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The impact of Ridomil Plus® (12% metalaxyl + 60% cuprous oxide) spray rate and frequency on the severity of potato late blight, caused by Phytophthora infestans, was evaluated during 1997 and 1999 in Dschang, Cameroon. A randomised complete block design was used in both experiments. In 1997, plots were sprayed with 0.6, 1.2, 2.4, or 4.8 kg a.i.ha^-1 of Ridomil Plus®, while in 1999, they were treated once, twice or thrice with 2.4 kg a.i.ha^-1 of the fungicide. In both trials control plots were not sprayed. Late blight intensity was recorded weekly and yields were obtained at harvest. Fungicide applications reduced late blight intensity and consequently increased potato tuber yields. Marketable yields were significantly higher in the resistant variety, Cipira (22.4 - 34.8 t ha^-1), than in the susceptible variety, Tezelfeu (4.0 - 12.0 t ha^-1). A significant quadratic relationship existed between the rate or number of fungicide applications and tuber yields. Economic analyses indicated that two sprays of the fungicide at 2.4 kg a.i. ha^-1 are necessary for a good late blight protection programme in potato. This spray regime resulted to increases in net returns of US$ 1947 to 3327 ha^-1. Results also suggest that high rates or frequencies of fungicide applications do not necessarily lead to better economic benefits.

Key Words: Economic gains, fungicide application, Phytophthora infestans, Solanum tuberosum

L'impact de la dose et la fréquence des traitements au Ridomil Plus® (12% métalaxyle + 60% oxyde cuivreux) sur la sévérité du mildiou de la pomme de terre, causé par Phytophthora infestans, a été évalué en 1997 et 1999 à Dschang, Cameroun. Les essais ont été réalisés en blocs complètement randomisés. En 1997, des parcelles ont été traitées au Ridomil Plus® à la dose de 0.6, 1.2, 2.4, ou 4.8 kg m.a.ha^-1, tandis qu'en 1999, elles ont été traitées une, deux, ou trois fois au même fongicide à la dose de 2.4 kg m.a.ha^-1. Dans les deux essais, les parcelles témoins n'ont pas été traitées. L'intensité du mildiou a été évaluée hebdomadairement et les rendements ont été obtenus à la récolte. Des traitements fongicides ont significativement réduit l'intensité du mildiou et par conséquent augmenté le rendement en tubercules. Des rendements commercialisables ont été significativement plus élevés chez la variété résistante, Cipira (22.4 - 34.8 t ha^-1), par rapport à la variété sensible, Tezelfeu (4.0 - 12.0 t ha^-1). Une corrélation quadratique significative a été mise en évidence entre la dose ou la fréquence de traitement fongicide et le rendement en tubercules. Les analyses économiques indiquent que deux traitements fongicides à 2.4 kg m.a. ha^-1 sont nécessaires dans un programme de protection contre le mildiou de la pomme de terre. Ce régime de traitement a abouti aux augmentations en revenus nets de l'ordre de US$1947 à 3327 ha^-1. Ces résultats suggèrent également qu'en culture de la pomme de terre, l'emploi des doses ou des traitements intensifs des fongicides n'est pas nécessairement rentable.

Mots Clés: Rentabilité économique, application fongicide, Phytophthora infestans, Solanum tuberosum

Potato (Solanum tuberosum L.) is one of the most intensively cultivated solanaceous vegetables in Cameroon. The crop fits well in the multi-cropping systems of peasant farmers because of its relatively short duration in the field (3-4 months) compared to 6 months or longer for other roots and tuber crops (Aighewi and Lyonga, 1989). Annual production in Cameroon is estimated at about 150,000 tonnes (USAID, 1982). Although it is a subsistent crop, potato is exported to neighbouring countries, such as Central African Republic, Gabon and Equatorial Guinea (Fontem, 1998).

Potato yields are generally low predominantly because of disease problems. The most important disease in Cameroon is late blight caused by Phytophthora infestans (Mont.) de Bary (Fontem, 1991,1995a,b, 1998; Fontem and Aighewi, 1991). Yield losses caused by late blight foliage infection in Cameroon have been estimated to be as high as 71 % (Fontem and Aighewi, 1993; Fontem, 1998).

The most feasible method to manage a plant disease is simply to grow resistant varieties. However, efforts to manage late blight in Cameroon have had little success because few growers have access to resistant varieties. Consequently, most farmers grow traditional varieties that are very susceptible to late blight and tend to control the disease with intensive fungicide applications, irrespective of spray rates or schedules recommendations (Fontem and Bouda, 1996; Teugwa Mofor et al., 1996). There is, therefore, an urgent need to identify cost-effective late blight management strategies for farmers.

Samoucha and Cohen (1989) reported that a mixture of systemic and contact fungicides controls late blight more effectively than the individual fungicides. In our previous investigations, Ridomil Plus® (12% metalaxyl + 60% cuprous oxide) or Ridomil MZ (12% metalaxyl + 60% mancozeb) was reported to be more effective in late blight control than weekly sprays of contact fungicides, maneb, mancozeb or cupric hydroxide (Fontem and Aighewi, 1991, 1993; Fontem, 1998). This study was carried out to determine an appropriate rate and frequency of Ridomil Plus treatments for late blight management in potato.

Cultural procedure. Field experiments were conducted during the 1997 and 1999 growing seasons in Dschang (1400 m), Cameroon. The land used had not been grown with any solanaceous crop for the past three years. The trials were designed in four randomised complete blocks. Individual plots, measuring 5.0 x 4.8 m, were separated by a potato-free zone 3-4 m wide to limit interplot interference.

A late blight-susceptible potato variety, Tezelfeu and a late blight-resistant variety, Cipira (Martin et al., 1994) were used in 1997, while Tezelfeu was used alone in 1999. In either experiment, seed tubers were planted in April and plants were spaced 0.8 m apart with 0.3 m between plants in a row. In either year, plots were fertilised with 400 kg ha^-1 N-P-K in bands at planting. A foliar fertiliser, Fertigofol 313 (4 L ha^-1), was applied on leaves one month after planting. An insecticide-nematicide, carbofuran (Furadan 10G, 3 kg a.i. ha^-1), was broadcast after planting but before emergence. Plots were hand-weeded and hilled as needed.

Plots were harvested at maturity, when the vine was dead. Tubers were harvested from the inner rows of each plot. They were sorted, counted and weighed and marketable yields were expressed in metric ton fresh wt ha^-1.

Fungicide protection. In both trials, fungicide protection with Ridomil Plus® (72 WP) was applied using a Solo knapsack sprayer (Solo Kleinmotoren GmbH, Sindelfingen, Germany) that delivers about 700 l ha^-1 at a maximum pressure of 4 kg cm-2 using a single flat fan nozzle. Control plots were not sprayed.

The effect of Ridomil Plus spray rate on potato late blight severity was evaluated during 1997. Plots were treated twice with Ridomil Plus at 0.6, 1.2, 2.4 or 4.8 kg a.i. ha^-1 to determine the optimum spray rate. The first treatment was applied at first foliar symptoms (28 days after planting, DAP), while the second was applied three weeks later.

The effect of the fungicide spray frequency on late blight severity was evaluated during 1999. Results from the 1997 experiment showed that the optimum spray rate was 2.4 kg a.i. ha^-1. Plots were then sprayed once, twice or thrice with Ridomil Plus at this rate to determine the optimum spray frequency. The first spray was applied at the first foliar symptoms (35 DAP) and a 21-day spray schedule was observed for the second and third sprays.

Disease evaluation. Crops were inoculated by naturally-occurring inocula in the field. In both trials, disease severity (proportion of leaf area diseased) was scored weekly on five randomly-selected plants in the centre rows of each plot with the aid of a modified Horsfall-Barratt (Berger, 1980) rating scale. Seven weekly ratings, initiated from first foliar late blight symptoms, were scored for each plot. The Horsfall-Barratt severity ratings were converted to disease proportions with the conversion tables of Redman et al. (1967). Values for standardised area under disease-progress curves (SAUDPC) were calculated from the severity data according to the formula used by Campbell and Madden (1990).

Data analyses. Disease and yield data were analysed by analysis of variance (ANOVA) and contrasts using an MSTAT-C statistical package (Michigan State University). The ANOVA was performed to test significant differences among treatments, while contrasts analyses were conducted to test linear, quadratic or cubic trends of disease intensity and yield over fungicide rate or frequency.

Economic analyses were conducted for costs and returns to the different fungicidal treatments. Total returns were the value of the marketable yield obtained in each treatment. A unit farm gate price of $ 0.30 kg^-1 of potato was applied on marketable yields to estimate the total returns. The cost of each fungicide and its application was also assessed. The cost of applying the fungicide with a knapsack sprayer was estimated at US$ 9.0 ha^-1 (US$ 3.0/man-day x 3 man-days/ha). A net return was determined for each treatment by deducting the cost of fungicide protection from the total return. The net increase in revenue due to fungicide treatment was assessed by deducting the net return from the unsprayed plot from that obtained in each sprayed plot to determine the most economically efficient fungicide spray regime.

Disease progress curves varied according to the fungicide regime and late blight progress was slower in treated plots than in the control.

Effect of Ridomil Plus spray rate on late blight severity. The effect of the different rates of Ridomil Plus on late blight progress curves on Tezelfeu and Cipira are presented in Figures 1 and 2, respectively. In both varieties, late blight severity varied with the rate of the fungicide sprayed on the crop. Late blight was more severe on Tezelfeu compared to Cipira. At harvest all the unsprayed plots of Tezelfeu had heavy late blight infections (disease severity = 1) while those of Cipira had few infections (disease severity = 0.23).

An analysis of variance showed a highly significant (P < 0.001) effect of spray rate on SAUDPC and marketable yields. The SAUDPC value decreased with increase in the rate of Ridomil Plus, the reduction being more significant on the susceptible than the resistant variety. Contrast analyses showed a significant quadratic effect of the fungicide spray rate on SAUDPC for both varieties (Fig. 3). When a high rate (4.8 kg a.i. ha^-1) of the fungicide was sprayed on crops, phytotoxicity, characterised by leaf deformation was observed on both varieties.

In variety Tezelfeu, plots sprayed with the highest rate of the fungicide (4.8 kg a.i. ha^-1) produced a lower yield than those that received 2.4 kg a.i. ha^-1 (Fig. 2), presumably due to the phytotoxicity associated with high rate usage. Contrast analyses also showed a highly significant (P < 0.001) quadratic effect of fungicide spray rate on tuber yields for both varieties (Fig. 4). Yields were significantly low on Tezelfeu (4.0 - 12.0 t ha^-1) compared to Cipira (22.4 - 34.8 t ha^-1).

Effect of Ridomil Plus spray frequency on late blight severity. The effect of Ridomil Plus spray frequency on late blight progress on Tezelfeu is presented in Figure 5. Disease severity decreased with increase in the number of the fungicide applications on the crop. Late blight intensity was high in unsprayed plots compared to those that received two or three sprays.
An analysis of variance revealed a highly significant (P < 0.001) effect of spray frequency on SAUDPC and marketable yields. Values for SAUDPC decreased with increase in the number of fungicide applications (Fig. 5). There was no significant difference in SAUDPC when plots were sprayed twice or thrice.

The amount of marketable tuber yields varied inversely with SAUDPC. Contrast analyses revealed a highly significant (P < 0.001) quadratic relationship between the number of fungicide applications and SAUDPC or tuber yields (Fig. 6). An increase from two to three sprays did not result in a significant increase in yields.

Economic appraisal of fungicide management. Both cost and benefit derived from late blight protection increased with the rate and number of fungicide applications. Total and net returns were higher for Cipira than for Tezelfeu because of the higher yields recorded for the former. Net returns were higher in plots exposed to high rates of the fungicide. Increase in net return was highest for Cipira plots sprayed with the highest rate (4.8 kg a.i.ha^-1), while in Tezelfeu plots, the value obtained from plots sprayed with 4.8 kg a.i.ha^-1 was lower than that recorded from 2.4 kg a.i. ha^-1-sprayed plots (Table 1 ). A net return of US$ 1947 was obtained in plots sprayed twice, while those sprayed once or thrice produced lower returns (Table 2).

Fungicide applications reduced late blight intensity and consequently increased potato tuber yields. Results indicate that to select a fungicide for late blight control, a farmer may consider the rate and number of applications needed to achieve an adequate control. Economic analyses revealed that two sprays of Ridomil Plus at 2.4 kg a.i. ha^-1 are necessary for an adequate late blight protection programme in potato. This spray regime was profitable and resulted in increases in net returns of US$ 1947 to 3327 ha^-1. Similar studies have revealed the economic benefits of oxadixyl-mancozeb and metalaxyl-mancozeb sprays on potato late blight control in India (Prasad, 1994; Sharma, 1994).

A significant quadratic relationship existed between the rate or number of Ridomil Plus applications and potato yields, suggesting that high rates or frequencies of fungicide applications do not necessarily lead to better economic benefits. High rates of pesticide usage usually lead to wastage and phytotoxicity (Fontem, 1998). In Cameroon, late blight protection in potato and tomato is heavily dependent on fungicide applications (Fontem et al., 1998). In potatoes, farmers usually apply 10 - 20 fungicide sprays per crop cycle (Fontem and Bouda, 1996; Teugwa-Mofor et al., 1996). However, high yielding late blight-resistant varieties, such as Cipira (Martin et al., 1994), may be used to reduce the high dependence of farmers on costly fungicides.

The results show that potato late blight infections may decrease tuber yields and consequently net farm incomes if not checked by an appropriate management method. Moreover, resistant varieties, such as Cipira, and two foliar sprays of Ridomil Plus® at a 21-day interval may be incorporated into integrated pathogen management (IPM) schemes for potato late blight.

This study was financed by a grant from the International Foundation for Science. We are grateful to Nicholas Morfaw for his valuable assistance in the field.

TABLE 1. Cost and return analysis for potato production under late blight protection using different Ridomil Plus spray rates.

Fungicide rate (kg a.i.ha^-1)	Total return^x		Cost of fungicide protection^y	Net return^z		Increase in net return
Fungicide rate (kg a.i.ha^-1)	Cipira	Tezelfeu	Cost of fungicide protection^y	Cipira	Tezelfeu	Cipira	Tezelfeu
Unsprayed	6,720	1,200	0	6,720	1,200	0	0
0.6	7,290	2,100	60	7,230	2,040	510	840
1.2	8,040	2,790	101	7,939	2,689	1,219	1,489
2.4	10,230	3,600	183	10,047	3,417	3,327	2,217
4.8	10,440	3,060	348	10,092	2,712	3,372	1,512

All figures in US$ ha^-1, US$ 1 = 600 FCFA.
^xMarketable yield (kg ha^-1) x $ 0.30 kg^-1.
^yCost of protection = (Cost of fungicide + cost of spraying) x 2 applications per cycle
^zNet return = Total return - cost of protection

TABLE 2. Cost and return analysis for of potato cultivation under late blight protection using different Ridomil Plus spray frequencies

Fungicide spray frequency	Total return^x	Cost of fungicide protection^y	Net return^z	Increase in net return
Unsprayed	1,050	0	1,050	0
Once	2,190	92	2,098	1,048
Twice	3,180	183	2,997	1,947
Thrice	3,240	275	2,965	1,915

All figures in US$ ha^-1, US$ 1 = 600 FCFA
^xMarketable yield (kg ha^-1) x $ 0.30 kg^-1
^yCost of protection = Cost of fungicide + cost of spraying
^zNet return = Total return - cost of protection

Figure 1. Effect of Ridomil plus spray rate on the progress of potato late blight on cultivar Tezelfeu.

Figure 2. Effect of Ridomil plus spray rate on the progress of potato late blight on cultivar Cipira.

Figure 3. Effect of Ridomil plus spray rate on standardised areas under late blight progress curves (SAUDPC) on two potato varieties.

Figure 4. Effect of Ridomil plus spray rate on the marketable potato yield of two varieties of potato.

Figure 5. Effect of Ridomil plus spray frequency on the progress of potato late blight.

Figure 6. Effect of Ridomil plus spray frequency on standardised area under disease-progress curve (SAUDPC) and marketable potato yield.

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