Six years of results from a potato rotation and green manure trial in Tasmania, Australia

Irrigated cropping of vegetables and poppies (Papaver somniferum) is an important farming system in Tasmania. Green manure crops are often grown between cash crops to provide ground cover and soil organic matter and to retrieve subsoil nitrogen. A field trial was started in 2006 to assess the cumulative effects of three autumn-winter land uses: (a) fallow, and (b) annual ryegrass and (c) brassica (BioQure BQ Mulch^TM) green manures on cash crop production, soil-borne disease inoculum, and other soil properties. The three green manure treatments were established each autumn in a randomized block design. A cash crop was grown over all plots in each spring-summer period except in 2009 when plots were split to accommodate two cash crops, potatoes and onions. Because potatoes were also grown over the whole trial in 2006 and 2012, the split in 2009 allowed 3- and 6-year potato rotations to be compared. Concen¬trations of DNA in soil of the pathogens causing powdery scab, common scab, black scurf and stem canker in potatoes were measured each year, while soil carbon and other chemical properties were measured in 2006, 2009 and 2012. In potato ‘Desiree’ the 3-year potato rotation resulted in more powdery scab and lower total tuber yield than the 6-year potato rotation, the first report of such an effect for this disease. Concentrations of DNA of Spongospora subterranean, the cause of powdery scab, increased in the soil each time potatoes were grown. Concentrations of DNA of the other pathogens were little affected by the presence of potatoes or green manures, but DNA of Rhizoctonia solani AG3 was less on average in fallow compared to green manure plots. Soil carbon decreased across the whole site during the experiment but there was no statistically significant effect of green manures on soil carbon or cash crop production.