Detection and assessment of Armillaria in young conifer plantations of northwestern Ontario and northeastern China

Abstract

Methods for detecting and evaluating Armillaria in plantations were compared in a series of studies. The purposes of the studies were to standardize the Armillaria trapping technique, and to determine if it could be used in practical forest management to monitor and evaluate Armillaria root rot hazard in plantations. Trapping involves burying a removable substrate in the soil for infection by rhizomorphs (RMs). The fungus reacts to the trap by rapidly colonizing the substrate. The distribution of Armillaria is then inferred from the locations of infected traps. In a study of entrapment methods, spruce [Picea sp.) and poplar [Populus sp.) trap logs were compared with each other and with mesh bags filled with conifer bark. Potato tuber (Solarium tuberosum) traps were unsuccessful. Bark bags were the most successful traps in terms of sensitivity, clarity of infection, and ease of interpretation, but they were more difficult to prepare and install than trap logs. Both species of trap logs detected similar levels of Armillaria prevalence. However, the spruce logs were generally easier to evaluate. Some inconsistencies in detection may be resolved by further refinements in trap preparation. In a study of young plantations on recent cutovers and one undisturbed, mature spruce stand, estimates of the distribution of Armillaria based on various indicators were compared. Trap logs detected Armillaria in all plots including the mature spruce plot which was mossy and water-logged. The percentages of plot area subjected to Armillaria impact were estimated to be 3-21% using dead trees, 16-54% using residual stand material, and 12-69% using positive trap logs. A comparison of these estimates showed that Armillaria RMs were much more prevalent than was indicated by the dead planted trees. These estimates plus a survey of healthy and infected trees showed that stump presence alone was a poor indicator of potential damage from Armillaria root rot. Mortality surveys were used to augment the trap results. Although current levels of mortality were high (4.8% spruce, 3.6% Larix sp.), it was suggested that the trees may have been predisposed to Armillaria attack by stresses such as root deformity. To determine the utility of the trapping technique by forest managers unfamiliar with it, the trap bag technique was introduced to a forest management unit in northeastern China. The traps tested in a Pinus koraiensis plantation were superior to soil samples for evaluating the presence of viable RMs. Persons with little or no experience in identifying Armillaria learned to recognize the fresh, abundant RMs quickly and confidently. It was concluded that the trap methods described can be used at the management level, but that they should be used in association with sound advice regarding the role of Armillaria in overall plantation health.