ZHENG ZHAO¹, DAKAI ZHANG¹, HONG TAN¹, JINHUO PENG¹ and ZHINONG MAO¹

¹Dalian Animal and Plant Quarantine Service, Dalian, Liaoning 116001, PR China

Background and objectives
Tilletia foetida shows tolerances to several quarantine treatments (such as irradiation and fumigation) similar to some important smut pathogens whose germination takes a long time and requires special temperature levels. The purpose of this study is to discover the heat tolerance of T. foetida and explore the possibilities for using it as an indicator pathogen in future heat-treatment experiments.

Materials and methods
Dry heat treatments: teliospores of T. foetida were treated for 100 and 200 min at temperatures of 90°C; for 50 and 100 min at 100, 110 and 120°C; for 20 and 40 min at 125 and 130°C; for 20 min at 135°C; and for 10 min at 140°C.

Moist heat treatments: teliospores were treated at 70°C for 60, 120, 180 and 240 min at RH 70%; for 40, 80, 120 and 160 min at RH 75%; for 40, 60, 80 and 100 min at RH 80%; for 10, 20, 30 and 40 min at RH 85%, and for 10, 15, 20 and 25 min at RH 90%. At 75°C, treatments were for 50, 100, 150 and 200 min at RH 65%; for 40, 60, 80 and 100 min at RH 70%; for 20, 30, 40 and 50 min at RH 75%; for 5, 10, 15 and 20 min at RH 80%, and for 3, 6, 9 and 12 min at RH 85%. At 80°C, treatments were for 50, 100, 150 and 200 min at RH 55%; for 15, 20, 25 and 30 min at RH 65%; for 6, 9, 12 and 15 min at RH 70%; for 2, 4, 6 and 8 min at RH 75%, and for 1, 2, 3 and 4 min at RH 85%. At 85°C treatments were for 40, 60, 80 and 100 min at RH 60%; for 15, 20, 25 and 30 min at RH 65%; for 6, 9, 12 and 15 min at RH 70%; for 3, 6, 9 and 12 min at RH 80%, and for 3, 6, 9 and 12 min at RH 90%.

In each treatment, the longest time during which teliospores were not completely killed was considered as the longest survival time (LST), and the shortest time within which teliospores were completely killed was considered as shortest death time (SDT). The thermal death time (TDT) at each temperature was given by extracting the square root of the product of LST and SDT.

Results and conclusions
The germination rate of control teliospores was 67.67%. The teliospores of T. foetida were not completely killed in the dry heat treatments at temperatures of 90, 100 and 110°C. No teliospores germinated in the dry heat treatments at temperatures of 120, 130, 135 and 140°C. The disinfection time for teliospores in the dry heat treatment at 125°C was between 20 and 40 min.

Some teliospores of T. foetida survived in moist heat treatments as follows: at 70°C for 60 and 120 min at RH 70%, for 40 min at RH 75%, for 10 min at RH 85%, for 10 min at RH 90%; at 75°C for 50 min at RH 65%, for 5 min at RH 80%, for 3 min at RH 85%; at 80°C for 50 min at RH 55%, for 1 min at RH 85%. No teliospores germinated in the other moist heat treatments. Thus nine TDTs were obtained: 146.97 min at 70°C and RH 70%; 56.57 min at 70°C and RH 75%; 14.14 min at 70°C and RH 85%; 12.24 min at 70°C and RH 90%; 70.71 min at 75°C and RH 65%; 7.07 min at 75°C and RH 80%; 4.24 min at 75°C and RH 85%; 70.71 min at 80°C and RH 55%; 1.41 min at 80°C and RH 85%.

Conditions of moist heat treatment can be converted into an increment of the specific enthalpy based on temperature and relative humidity [1]. TDT curve, log(TDT)=3.3959-0.0031¦¤H, was made by linear regression of the logarithm of TDT and the specific enthalpy at corresponding temperature and relative humidity, and the correlation coefficient r=-0.9153.

The TDT curve is helpful to evaluate and predict the lethal effect of moist heat on the teliospores. Comparing the TDT curve of T. foetida with that of the target pathogen made in the same way will indicate which pathogen is more tolerant to moist heat treatment and whether T. foetida can be used as an indicator pathogen in heat-treatment experiments.

References
1. Tanabe K, Kawai A, Sugimoto T, 1990. Research Bulletin of the Plant Protection Service, Japan 26, 57-60.