This review summarizes the key research findings pertaining to sorghum (Sorghum bicolor) and fall armyworm (Spodoptera frugiperda) and identifies some of the research gaps that require immediate to long term interventions. This review focuses on but is not limited to, sorghum the crop, its origin and production, socio-economic importance of sorghum, pests of sorghum, fall armyworm and its life cycle, the economic impact of fall armyworm in sorghum, control and management strategies of fall armyworm, and insect resistance mechanisms in plants.
Sorghum belongs to the grass family, Gramineae. The center of origin and domestication for cultivated sorghum is considered to be the northeastern part of Africa, most likely in the modern Ethiopia and Sudan regions where cultivation started approximately 4000 – 3000 BC (Dillon et al. 2007). Sorghum is adapted to a wide range of environments throughout Africa, spreading from the highlands of Ethiopia to the semi-arid Sahel. The crop then spread to India and China and eventually came into Australia.
Cultivated sorghums of today arose from the wild Sorghum bicolor subsp. arundinaceum (Vara & Staggenborg, 2009).
There are many varieties of sorghum ranging in color from white through red to brown. Traditional varieties are open-pollinated from which rural farmers retain seed for planting in the next season (Food Security Department, 1999). Sorghum consists of 25 recognized species that are classified morphologically into five subgenera: Chaetosorghum, Heterosorghum, Parasorghum, Stiposorghum and Eusorghum (Price, Hodnett, Burson, Dillion, & Rooney, 2005). Cultivated sorghum belongs to the subgenus Eusorghum. The species belonging to the genus sorghum form a remarkably homogeneous group, as far as their chromosomes are concerned (Bhattacharjee, 1957).
The complexity of the genus sorghum is reflected in the chromosome and number of the species belonging to the different subgenera. The lowest haploid chromosome number found in Parasorghum and Stiposorghum is five and most polyploid species are autopolyploids in which chromosome number is built by units of ten (i.e. 2n = 10, 20, 30, 40). Ten is the lowest haploid chromosome number in Eusorghum, the polyploid species are allopolyploids3 and chromosome number is built by units of twenty (i.e 2n = 20, 40). Both Chaetosorghum and Heterosorghum are 2n = 40 allopolyploids (Office of the Gene Technology Regulator, 2017).
Sorghum leaves are typically green, grasslike and flat, and not as broad as maize leaves. Sorghum plants have a leaf area smaller than that of maize. The leaf blade is long, narrow and pointed. The leaf blades of young leaves are upright but the blades tend to bend downwards as leaves mature (Truong, McCormick, Rooney, & Mullet, 2015).
Sorghum (Sorghum bicolor) is ranked 5th in the world in terms of importance among the cereal grains. But in spite of its importance, only marginal progress has been made in developing countries in improving its yields. Because of the low market value of sorghum in these countries, researchers have so far paid little attention to this crop (Orr, Mwema, Gierend, & Nedumaran, 2016). The national average yields range from 0.6 to 1.5 t/ ha for African countries as compared to a mean yield of 4.3 t/ha in the USA (Mburu, et al., 2009).
In Malawi, sorghum is an important staple food in the Shire Valley and a food security crop in other marginal rainfall areas. Smallholder sorghum average yield is about 600 kg/ha. There is scope to increase yields up to 3,000 kg/ha with improved varieties grown and produced under good management. Varieties that are grown in Malawi include Pilira 1, Pilira 2, PN 3, Gwiramtina, kaya, Sinakhomo, Makolokoto, Acc 967. Farmers also grow the tall unimproved sorghum varieties like Thengalamanga, Kasonthe, Masotongo, Dikwa and Kawaladzuwa (Ministry of agriculture, 2012).
Sorghum, apart from being a subsistence crop, is an important commercial and export crop for the United States of America, Australia, and Argentina (Deb, Bantilan, Roy, & Parthasarathy, 2004). In these countries dwarf hybrid varieties are grown and harvested mechanically, predominantly for livestock feed. The trade-in sorghum is small compared with the major grains such as wheat, maize, barley and rice. The main importers of sorghum are Japan, Mexico, and Venezuela. Within most developing countries, the sorghum crop rarely reaches the market. It is grown for home consumption unless there is a bumper crop, or if cash is needed (Food Security Department, 1999).
In many parts of the world, sorghum is utilized in food products such as porridge, unleavened bread, cookies, cakes, couscous, and malted beverages. Traditional food preparation of sorghum is quite varied. Boiled sorghums are one of the most basic uses and small, corneous grains are normally desired for this type of food product. The whole grain may be ground into flour or decorticated before grinding to produce either a fine particle product or flour, which is then used in various traditional foods. The seed is used as food, in brewing beer, sorghum malt and meal (Department of Agriculture, Forestry and Fisheries, 2010).
Sorghum is also an important animal feed, with a nutritional feeding value that is equivalent to that of maize. Sorghum can be processed to further improve its feed value and techniques such as grinding, crushing, steaming, steam flaking, popping and extruding have all been used to enhance the grain for feeding (Deb, Bantilan, Roy, & Parthasarathy, 2004). The products are then fed to beef and dairy cattle, laying hens and poultry and pigs, and are used in pet foods. Sorghum serves as an important summer fodder where temperatures are high and rainfall is insufficient for maize. The most important use is for silage or green siling, or for hay when grown under irrigation in very dry areas (Orr, Mwema, Gierend, & Nedumaran, 2016).
Industrial uses for sorghum include wallboard and biodegradable packaging materials and it can also be processed interchangeably with maize for the production of ethanol (Department of Agriculture, Forestry and Fisheries, 2010).
Sorghum grain has moderately high levels of iron (> 40 ppm) and zinc (> 30 ppm) with considerable variability in landraces (iron > 70 ppm and zinc >50 ppm). Both micronutrients help reduce stunting. The protein and starch in grain sorghum are more slowly digested than other cereals, which is beneficial for diabetics (Department of health, 2017).
Sorghum is attacked by weeds, disease, and insect pests. Weed control during the first six to eight weeks after planting is crucial, as weeds compete vigorously with the crop for nutrients and water during this period (Barber, Scott, & Norsworthy, 2015). The root parasite Striga asiatia (L.) Kuntze or witchweed (rooibos) can damage the crop and mainly occurs under low input farming conditions. Rotation with cotton, groundnut, cowpea and pigeon pea will reduce the incidence of Striga. Hand pulling the plants before flowering also helps reduce Striga populations (Orr, Mwema, Gierend, & Nedumaran, 2016).
Many major diseases cause substantial grain loss in sorghum under different environments and among these are seedling rot diseases, downy mildew, anthracnose, loose and covered kernel smuts, head and long smuts, and other fungal diseases affect grain during its development and contribute to the low yield (Satyagopal, et al., 2014).
Insects attacking sorghum can be grouped into soil pests (wireworms, grubs, and rootworms), foliage feeders, (Greenberg, aphids, bugs, armyworms, grasshoppers, and mites), stem feeders (stem borers, shoot fly), gearhead feeders (midge, head bugs, bollworm, blister beetles, and head caterpillars), and storage pests. Yield losses caused by these pests are often substantial but have seldom been quantified (Sharma, Taneja, Kameswara, & Prasada, 2003). It is reported that the pests, for example, fall armyworm can cause losses of up to 100% in production if not managed correctly (Michelotto et al., 2011).
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