Effect of aqueous extracts of Thevetia peruviana K. seeds on the control of late blight and pest insects of Solanum tuberosum L. in Cameroon

Greener Journal of Agricultural Sciences

Vol. 9(1), pp. 14-22, 2019

ISSN: 2276-7770

DOI Link: http://doi.org/10.15580/GJAS.2019.1.121418176

http://gjournals.org/GJAS

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Effect of aqueous extracts of Thevetia peruviana K. seeds on the control of late blight and pest insects of Solanum tuberosum L. in Cameroon

Sylvere Landry DIDA LONTSI¹, Zachee AMBANG^1*, Champlain Lordon DJIETO², Becaire Cedrik CHEDJOU WOUOM¹, Alain HEU¹, Angele NDOGHO PEGALEPO¹

¹ Laboratory of Phytopathology and Microbiology, Department of Plant Biology, Faculty of Science, University of Yaoundé I, Box. 812, Yaoundé-Cameroon

² Laboratory of Zoology, Department of Animal Biology and Physiology, Faculty of Science, University of Yaoundé I, PO BOX.812 Yaoundé, Cameroon.

ARTICLE INFO

ABSTRACT

Article No.: 121418176

Type: Research

DOI: 10.15580/GJAS.2019.1.121418176

In the perspective of effective control of diseases and pests of potatoes (Solanum tuberosum), a study carried out in the Highland West Region of Cameroon permitted to evaluate the effect of aqueous extracts of Thevetia peruviana seeds (AETPS) on the development of late blight caused by Phytophthora infestans and the population of insect pests. Two potato varieties (local (V1) and one improved variety "Cipira" (V2) were used. Five treatments: AETPS (T1), Bravo 720 fungicide (T3), Decis 5 EC insecticide (T4), mixture of the three substances (T2) and the control (T0) were tested using a double factorial system with completely randomized blocks. The evolution of the disease, insect population and tuber yield were evaluated and compared according to different treatments. The results show that a main disease identified in field and laboratory was mildew. 88 pests were collected and 9 pest families were identified. The Coccinelidae family was most represented (23, 86%). The incidence and severity of late blight in the field were respectively reduced to 22.77 and 76.05 % by the AETPS at 50 g/l, as the synthetic fungicide (T3). The number of insects decreased after treatment but remained relatively stable in control. The insecticidal power was maximum 1 h after application of the decis contrary to the AETPS of with maximum was between 24 and 48 h. The commercial yields obtained in the AETPS treatment were greater than 7.5 t/ha compared to 0 t/ha obtained in the controls (T0). Extracts of T. peruviana can therefore be used as anti-parasite substances in crop protection.

Submitted: 14/12/2018

Accepted: 20/12/2018

Published: 31/01/2019

*Corresponding Author

AMBANG Zachee

E-mail: zachambang@ yahoo.fr

Keywords: late blight, pests, Thevetia peruviana, aqueous extracts

ABBREVIATIONS:

AETPS: aqueous extracts of Thevetia peruviana seeds

DAS: days after sowing

WAS: week after sowing

INTRODUCTION

The potato (Solanum tuberosum L.) is a tuberous herbaceous plant native to Latin America. Its production has increased significantly since 1980 up to these days. It rose from 240 million tonnes on 19 million hectares in 1980 to 376 million tonnes in 2013 on 18 million hectares (Anonymous 2013), giving its the fourth place among the cultivated plant after maize, wheat and rice. In agricultural practice, the production cycle of the potato is mainly vegetative, product tubers forming both an asexual reproductive organ, food part of the plant and also a raw material for industrial processing (Ellissèche, 2008). More than 1.5 million hectares are cultivated in Africa (Anonyme, 2007), but in spite of the increase of potato production in the tropics, yields are generally low and are between 3 and 11 t/ha, compared to those of European countries which varied from 25 t/ha to 60 t/ha. Several factors contribute to this decline in production. Some producers continue to use local material or varieties with low potential of production; the prevalence of certain diseases and pests such as mildew, causes almost 60 to 80 % loss of total production (Fontem et al., 2003). Several methods are recommended to fight against this scourge. Mildew caused by Phytophthora infectans (Mont. De Bary) is one of the most destructive disease of potato worldwide (Fontem et al., 2005). Thousands of documents are published each year on P. infestans (Fry et al., 2015). Despite all this research, the late blight pathogen continues to cause major losses on potato and tomato in worldwide (Alkher et al., 2015). The chemical protection is the mostly used and effective but is expensive and polluting. There are recent researches which report the importance of plant extracts in agriculture as pesticidal used to protect crops (Mullah and Islam, 2007; Ambang et al., 2007). Therefore, the search of alternatives to chemicals products such as the use of plant pesticides are promising ways for a reasonable and sustainable agriculture. For a decennia now, researchers have focused their studies on the action of extracts from Thevetia peruviana (Oji and Okafor, 2000) on fungi phytopathogens (Ngoh Dooh et al., 2014), bacteria (Saxena and Jain, 1990; Obasi & Igboechi, 1991), parasites and rodents (Ambang et al., 2002; Chougourou et al., 2012). The yellow laurel (Thevetia peruviana (Pers.) K. Schum) is a small tree belonging to the Apocynaceae family and is used as an ornamental plant in Cameroon. In recent years have been considered a source of biopesticides in crop protection. The general of this work is to evaluate the effect of aqueous extract of T. peruviana seeds on the development of late blight and potato pests.

MATERIAL AND METHOD

Material

The plant material consisted of two varieties of potato (local (V1) and improved variety "Cipira" (V2)) and Thevetia peruviana seeds. Chemical material used were Bravo 720 fungicide and Decis 5 EC insecticide. Several other materials were used.

Methods

Obtention of aqueous extracts and plant treatment

The fruits of Thevetia peruviana collected in Yaounde have been peeled, the cores obtained were crushed, weighed and the paste were introduced in tap water, kept for about 12 hours and filtered with the muslin tissue. This solution was directly applied in the field by adding soap (Blue) used as wetting. Chemicals pesticides were used at the recommended doses. Their application was done from the 21^stweek after sowing (WAS) with a weekly frequency, then we made 9 applications.

Experimental apparatus

The experimental design was bifactorial system arranged in 03 completely randomized blocks. Each block comprises 5 plots representing the different treatments tested (T0= control; T1 = Aqueous extracts (50 g/l); T2 = aqueous extract (20 g/l) + Decis 5 CE (0.5 ml/l) + Bravo 720 (0.8 ml/l); T3 = bravo 720 (1.7 ml/l); T4= Decis 5 CE (1.2 ml/l).

Identification of the disease and inventory of different pest families on potato

When symptoms of the suspected disease appeared, the organs attacked were removed and brought back to the laboratory. There, stem and infested leaves were washed several times with tap water and then cut into small fragments of 2 to 4 cm² for the sheets and 0.56 to 0.84 cm³ for the stems. The pieces are soaked in alcohol for 1 minute, rinsed twice with distilled water, and fed in Petri dishes containing paper moistened with water. 04 days after incubation, the mycelium developed was removed and cultured in new petri dishes containing PDA medium (Potato Dextrose Agar). The mycelial strain was then maintained by transplanting to allow purification of the fungus (Djocgoué, 2008). These pure strains are identified by a microscopic observation using an identification key.

The pests were collected in boxes containing alcohol and returned to entomology laboratory where they were identified using dichotomous keys.

Evaluation of the disease

The evaluation of the plant infection by the disease was focused on incidence and severity evaluated the 6^th; 8^th; 10^th weeks after sowing. The incidence or rate of expansion of the disease which expresses the frequency of occurrence of the disease on the plants in a plot was determined according to the formula: (Chumakov and Zaharova, 1990)

Where: I = Incidence of the disease; n = Number of infected plant s in the field and N = Total number of plant s in the parcel

The severity of the disease or the infection intensity of the disease was evaluated on the plants for each plot. It was estimated according to the proportion occupied by the characteristic symptoms of the disease on the aerial part of the plants. It was calculated according to the formula:

Where: S= Severity of infection; a= Number of sick plants; b= Degree of infection corresponding to diseased plants; N= Total number of diseased plants; The degree of infection noted according to the standard evaluation scale graded from 0 to 4

Evaluation of the effect of aqueous extracts on the population of insect pests

The insecticidal power of the extracts was evaluated by counting insects before and after spraying at time intervals (1, 24 and 48 hours). Counting is done on each plot (avoiding edge lines) (Ambang et al., 2002). The values obtained were compared between the two insecticide treatments (T1 and T4) compared to the absolute control (T0). The insecticidal power was determined according to the formula: P X 100

Or: P = insecticidal power; A = number of insects before treatment; B = number of insects after treatment

Evaluation of the yield

Commercial Tuber yield were determined at 120 days after sowing (DAS). Two parameters were taken into account, the number of tubers per plant for each variety and the weight of the tubers. The number of tubers per plant was evaluated by the exhaustive count of healthy tubers of each plant per plot. Then tubers were weighed using a precision balance 1g and mass estimated per hectare.

Statistical analysis

The results obtained were analysed in the R software that uses the standard of the variance analysis method (ANOVA). For tests (Tukey, Student - Newman- Fischer), the level of significance was evaluated at the 5 % threshold. The Excel 2010 spreadsheet was used for the neck layouts curves, histograms and tables.

RESULTS

Diseases identified in field

The main disease identified in the field from the visual observation method was downy mildew (Fig.1a et b) of which the causative agent Phytophthora infestans was demonstrated in the laboratory (Fig. 1c).

Insects pests

The potato pests inventoried in the study site are very diverse. The dichotomous keys used made it possible to inventory individuals belonging to four orders (Coleoptera, Hemiptera, Orthoptera and Homoptera) and nine families (Chrysomelidae , Coccinelidae , Pentatomidae , Tettigonidae , Scarabae idae , Pigomorphidae , Acrididae , Ciccadellidae and Oedemeridae ). (Table I). The family most represented is Coccinelidae (21) followed by that of Pentatomidae (14). Beetles and Orthoptera are the orders of the most abundant insect pests.

Table I. Diversity of insect pests encountered in the field

Ordre	Famille	Abondance
Beetle	Chrysomelidae	13 (14.77)
Beetle	Coccinelidae	21 (23.86)
Hemiptera	Pentatomidae	14 (15.90)
Orthoptera	Tettigonidae	10 (11,36)
Beetle	Scarabaeidae	6 (6,81)
Orthoptera	Pigomorphidae	4 (4,54)
Orthoptera	Acrididae	8 (9.09)
homopteran	Ciccadellidae	7 (7,95)
Beetle	Oedemeridae	5 (5.68)

Individuals of some families of insect pests encountered in the field of potato plants, because damage to the aerial parts of the plants (Fig.2a). Some of these families preferentially cause damage to the leaves (Fig.2b and 2c) and on the stems (Fig.2d).

Table II. Evolution of the incidence of the disease (%) on the seed potatoes according to the treatments.

treatments	variety	Time (weeks)
treatments	variety	6 WAS	8 WAS	10 WAS
T0	V1	83, 00 ± 17.00 a	100, 00 ± 0.00 a	100.00 ± 0.00 a
T0	V2	80.33 ± 9.23 a	100.00 ± 0.00 a	100.00 ± 0.00 a
T1	V1	35.66 ± 21.12 b	88.66 ± 12.66 b	91.33 ± 8.50 a
T1	V2	38.66 ± 12.66 b	74.66 ± 14.43 bc	77.33 ± 9.81 b
T2	V1	49.66 ± 21.73 b	83, 00 ± 8.00 bc	97.00 ± 5.19 a
T2	V2	41.66 ± 14.43 b	69.33 ± 9.81 c	74.66 ± 8.50 b
T3	V1	46.66 ± 9.81 b	85.66 ± 4.61 b	94.00 ± 5.19 a
T3	V2	41.33 ± 8.50 b	69.00 ± 5.19 c	80.33 ± 4.61 b
T4	V1	55.00 ± 12.76 b	88.33 ± 4.61 b	97.00 ± 5.19 a
T4	V2	36.00 ± 1.76 b	83.00 ± 17.00 bc	91.66 ± 14.43 a

The averages followed by the same letter in the same column are not significantly different at the 5% threshold according to the Student- Newman-keuls test.

Table III. Evolution of the severity of late blight on potato plants according to treatments and varieties.

Treatments	Variety	Time (weeks)
Treatments	Variety	6^th WAS	8^th WAS	10^th WAS
T0	V1	30.25 ± 12.06 a	100.00 ± 0.00 a	100.00 ± 0.00 a
	V2	30.55 ± 14.61 a	100.00 ± 0.00 a	100.00 ± 0.00 a
T1	V1	6.88 ± 5.06 b	13.45 ± 2.87 c	30.22 ± 3.82 c
	V2	7.13 ± 1.52 b	9.05 ± 2.01 d	23.95 ± 3.81 de
T2	V1	12.38 ± 2.48 b	17.61 ± 4.02 b	36.42 ± 2.50 b
	V2	9.54 ± 1.50 b	11.08 ± 3.19 cd	26.03 ± 4.58 d
T3	V1	7.69 ± 2.03 b	11.95 ± 1.5 cd	24.49 ± 3.33 de
	V2	5.78 ± 1.57 b	8.54 ± 1.17 c	20.71 ± 1.98 e
T4	V1	12.03 ± 1.05 b	16.17 ± 0.84 bc	39.05 ± 2.36 b
	V2	13.41 ± 1.43 b	15.63 ± 3.40 bc	31.97 ± 2.66 c

Means followed by the same letter in the same column are not significantly different at the 5% according to the Student test -Newman- Keuls.

In fields from 6^th WAS all leaves and stems of plants in the control plots (T0) were severely infected with downy mildew (Fig.3a) in contrast with those of other plots (Fig. 3b, 3c, 3d and 3e). From 10^th WAS, all plants of plots T0 were decimated by Phytophthora infestans (fig 3f).

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Fig.3. Plants attacked by mildew on different plots 10^th WAS: a- witness (T0); b- aqueous extract (T1); c- aqueous extract + bravo + decis (T2); d- bravo 720 (T3); e- Dec 5 EC (T4); f: control.

Table IV. Variation of the number of pests by variety and treatments.

Treatments	Varieties	Before treatment	Duration after treatment
Treatments	Varieties	Before treatment	1 hour	24 hours	48h
T0	V1	19.50 ± 3.53 a	17.50 ± 0.70 c	17.00± 2.82 b	17.00± 4.24 b
T0	V2	15.50 ± 2.12 a	20.50 ± 0.70 c	22.50± 4.94 b	16.50± 4.94 b
T1	V1	15.00 ± 1.41 a	10.00 ± 1.41 b	3.00 ± 1.41 a	0.50 ± 0.70 a
T1	V2	13.00 ± 2.82 a	11.00 ± 1.41 b	0.00 ± 0.00 a	2.00 ± 1.41 a
T4	V1	12.50 ± 7.77 a	0.00 ± 0.00 a	1.00 ± 0.00 a	0.00 ± 0.00 a
T4	V2	12.50 ± 0.70 a	0.00 ± 0.00 a	0.00 ± 0.00 a	1.00 ± 0.07 a

Means followed by the same letter in the same column are not significantly different at the 5% according to the Student test -Newman- Keuls.

DISCUSSION

The appearance of late blight on young potato stalks 23 days after sowing, as well as the nine identified pest families, would indicate that climatic conditions during the experimental period were favourable for disease and insect pest development. In addition, the previous crop being an associated crop of beans and potatoes, the experimental site would be more favourable to the development of pests. Schaafsma et al. (2001) showed that the previous crop (source of the inoculum) and the climate respectively explained 21 and 48 % of the variation in deoxynivalenol (DON) levels produced by Fusarium graminearum in their similar study done on the corn.

The results obtained on the rates of disease expansion and on the intensity of the disease infection show that the aqueous extracts strongly reduced the development of the disease compared to the control; thus, showing the antifungal potential of this natural substance. Similar results were obtained by Ambang et al. (2007) who demonstrated that this extract reduced the incidence and severity of Cercosporia in peanuts from 32.61 % to 37.25 % compared with the control. Globally, all the phytosanitary products tested reduced the infection of the disease confirming the work of Habtamu et al. (2012) who found that synthetic pesticides reduced the incidence of late blight in potatoes from 38.66 to 59.52 %. Binyam et al. (2014) equally showed that synthetic pesticides combined with the genetic capabilities of varieties reduced the severity of late blight by 68-80 % in similar potato trials. However, at the last observation, high rates were recorded in all plots. The importance of plant contamination in the trial would be due not only to wind and high rainfall, but also to the vicinity of diseased plants as demonstrated by Williams et al. (1994b) in similar research on the epidemiology of late blight. Control plots showed 100 % severity in both varieties. This level of infestation including the tolerant variety (Cipira) could thus confirm the emergence of new strains of late blight in recent years as reported by George and Preston (2004) when searching ways to control downy mildew.

Both varieties were relatively susceptible to insect pests. Variance analyses showed that there was no significant difference (P <0.05) between varieties. The number of insects was totally reduced (100%) one hour after treatment with Decis 5 EC (T4). This confirms the efficacy of this insecticide reported in other plants of the tropics and subtropics like the work of Ambang et al. (2005) on corn weevils. At the same time, the number of insects decreases very little in the plots sprayed with aqueous extracts (T1) and after 24 and 48 hours, the plots T4 and T1 presented similar results showing an almost total reduction of insects. This could be explained by the fact that the toxins are released very slowly by the aqueous extracts and will act after a more evolved time unlike the synthetic insecticide (Decis 5 CE) as demonstrated by Ambang et al. (2002) in similar tests on larvae and imagos of Andrector ruficornis on Solanum tuberosum plants in Cameroon. The high insecticidal potential of the extracts could be due to the presence of toxic molecules such as terpenes and palmitic acids contained in the seeds as shown by Berhaut (1971), Gata-Gonçalves et al. (2003), Chougourou et al. (2012) in similar studies on yellow laurel.

The phytosanitary products tested greatly increased the yield compared to the control. These high yields in the different treatments are certainly related to the reduction of the diseases ensured by the aqueous extracts of the laurel seeds and the synthetic pesticides combined with the genetic capacities of the tested varieties. These results confirm those of Fontem et al. (2007), Muchiri et al. (2009), Girma et al. (2013) in similar research on the formulation of fungicides effective in the control of late blight. Cipira variety presented a high yield than the local variety. This would be due to the improved genetic capabilities of this variety to produce more tuber unlike the local varieties as described by the breeder. The best yields were observed in the T3 treatment (Bravo 720) and in the T2 treatment (mixture of extract + Bravo + Decis). This would result in the proven efficacy of synthetic fungicides registered for the control of late blight and increase the yields. The nil yields observed in both varieties in the control plots confirm the devastating effect of downy mildew, which can lead to a total yield loss, as stated by George and Preston (2004) in similar research on apple blight.

CONCLUSION

The aqueous extract of the yellow laurel seeds (Thevetia peruviana) were tested on two potato varieties (Solanum tuberosum L.), with the aim of determining its impact on the control of downy mildew (Phytophthora infestans) and insect pests of potato in the field. The aqueous extracts of T. peruviana significantly reduced the course of the disease and the population of insect pests compared to the control (To) which contained no application of phytosanitary product. Likewise, this extract increased the yield relative to T0 thus demonstrating its potential against cultivated plants nuisances. The nil yields observed even in Cipira variety which is an improved variety, could show the emergence and the virulence of new strains of late blight during recent years. This represents a serious challenge for potato growers and breeders. It is therefore necessary to intensify biological options to control this disease, including resistant varieties and plant pesticides such as laurel.

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Treatments	Varieties	Number of tubers per plant	Yield in tonnes per hectare (t/ha)
T0	V1	0.00 ± 0.00 a	0.00 ± 0.00 a
T0	V2	0.00 ± 0.00 a	0.00 ± 0.00 a
T1	V1	6.94 ± 0.12 b	0 7.6 ± 0.19 b
T1	V2	8.41 ± 0.79 c	10.29 ± 0.01 c
T2	V1	7.52 ± 0.80 b	7.36 ± 1.11 b
T2	V2	10.52 ± 0.78 d	12.60 ± 0.99 d
T3	V1	7.72 ± 0.54 b c	7.85 ± 0.89 b
T3	V2	10.16 ± 0.38 d	12.68 ± 0.06 d
T4	V1	6.85 ± 0.25 b	7.11 ± 0.22 b
T4	V2	7.33 ± 0.08 b	9.25 ± 0.53 c