Synchytrium solstitiale: reclassification based on the function and role of resting spores.

Studies were made about resting spores of Synchytrium solstitiale, a chytrid that causes false rust disease of yellow starthistle (YST). During evaluation of this fungus for biological control of YST, a protocol for resting spore germination was developed. Details of resting spore germination and study of long-term survival of the fungus were documented. Resting spores from dried leaves germinated after incubating them on water agar at least 7 d at 110-15 C. Resting spores were viable after storage in air-dried leaves more than 2 y at room temperature, suggesting they have a role in off-season and long-term survival of the fungus. Each resting spore produced a single sorus that contained a single sporangium, which on germination released zoospores through a pore. YST inoculated with germinated resting spores developed symptoms typical of false rust disease. All spore forms of S. solstitiale have been found to be functional, and the life cycle of S. solstitiale has been completed under controlled laboratory and greenhouse conditions. Resting spore galls differ from sporangial galls both morphologically and biologically, and in comparison, each sporangial gall cleaves into several soli and each sorus produces 5-25 sporangia that rupture during release of zoospores. For this reason S. solstitiale should be reclassified as diheterogallic sensu Karling (Am J Bot 42:540-545). Because resting spores function as prosori and produce an external sorus, S. solstitiale is best placed in into the subgenus Exosynchytrium.