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African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 245-250
Bekele Kassa and Hailu Beyene
Code Number: CS01054 ABSTRACT During the 1996 and 1997 seasons research was done at Holetta on the economic benefits of integrated management of late blight using genetic resistance and chemical (preventive and /or contact) methods. Varieties Tolcha (tolerant to late blight) and AL-624 (susceptible to late blight) were studied with five chemical controls during crop development. The chemical control started immediately after the first disease symptom was observed (41days after crop emergence) and chemical control was repeated after every 7 days for contact fungicides and 21 days for preventive fungicide. In 1997 AL-624 and Tolcha received 4 -5 and 2 applications, respectively, of the protectant fungicide whereas the preventive fungicide was applied 3 and 2 times on the susceptible and tolerant variety, respectively. In 1996 all contact fungicides were applied 5 times and the preventive fungicide 2 times on AL-624 while on Tolcha it was applied 3 and 1 time, respectively. Results showed that the use of preventive fungicide like Ridomil MZ 63.5% WP and contact fungicide such as Brestan 10, Chlorothalonil and Dithane M-45 can earn potato producers above the acceptable level (100%) of marginal rate of return (MRR). Thus, both genetic resistance and chemical control provide economic options for management of late blight. Key Words: Economic benefit, fungicides, genetic resistance, Phytophthora infestans, Solanum tuberosum RÉSUMÉ Pendant les saisons 1996 et 1997, une recherche a été conduite à Holetta sur les bénéfices économiques de la gestion integrée du mildiou en utilisant une résistance génétique et des contrôles chimiques (préventif et/ou de contact). Les variétés Tolcha (tolérante au mildiou) et AL-624 (sensible au mildiou) ont été étudiées avec cinq traitements chimiques pendant le développement de la culture. Le contrôle chimique a commnencé immédiatement dès que l' apparition des symptômes de la maladie a été observée (41 jours après émergence) et il a été répété après chaque 7 jours pour le fongicide de contact et 21 jours pour le fongicide préventif. Les cultivars AL-624 et Tolcha ont reçu en 1997 4-5 et 2 pulvérisations respectivement du fongicide protectant alors que le fongicide préventif a été appliqué 3 et 2 fois sur la variété sensible et tolérante respectivement. En 1996 tous les fongicides de contact ont été appliqués cinq fois et le préventif 2 fois sur AL-624 alors qu'il a été appliqué 3 et 1 fois respectivement. Les résultats ont montré que l' utilisation du fongicide préventif tel Rodomyl MZ 63.5% WP et le fongicide de contact tel que Brestan 10, Chlorothalonil ET Dithane M-45, les producteurs peuvent gagner au dèla du niveau acceptable (100%) du taux marginal du revenu (MMR). Ainsi, la résistance génétique et le contrôle chimique fournissent des options de gestion de contrôle du mildiou. Mots Clés: Bénéfice économique, fongicides, résistance génétique, Phytophythora infestans, Solanum tuberosum INTRODUCTION Late blight of potato, caused by Phytophthora infestans (Mont) de Bary, is the single most destructive disease of potato the world over (CIP, 1989). During the last two decades, this disease has increased globally (Fry and Win, 1997). The average global crop losses of all diseases combined was approximately 12.8 % of the potential production but potato alone was subjected to 21.8 % loss (James, 1981). In Ethiopia the disease caused 100 % crop loss on unimproved local cultivar, and 67.1% on a susceptible variety, Al-624 (Bekele and Yaynu, 1996). Use of resistant varieties is one of the main components of late blight management and is especially effective under tropical conditions (Shtienberg et al., 1994). However, the race-specific oligogenic resistance (CIP, 1989) in the existing released potato varieties can be rapidly broken down by compatible races of P. infestans rendering the varieties to be susceptible to the disease within a short period (Shtienberg et al., 1994). At a global level, the major approach to prevent late blight development has been application of fungicides (CIP, 1989). In Ethiopia, farmers frequently apply fungicides to control late blight but the economic benefit acruing from the fungicide spray have not been established. Therefore, the objective of this study was to assess the efficacies and economics of fungicide sprays in the management of potato late blight in Ethiopia. MATERIALS AND METHODS The experiment was conducted in 1996 and 1997 at Holetta, Ethiopia in a randomised complete block design of 2 x 5 and 2 x 6 factorial arrangement, respectively. Two varieties Tolcha (tolerant), and AL-624 (susceptible) (Yaynu et al., 1991) were used. The trial was planted on June 15 and June 17 in 1996 and 1997, respectively, which fell within the recommended planting dates for potato in the area (Gebremedhin and Berga, 1989). Each plot measured 6 m x 6 m, and were replicated thrice. Spacing between rows and plants were 0.75 m and 0.30 m, respectively. At planting, 138 kg P2O5 and 54 kg N ha-1 as Di-ammonium phosphate (DAP) were side-dressed. Four fungicides, Ridomil MZ (Metalaxyl + Mancozeb) 63.5 % WP, Mancozeb 80 %WP, Brestan 10, and Chlorothalonil 50 % EC which were previously screened and recommended for late blight control (Bekele and Yaynu, 1994) were tested for their economic benefit. However, in 1997 one additional fungicide treatment, i.e., one spray of Redomil MZ followed by two sprays of Mancozeb 80% WP was included. The rate used for all fungicides was 3.0 kg ha-1 except for Brestan 10 which was applied at 2.5 kg ha-1. The first fungicide application was done soon after the appearance of the first disease symptom on average 39 days after planting. A treatment without fungicide application was included as a control. In order to minimise the fungicide drift and other interferences between treatments each block was separated from adjacent blocks with three dense rows of oat (Avenae sativa). Late blight was scored based on percent leaf area infected at seven days interval starting from the onset of the first symptom (Jan, 1987). Percent disease severity was transformed using arc-sine transformation and subjected to analysis of variance (ANOVA) as suggested by Gomez and Gomez (1984) and the analysis was done using MSTAT-C statistical package. Disease data, seven readings in 1996 and six in 1997 were integrated into areas under the disease progress curves (AUDPC) following procedures outlined by Campbell and Madden (1990). In order to investigate the economics of fungicide use, gross revenue was calculated based on adjusted yield (less 25 %) using the prevailing prices of 0.50 Birr kg-1 and 1.50 Birr kg-1 for ware potato market. Spraying costs included fungicide and labour costs and these were used to calculate marginal rate of return following procedures of CIMMYT (1988). Combined analysis was not performed due to heterogeneous error terms between years. RESULTS AND DISCUSSION In both years plots sprayed with fungicides had lower incidence of late blight than unsprayed plots (Tables 1 and 2). Late blight appeared as early as 22 and 30 days after emergence on the susceptible variety AL-624 in 1996 and 1997, respectively. On the tolerant variety Tolcha, the disease symptom appeared after 28 days in 1996 and 40 days in 1997. Unlike on the tolerant, the development of units disease from primary foci was fast (r = 0.4711) on the susceptible variety. There were significant differences in the disease severity and percent control, among treatments in the susceptible variety AL- 624, unlike on the tolerant variety Tolcha. For example, the highest disease severity, as expressed by AUDPC, was 1542 in 1996 and 1618 in 1997 on the unsprayed susceptible variety as compared to 399 and 593, respectively, on the tolerant variety. Ridomil MZ - 63.5 % WP which is both systemic and protectant in action gave the best control (78.8%). On the other hand Chlorothalonil, Mancozeb and Brestan 10 did not differ significantly in respect to disease control, and gave 59.3, 43.0 and 46.8 % control, respectively. However, the three fungicides significantly (P< 0.05) controlled late blight when compared to the control plot. In 1997 the best control (90.2%) was recorded where Ridomil MZ spray was followed by Mancozeb and the poorest (22.3%) was in plots sprayed with Brestan 10. The remaining treatments gave control in the range of 57.5 to 82.3% (Table 2). Overall, fungicide application significantly (P<0.001) reduced the AUDPC which was negatively and significantly correlated with tuber yield (R2 = 0. 68** in 1996 and R2 = 0.597** in 1997). This complies with the previous findings (Bekele and Gebremedhin, 2000). Potato tuber yields differed significantly (P<0.05) between the treatments. In 1997 the control treatment of the tolerant variety gave significantly higher tuber yield (27 t ha-1) compared to the contact fungicide sprayed treatments (16.4 to 24.8 t ha-1). On the susceptible variety application of Ridomil MZ resulted the highest tuber yields (24.4 t ha-1 in 1996 and 15.6 t ha-1 in 1997) as compared to 5.2 t ha-1 in 1996 and 2.67 in 1997 t ha-1 in the unsprayed control plot. The other treatments gave yields in the range of 14.4 to 15.9 t ha-1 in 1996 and 9.0 to 13.8 t ha-1 in 1997. Basing on Government supplier prices, Ridomil MZ 63.5 % WP gave the highest net benefit with marginal rate of return of 192 % in 1996 and 487 % in 1997 (Tables 3 and 4). This treatment also gave the highest net benefit (7334 Birr) in 1996 season but because of its high price (201.76 Birr kg-1) it was dominated and rejected. Chlorothalonil and Brestan 10 gave net benefits of 5230 and 4838 in 1996, respectively, and 13292 and 5222 in 1997 with corresponding marginal rates of return of 248% and 451 % in 1996 and 171 % and 198 % in 1997. Mancozeb was dominated by the treatments that had lower cost that varied in both years. These results implied that use of Chlorothalonil, Ridomil MZ and Brestan 10 provided net returns of Birr 11.04, 7.26 and 8.60 in 1996, respectively. The corresponding values including Ridomil plus Mancozeb were 18.7, 11.6 12.7 and 7.8 in 1997, respectively. When one spray of Ridomil MZ was followed by two sprays of Mancozeb the net benefit (13505 Birr) was higher compared to the use of the contact fungicides alone but the marginal rate of return was lower (104 %). However, the rates are higher than the minimum acceptable marginal rate of return, which is assumed to be 100%. The difference in marginal rates of return for Ridomil MZ, Ridomil MZ plus Mancozeb and Mancozeb sprayed plot was probably due to the lower efficacy of Mancozeb in the control of the disease. However, through the combination of the two fungicides did not lead to a significantly increase in disease control reduction in frequency of application partially systemic fungicides (Ridomil MZ) could help to avoid the emergence of pathogen pathotypes resistant to Metalaxyl (Davidse et al., 1981; Deahl, 1992). We conclude that the fungicides Chlorothalonil 50% EC and Brestan 10 can be used to control late blight. Overall, Ridomil MZ 63.5 % WP gave effective control of late blight and the best return. Therefore, those potato growers who can afford to buy it can use it as an alternative fungicide in late blight control. Although the use of fungicides at government control prices level was economic, lack of experience in use of fungicides and availability of sprayers are obstacles that hinder the use of the technology. ACKNOWLEDGEMENT The authors are grateful to Ato Ejeta Tola and Fantahun Feleke for the field
assistance.
TABLE 1. Effect of different fungicide sprays on potato tuber yield in susceptible and tolerant varieties at Holetta in 1996
1PC = percentage control
Values followed with the same letter are not significantly differentat P=0.05 level TABLE 2. Effect of different fungicide sprays on tuber yield of susceptible and tolerant potato varieties at Holetta in 1997
1PC = percentage control
2CV% AUDPC= 23.50, Commercial tuber yield =20.56 * - Values followed with the same letter are not significantly different at P=0.05 level 3 The two chemicals were applied as a mixture but Ridomil MZ was used for only the first spray whereas the second and the third sprays were made with Mancozeb TABLE 3. Economics of fungicide use for late blight control on susceptible variety AL-624 at Holetta in 1996
D* = Dominated treatment
** - Ethiopian Birr; 8 ET- Birr is equivalent to one USD TABLE 4. Economics of fungicide use for late blight control on susceptible variety AL-624 at Holetta in 1997
D* - Dominated treatment, ** -Ethiopian Birr; 8 ET-Birr is equivalent to
one USD
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