseases of tomato as these diseases reach epidemic level in most cultivations. The present study was undertaken to investigate the efficacy of new fungicides as foliar spray to control leaf diseases of tomato. MATERIALS AND METHODS Identification of fungal diseases Disease affected leaf samples were collected from tomato plants in experimental sites, before spraying fungicides. Fungal species were identified by comparison of disease symptoms on the leaves, microscopic observations of conidia, sporangia of pathogen and culture characteristics on PDA, with published data (Anon., 1970 and 1993; Agrios, 1997; Blancard, 1992). Screening of fungicides Two experiments were conducted in 2004 during the rainy period at Rikillagaskada and Gannoruwa. Fungicides were evaluated to study the efficacy as foliar spray in controlling naturally infected leaf diseases of tomato under field conditions. In both experiments, three week-old nursery raised tomato seedlings of variety Thilina were transplanted in beds of 1.5m x 3m at a spacing of 80cm x 50cm as recommended by DOA (Anon., 1990). Hand weeding was done at 3 and 6 weeks after transplanting. Imidacloprid 200g/l SC was applied twice at nursery stage and four times in the field to control insects. The experiments were laid out in randomized complete block design with five replicates. Different fungicides were used as foliar sprays using a knapsack sprayer at 14 days interval. Experiment 1 Two concentrations of the fungicide, Difenoconozole 250 EC (Systemic fungicide) and single concentration of Pyraclostrobin 250 EC (Protective/curative fungicide) and Mancozeb 75 WP (Protective fungicide) were evaluated.
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Treatments:1 Difenoconozole 250 EC (application rate 50ml product/ha, dilution as 12 ml/10 l water) 2 Difenoconozole 250 EC (application rate 70ml product/ha, dilution as 16 ml/10 l water) 3 Pyraclostrobin 250 EC (application rate 350ml product/ha, dilution as 8 ml/10 l water) 4 Mancozeb 75 WP (application rate 900g product/ha, dilution as 20g/10 l water) 5 Untreated control Experiment 2 Two concentrations of Difenoconozole 250 EC (Systemic fungicides) and single contration of Dimethomorph/Mancozeb 90/600 WP (Systemic/protective fungicide) and Mancozeb 75 WP (Protective fungicide) were evaluated. Treatments:1 Difenoconozole 250 EC (application rate 50ml product/ha, dilution as 12 ml/10 l water) 2 Difenoconozole 250 EC (application rate 70ml product/ha, dilution as 16 ml/10 l water) 3 Pyraclostrobin 250 EC (application rate 350ml product/ha as 8ml/10 l water) 4 Dimethomorph/Mancozeb 90/600 WP (application rate 1800g product/ha, dilution as 45g/10 l water) 5 Mancozeb 75WP (application rate 900g product/ha, dilution as 20g/10 l water) 6 Untreated control In both experiments, spraying was started after the initiation of symptoms i.e. 30 days after planting and repeated thrice at 14 days interval. Disease severity on leaves was measured at 14 days interval during crop growth starting at 43 days after planting (DAP). Ten plants were randomly selected from each plot. All leaves were excised and disease scoring done on a 1-9 standard rating scale as given in Table 1. Disease severity index (DSI) was calculated using following formula. DSI (%) =
Total sum of numerical ratings x 100 Maximum disease rating x No. of observations
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Fruit yield of each plot was recorded at harvest. Values of percentage DSI were transformed to arcsine and data analyzed using the statistical package MSTATC developed by the Michigen State University, USA. Table 1. Rating scale for DSI of blight on leaves. Rating scale % Leaf area infected 1 3 5 7 9
<1 1-5 6-25 26-50 >50
RESULTS AND DISCUSSION Two fungal species were isolated from diseased tomato leaves. Phytophthora infestans was commonly isolated from tomato leaves in both experimental sites while Alternaria solani was isolated with lower frequency. Disease symptoms and culture characteristics of both pathogens are reported in Table 2. Irrespective of the fungicides sprayed, the DSI increased with age of crop (Table 3 and 4). This is because susceptibility to blight disease increases with the age of the crop (Anon., 1993). Table 2. Characteristics of fungal species associated with leaves of tomato. Fungal species Symptoms on leaves Characteristics of fungi Phytophthora
Initially indefinite spots on leaf then enlarge rapidly into brown lesions. Finally foliage becomes necrotic
Sporangia lemon shaped, hyaline, emerge through leaf stomata, size 25-38 x 12-20 Âľ
Alternaria solani
First small brownish black lesions on older leaves. Later spots enlarge rapidly, lesions are about 1 cm diameter. Concentric rings may be distinguished in brown portion of lesion.
Fungus grows rapidly on PDA. Mycelium is septate and branched and becomes dark with age. Conidia are beaked muriform, dark and borne singly. Size 50- 75 x 10-15 Âľ.
Efficacy of the fungicide treatments were reflected by low DSI and high fruit yield. Spraying of Pyraclostrobin 250 EC (8ml/10 l) and Difenoconozole 250 EC (16ml/10 l and 12ml/10 l concentrations) significantly reduced DSI and increased fruit yield compared to Mancozeb. Results showed that spraying of Pyraclostrobin 250 EC (8ml/10 l) and Difenoconozole 250 EC (16ml/10 l and 12ml/10 l concentrations) at fourteen days interval were equally effective in controlling late and early blight of tomato. Results further indicated that lower concentration (12ml/10 l) of Difenoconazole is sufficient to control the diseases. There were no significant differences in DSI between
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Mancozeb treatment and the control at 43, 57 and 71 DAP and fruit yield. They were significantly different only at 85 DAP. Although Mancozeb is recommended as a protective fungicide in controlling blight (Nene and Thapliyal, 2000) it can be easily washed off from foliage. However, Pyraclostrobin and Difenoconazole were highly effective though they are systemic and protective/curative fungicides respectively. Therefore, Pyraclostrobin and Difenoconozole cannot be easily washed off from foliage by frequent rains and dew during the rainy period. This may attribute to the high efficacy of Difenoconazole and Pyraclostrobin against blight of tomato. Table 3. Disease severity index and fruit yield in different treatments of experiment 1. Fungicides Disease severity index (%) Fruit yield 43 DAP 57 DAP 71 DAP 85 (kg/10 plants) DAP Difenoconazole, 1.99 25.11 46.22 49.07 11.4 a 12ml/10 l (7.92)a (29.95) a (42.7) a (44.49) a Difenoconazole, 1.80 24.66 45.60 45.77 12.5 a a a a a 16ml/10 l (7.41) (29.61) (42.32) (42.58) Pyraclostrobin, 1.77 25.33 43.32 44.62 11.9 a a a a a 8ml/10 l (7.74) (28.87) (35.55) (41.92) Mancozeb, 5.99 40.42 53.97 56.88 9.5 b b b b b 20g/10 l (13.98) (39.44) (47.3) (49.0) Untreated 7.80 49.77 72.16 75.79 7.6 c b b c c control (16.17) (44.87) (57.88) (61.72) Values with same letter in a column are not significantly different at p=0.05 level (DMRT) Values within parenthesis are arcsine values, DAP â&#x20AC;&#x201C; Days after planting
Results of experiment 2 showed that all tested fungicides significantly reduced DSI at 43, 57, 71 DAP compared to untreated control. Under high disease pressure at 85 DAP, both concentrations of Difenoconazole and Pyraclostrobin significantly reduced DSI and increased fruit yield compared to other tested fungicides, Dimethomorph/Mancozeb and Mancozeb (Table 4). Although Dimethomorph/Mancozeb 90/600 WP is a systemic/protective fungicide, it was less effective in controlling disease under high disease pressure at later stages of crop growth. Continuous rains were experienced in 2nd experimental site during crop growth (data not present). Highest DSI (97.7%) was recorded in the untreated control while all tested fungicides i.e. Pyraclostrobin, Dimethomorph/Mancozeb, Difenoconazole and mancozeb showed over 77% of DSI at the maturity of the crop. This suggests that nearly 100% disease infection is possible during wet weather, if left untreated.
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Table 4. Disease severity index and fruit yield of tomato crops in different treatments of experiment 2. Fungicides Disease severity index (%) Fruit yield 43 DAP 57 DAP 71 DAP 85 DAP (kg/10 plants) 11.1 34.2 52.7 82.5 7.1 a (19.46) a (35.62) a (46.37) a (65.44) a 11.1 32.4 41.6 81.0 6.8 a a a a a (19.46) (34.37) (39.91) (64.36) 7.6 23.6 38.5 77.1 7.6 a a a a a (13.62) (27.78) (37.95) (62.52) 9.7 27.7 49.9 94.4 5.0 b a a a b (14.56) (31.64) (44.98) (78.92) 10.6 27.3 44.4 89.8 5.4 b a a a b (14.85) (31.56) (41.77) (74.66) Untreated control 37.7 52.6 76.7 97.7 4.2 c b b b b (37.44) (46.35) (59.45) (81.28) Values with same letter in a column are not significantly different at p=0.05 level (DMRT) Values within parenthesis are arcsine values, DAP â&#x20AC;&#x201C; Days after planting Difenoconazole, 12ml/10 l Difenoconazole, 16ml/10 l Pyraclostrobin, 8ml/10 l Dimethomorph/Man cozeb, 5g/10 l Mancozeb, 20g/10 l
CONCLUSIONS Fungicides - Difenoconazole 250 EC (12ml/10 l or 16ml/10 l) and Pyraclostrobin 250 EC (8ml/ 10 l) are more effective for the management of blight diseases of tomato than the presently recommended fungicide Mancozeb 75 WP (20g/10 l). REFERENCES Anon., 1970. Descriptions of Pathogenic fungi and bacteria, CMI Publications, UK. Anon., 1990. Crop Recommendation Techno-guide, Published by Department of Agriculture, Sri Lanka, 86-92. Anon., 1993. Compendium of tomato diseases, Edited by J.B. Jones, J.P. Jones, R.E. Stall and T.A. Zitter, Amarican Phytopathological Society, USA, 9-25. Anon., 1997. Pesticide Recommendation, Published by Department of Agriculture, Sri Lanka, 38-55. Agrios, G.N. 1997. Plant pathology, 4th Edition, Academic press, USA, 274-278 and 300-303. Blancard, D. 1992. A colour atlas of tomato diseases, Wolfe Pub. Ltd. UK, 60-65. Nene, Y.L. and P.N. Thapliyal. 2000. Fungicides in plant disease control, 3 rd Edition, Oxford and IBH Publishing Co. LTD, New Delhi.