Heat Treatments 2031 In Vitro Studies

There is considerable variation in sensitivity to high temperature among various fungi [14]. Vegetative cells and conidia of most fungi are inactivated when exposed to a temperature of 60°C for 5 to 10 minutes in vitro [35]. Spore germination and germ tube elongation in vitro was inversely related to the duration of exposure or the range of temperature used [36]. Hot water treatments were found to be ineffective in killing dormant spores [37,38]. Ranganna et al. [39] reported that bacterial infection (Erwinia caratovora) was more sensitive to hot water treatments than was the fungal pathogen Fusarium solani. Colletotrichum gloeosporiodes was more heat sensitive than Dothiorella dominicana in mango fruits [40]. Botrytis cinerea was found to be more susceptible to hot water treatment than Alternaria alternata in sweet bell pepper [36]. The effective time to kill 50% of the spores (ET50 germination) for B. cinerea was 3.2, 1.5, and 0.8 minutes at 45, 50, and 55°C, and for A. alternata was 8.8, 4.2, and 1.4 minutes, respectively, at those temperatures. The ET50 for germ tube elongation for B. cinerea was 2.6, 0.9, and 0.5 minutes at 45, 50, and 55°C, and for A. alternata was 7.2, 2.5, and 1.6 minutes at the same temperatures [36]. Percentage spore germination of A. alternata and F. solani was inversely proportional to the length of exposure to 55 and 60°C. Exposing fungal spores of A. alternata and F. solani to 60°C for about 15 seconds, in vitro, resulted in 48 and 42% reduction in spore germination, respectively [27]. The ET50 for A. alternata was 25 and 16 seconds at 55 and 65°C, respectively, whereas for F. solani the ET50 was 18 seconds at 60°C. None of the temperature/time regimes tested completely inhibited spore germination, although A. alternata was slightly more susceptible to heat treatment than F. solani [27]. A minimum exposure period of 20 seconds at 56°C was required to inhibit Penicillium digitatum spore germination in vitro [30]. No surviving spores of B. cinerea were observed after 15 minutes at 45°C [7]. Monilinia fructigena was more sensitive and a thermal treatment of 3 minutes at 45°C resulted in complete spore inactivation [7]. In vitro studies showed Monilinia fructicola to be more sensitive than Penicillium expansum to high temperature (60°C for 20 seconds) [41].

The viability of five pathogens was decreased by treatment with hot water when tested in vitro. Polyscytalum pustulans was most sensitive and Rhizoctonia solani least sensitive. The temperatures that killed 50% (LT50) and 95% (LT95) of spores of Penicillium digitatum in 15 seconds were about 5.2°C higher than those for arthrospores of Geotrichum citri-aurantii, and those for spores of P. digitatum in 30 seconds were about 3.4°C higher [34].

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