A SENSOR FOR MONITORING MOISTURE OF WHEAT RESIDUES - APPLICATION IN ASCOSPORE MATURATION OF PYRENOPHORA-TRITICI-REPENTIS

An electrical impedance sensor was constructed for monitoring water content and water potential (psi) 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 psi of the elements increased. Readings for sensors made with 8-mo-old straw were lower at all water contents and at most psi 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 psi of the residues greater-than-or-equal-to -2.5 MPa.