Identity of The Sorghum Anthracnose Pathogen
Colletotrichum graminicola (Ces.) Wilson (Mordue, 1967) has previously been regarded as the main species of Colletotrichum causing anthracnose on cereals, most notably maize, sorghum and cultivated and wild grasses (Holliday, 1980). Pathogens affecting these different plant species were originally considered as different forms of the same fungal species (von Arx, 1957; Holliday, 1980). However, pathogenicity studies with isolates from maize and sorghum indicated strict host specificity (LeBeau, 1950; Dale, 1963; Jamil and Nicholson, 1987), implying that the forms of C. graminicola that attack maize and those that attack sorghum are distinct, being either formae speciales or even different species. This view is supported by studies on their morphology and genetic composition. Sutton (1968) has also reported that the structure of appressoria produced in vitro by isolates from sorghum and maize were different. Isolates from maize produced appressoria that were more irregular in outline than those produced by isolates from sorghum. On this basis, Sutton regarded them as two species; isolates from maize being designated C. graminicola, whilst those from sorghum were designated C. sublineolum (Sutton, 1980). More recently, support for the distinction of these two species has come from biochemical and molecular studies of isozyme variation (Huguenin et al., 1982; Ali et al., 1989), rDNA sequence analysis (Sherriff et al., 1995), mating tests, analysis of RFLPs of mitochondrial DNA and random polymorphic DNA (Vaillancourt & Hanau, 1992). It is important to recognise that there are two distinct species of Colletotrichum attacking maize and sorghum because, even though these two species share similar morphological characteristics, other parameters such as their mode of infection and sporulation, host specificity, genetic compatibility, epidemiology and sensitivity to fungicides, may vary. Unfortunately, to date C. sublineolum has not been widely accepted as a distinct species and is still often referred to in the recent literature as C. graminicola by many authorities.

The Genus Colletotrichum
The genus Colletotrichum is a member of the order Melanconiales of the class Coelomycetes (Hawksworth, 1983). It has a wide host range and a worldwide distribution, but is most important in the tropics. Diseases caused by the genus Colletotrichum have had a substantial impact on world agricultural production through their capacity to cause economic losses on a number of important cereal, legume, fruit and cash crops (Waller, 1992). The ability of many Colletotrichum species to cause latent or quiescent infections places them among the most important of post-harvest pathogens.

Taxonomy of the genus Colletotrichum
The genus Colletotrichum was first described by Tode in 1790 under the name Vermicularia, but was later established as Colletotrichum by Corda in 1837. Despite the original clear description and excellent illustrations provided by Corda, the genus has been re-described under several different names, the two most common of which are Vermicularia and Gloeosporium. The names Colletotrichum and Vermicularia were used interchangeably during the 19th and early 20th centuries for a range of species now accepted in the genus Colletotrichum (Sutton, 1992). Clements and Shear (1931) distinguished Colletotrichum from Vermicularia by the presence of marginal setae in Colletotrichum, compared with setae dispersed throughout the conidiomata in Vermicularia. However, Duke (1928) demonstrated that conidiomatal structure and form, the presence/absence of setae and their arrangement within the acervulus are extremely variable and of no significance at the generic level. This resulted in a large number of species being transferred from Vermicularia to Colletotrichum. There were also problems in distinguishing Colletotrichum from the morphologically similar Gloeosporiums, as although Gloeosporium species were not thought to produce setae, some could produce setae on certain substrates (Baker et al., 1940). The names of some the Gloeosporium species have now been transferred to Colletotrichum as distinct acceptable species but the majority have been reduced to synonoymy with the already well-known species.

Morphological characters such as shape and size of conidia, setae and appressoria have been used, together with host origin, to define Colletotrichum species, although excessive reliance on the latter has led to a proliferation of unnecessary names. Using this system around 900 ‘species’ have been assigned to Colletotrichum to date (Sutton, 1992). However, more recently, host origin has been deemed less important and the number of species was reduced to 11 by von Arx (1957), and to 25 by Sutton (1980), who later increased this number to 37 on the basis of host specificity (Sutton 1992). The difficulties in species discrimination have resulted in some confusion in taxonomic grouping, for example von Arx (1970) recognised eight host-specific species or ‘types’ in C. gloeosporioides, whilst Sutton (1980) recognised species such as C. gloeosporioides, C. dematium, and C. capsici as ‘group species’. Colletotrichum species can be conveniently divided into two classes, one with straight conidia, and the other, with falcate conidia (Table 1.1). Of the few species of Colletotrichum which undergo a sexual mode of reproduction (designated in the teleomorphic genus Glomerella), ascal states have frequently been associated with species producing straight conidia (Table 1).

Many problems remain in providing a workable taxonomy for this group. However, molecular biology is providing new insights into systematics, particularly in the delimitation of species and in defining inter- and intraspecific relationships (Bruns et al., 1991). The most significant advances in taxonomy have been obtained from approaches based on the analysis of nucleic acids. These include comparisons of DNA sequences, analysis of restriction fragment-length polymorphisms (RFLPs), and the use of polymerase chain reaction (PCR)-based techniques to assess random amplified polymorphic DNAs (RAPDs) (Welsh and McClelland, 1990; Williams et al., 1990).

Table. 1. A representative sample of species in the genus Colletotrichum, their associated Glomerella teleomorphs, and an indication of host range (adapted from Skipp et al., 1995).

RFLP analysis has recently been used to determine the genetic relationship within C. gloeosporioides isolates which cause anthracnose on Stylosanthes species (Braithwaite et al., 1990). Mills et al. (1992), used RAPD analysis to assess the molecular variation of 39 C. gloeosporioides isolates from different hosts, dividing this group into several sub-groups. Sherriff et al. (1994) used DNA sequence analysis to distinguish one group, consisting of C. lindemuthianum, C. malvarum, C. obiculare, and C. trifolii, from all other Colletotrichum species. They also verified the distinction between species with falcate conidia (Table 1).

Genomic markers are proving to be of practical value for identification and diagnosis, providing a quick method for characterising variations and identifying genotypes within pathogen populations. By regularly sampling field material, researchers can follow the shifts in the genetic makeup of pathogen populations, which can then provide a dynamic picture of the interactions between host and pathogen types (Guthrie et al., 1992).

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Colletotrichum Diseases

1. Economic Importance on Agricultural Crops
   Colletotrichum diseases have been reported on a wide range of hosts. However, the most economically important of these are species attacking cash crops such as cotton, coffee, beans and tropical fruit, such as bananas, mangoes and papayas (Waller, 1992). Losses caused by Colletotrichum species mostly occur as a result of the direct reduction of quality and or quantity of the harvested yield. This is most noticeable and severe when Colletotrichum directly attacks the harvested portion of the crop, often the fruit. Wider economic consequences are also linked with other factors including the poor return on investment, and the cost of chemical control. Many export crops play a crucial role in the economy of small, developing countries, and disease losses can have a significant impact at the macro-economic level (Waller, 1988). At the micro-economic level, reductions in yield losses have a significant effect on the lives of people, many of whom are subsistence or small-holder farmers, or depend on local markets for food.

2. Epidemiology of Diseases Caused by Colletotrichum Species
   Environmental factors play a major role in the development of Colletotrichum diseases. For example, Lenné (1992) reported that the optimum conditions for the development of the most important species of Colletotrichum affecting legumes were temperatures of 18-28 °C with relative humidities of greater than 90%. In general, the primary factor influencing the development of disease is the availability of water, as high humidity is essential for the sporulation of most Colletotrichum species. Other key factors such as temperature, often interact with factors such as leaf surface wetness, humidity, light or competitive microflora (Rotem et al., 1978: Royle and Butler, 1986). However, in tropical species such as C. gloeosporioides, the duration of surface wetness appears to have the most direct influence on germination, infection and growth. Mist or dew can also play an important role in extending the period of wetness following rainfall, and may be important in increasing the rate of lesion spread (Dodd et al., 1992).

    

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