Notes on Puccinia graminella, a rust of stipoid grasses in South America and a potential biocontrol agent for Nassella neesiana in New Zealand and Australia

Abstract

Puccinia graminella is one of several rusts considered as potential biocontrol agents for the grass weed Nassella neesiana in Australia and New Zealand. While Uromyces pencanus was chosen as the most promising candidate, results of our studies on P. graminella in Argentina (where the rust and grass are native) could be useful if a complementary agent is eventually needed. P. graminella produces aecidioid urediniospores, which morphologically resemble aeciospores but behave like the urediniospores of other rusts i.e. they are repetitive and the main (perhaps only) source of new infections. To improve the consistency of previous laboratory results, we aimed to locate accessible sources of spores in the field, and to determine optimal conditions for their collection, production and storage in the laboratory. Aecidioid urediniospores were found to germinate most readily at temperatures between 10 and 15°C. It was possible to store them in a refrigerator (4°C) or a freezer (−70°C), but already-low germination rates decreased over time using both methods. The appearance of host leaves affected inoculum quality, with green healthy leaves yielding aecidioid urediniospores with good germination rates. Best results in inoculation tests were obtained when spores and plants came from the same site. The presence of qualitative resistance in the pathosystem is suspected. P. graminella is not easy to manipulate in the laboratory, its impact on Nassella neesiana in the field is variable and the nature of its life cycle remains elusive. Remaining gaps in knowledge are identified and future directions for research are discussed.