Management of Rust on Mint

Management of Rust on Mint Mint rust, caused by the fungus Puccinia menthae, infects Scotch spearmint and Native spearmint in central Washington. Peppermint, Scotch spearmint and Native spearmint are infected in western Oregon. Two principal types of races of P. menthae have been recognized. One type infects Native spearmint but not peppermint and is called native spearmint rust; the other, peppermint rust, infects peppermint but not Native spearmint. Both groups of races infect Scotch spearmint, but native spearmint rust is more aggressive on Scotch spearmint than is peppermint rust. High temperatures, such as those encountered in the Columbia Basin of central Washington, are limiting for peppermint rust, but not for native spearmint rust. Wet, cloudy weather and prolong leaf wetness from irrigation, dew, and rainfall favor rust development. Defoliation and a reduction in essential oil content of mint plants results from severe rust infection. Rust is first seen in early spring and appears as compact series of light-yellow blister-like pustules, called aecia, on plant stems and leafstalks. Early spring infections are systemic in spearmint and infected plants are twisted and distorted, and stems easily break. Infected plants in early spring are usually sparse in occurrence but are early sources of inoculum. Rust spreads by air-borne spores from aecia during late spring to nearby plants and produce golden to cinnamon brown pustules, called (uredinia) on stems the undersides of leaves. Air-borne spores from produce additional uredinia and the disease is recycled every 8 to 10 days when favored by moisture on foliage from rain, dew or sprinkler irrigation. When these pustules become numerous, the infected leave curl, turn yellow and then brown, and drop from plants. In late summer and fall, pustules on mint stubble and foliage become dark chocolate brown. This is the overwintering stage (telia) of the rust fungus and is the source of new infections the following spring. Rust from escaped mint and non-cultivated mint can also be a source of rust infection in the spring. Fields should be monitored regularly for rust through out the season. Infections in late fall will indicate potential infections in the spring. Early spring application of a light concentration of liquid nitrogen (UN 32) on Scotch and Native spearmint (mint should be less than 1 inch in height) will burn systemically infected shoots and destroy aecia. This will eliminate the spores that potentially give rise to later rust outbreaks. Applying UN 32 at 100 lb available nitrogen/acre after the first cutting on scotch and native spearmints will also reduce inoculum levels and facilitate rust control for the following crop. Fungicides can be effective tools in managing mint rust. Timing of applications should be based field monitoring, crop growth stage, and on recent past weather and forecasted weather. Registered materials include Rally 40W, Headline and Amistar. Fungicide should be initially applied before the disease can be easily found in a field. Repeated applications will be needed when rust pressure is high due to favorable weather and moderate to high inoculum levels. Rally must be applied 30 days before harvest. Headline has a 14 day minimum time from application to harvest, and Amistar can be applied up to 7 days before harvest. Headline and Amistar are Group 11 fungicides and, for fungicide resistance management should be alternated with a fungicide of a different mode of action. Do not make more than three applications of Group 11 fungicide per season. Headline and Amistar are also effective against powdery mildew. Read and follow chemical label when making fungicide applications. Rust can be severe on mint propagated in greenhouses due to prolong leaf wetness and high humidity. Such rust outbreaks are usually initiated from systemic infections and pustules on rhizomes and stem cuttings used in propagation. Rust on rhizomes can be eliminated by immersing rhizomes in water at 113 F (34 C) for 10 minutes before planting. Source materials should be inspected and be rust-free before propagating stem cuttings. Monitor stem cuttings closely for rust and discard infected plants promptly. Of course, greenhouse material should be rust-free before transplanting to the field.