Global Journal of Biology, Agriculture & Health Sciences

Evaluation of Taro Genotypes against the Most Virulent Isolate of Phytophthora colocasiae in Wolaita and Kembata Tembaro Zones of Southern Ethiopia

Zerhun Tomas^1*, Elfinesh Shikur² and Alemar Said^{1 1}Department of Plant Pathology, Southern Agricultural Research Institute, Areka Agricultural Research Center, Areka, Ethiopia
²Department of Plant and Horticultural Science, College of Agriculture, Hawassa University, Hawassa, Ethiopia
^*Correspondence: Zerhun Tomas, Department of Plant Pathology, Southern Agricultural Research Institute, Areka Agricultural Research Center, Areka, Ethiopia, Tel: +251925708127, Email:

Received: 04-Sep-2022, Manuscript No. GJBAHS-22-17925; Editor assigned: 07-Sep-2022, Pre QC No. GJBAHS-22-17925; Reviewed: 21-Sep-2022, QC No. GJBAHS-22-17925; Revised: 28-Sep-2022, Manuscript No. GJBAHS-22-17925; Published: 05-Oct-2022, DOI: 10.35248/2319-5584.22.11.147

Introduction

Taro (Colocasia esculenta) is a perennial tropical starchy root crop which belongs to the Araceae family [1]. It originated from South East Asia and later spread into other parts of the continent and Africa of tropical climatic settings [2]. It mostly growth where the annual rainfall exceeds 2000 mm and it grows best under hot and wet conditions with temperatures above 21°C. It is sensitive to frost and it is therefore a lowland crop [3]. Taro has both medicinal and nutritional uses as it is used as food for man and animal feed [4]. The crop is a good source of income to its producers to the extent that some subsistence farmers generate enough revenue from taro production to take care of basic family needs [5]. Despite the importance of taro, the major constraint to its production is Taro leaf blight. It is one of the major important economic diseases of taro because it reduces corm yield of up to 50% [6] and leaf yield of up to 95% in susceptible genotypes [3]. Phytophthora colocasiae causes corms to rot both in the field and in storage, and this has led to heavy storage lost [7]. Taro Leaf Blight Disease (TLBD) is characterized by large necrotic zonates spot on the leaves often coalescing to destroy large areas of leaf [8]. The margin of the lesion is marked by a white powdery band of sporangia and numerous droplets of orange or reddish exudates [9]. Phytophthora colocasiae originated in South East Asia [8] and is widely distributed throughout the tropical regions of the world [10]. This study was conducted to investigate test for virulence and pathogenicity of P. colocasiae under greenhouse conditions.

Materials and Methods

Description of experimental sites

The laboratory and green house experiments were done at Areka Agricultural Research Center Plant Pathology Laboratory. It is located 300 km South West of Addis Abeba at an altitude of 1833 meters above sea level with latitude and longitude of 703’25’’N and 37040’52’’E, respectively.

Collection, isolation and identification of fungi isolates

Survey was carried out at 27 Taro growing farmer’s fields at 5 km-10 km interval in 2017 cropping season. From them 15 representative taro leaf blight samples collected were surface-sterilized with 0.5% sodium hypochlorite solution for 60 sec and rinsed three times in sterile distilled water. Surface-sterilized leaf fragments were dried on sterile filter paper in a laminar flow hood and for each sample four leaf fragments were transferred into sterilized Petri dishes containing solidified cool Potato Dextrose Agar (PDA) medium amended with antibiotics (penicillin, rifampicin and nystatin). Then, the Petri dishes were labelled and placed in an incubator at temperature of 22°C-26°C [3]. After 2-3 days, the culture was sub cultured in to new Petri dishes to obtain pure culture of isolates and the isolated fungi were designated as A-O. Then isolated fungi were identified as Phytophthora colocasiae based on its mycelia and sporangial characters using standard mycological keys [11,12].

Virulence test of fungi isolates

Virulence tests of 15 P. colocasiae isolates were done by using detached leaf disc method. One 60 mm diameter leaf disc was taken from youngest, fully expanded and disease free, local taro cultivar (Molia) and placed into 90 mm diameter Petri dishes containing water amended with 150 μg/litre kinetin. Each leaf disc was inoculated in the center with each isolate of 6 mm diameter agar plug taken from 2 day old cultures of P. colocasiae. The inoculated leaf discs were incubated for 4 days at 25°C in the dark. Then mean lesion diameter was measured to identify virulent isolate [13]. From the 15 P.colocasiae isolates inoculated on the detached leaf disk of taro plant, isolate A, collected from Sodo Zuria produced extensive necrotic lesions on the leaf and the pathogen was found to be most virulent when compared with the other fourteen isolates, followed by isolates from Damot Gale (C) and Bolosso Sore (J) with mean lesion diameter of 46 mm, 37 mm and 35 mm, respectively.

Inoculums preparation

Sporangia suspension was prepared from 21 days old culture of different P. colocasiae isolates, by flooding the surface of the growing colonies in each Petri dish with 5 ml of sterile distilled water and dislodging the sporangia with a small sterile brush. The suspension was then centrifuged for 3 minutes and the supernatant filtered through a 2 layered sterile muslin cheesed cloth. A sporangia suspension (inoculums) of each isolate was adjusted with the aid of haemocytometer to concentration of 3 × 10⁴ sporangia/ ml of sterilized water. The inoculums were put in a refrigerator at a temperature of 4°C for 30 minutes to stimulate liberation of zoospores and a drop of Tween 80 (25 μl) was added to each sporangia suspension as a surface wetting agent [14].

Evaluation of taro genotypes against virulent

Phytophthora colocasiae isolate

One improved cultivar of taro, Bolloso-I and four local cultivars, Yeda, Dolka, Molia and Yiteria, which are commonly grown in the study area were obtained from taro accessions maintained at AARC were tested for their reaction against a virulent P. colocasiae isolate. For this purpose plants were raised in plastic pots (one seedling per pot) and kept in greenhouse condition. Forty nine days after planting youngest fully expanded taro leaves were inoculated with the most aggressive P. colocasiae isolate identified by virulent test at the concentration of 3 × 10⁴ sporangia/ml of sterilized water (adjusted with the aid of haemocytometer). Inoculation was done by injecting 3 ml of sporangia suspension into three spots on the leaves of taro plant using sterile hypodermic syringe (control plants were also inoculated with 3 ml of sterile distilled water). After inoculation plants were kept in greenhouse and watered and sterilized as needed to maintain normal growth. Treatments were replicated three times and arranged in CRD. Then three days after inoculation mean lesion diameter was measured at 24 hr interval for six consecutive days by using ruler.

Data analysis

Data collected were subjected to Analysis of Variance (ANOVA) using SAS computer software program and significant means were compared using Duncan Multiple Range Test (DMRT) at 99% level of probability.

Results and Discussion

Evaluation of taro genotypes against virulent P. colocasiae isolate

The reactions of taro cultivars tested against P. colocasiae (isolate A) are presented in Table 1. Five cultivars were evaluated against one virulent isolates of P. colocasiae (isolate A) under greenhouse conditions. The result of the study indicated that all the cultivars tested were affected by the P. colocasiae. There was highly significant difference in Mean Lesion Diameter (MLD) among cultivars tested. Bolloso-I had resistant reaction with MLD of 22 mm, whereas two cultivars, Yeda (MLD of 35 mm) and Dolka (MLD of 30 mm) showed moderately resistant (MR) reaction. Cultivars Molia (MLD of 50 mm) and Yiteria (MLD of 65 mm) showed susceptible and highly susceptible reactions, respectively. Similar research evaluated six taro cultivars against P. colocasiae isolate [15]. His result revealed that out of six cultivars evaluated none of them were found to be immune to the isolate, while only one cultivar had resistant reaction to the disease with MLD of 20.5 mm, and three cultivars with MLD of 32.5 mm, 32 mm and 30 mm, respectively showed moderately resistant reaction. The remaining two cultivars showed susceptible reaction. In another study 38 promising taro lines were evaluated against P. colocasiae and their result revealed that 11 isolates were found to be immune [16]. Others isolate 4, 7, 7 and 9 showed resistant, moderately resistant, moderately susceptible and susceptible reaction to leaf blight, respectively. Identification of resistant sources is considered as an important factor in determining the breeding methodology to be adopted for incorporating resistance. The findings of the present research will serve as source of information for resistance breeding of taro against Phytophthora leaf blight.

Conclusion and Recommendation

Five Taro cultivars were screened against one virulent isolates of P. colocasiae (isolate A) under greenhouse conditions. The isolate caused lesions, on inoculated leaves. The result of the study indicated that more or less all the cultivars tested were infected by the pathogen and none of them were found to be immune to P. colocasiae. Bolloso-I was resistant with MLD of 22 mm, whereas two cultivars, Yeda (MLD of 35 mm) and Dolka (MLD of 30 mm) showed moderately resistant (MR) reaction. Cultivars Molia (MLD of 50 mm) and Yiteria (MLD of 65 mm) showed susceptible and highly susceptible reactions, respectively. The result of the present study revealed that importance of Phytophthora leaf blight of taro in Southern Ethiopia. It also demonstrates the role host resistance may play in integrated management of the disease. Future research should be directed towards evaluating additional taro genotypes against P. colocasiae. Since, this is the first study on the taro leaf blight pathogen in Ethiopia, will provide base line information for future research on pathogen characterization, yield loss assessment and management study.

References

1. Graf AB. Tropica 4^th ed Rehrs Company, East Rutherford, NJ; 1992.
2. Rao VR, Matthews PJ, Eyzaguirre PB, Hunter D. The global diversity of Taro: Ethnobotany and conservation. 2010 2-5.

   [GoogleScholar]

3. Scot N, Brooks F, Teves G. 2011. Taro leaf blight in Hawai’ I. Plant Disease. 71: 1-14.
4. Rochani A. The role of taro (Colocasia esculenta) in live hood of local people in the Ayanna Sub-district Sorong Iran Jaya Province. In the Second Taro Symposium, Manokwari, Indonesia. 1994:23-24.
5. Vinning G. Select markets for taro, sweet potato and yam. RIRDC Project No. UCQ-13A. 2003.
6. Singh D, Guaf J, Okpul T, Wiles G, Hunter D. Taro (Colocasia esculenta) variety release recommendations for Papua New Guinea based on multi‐location trials. N Z J Crop Hortic Sci. 2006;34(2):163-171.

   [Crossref] [Google Scholar]

7. Brunt J, Hunter D, Delp C. A bibliography of taro leaf blight. Secrtariat of the Pacific Community: New Caledonia. 1-10.
8. Zhang KM, Zheng FC, Li YD, Ann PJ, Ko WH. Isolates of Phytophthora colocasiae from Hainan Island in China: Evidence suggesting an Asian origin of this species. 1994;86:108-112.

   [Crossref] [Google Scholar]

9. Bandyopadhyay R, Sharma K, Onyeka TJ, Aregbesola A, Kumar PL. First report of Taro (Colocasia esculenta) leaf blight caused by Phytophthora colocasiae in Nigeria. Plant Dis. 2011;95(5):618.

   [Crossref][Google Scholar][PubMed]

10. Commonwealth Mycological Institute. Distribution maps of plant diseases. Common wealth Agricultural.
11. Gerettson-Cornell L. A compendium and classification of the species of the genus Phytophthora de Bary by the canons of traditional taxonomy. Technical Paper-Forestry Commission of NSW. 1989;(45):103.

    [Google Scholar]

12. Ristaino JB. A Lucid key to the common species of Phytophthora. Plant Dis. 2012;96(6):897-903.

    [Crossref][Google Scholar][PubMed]

13. Tyson JL, Fullerton RA. A leaf disc assay for determining resistance of taro to Phytophthora colocasiae. N Z Plant Prot. 2015;68:415-419.

    [Crossref][Google Scholar]

14. Fokunang CN, Mbong G, Manju E, Tembe E, Hanna R. Screen house and field resistance of taro cultivars to taro leaf blight disease (Phytophtora colocasiae). Br Biotechnol J. 2016;15(1):1-15.

    [Crossref] [Google Scholar]

15. Singh L. Prevalence, Perpetuation and Management of Phytophthora Blight of Colocasia.
16. Dey TK, Ali MS, Bhuiyan MK, Siddique AM. Screening of Colocasia esculenta (L.) schott lines to leaf blight. J Root Crops. 1993; 19(1):62-65.

    [Google Scholar]