Studies on common scab of potato

This thesis includes studies on: 1) characteristics of streptomycete isolates from common scab lesions of potatoes collected from across the Tasmanian potato cropping region; 2) scab conduciveness of soils from that region and ; 3) effects of green manures on common scab of potato. Most of 94 streptomycete isolates fitted into four groups based on morphological and physiological characteristics. Groups were: Streptomyces scabies (25 stains); S. violaceusniger (11); Streptosporangium spp. (3); and strains resembling Streptornyces halstedii (29). Some of the remaining 26 ungrouped strains resembled S. scabies. Most S. scabies strains could grow at pH 4.5. Pathogenicity of strains was evaluated in four ways. In a potato disk assay, all S. violaceusniger strains, S. scabies reference pathogen strain #32 and five S. scabies-like ungrouped strains showed clear signs of pathogenicity. Media from oatmeal-broth (OMB) cultures of S. scabies and S. violaceusniger strains inhibited radish seedling growth, while S. violaceusniger strains 54/3 and 75/1-1 killed seedlings. Surface necrosis occurred within 24 hours of dipping potato minitubers in media from OMB cultures of S. violaceusniger. Three S. scabies strains (including #32) also produced darkening of lenticels, indicating necrotising potential. S. violaceusniger strains produced nigericin while strains 54/3 and 75/1-1 also produced geldanamycin, probably accounting for their virulence in pathogenicity assays. Thaxtomin A production by strain #32 was also confirmed. Scab conduciveness of 36 Tasmanian potato cropping soils was compared in a glasshouse trial. Linear regressions showed no association between scab severity and any of 12 individual soil chemical properties. However, scab was much less likely below a threshold pH or when the concentration of exchangeable cations Ca', Mg' and K+ (considered together as milliequivalents) were below a threshold value. In glasshouse studies, green manures did not increase scabbing and in one pot trial a broad-bean manure reduced scab severity. Green manures were associated with increased cellulase activity and electrical conductivity (EC) of soil in the absence of added fertiliser plus, in some trials, increased tuber yields. Added streptomycete antagonists did not noticeably effect scab severity. In field trials at two sites, lupin or ryegrass green manures did not effect scab severity, while increased scab with an oat manure at one site was associated with increased soil EC. Increased scab with a canola green manure, evaluated at only one site, was associated with reduced soil microbial activity as assessed by FDA hydrolysis. At field sites, green manures did not affect tuber yield, soil cellulase activity or pH.