Tomato diseases, quality, yield and pesticide use : a field study in Nicaragua

Sammanfattning: Nicaraguan farmers have for a long time struggled with tomato production. The primary problem has been Begomovirus infection, which devastated the tomato production in the country in the 1990's. In the battle against the Begomovirus other problems in the native tomato production have been neglected. The overall goal with these projects was to identify common tomato diseases and to evaluate their effect on yield and quality of the tomato fruits at different nitrogen levels. This report contains two separate theses (chapter 2 and 3). The results of the studies are based on the same field experiments. To increase the understanding about local farming conditions (chapter 1), interviews were made at two locations. During the interviews the farmers were asked questions about the farms, their production and pesticide use. Today, the farmers overuse chemical products to control different pests. The Nicaraguan farmers in general have a strong belief that pesticides are the solution of all their agricultural problems. The thesis in chapter 2 deals with the effect of nitrogen on some diseases of tomato. The symptoms that appeared in the field experiment indicated that the plants were not infected by the suspected pathogens and lab results showed that symptoms on the plants were caused by two other diseases: bacterial wilt and powdery mildew (causing agents: Ralstonia solanacearum and Leveilulla taurica). The results showed that plants fertilised with double the normal amount of nitrogen were significantly more resistant to powdery mildew. There was also a significant difference between the varieties in severity of powdery mildew. Regarding bacterial wilt the results are ambiguous, one of the field experiments show no difference between the treatments but in the other experiment there was a considerably higher rate of infection in the to highest levels of nitrogen fertilisation. The aim of the study in chapter 3 was to evaluate the effects of nitrogen fertilization on the yield and quality of the tomato fruits and to evaluate the relations between the incidence of diseases and nitrogen fertilization on one hand and fruit quality on the other. The harvest was divided in good (marketable) and bad (not marketable) fractions. To evaluate the fruit quality, taste tests were carried out on the two first harvests and laboratory analyses of acid (% titratable acids) and sugar content (ºBrix) were measured on the second harvest. The results showed that the unfertilized plots had significantly lower foliar nitrogen content. The total harvest levels of the experiment were in similar range as the average harvest for the region (12-18 t/ha), but the marketable fraction was only between 39 and 50 % of the total yield. In the taste test, one of the varieties was significantly the most preferred variety, independent of fertilizing level. The high not marketable fraction was probably due to poor pollination and fruit set caused by high temperatures and heavy infections of Begomovirus and other diseases. It is also very important to identify the pathogen to know which diseases that are present in the field and how to treat them properly. A solution to many disease and quality problem could be a break in tomato production some time during the year to create a host free period for diseases and vectors.