PHYSIOLOGICAL CHANGES ASSOCIATED WITH PYTHIUM ROOT ROT IN HYDROPONIC LETTUCE

Whole plant and leaf level alterations accompanying Pythium infection were examined. Hydroponic lettuce (Lactuca sativa L.) inoculated with Pythium dissotocum displayed cultivar-specific changes in primary productivity associated with disease progression. ‘Bella Green’, a susceptible cultivar, showed inhibited whole plant photosynthesis (measured as NCER, net carbon exchange rate) and C accumulation, resulting in 35-40% growth reductions (i.e. leaf area, shoot dry weight). Source leaf gas exchange and export of labeled ¹⁴C were reduced as early as 7 days after inoculation (DAI). In contrast, the more resistant cultivar ‘Buttercrunch’ showed signs of rapid recovery following inoculation, with plentiful root regrowth, minimal alterations in whole plant NCER and C gain, and no evidence of growth inhibition at final harvest. ‘Buttercrunch’ displayed no differences in leaf gas exchange or export of fixed ¹⁴C associated with Pythium infection; however, the allocation pattern was markedly different between 14 and 21 DAI, indicating that roots were a more dominant sink early in the infection process until resistance was established. These physiological parameters may be useful in early diagnosis of root disease and more accurate than visible symptoms like root browning in predicting disease severity.