Exploring the life cycles of three South American rusts that have potential as biocontrol agents of the stipoid grass Nassella neesiana in Australasia

Three rusts, Puccinia nassellae, Uromyces pencanus, and Puccinia graminella, are being studied as potential biological control agents for Nassella neesiana (Chilean needle grass) in Australia and New Zealand. An understanding of the life cycle of a pathogen is desirable before its release as a biocontrol agent is considered, to enable the assessment of the risks involved in such a release. Field observations and experiments have been carried out to elucidate the life cycles of these three pathogens. Puccinia nassellae cycles as urediniospores and produces dormant teliospores. Dormancy of teliospores has been broken through manipulation in the laboratory, but resulting basidiospores have failed to infect N. neesiana plants under the conditions tested. Uromyces pencanus cycles as urediniospores and its telia appear to have lost the capacity to produce basidiospores. Aecia have been reported for this rust in the literature. However, evidence is provided that these aecia in fact belong to the life cycle of P. graminella. Puccinia graminella produces only aecia and telia. The aeciospores have been shown to be repetitive (aecidioid urediniospores). Teliospores germinate directly without a dormant phase, but resulting basidiospores failed to infect N. neesiana plants under the conditions tested. The role of teliospores in the life cycle of all three rusts remains unknown. Although the autoecious nature of their life cycles has not been proven experimentally, neither is there any evidence that they are heteroecious. Practical and theoretical implications of these findings are discussed.