Susceptibility of Lenticels within the Stem Depression of Tomato Fruit to Bacterial Soft Rot

Abstract

The postharvest development of bacterial soft rot in tomato fruit caused by Pectobacterium carotovorum is herein linked with inoculation of lenticel-like apertures located around the stem attachment. Stem scars misted with aqueous cell suspensions of P. carotovorum (1 × 106 CFU/ml) or briefly (5 s) immersed in the suspension were likely to become infected during subsequent storage, with a disease incidence exceeding 70% within 7 days at 24°C. Water soaking was initially observed beneath the fruit surface at the juncture of radial walls of endocarp tissues and a corky ring around the stem attachment. If fruit were swirled for 1 min in chlorine at 150 ppm, pH 6.5, within 5 s after inoculation, lesions did not occur. However, if the chlorine treatment was delayed by 120 s, lesions occasionally developed. A 60-min delay usually assured that decay would occur before fruit ripened (within 5 days). Delaying the chlorine wash by 18 h, as would occur if commercial harvests were held overnight prior to packing operations, led to the same incidence of bacterial soft rot as observed among control fruit. Inoculation of fruit with attached stems did not lead to as much decay during storage compared with stemless fruit, suggesting that an intact calyx physically reduced lenticel exposure to external aqueous cell suspensions. Tomato fruit exposed to uncontrolled free moisture during or shortly after harvest have a high potential for postharvest decays and the likely site for infection development is the area surrounding the stem attachment.