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DISCUSSION
Ergot is essentially a replacement disease (Tulasne 1853; Luttrell 1980), i.e. pollen of the host is replaced by spores of the pathogen, pollination by penetration, ferti lisation by infection, and seed development by formation of sclerotia. Fertilisation usually renders flowers resistant to ergot infection (Burton & Levebvre 1948; Fuentes et al. 1964; Futrell & Webster 1965; Willingale et al. 1986), implying that viable inoculum of the pathogen has to be present immediately prior to, or at the time of, pollination. It is also obvious that a host in which the glumes do not open at all, or ones in which they are open for a short period of time, would provide only slight opportunity for infection (Campbell 1957).
Although c. cyper; propagates mainly vegetatively (Tumbleson & Kommedahl 1961; Garg et al. 1967; Jansen 1971 ; Stoller et al. 1972; Mulligan & Junkins 1976; Stoller & Sweet 1987), it is a prolific self-pollinating flower producer (Brady 1962; Thullen & Keeley 1979). However, less than 5 % (0-19 %) ofthe flowers that are formed normally produce seed (Thullen & Keeley 1979). This semi-celibate nature of the weed was also evident from the relative paucity of C. cyperi sclerotia in inflorescences of C. escu/entus in the field. Each inflorescence, comprising up to 2500 florets (Thullen & Keeley 1979), on average contained only 5-15 (maximum 35) ergot sclerotia. The dearth of susceptible target organs in florets of C. esculentus, together with the tendency towards delayed anthesis inherent to the species, probably was one of the reasons why artificial infection with C. cyperi could not be attained in the greenhouse. Direct inoculation into the florets (Thakur et al. 1983) or clipping away the tips of the glumes before spraying (Campbell 1957) could have been attempted but were deemed unpractical due to the small size of the florets.
As indicated above, ergot infection tends to mimic the pollination process. With species such as C/aviceps fusiformis Loveless, Claviceps paspa/i F. Stevens & J.G. Hall and Sphacelia sorghi McRae, this pseudo-pollination process is the primary means of infection and direct penetration of ovaries is rare (Willingale et al. 1986; Willingale & Mantle 1987; Frederickson & Mantle 1988}. Direct infection of ovaries is more common in Claviceps purpurea (Fr.:Fr.) Tul. (Frederickson & Mantle 1988), but spore germination, penetration and hyphal growth down the stigma by this ergot species also closely follows the path taken by pollen. After germination and penetration hyphae grow between the cortical cells of the stylodial axis, and within the electron-dense matrix of the conspicuous pollen transmitting tract until they reach and enter the ovary (Willingale et al. 1986; Willingale &Mantle 1987). With C. cyperi, however, penetration of the stigmata and stylodia was not observed although extensive superficial colonisation of these organs occurred. It therefore seems if direct penetration through the ovary wall is the primary path of infection in nut sedge ergot, which is in accordance with the apparent illadaptedness of the host to sexual reproduction. Concerning the greater similarity in the mode of infection of C. cyperi to C. purpurea than to C. Fusiformis or C. paspali, it is interesting to note that the sclerotia produced by C. cyperi, like those of C. purpurea (Luttrell 1977), are more complex than those of C. fusiformis.
Colonisation of ergot honeydew and sclerotia by saprophytic fungi is a common phenomenon. However, considering the abundance of nutrients in these substrates, it is surprising that the taxa reported from them are limited to species in the genera Cerebella, Cladosporium, Epicoccum, Fusarium, as well as some unidentified yeasts (Ajrekar 1926; Rhind 1928; Gon~lves 1937; Chalaud 1940; Langdon 1942; Simpson & West 1952; Theis 1952; Schol-Schwarz 1959; Loveless 1964; Mantle 1965; Futrell & Webster 1966; Cunfer 1975; Mower et al. 1975; Cole et al. 1981; Frederickson & Mantle 1988; 8andyopadhyay et al. 1990, 1998; Ali et al. 1996; Pazoutova & Kolinska 1999; Blaney at a/. 2000). It is therefore likely that Claviceps species have evolved certain
defence mechanisms which protect them from other fungi, as previously proposed by Mower et al. (1975) and evident from the in vitro inhibitory effect of C. cyperi on C. cladosporioides and F. heterosporum in the present study. Antimycotic activity has not been reported for other Claviceps species, but is probably L;UII II 11011 lo the genu5. However, despite the presence of such or alternative defence mechanisms inherent to Claviceps, most reports indicate that honeydew-colonising fungi suppress the formation of ergot sclerotia, as was also observed with C. cladosporioides in the present study. Although Cladosporium species are frequently associated with ergot (Futrell & Webster 1966; Frederickson & Mantle 1988; Bandyopadhyay et al. 1990), this is the first record of C. cladosporioides specifically from honeydew. F. heterosporum, on the other hand, is probably the most commonly reported Fusarium species from ergot (Futrell & Webster 1966; Cunfer 1975; Mower et al. 1975; Cole et al. 1981; Ali et al. 1996; Raybould et al. 1998). Indeed, in South Africa it has been recorded exclusively on ovaries of no less than 17 different poaceous hosts infected with either ergot or smut fungi (Doidge 1950). Both C. cladosporioides and F. heterosporum are mycotoxigenic species. The most important toxin produced by C. cladosporioides is emodin (Daunter & Greenshields 1973; Jacyno et al. 1993), whereas F. heterosporum has been reported to produce fusaric acid (Bacon et al. 1996) and various trichotecenes (Cole et al. 1981). The primary symptom associated with trichothecene toxicosis (Marasas et al. 1984) is feed refusal, while emodin has a diarrheagenic effect (Wells et al. 1975). Considering the prolific growth of C. c1adosporioides and F. heterosporum on and in ergotised nut sedge honeydew, it is likely that the above mycotoxins were produced in significant quantities in the honeydew, and probably induced toxic effects additive to those of the ergot alkaloids, particularly reduced food intake and loss of body mass. Emodin furthermore has mutagenic activity (Brown & Brown 1976; Wehner et al. 1979), implying that prolonged intake of the compound could induce cancer. Although it has not been confirmed, there is also a possibility that emodin, like aflatoxin for instance (Raisbeck et al. 1991). can be excreted in a still toxic form in milk destined for human consumption.
MATERIALS AND METHODS
Sclerotia of C. cyper; collected between 1997 and 2000 from ergotised nut sedge at localities where bovine ergotism occurred (Table 5. 1) were analysed for ergot alkaloids by Meadows Cape in Paarl, Western Cape Province, according to the HPLC method of Rottinghaus et al. (1 993), following extraction of the sclerotia as described by Scott et al. (1992). Sclerotia collected in 1997, 1999, 2002 and 2003 were tested in 2003 for ergopeptines as described in 5.2.3 and 5.2.4. A preliminary experiment was also done in which C. cyper; PREM 56618 was grown for 8 weeks at 22 °C in the dark on Mantle’s alkaloid medium (Mantle 1973) solidified with 1.5 % agar, and the cultures analysed by HPLC for a-ergocryptine at the Council for Scientific and Industrial Research in Pretoria.
Following the above, in vitro production of alkaloids by C. cyper; in liquid medium was attempted according to the procedures described below.
CHAPTER 1 GENERAL INTRODUCTION
CHAPTER 2 SYMPTOMATOLOGY AND MORPHOLOGY OF CLAVICEPS CYPERI ON YELLOW NUT SEDGE IN SOUTH AFRICA
Abstract
2.1 INTRODUCTION
2.2 MATERIALS AND METHODS
2.3 RESULTS
2.4 DISCUSSION
2.5 REFERENCES
CHAPTER 3 THE SPHACELIA STATE OF CLAVICEPS CYPER/IN CULTURE
Abstract
3.1 INTRODUCTION
3.2 MATERIALS AND METHODS
3.3 RESULTS
3.4 DISCUSSION
3.5 REFERENCES
CHAPTER 4 MODE OF INFECTION OF CYPERUS ESCULENTUS BY CLA VICEPS CYPERI
Abstract
4.1 INTRODUCTION
4.2 MATERIALS AND METHODS
4.3 RESULTS
4.4 DISCUSSION
4.5 REFERENCES
CHAPTER 5 ERGOT ALKALOIDS PRODUCED BY CLAVICEPS CYPERI
Abstract
5.1 INTRODUCTION
5.2 MATERIALS AND METHODS
5.3 RESULTS
5.4 DISCUSSION
5.5 REFERENCES
CHAPTER 6 MOLECULAR SYSTEMATICS OF CLAVICEPS CYPERI AND OTHER SOUTH AFRICAN CLAVICEPS SPECIES
Abstract
6.1 INTRODUCTION
6.2 MATERIAL AND METHODS
3 region
6.3 RESULTS
6.4 DISCUSSION
6.5 REFERENCES
CHAPTER 7 GENERAL DISCUSSION
