Synchytrium is a large genus of plant pathogens within the phylum Chytridiomycota. Species are commonly known as false rust or wart disease. Approximately 200 species are described, and all are obligate parasites of angiosperms, ferns, or mosses. Early species were mistakenly classified among the higher fungi (Ascomycota or Basidiomycota) because of their superficial similarity to the rust fungi. Anton de Bary and Mikhail S. Woronin recognized the true nature of these fungi and established the genus to accommodate Synchytrium taraxaci, which grows on dandelions, and S. succisae, which grows on Succisa pratensis. Synchytrium taraxaci is the type of the genus. The genus has been divided into 6 subgenera based on differences in life cycles.
Morphology Members of Synchytrium are endobiotic, holocarpic, and inoperculate. This means Synchytrium species grow inside of the host cell (endobiotic), produce structures other than a zoosporangium (holocarpic), and do not release zoospores through a lid-like structure (inoperculate). Zoospores of other members of Chytridiomycota typically give rise to one zoosorangium or a polycentric thallus capable of producing many zoosporangia. In Synchytrium, the zoospore nucleus divides many times with each daughter nucleus giving rise to a zoosporangium. This produces a cluster of clonal zoosporangia, often enveloped with a membrane. This cluster is called a sorus. The zoospore can give rise to the sorus directly or it can act as a prosorus. The difference is demonstrated in the life cycle, which is discussed below.
Life cycle Most species share the same initial developmental stages. The released zoospores swim until they find a suitable host and will occasionally use amoeboid movement to better orient themselves to a host plant cell. After the zoospore attaches to a host cell, a narrow germ tube forms and penetrates the host cell, which is usually an epidermal cell. An exception to this is S. minutum; it uses the stomata to enter the host plant and penetrate a sub-epidermal cell. After penetration, the zoospore cytoplasm flows into the host cell. The Synchytrium nucleus travels toward the host cell nucleus and becomes enveloped in host cytoplasm. After this point, differences arise among Synchytrium species. Species fall into one of two broad categories: short cycled and long cycled. Short cycled species follow one of two lines of development: sori, sporangia, zoospore or resting spore, sori, zoospore. Long cycled species follow a general pathway of prosori/sori, sporangia, zoospore, resting spore, prosori/sori, sporangia, zoospore. The nuances in life cycles are used to delineate the subgenera.
Microsynchytrium Species in this subgenus are long cycled and begin as a uninucleate thallus that functions as a prosorus. Basically, the primary nucleus of the parasite grows within the host cytoplasm. At a point, it will produce a new germ tube and exits out of the envelope. It then divides numerous times with each daughter nucleus partitioned into a developing sporangium. An envelope forms around the cluster of sporangia and the cluster becomes a sorus. The sporangia release zoospores that infect other cells. These develop into resting spores that will overwinter. Upon germination, the resting spores function as prosori. Karling included the genus Micromyces within this subgenus while other authors do not.
Mesochytrium Species in this subgenus develop in a similar fashion as those in subgenus Microsynchytrium, except that the resting spore functions as a sporangium during germination. In these species, the zoospores can develop into either a prosori, as in Microsynchytrium, or they can fuse to form a flagellated zygote. The zygote infects a host cell and becomes a resting spore. Synchytrium endobioticum is included in this subgenus.
(Eu)Synchytrium This subgenus is referred to as Synchytrium or Eusynchytrium. Species in this group do not form prosori. The sorus forms directly from the zoospore nucleus. Several generations can be produced during the spring and summer. Resting spores are developed in the fall and winter. Upon germination, the resting spore acts as a sporangium. The type, Synchytrium taraxaci, is placed in this subgenus.
Exosynchytrium These species develop in a similar fashion as Eusynchytrium except that the resting spore acts as a prosorus upon germination.
Pycnosynchytrium This subgenus is a "dumping ground" for species with incompletely known life cycles. It would seem that the primary nucleus forms a resting spore that acts as a prosorus upon germination. However, these species will need to be more closely examined for proper placement.
Woroniella Species in this group are short cycled. The zoospore nucleus forms a sorus. Resting spores are either unknown or truly absent. To date, sexual reproduction is only described in four species: Synchytrium endobioticum, S. fulgens, S. macroporosum, and S. psophocarpi.
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