Detection of bark lesions caused by Phytophthoracinnamomi in Eucalyptusmarginata with the plant impedance ratio meter and the Shigometer
A new instrument, the plant impedance ratio meter (PIRM), has been used to determine the extent of lesions caused by Phytophthoracinnamomi Rands in Eucalyptusmarginata Sm. The performance of the PIRM, which measures electrical admittance (the inverse of impedance) of plant tissues at two frequencies (from which an impedance ratio is calculated), was evaluated and compared with that of the Shigometer. The electrical admittance measured at 1 and 10 kHz (PIRM) and the electrical resistance (Shigometer) of healthy tissue varied with both the depth of tissue probed and the water status of stems. However, the impedance ratios (calculated from the admittance values) remained relatively constant for healthy tissue and changes were independent of depth of probing. Hence, changes in ratios indicated a change in tissue condition or necrosis rather than changes in either tissue water content or depth of probing. The impedance ratios recorded for healthy bark tissues were consistently higher than those for the P. cinnamomi lesions in E. marginata. Trends in electrical resistance measured across the boundaries of the lesions with the Shigometer were variable depending on lesion age. The PIRM was used successfully to detect P. cinnamomi lesions in E. marinate and lesion fronts were predicted to an accuracy of ± 7.2 mm (n = 150), lesions being up to 1.0 m long at the time stems were harvested.