Potato Grower

50 POTATO GROWER | DECEMBER 2018 Do some potato-growing soils suppress powdery scab? What's in Your Dirt? A research project in New Zealand is determining if different field soils affect the development of powdery scab on potatoes, and whether soil physical, chemical and/or biological characteristics influence this important disease. The project is developing new knowledge that may provide a basis for manipulating soil factors to reduce the harmful effects of the powdery scab pathogen. Powdery scab can severely reduce the quality and marketability of seed, fresh market and processing potatoes. The powdery scab pathogen, Spongospora subterranea, also reduces tuber yields by disrupting root function—water and nutrient uptake—in actively- growing potato plants and causes severe galling on roots. A research initiative that began in March 2016 aims to identify soil factors that influence the development of powdery scab in potato crops. The study involves scientists at the New Zealand Institute for Plant & Food Research Limited. Previous results from a long-term potato/onion trial carried out in the Pukekohe vegetable-growing region near Auckland, New Zealand, indicated that continuous potato cropping over 10 growing seasons did not result in increased incidence or severity of powdery scab on harvested potatoes. This suggested that the trial site soil was "suppressive" to the powdery scab pathogen. Multiple potato cropping is usually associated with severe outbreaks of powdery scab, particularly in potato cultivars that are susceptible to the disease and to Spongospora root infection. PHASE I A cross-discipline team is working on the project, including plant pathologists, soil scientists and molecular biologists. In the study's first phase, 12 field soils (including the soil from the 10-year Pukekohe trial site) have been evaluated for disease "conduciveness." Their physical, chemical and biological characteristics are being determined. The soils have been chosen to represent different soil types and differences in potential to support or suppress soil-borne pathogens of potato. An extensive greenhouse pot trial has been completed, where the different soils were each placed in large pots which were then either inoculated with Spongospora or left uninoculated. The pots were each planted with a seed tuber of a powdery scab-susceptible potato cultivar. Root galling and powdery scab on harvested tubers, and plant productivity measurements (shoot growth and tuber yields), were assessed during the following 20 weeks to crop maturity. The 12 field soils were also assessed in detail for their physical and chemical characteristics, using standard soil science methods. Samples were also tested for potential soil-borne potato pathogens using the PreDicta Pt service provided by the South Australian Research and Development Institute, and DNA has been extracted from samples of the soils. This is being characterized using gene sequencing technologies to determine the microbial population profile of each soil. The 12 soils differed in several physical and chemical factors. Soil texture was different (clay content ranged from 10 to 60 percent), and organic matter content varied (ranging from 2 to 14 percent). The soils differed in fertility and nutrient availability; Olsen tests for phosphate availability gave results from 30 to 260 milligrams of phosphorus per kilogram of soil. In the pot trial, root galls occurred only on inoculated plants, and mean numbers of root galls on these plants ranged from less than one up to 11. Mean marketable tuber yields ranged from 1 pound per plant from one inoculated soil, to 3.8 pounds per plant from a different, non-inoculated soil. Spongospora inoculation influenced the severity of powdery scab on tubers harvested from some of the soils, but had little or no effects for others. The soil containing Diggin' In STORAGE| By Richard Falloon, The New Zealand Institute for Plant & Food Research Limited Severe powdery scab on a potato tuber harvested from the pot trial. Photo courtesy of Peter Wright Spongospora root galls on a field-grown potato plant. Photo courtesy Richard Falloon

• Cover