Agronomy and Disease Management
Sorghum is rated fifth in importance among the world’s cereals, with 61,787 million tonnes being produced in 1988. In Africa, where annual cereal production amounts to some 89 million tonnes, sorghum is second in importance only to maize, and accounts for 14% of the total cereal production, or 15,280 million tonnes per annum (Chantereau and Nicou, 1994).

Although the developing countries of the semi-arid tropics account for 80% of the total world area sown with sorghum (Williams et al., 1980), most of this is grown on a relatively small scale by small-holder farmers. The landraces in each area were developed as a result of a continual process of artificial selection by these farmers, who keep the seed from the sorghum varieties which performed best for planting in the next season. As a result, discrete populations of plants which were ideally suited to their environment and in equilibrium with the local pest and disease populations evolved. Although low yielding compared with the modern hybrids, these local landraces satisfy the balance of requirements of the small-holder farmers, who rate the desire for consistency of harvest and food security as highly as yield. Thus a lower level of production which is consistent from season to season is preferable to a "boom-and-bust" cycle of surplus and crop failures which would place the farmer and his family in a hazardous position. However, the situation today is quite different for most small-holder farmers. Increasing population pressures, with the need for production surpluses to provide a cash income, have necessitated the introduction of higher yielding cultivars. However, attempts to get small holder farmers to adopt the new cultivars has usually met with limited success. This is due to a number of reasons. Firstly, the improved cultivars usually require greater cash inputs to buy fertiliser and seed (which is frequently unavailable), to obtain the higher yields. Secondly, the quality e.g. taste and milling quality of the local landraces is often preferred by local consumers to the improved cultivars, and finally the improved varieties are often more prone to disease and attack by endemic pests. In areas where high yielding cultivars have been successful, this success has usually been short lived, with the rapid build up of diseases, such as anthracnose, as resistance breaks down within a couple of seasons due to evolution of more virulent races.

In many cases the deployment of host plant resistance is the only practical means of controlling sorghum diseases in the low input agricultural systems characteristic of the developing world. It is therefore important to ensure that resistance genes to endemic pathogens are incorporated into improved cultivars before release. For example, in Texas a new high-yielding commercial hybrid introduced in 1965 was abandoned in 1968 because of anthracnose (Reyes et al., 1969). Furthermore, it is important that this resistance is broad based, both in terms of spectrum of diseases and in terms of genetic basis. This is by no means a simple task, as resistance to different pathogens may be determined by different genes, which would all have to be incorporated into the improved cultivars. Polygenic or horizontal forms of resistance are less likely to break down within a couple of growing seasons. An additional factor is the effect of moving germplasm between regions with widely differing climatic conditions, and endemic pathogen races. It is also important to ensure that the resistance is sufficiently stable across environments (Rosenow and Frederiksen, 1982). In summary therefore, sorghum cultivars combining durable resistance to a spectrum of diseases with acceptable yields and other agronomic characteristics would be of immediate benefit to subsistence and small-holder farmers who cannot afford to control disease by chemical means.

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Sorghum Anthracnose
Sorghum anthracnose was first reported in 1902 from Togo, West Africa (Sutton, 1980), and has since been observed in most of the regions of the world where sorghum is grown. However, it is more prevalent in warm, humid areas (Tarr, 1962; Pastor-Corrales and Frederiksen, 1980), where grain yields from susceptible cultivars may be reduced by up to 50% during severe epidemics (Harris and Cunfer, 1976).

Economic Importance
Sorghum anthracnose, is one of the most important sorghum diseases, limiting grain production in most of the areas in which sorghum is grown, i.e., the semi-arid tropics, the south-eastern USA and Latin America (Harris and Sowell, 1970; Bergquist, 1973; Pastor-Corrales and Frederiksen, 1979). It is the most serious disease of grain sorghum in Latin America, and in Brazil, Venezuela and Guatemala, it is one of the main yield-limiting factors (Williams et al., 1980). The common factor among these sorghum growing regions with prevalent anthracnose is frequent rainfall, particularly during the later stages of plant growth (Frederiksen, 1984).

The extent of damage or yield loss due to anthracnose is usually related to: (a) the degree of host susceptibility to anthracnose, (b) the environment, (c) the aggressiveness of the pathogen, and (d) the physiological status of the host. With regard to the environment (i.e. the geographical region where a cultivar was developed), Pastor-Corrales and Frederiksen (1979) reported that many of the sorghum cultivars showing resistance in Georgia (USA) were susceptible to pathogen strains endemic to Brazil when planted there; although cv. Redlan, which is normally susceptible in Georgia, was resistant in Brazil. With regard to the aggressiveness of the pathogen, Cardwell et al., (1989) reported that cultivars TAM 428 and SC414-12E showed resistance to isolates from Texas, but susceptibility to isolates from Georgia and Puerto Rico (Caribbean), when tested under standardised conditions. Finally host tissue maturity or senescence is extremely important. Sorghum senescence is sequential from the base of the plant up (Duncan, 1984). This sequential senescence appears to be a factor in foliar anthracnose. It has been observed that the lower leaves are more subject to colonisation by C. sublineolum (Frederiksen, 1984). Snyder and Nicholson (1990) reported that young sorghum plants of both genetically resistant and susceptible cultivars were resistant to anthracnose, but that the susceptible cultivars became progressively more susceptible as they matured.

Under optimal conditions for pathogen development, large yield losses can occur in susceptible cultivars. Harris et al. (1964), estimated yield losses in susceptible cultivars to exceed more than 50% in a severe anthracnose epidemic in Georgia. More recently Ferreira and Warren (1982) estimated grain losses caused by anthracnose to reach 88.7% in the highly susceptible cultivar IS-4255; 42% in the intermediate RS-671, whereas little yield loss was reported from resistant cultivars such as IS-9189 and IS-9569. These examples highlight the importance of disease resistant germplasm for controlling this pathogen. Chemical control of anthracnose on sorghum is not practical or economic, particularly in areas of the world where sorghum is grown by subsistence and small-holder farmers who cannot afford cash inputs. Unfortunately, effective host resistance breaks down because of the rapid evolution of new races, or pathotypes in pathogen populations. Consequently, long term control of anthracnose depends on the management of host resistance genes, manipulating the host-pathogen environment, and reducing inoculum sources i.e., destruction of debris or susceptible collateral weed hosts such as S. halepense (Frederiksen, 1984; ICRISAT, 1983).

Disease Symptoms
The term anthracnose literally means ‘like coal’ and is used in an aetiological sense for diseases caused by species in the genus Colletotrichum. Anthracnose on sorghum can affect the leaves, stems, peduncles, panicles and the grain, either separately or all together. Often these different aspects of anthracnose appear to be different diseases, and some authors speculate that they are caused by different strains of pathogen. The stem infection is often referred to as red-rot, since it is associated with reddening of the stem tissues. However, red-rots are not solely attributed to anthracnose, since other pathogens i.e., Fusarium moniliforme, Macrophomina phaseolina and some insects can incite systemic red-colour symptoms, which are produced by the host as a reaction to wounding.

A                          B

Classical foliar symptoms of anthracnose (A) are circular-elliptical spots up to 5 mm in diameter formed on the leaves and leaf sheaths. As the spots age, their centres becomes greyish to dark straw-coloured with wide margins that vary in colour from red to tan to blackish purple, depending on the sorghum cultivar. At the same time, numerous acervuli with prominent black to dark brown setae appear. Setae are interspersed among conidiophores in the acervuli, and under humid conditions, creamy to salmon-pink spores masses are produced in profusion (LeBeau et al., 1951; Tarr, 1962; Doggett, 1988). On resistant sorghum cultivars, anthracnose symptoms (B) are characterised by small pigmented (from reddish to tan to blackish purple) flecks, with some coalescence but no lesions or acervuli are produced and further development of the plant is not affected.

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