A Sensor for Monitoring Moisture of Wheat Residues: Application in Ascospore Maturation of Pyrenophora tritici-repentis. J. M. C. Fernandes, Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1. J. C. Sutton, and T. D. W. James. Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Plant Dis. 75:1101-1105. Accepted for publication 6 May 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1101.

An electrical impedance sensor was constructed for monitoring water content and water potential (ψ) of wheat straw residues. The sensor consisted of two stainless-steel sewing needles mounted parallel on a pair of insulated terminals and inserted into the straw. Excitation voltage (V_x = 500 mV) was applied to the sensor from a data logger, and residual voltage from the sensor (V_s) was measured. Electrical impedance of the sensor was recorded as V_s/V_x. Five of the sensors performed similarly when evaluated under alternating dry and humid conditions in a moisture apparatus and under field conditions. Readings for sensors made with straw collected in the field at 0, 2, and 8 mo after wheat was harvested decreased logarithmically with declining water content of the straw elements and increased exponentially as ψ of the elements increased. Readings for sensors made with 8-mo-old straw were lower at all water contents and at most ψ values (except near –1.2 to –2.5 MPa and –40 to –50 MPa) than were those for sensors made with 0- or 2-mo-old straw, which did not differ significantly. Sensors were used to monitor residue moisture in relation to ascospore maturation of Pyrenophora tritici-repentis in wheat stem residues kept in a controlled moisture apparatus or in the field. Significant linear relationships were found between numbers of mature asci in pseudothecia and accumulated hours when ψ of the residues ? –2.5 MPa.