The Influence of Nitrogen on Culturable Phyllosphere Microorganisms and the Incidence of Botrytis cinerea in Postharvest Leafy Vegetables

Lettuce (<i>Lactuca sativa</i>), pak choi (<i>Brassica rapa</i>), and basil (<i>Ocimum basilicum</i>) were grown in hydroponic NFT systems under four nitrate levels (80-180 mg L^-1 N). We measured natural microbial contamination by plating nutrient-solution samples and leaf washes to obtain colony-forming unit (CFU) counts of bacteria and fungi. Separately, postharvest leaves were artificially inoculated with <i>Botrytis cinerea</i> and stored at 22 °C or 4 °C for 7 days to assess gray mold. In lettuce, high N (180 mg L^-1) markedly increased culturable microbial loads in both solution and leaves, whereas pak choi microbial counts remained low at all N levels. Basil showed a non-linear response: CFU counts peaked at moderate N (120 mg L^-1) and were lower at both deficit and excess N. At 22 °C, gray mold severity in pak choi increased with N; leaves fertilized at N150-180 suffered about 1.5-2 times greater lesion area than those at N80. By contrast, lettuce exhibited the worst decay under N deficiency: N80 leaves developed the largest lesions by 4-7 DPI, while moderate N (120-150 mg L^-1) minimized disease progression. Basil was highly susceptible under warm storage: all N levels reached near-total decay by 7 days, though N120 delayed early infection slightly. Refrigeration (4 °C) greatly suppressed lesion development in lettuce and pak choi, although high-N pak choi still showed ~20-30% more infected area than low-N after 7 days. Basil, however, suffered chilling injury at 4 °C, and all refrigerated basil leaves decayed severely (regardless of N). These results indicate crop-specific nutrient and storage strategies: avoid excessive N in pak choi, maintain balanced N in lettuce, and handle basil with non-chilling methods to reduce postharvest gray mold.