Some effects of crown rust ( Puccinia coronata Corda) on the growth of two ryegrass varieties in New Zealand

Some effects of crown rust ( Puccinia coronata Corda) on the growth of Grasslands Ruanui perennial ryegrass ( Lolium perenne L.) and Grasslands Ariki ryegrass ( Lolium ( multiflorum × perenne ) perenne ) were studied in a plot trial. Half the plots were kept relatively free of rust by spraying with a mixture of maneb and nickel sulphate, while the remainder were allowed to become rusted both from natural infection and from inoculation with urediniospores of crown rust. The disease caused mean reductions of up to 36% in green ryegrass yield, 30% in leaf area index, and 20% in tiller number. These effects appeared to be the result of widespread death of ryegrass leaves as shown by the increases of up to 91% in yellow ryegrass and 46% in brown ryegrass in the rusted plots. Reductions in the yield of green ryegrass were greater in Ruanui perennial ryegrass (maximum reduction of 53%) than in Ariki ryegrass (21%); in the 4 in. mowing treatment (41%) than in the 1 in. mowing treatment (30%); and in the late summer (36%) than in the autumn (29%). These differences were attributed to differences in the severity of crown rust infection. As the severity of the rust infection in the plots was considered to be equivalent to a severe infection in the field, it was concluded that crown rust is an important factor limiting pasture production in this environment in the summer and autumn. The results are discussed in relation to the quantitative assessment of the effects of pasture plant diseases; the incidence of facial eczema; grazing management; and possible control measures.