Biology and chemical ecology of Spongospora subterranea during resting spore germination : towards a germinate/exterminate control approach for Spongospora diseases of potato

Resting spores are important for the spread and survival of Spongospora subterranea, the causative agent of potato powdery scab and root disease. However, resting spores must germinate and release zoospores (the infective agents) to cause infection. Understanding of the germination process and factors is, therefore, important. The knowledge can potentially have implications for resting spore-inoculum management and Spongospora disease control. The biology and ecology of S. subterranea during resting spore germination is not well understood. Knowledge of factors influencing resting spore germination have been limited. This thesis studied the properties of S. subterranea resting spores and investigated some chemical factors stimulating resting spore germination. The role of germination-chemical stimulants was also examined. This study used a combination of tomato-plant and zoospore in vitro bioassays, light microscopy, a targeted hydrophilic interaction ultra-high performance liquid chromatography‚Äö-mass spectrometry metabolomic approach, and a greenhouse chemical-soil treatment study. Some of S. subterranea resting spores exhibited dormant spore characteristics, which required specific stimuli to germinate. Although a proportion retains constitutive dormancy characteristics, chemical germination-stimulants were found in potato root exudates and in Hoagland's solution. The low-molecular weight organic amino acids ‚Äö- tyramine and L-glutamine ‚Äö- and sugars ‚Äö- cellobiose and L-rhamnose ‚Äö- compounds stimulated resting spore germination at 0.1 mg/ml solution. The release of these compounds in potato roots were influenced by the plant's physiology and growth conditions. Hoagland's solution contains Iron-EDTA, which stimulated resting spore germination. Germination stimulant Fe-EDTA, in Hoagland's solution, in the presence a susceptible host plant, enhanced root infection. However, Fe-EDTA and Hoagland's solution added into S. subterranea-infested soil, a month prior to planting, reduced pathogen inoculum DNA levels in the soil. Further field studies underpinning the use of germination stimulant compounds could lead to a novel, safe and sustainable chemical approach for the management of S. subterranea inoculum in the soil and thus will augment other Spongospora disease control measures. This thesis advances our understanding of S. subterranea biology and chemical ecology during resting spore germination. Substantial development is now beginning to be made in the facet of resting spore germination biology and chemical ecology, which are important aspects of Spongospora disease epidemiology and disease control development. There remains much to be learned, but the knowledge presented here will encourage additional studies and research efforts. Powdery scab, Spongospora root infection, plasmodiophorid, chemical ecology, potato metabolomics, HILIC UHPLC-MS, resting spore germination, spore dormancy.