Soil-borne pathogens are cryptic killers causing pre- and post-emergence mortality of large numbers of plants. Their effects are most pronounced on seedlings (far right) but they may also cause saplings (immediate right) and even trees to die (e.g. Sudden Oak Disease caused by Phytophthora ramorum). Plants with damping-off appear wilted. Close examination reveals necrotic stem and root tissue. A soil-borne disease (Phytophthora infestans a close relative to Pythium) was also responsible for the Irish Potato Famine (1845-1849). These pathogens can spread vegetatively with mycellium (microscopic threads), lay dormant with various spores, or swim through solution with motile zoospores. To see movies of zoospores being released and swimming visit The American Phytopathological Society.
The movie clip below shows the growth of black cherry (Prunus serotina) seedlings on two Petri plates. The plate to the left is a control plate and the seedlings grow normally. The seedlings in the plate to the right were inoculated with an isolate of Pythium sp. (look for grass blade segment from a grass blade culture during the "start of disease" segment) from Mammoth Cave National Park, KY. This isolate was previously determined to be highly virulent and caused root necrosis in 3 days. The sequence spans 10 days and the development of necrotic tissue was retarded relative to pathogenicity experiments. In this studio experiment, the diseased plants remain alive but stunted and necrotic. Being alive is probably an artifact of their growing conditions. The plants were grown in a closed chamber with humidity near saturation. Even the slightest bit of water stress would likely cause them to die with characteristic damping-off symptoms (i.e. wilting).
This particular sequence was taken using a Canon Digital Rebel tethered to a laptop running the program RemoteCapture 2.7. The photo interval was 60 minutes and the footage was shot over 10 days.
