A combining ability analysis of cassava manihot esculenta Crantz genotypes to anthracnose disease

American Journal of Applied Sciences, Jan, 2009 by O.F. Owolade, A.G.O. Dixon, S.R. Akande, S.A. Olakojo

INTRODUCTION

The tropical root crop cassava (Manihot esculenta Crantz) is the third most important source of calories for human food in the tropics after rice and maize. Over 600 million people depend on cassava in Africa, Asia, and Latin America. Cassava is grown by poor farmers, many of them are women. For these people, the crop is vital for both food security and income generation (1). In spite of the importance of this crop as a famine and food security plant, it is constantly threatened by production constraints such as drought, low yielding local cultivars, lack of good quality planting materials, land tenure, pests and diseases (2).

Of all the diseases found on cassava, Cassava Anthracnose Disease (CAD) caused by Colletotrichum gloeosporioides Penz f sp. manihotis Chev is the most important fungal disease of cassava in the field (3). The most outstanding effect of the disease is its ability to cause severe stem damage causing canker on stem, wilting of leaves and diebacks. Badly infected stems become brittle and break easily under strong winds. The overall effect of these is the reduction in yield and in the amount of healthy plantable stems available to the farmers. The frequency with which the disease is encountered in cassava in African has been a matter of concern to many workers (4), (5) reported that between 80-90% of local cultivars were rated as severely infected in Zaire and Congo respectively (6) also observed that the causal organism of CAD was found on cassava stems from all the humid and the sub-humid agro-ecological zones of Nigeria, just as (7) reported a high incidence of CAD across the countries of the rainforest and transition forest zones. In spite of all these reports of widespread distribution of CAD in Africa and the progress made in resistance breeding to Africa Cassava Mosaic Virus (ACMV), Cassava Bacterial Blight (8), (9) CAD is rarely taken into consideration in the breeding programme. The spotlight has, therefore, shifted to host plant resistance since it is acknowledged that, resistant cassava varieties could potentially form the basis of sustainable management strategies for cassava diseases (8), (9), (10). The selection of resistant varieties and continuous breeding programme for disease resistance appears to be the efficient means of controlling CAD. Although little work has been done on resistance to CAD and determination of mode on inheritance. Studies in these areas will assist the breeder in formulating an efficient strategy for incorporating the resistant genes into high yielding improved and stable varieties. The overall objective of this work is to contribute to the development of stable anthracnose resistance in cassava. The specific objective of this study was to evaluate the relative importance of general and specific combining ability for resistance to CAD.

MATERIALS AND METHODS

Genetic experiments: The study was carried out on the experimental fields of the International Institute of Tropical Agriculture (IITA) at Ibadan in Nigeria. The genetic materials were evaluated using the complete diallel mating Scheme during 2004 and 2005 growing seasons. The genotypes were selected based on plant vigour (PV), Flowering Ability (FA) and Sprouting Ability (SA).

All possible crosses involving 9 parents with various degrees of resistance to CAD, (resistant, moderately resistant and susceptible varieties) were made on IITA research field in Ubiaja, Edo State, Nigeria in 2002 by hand pollination and the seeds were made available by cassava breeding unit at IITA for the study. The seeds were planted in pots in nursery prior to transplanting in the field, and watered twice daily for three weeks, and then once a day until the seedlings were established. Established seedlings were transplanted in the field at the same time to produce woody cuttings for the study. Mature stakes (25 cm long) of the parents were planted at the beginning of the rainy season (June 2003). A randomized complete block design with three replicates was used. Each plot consisted of a minimum of 40 plants spaced 0.5m apart in rows (ridges 30 cm high and 10 m long) and was spaced 1m apart, giving a plant population of 20,000 plants per hectare. No fertilizer or herbicide was applied during the course of the experiment, and hand weeding was done when necessary. The parents and their [F.sub.1]'s hybrids were evaluated under rain fed conditions in an area known for CAD epidemics in the 2004 and 2005 planting seasons at IITA's research farm in Ibadan, Nigeria, for their reactions to CAD 12 MAP. The improved cassava genotypes TMS I30572 (highly susceptible), TMS 91/02324 (moderately resistant), a moderately resistant landrace TIME 117 (Isunkankiyan) and a susceptible landrace TIME 1 (Antiota) were included as checks.

Individual plants were examined for symptom severity using the parameters and method as adopted by (11).

Genotypes were partitioned into variation due to lines (parents and crosses) and checks using the GLM procedure in Statatistical Analysis System (SAS). Analysis of variance for the crosses was based on Griffing's method 2, model 1 for fixed genotypes (12) and the linear model (13). The analysis was performed on individual environments using the diallel-SAS programme written by (14) and a combined analysis over environments using the diallel-SAS programme written by (15).