THE LONG-TERM EPIDEMIC OF CLAVICEPS-PURPUREA ON SPARTINA-ANGLICA IN POOLE-HARBOR - PATTERN OF INFECTION, EFFECTS ON SEED PRODUCTION AND THEROLE OF FUSARIUM-HETEROSPORUM

An epidemic of the ergot fungus (Claviceps purpurea (Fr.) Tul), a non- systemic floral pathogen, appeared in populations of Spartina anglica C. E. Hubb in Poole Harbour in the 1980s. Over 70% of inflorescences w ere infected between 1985 and 1995. Between 1983 and 1995, there was n o consistent pattern in the rank order of sites with respect to the pr oportion of inflorescences infected, and variation in the amount of in fection was not related to inflorescence density. Among years, there w as significant variation in the total biomass of ergot per inflorescen ce, but no significant difference in the number of ergots per inflores cence. Log-log regressions of total weight of ergot per inflorescence against the number of ergots per inflorescence showed that in all year s resources for each additional ergot were limited. There was no stati stically significant difference between the number of seed set on infe cted and uninfected inflorescences in 1985 or 1995. Further analysis s howed that, compared with uninfected inflorescences, there was higher seed output from inflorescences with fewer than 10% of spikelets infec ted, which balanced reduced seed output from heavily infected inflores cences. At high levels of ergot infection, a lower proportion of uninf ected spikelets set seed, compared with spikelets on uninfected inflor escences. This suggests that conditions which favour ergot growth are detrimental to seed production. Compared with uninfected inflorescence s, mean and total seed weights were significantly lower in inflorescen ces with > 10% of spikelets infected. The fungus Fusarium heterosporum Nees ex Fr. was found in association with the ergots on Spartina. The re was a significant positive correlation between the average severity of Fusarium infection and the number of ergots on an inflorescence. T here was a non-significant negative correlation between severity of Fu sarium and mean ergot weight, when ergot number per inflorescence was held constant. These data suggest that any negative effect of Fusarium on Claviceps is small. The high, and temporally and spatially uniform levels of ergot infection probably result from genetic uniformity of Spartina and the lack of zonation in the salt marshes of Poole Harbour . This situation is likely to persist for the foreseeable future.