2005 Annual Report 4d.Progress report. This report serves to document research conducted under a specific cooperative agreement (SDA) between ARS and North Carolina State University. Additional details of this research can be found in the report for the parent CRIS 3607-21000-008-00D “Develop New or Improved Method of Hydroponic and Greenhouse Production and Pest Protection.” This SCA is supported by a PhD graduate student employed at North Carolina State University. The objectives of this project are. 1)to identify the environmental signal that initiates sudden pH decline (SPD) syndrome in geranium and to learn how to control that signal,. 2)to identify parent germplasm in order to establish the genetic relationships between susceptible cultivars, and. 3)to develop linkages between SPD in geranium with SPD in marigold and the reciprocally related problem of excess pH increase in other bedding plant species. Geranium growers in the southeast US have been surveyed to assist in developing a list of geranium cultivars known to be susceptible and resistant to SPD. Based on this information, four cultivars have been selected (two susceptible and two resistant) for laboratory studies. Under laboratory conditions, no difference was found among the cultivars in their susceptibility to SPD. Because of this, future studies will only include two commercial geranium cultivars. Iron or phosphorus deficiency, as well as high ammonium nitrate ratios resulted in lower pH than control plants. High light or temperature also stimulated pH decline, possibly due to higher growth rates. Companion hydroponic studies verified these results. Current and future studies will determine the importance of interactions between nutrient and environmental stimuli as well as the extent of this phenomenon in various Pelargonium spp.