Abstract: The present invention is directed to the method and apparatus for monitoring the electrical resistance characteristic of tree tissue which undergoes a step change of resistance at the "freezing" point. The resistance of the tissue is monitored by converting the resistance magnitude sensed to an electrical signal whose frequency is controlled in accordance with the magnitude of the sensed resistance. The frequency of the electrical signal is detected and a control signal generated in accordance with a predetermined frequency change in a set time period which is accordingly indicative of the step change of tissue resistance. The control signal is thus adapted to activate an alarm device or provide some other indication so that precautionary measures can be taken to protect trees, and more particularly fruit bearing trees from crop damage.