Equally important to development of yield and quality potential of energy sorghum is the inherent protection of that yield potential. Thus, breeding for tolerance to both abiotic and biotic stress is critical for adaptation and productivity.

Abiotic stress is defined as a yield limiting factor caused by a non-biological source. Examples of abiotic stress include but are not limited to temperature, moisture and soil fertility. Of these factors, Boyer [92] estimated that drought was the largest single factor in reducing grain sorghum yield. The same situation will be true in bioenergy sorghum cultivars, especially since production will be reliant on rainfall and not irrigation. Drought stress is a complicated condition that initiates the expression numerous genes in a myriad of signal transduction pathways [93]. Drought tolerance has been studied for many years and several different mechanisms of tolerance have been described. Stay-green is a post­flowering drought tolerance trait that can be described as delayed senescence. Therefore, it can mitigate effects of terminal drought stress on the final yield processes including grain fill and stalk development. This trait is of special interest to breeders of dual purpose sorghum varieties [94]. The stay-green trait includes modifications in stalk properties that affect biotic stresses such as charcoal stalk rot [56], and lodging [62]. Stay-green plants tend to be superior in digestibility by ruminant animals due to higher basal stem sugars [95]. Stay- green may be of particular importance in sweet sorghum as sugar accumulation is optimized during grain fill. A second and less well understood drought tolerance mechanism is pre­flowering drought tolerance. This is tolerance prior to flowering and is physiologically different than stay-green. In fact, only a few sorghum genotypes possess both types of tolerance. Additional studies are necessary, but this tolerance may be of particular value in biomass sorghum hybrids since they do not enter the reproductive growth phase.

Aluminum toxicity can limit sorghum production in acidic soils, which are common in the humid tropics [96]. Genetic variability for aluminum tolerance has been observed [96,97] and is commonly used in sorghum breeding programs in Brazil. In fact, Magalhaes et al. [98] identified and cloned the aluminum tolerance gene AltSB5. Nitrogen use effi­ciency (NUE) in cereal crop production has been estimated to be at about 33% worldwide [99]. However, for lignocellulosic bioenergy production new experimental directions that emphasize improving biomass yield rather than grain production will need to be taken. Currently, experiments are being conducted using sweet sorghum to indentify genotypes and QTL that can improve NUE.

Biotic stresses are defined as yield limiting factors of a biological source, typically either an insect pest or a disease pathogen. There are numerous pathogens and pests that can reduce yield and quality of all types of sorghum. However, the relative importance of each biotic stress differs based on the type of sorghum and location of production. For example, biomass sorghum will generally be managed to eliminate or minimize reproductive growth. Consequently, pests and pathogens of reproductive growth are much less important. If grain is important, then breeding for resistance to midge and shoot fly are important because these pests reduce grain yield [100-103].

For energy sorghum, insects of greatest interest to breeders are those that affect harvested plant organs. For biomass sorghum, two of the most important insect pests include green — bugs, which stunt growth in young plants, and the borers because they affect stalk integrity. Tunneling in the stalk can disrupt vascular tissues causing nutrient or water deficiencies in addition to general weakening of stalks and subsequent lodging [64]. Additionally, stalk borer damage causes wounds that provide entry points for stalk rot pathogens [104]. Grain sorghum crops in the United States are not generally seriously infested with stalk boring insects, but in energy cultivars these may become very serious crop pests. Genetic variation for resistance to selected stem borer has been reported but it is not complete and breeding may be difficult [101, 105, 106]. Effective insecticide and management practices, as well as transgenic sources of resistance, will likely be critical for controlling these pests.

Among the most important sorghum diseases are anthracnose Colletotrichum subline — olum, and downy mildew Peronosclerospora sorghi (Weston and Uppal) C. G. Shaw. Con­siderable effort has been put into developing cultivars that are resistant to these pathogens. Anthracnose is one of the most economically important sorghum diseases [107,108] and it is likely that this disease will become more important as production of energy sorghum increases. Anthracnose can affect all above ground plant organs and is among the most seri­ous pathogens of sorghum [109]. At any one time, pathogen populations can be made up of many pathotypes making breeding for resistance particularly challenging [63]. Estimated yield losses due to anthracnose alone have been reported in excess of 50% [110]. Gene-for — gene relationships have not been conclusively demonstrated and, therefore, assignment of races or pathotypes has been difficult [108]. However, genetic variation for resistance has been reported [109, 111].

Another pathogen that affects sorghum production worldwide is downy mildew. Genetic variation for resistance to this pathogen has also been reported [112-114]. Multiple races of downy mildew that infect sorghum have been identified and, consequently, resistance genes have come from multiple sources [113]. Resistance to single races of this pathogen is reported as simply inherited and, therefore, resistance to the pathogen in general is described as oligogenic in most cases [115]. For example, resistance to the ICRISAT Centre race has been described as fitting the expected ratio of a two locus model with complementary and inhibitory interactions [114]. Thus, it is feasible to pyramid resistance genes for multiple races as new ones are identified. Oh etal. [116], linked RFLP (restriction fragment length polymorphism) markers to resistance for pathotypes 1 and 3. However, the fragment patterns could not be reconciled with original mapping cross making it impossible to locate the QTL. Therefore, more research is required to develop useful marker assisted selection schemes for downy mildew.

7.2 Summary and Conclusions

Sorghum is among the most versatile of crop species with wide environmental adaptation and a diversity of end uses. The successful breeding history of grain and forage sorghum demonstrates that biomass and sweet sorghum hybrids can be developed using the same approaches. The genetic resources available to sorghum breeders, contained in both the cur­rently utilized breeding germplasm, as well as the extensive public germplasm collections, represent a rich repository of genetic variation, which is necessary for breeding progress. There remain significant needs and opportunities to develop the production logistics and management schemes to fully integrate sorghum and other grass genera into biomass pro­duction schemes that fit the needs of processing facilities.