Role of seed and aphid vectors in proliferation of tomato virus diseases on small holder farms in Kasese District, Uganda
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In Uganda, tomato productivity is very low due to poor agronomic practices, lack of improved varieties and damage caused by insect pests and diseases. Studies to evaluate factors influencing the increasing incidence of virus diseases on tomato were conducted in 2013 in order to develop recommendations for strategies to effectively manage these diseases with Kasese district as a case study. A biological survey of 50 tomato farms was implemented. Results showed that incidence of virus diseases ranged from 60- 100%. Tomato fields planted with farm-saved seed recorded significantly higher virus incidence (81.7%) and severity scores (89%) than those with seed from agro-dealers (59.9%) and (75.6%), respectively. Of the two tomato varieties grown in the area, virus incidence on variety ‘Cal J’ (78%) was higher than ‘Tengeru 97’ (61%). A total of 400 symptom bearing (symptoms typical of Tomato mosaic virus (ToMV), Cucumber mosaic virus (CMV) &Tomato yellow leaf curl virus (TYLC) such as mosaic, mottling, chlorosis, distortion & dwarfing of leaves, fruits and entire plants) tomato & 60 weed leaf samples occurring within 10 meters of farmers’ tomato fields were assessed for aphid and virus diseases, and serologically tested using DAS-ELISA for ToMV and CMV. Incidence of virus diseases in tomato leaf samples was: ToMV (90%), CMV (77.38%), mixed infection (86%); for weeds: ToMV (100%), CMV (72.22%), mixed infection (72.22%). Farm saved and seed from one seed company tested positive for ToMV and Potyviruses in DAS-ELISA. Due to the established seriousness of infected seed, an on-farm trial was set-up to establish the effect of planting cleaned seed (using dry heat at 700C for 48 hours) and seedling protection (using fine netting for 30 days from sowing) on occurrence of key tomato viruses and plant productivity. The design was a RCBD with three replications of four treatments: (i) clean seed / protected nursery; (ii) clean seed / unprotected nursery; (iii) farm saved seed / protected nursery; and (iv) farm saved seed / unprotected nursery. The trial was implemented in two seasons. Data were collected on aphid population dynamics, incidence and severity of virus disease, tomato growth parameters on 10 randomly selected plants in each plot at 30, 60 and 90 days after sowing; as well as on and fruit yield of tomato. Clean seed and nursery protection had significant effects on plant height, number of leaves, leaf area, number of branches, number of flowers, number of fruits and on apterous aphid occurrence as well as on virus symptom severity (P<0.05). Cleaned seed and nursery protection reduced virus severity and consistently enhanced tomato growth. As such, the high incidence of viruses on small holder tomato fields in Kasese was sustained by growing ToMV and Potyvirus infected seed, susceptible varieties, and the presence of infected weeds in the crop’s vicinity. Planting cleaned seed and seedling protection can offer the first line of defence against losses caused by these viruses on tomato. Farmer awareness of these issues may be crucial in haltering the rate of increase in virus disease infections on tomato.