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

Authors: Bruckart, William; Eskandari, Farivar; Widmer, Timothy

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2011
Publication Date: 4/11/2011
Citation: Bruckart, W.L., Eskandari, F., Widmer, T.L. 2011. Synchytrium solstitiale: reclassification based on the function and role of resting spores. Mycologia. DOI: 10.3852/10-286.

Interpretive Summary: Yellow starthistle is a very important weed in the western United States. It can be infected by a fungus called Synchytrium solstitiale, which causes False Rust Disease and which is being evaluated for biological control of yellow starthistle. The fungus produces distinct orange swellings (galls) that are easy to germinate and that produce many motile spores. The fungus also produces resting spores, and these have not been studied because they are difficult to germinate. This paper describes a way to get resting spores to germinate and what happens after plants are inoculated by them. Because it was possible to study germinating resting spores, it was possible to learn about how the spores germinate and to compare this with germination of the orange galls. It was learned that resting spores germinate differently than galls, so we recommend that classification of the fungus is changed to reflect this. Also, it was learned that resting spores would survive for over 2 years in dried leaf tissue in the laboratory, suggesting that they are key to survival of the fungus between seasons and, possibly, for several years in nature. Finally, it was learned that motile spores produced either from resting spores or the orange galls cause the same disease reaction in plants.

Technical Abstract: Resting spores of Synchytrium solstitiale were studied and protocol for spore germination was developed during evaluation of this fungus for biological control of yellow starthistle (YST). Details of resting spore germination and study of long-term survival of the fungus were documented. Resting spores removed from dried leaves germinated after incubation on water agar for at least 7 d at cool (10 – 15 C) temperatures. Each resting spore produced a single sorus that contained a single sporangium, which upon germination, released zoospores. YST inoculated with germinated resting spores developed symptoms typical for false rust disease. All spore forms of S. solstitiale have now been found to be functional, and the life cycle of S. solstitiale has been completed under controlled laboratory and greenhouse conditions. It is now known that resting spores differ from galls, both morphologically and in content. For this reason, S. solstitiale should be re-classified as diheterogallic, sensu Karling. Resting spores were viable after storage on dried leaves for over 2 y at room temperature, suggesting they have a role in off-season and long-term survival of the fungus.