Studies on Puccinia coronata var. coronata and other recently observed rust fungi in Minnesota

Abstract

In the spring of 2017, a prolific crown rust fungus was observed on the highly invasive glossy buckthorn (Frangula alnus) around Central Park, Roseville, MN. Field observation and greenhouse studies established the grass host as another invasive species, reed canarygrass (Phalaris arundinacea), and sequencing identified the rust fungus as Puccinia coronata var. coronata sensu stricto (Pcc), a taxon of likely Eurasian origin not previously known in Minnesota. Curiously, this new pathogen appeared to have a desirable effect locally, strongly affecting only two invasive plant species. In a research project that began in 2019 and was funded in 2020 by the Minnesota Invasive Terrestrial Plants and Pests Center, we pursued three goals, which correspond to the first three chapters of this dissertation: 1) to determine the distribution of Pcc in Minnesota and North America; 2) to assess its host specificity on potential buckthorn and grass hosts; and 3) to evaluate its potential as an augmentative biological control agent of one or both of its invasive hosts. We report Pcc across the range of glossy buckthorn in the Midwest and Northeastern U.S. but find that it is absent on susceptible reed canarygrass outside the range of glossy buckthorn within Minnesota. Cereal crop and turfgrass species were highly resistant to Pcc, but other grass and buckthorn species were susceptible, including some native North American species. In greenhouse trials, Pcc significantly reduced the height and biomass of both reed canarygrass and glossy buckthorn, supporting its use as a possible biological control agent of one or both of its hosts, although non-target effects and deployment strategies would require further consideration. Chapter four describes a similarly designed study around another crown rust fungus, Puccinia digitaticoronata, which we confirm for the first time in North America. We investigate its relation to other crown rust fungi and its pathogenicity on grass and buckthorn species. In greenhouse studies, the popular turfgrass species Kentucky bluegrass (Poa pratensis) is broadly susceptible, along with numerous other native and weedy Poa spp. Common buckthorn (Rhamnus cathartica), another widespread invasive species, is an aecial host of the rust fungus and likely facilitates its sexual cycle locally. Chapter five combines the results of two small projects published as plant disease notes: first reports of Puccinia glechomatis, a rust of creeping Charlie (Glechoma hederacea) in Minnesota and of a Puccinia sp. causing rust of lemongrass in Minnesota.