Resistance of selected cassava genotypes to whiteflies in Malawi.

Abstract

Cassava is an important staple food and cash crop for more than 30% of the total population in Malawi. Whiteflies, Bemisia tabaci and Bemisia afer, are the key pests affecting productivity of the crop in Malawi. B. tabaci acts as a vector of plant viruses, which cause the two most devastating diseases: cassava brown streak disease (CBSD) and cassava mosaic disease (CMD). In susceptible cultivars, each disease causes yield reduction of up to 70%. Together with B. afer, B. tabaci inflicts physical damage on plants from feeding and excretions, hence reducing plant respiration and photosynthetic ability that leads to additional yield loss. Over the years, research to mitigate CMD and CBSD has focused on developing cassava varieties with resistance to the viruses that cause these diseases. The diseases, however, continue to spread widely in Malawi due in part to break down in host resistance to the viruses. For this reason, sustainable disease management also requires research on management of the vector. This study was, thus, conducted to identify cassava genotypes with resistance to whiteflies and establish mechanisms underlying their resistance in Malawi. Twenty-eight (28) cassava genotypes were evaluated in the field for two seasons, 2015/2016 and 2016/2017, in five locations (Lilongwe, Salima, Nkhata Bay, Karonga and Zomba districts) that represented major cassava agro-ecologies of Malawi, in order to investigate their reaction to whiteflies. Experiments were laid out in a randomized complete block design with three replications. Following the field trial in the 2015/2016 season, 14 genotypes were selected for further assessment in a screen house at Bvumbwe Agricultural Research Station. The genotypes were evaluated twice from November 2016 to August 2017, in order to determine the behaviour and biology of whiteflies in choice and no-choice experiments. These experiments were also set up in a randomized complete block design with three replications. Data for both studies were analyzed by GenStat statistical package (14th Edition). Genetic variability was exhibited among the cassava genotypes in terms of resistance to whiteflies. Based on mean field reaction to whitefly populations, resistant genotypes (1.2 - 2.07 mean whiteflies) included ‘Pwani’, ‘Mkumba’, ‘Shibe’, NAROCASS 1, ‘Colicanana’, and ‘Kizimbani’. Genotypes with moderate resistance (2.18 - 5.50 mean whiteflies) were: NASE 18, NASE 3, ‘Yizaso’, ‘Mkuranga-1’ and ‘Albert’. ‘Kibandameno’, ‘Kipusa’, ‘Sauti’, ‘Phoso’ and ‘Kalawe’ were identified as susceptible genotypes (>5.50 mean whiteflies). Petiole colour was the morphological characteristic that significantly affected (r = -0.41; P ≤ 0.05) whiteflies preference under field conditions. Temperature (r = 0.37, P<0.001) and rainfall (r = -0.05, P<0.001) were among the key weather factors influencing whitefly population dynamics. Salima and Karonga were the most informative locations and, hence, best suited for whitefly resistance screening. The genotypes ‘Mbundumali’, ‘Pwani’ and ‘Albert’ exhibited antixenotic and antibiosis resistance mechanisms to whiteflies. ‘Orera’, NASE 3 and ‘Tajirika’ genotypes had high levels of antibiosis while NAROCASS1 revealed high levels of antixenosis. Antixenosis was characterized by low nymph and adult whitefly populations. Antibiosis was, in contrast, characterized by low oviposition preference, high egg mortality and low numbers of adults developing from nymphs which resulted in low survival rates. Identification of cassava varieties with inherent resistance against cassava whiteflies indicates a potential for enhancing CMD and CBSD management in Malawi through both promotion of these varieties with farmers and their use in improvement programs.