High temperature limits the growth and fruit yield of tomato. Five experiments were conducted to better understand the physiological and genetic response of tomato to high-temperature and Fusarium wilt disease. The first experiment optimised a method for identifying heat stress tolerant genotypes. The second experiment identified traits that expressed across environments and that could be useful for the development of high temperature tolerant cultivars. The third identified candidate genes controlling important traits linked to heat stress tolerance. The fourth experiment identified genetic variation for Fusarium wilt resistance and the fifth assessed the impact of heat stress on Fusarium wilt incidence.
Electrolyte leakage was a key trait for differentiating genetic materials for response to high temperature and this was used in all subsequent evaluations. Histological studies on selected lines varying for electrolyte leakage confirmed that pollen development was significantly impaired by heat stress. High temperature reduced numbers of fruit, flower fruit set ratio and fresh fruit weight. Organic metabolites also varied and changes in these metabolites was greater in heat tolerant genotypes, except for fructose and sucrose, where sensitive genotypes produced elevated levels. Molecular markers significantly associated with these traits were identified under heat stress; however, they need to be validated in other materials before utilization in tomato breeding. Significant genetic variation for resistance Fusarium solani was observed and disease incidence was strongly associated with changes in citric acid, sucrose and L-proline. Significant temperature x disease treatment interactions were also observed for important traits and higher temperatures increased disease severity. Concurrent selection for improved disease and heat tolerance is possible as the disease severity index and the heat stress response were not correlated.