FAW BIOLOGY--FROM MY THESIS EXCERPTS

 KEY INSECT PESTS AND DISEASES OF SORGHUM AND THEIR MANAGEMENT OPTIONS

 

Summary

Sorghum S. bicolor is an important dryland crop, used for food, feed and biofuel in South Sudan. It accounts for 76% of total cereal production in the country (FAOSAT, 2017). Despite being tolerant to pests and diseases, many invasive lepidopteran insect pests and diseases have emerged in the past and will continue to emerge to constrain sorghum production, hence, threatening food security.

 

 Introduction

 

Sorghum (Sorghum bicolor (L) Moench) is one of the most important cereal crops in semi-arid tropics (SAT). It is an important diet for more than 500 million people in over 30 countries, especially in arid and semi-arid (ASA) regions of Africa (ICRISAT, 2015), Asia and Central America. Its production is, however, constrained by biotic and abiotic problems. It is infested by about 150 insect pests, which results in an annual loss of over $1billion in semi-arid regions of the world to ICRISAT (1992). The key insect pests of economic importance of sorghum include; fall army worm (Spodoptera frugiperda), species of stalk borers, sorghum midge (Stenodiplosis sorghicola), shoot fly (Antherigona soccata) and parasitic weed (Striga). While the major diseases include; anthracnose (Colletotrichum graminicola) and leaf blight (Exserohilum turcicum) (Ngugi et al., 2002).

Insect pests and diseases continue to compete with humans for sorghum crop, and the knowledge of both old and new pests is important as the crop has received increasing attention. Therefore, it is the purpose of this paper to review the current situation of insect pests and diseases of sorghum and their management options.

 

Key insect pests of sorghum

 

Fall armyworm S. frugiperda (Smith):

FAW is a quarantine pest which is indigenous to tropical and subtropical Americas (CABI, 2017). It is reported to infest 350 plant species, belonging to 27 families in the Americas, preferring wild and cultivated grasses (Goergen, 2017). It was reported in 2017 in South Sudan at Magwi County in Eastern Equatoria State (FAO, 2017).

Damage symptoms

The first signs of FAW infestation are the small scratches made by first instars. They feed superficially on one side of the leaf, whorl and panicle, ragging the leaves. Mature larvae break the stem at seedling stage of growth and development, leading to reduced plant stands (FAO, 2017).

 

Economic impact

According to Andrew (1988), the leaf and panicle feeding damages in sorghum cause yield losses that range from 55 to 80% in Brazil. FAO (2017) reported that US $600m is spent yearly on the management of FAW in Sorghum in Brazil. In Eastern Africa, much attention is being paid to the loss it causes to maize but not much is known about the pest impact on sorghum. However, the estimates by CABI (2017) indicate that, if there are no proper control measures put in place, the potential impact of FAW in maize production on the continent could lead to yield losses between 2.5 to USA $ 6.2 billion per year of the total expected value of USA $ 11.6 billion per year. The yield loss due to FAW infestation in sorghum in Africa is not yet clear, however, the Intergovernmental Panel on Climate Change puts the yield losses roughly above 31% per year (Goergen, 2017).

Spotted stem borer (Chilo partillus (Swinhoe):

Spotted stem borer is one of the important pests of cultivated cereals like maize Zea mays, sorghum Sorghum bicolor and pearl millet Pennisetum glaucum (Reddy et al., 1988).

Damage symptoms

The first symptom of attack is the irregular shaped pinholes, caused by the early instar larvae feeding in the whorl, in turn cause elongated lesions on the leaves (CABI 2007). In young plants the shoot can be killed, causing a “dead heart”. In older plants, where internode elongation has started and the growing point has moved upwards, the larva feeds inside the stem, causing extensive tunnelling. Damage to inflorescences may interfere with grain formation, causing chaffy heads in sorghum.

 

Economic impact

The grain yield loss by C. partillus depends on the pest population density at the time of attack, crop age, and variety (Karaya et al., 2009). It causes the economic losses of up to 85%, estimated at US$334 million annually in semi-arid and tropical regions (SAT) (ICRSAT 1992).

Maize stalk borer, Busseola fusca (Fuller): 

Maize stalk borer is one of the most destructive insect pests that attacks mostly cereals, including sorghum.

Damage symptoms:

Leaf feeding reduces translocation, ear damage, lodging, initial leaf senescence and in severe cases a complete crop failure (Naz et al. 2003, Gupta et al. 2010). Larval damage causes formation of dead hearts (death of central growing tip), exit holes and tunnel in main stem that causes stunted plant growth and favours secondary infection of fungus and bacteria (Songa et al. 2001).

 

Economic impact

Stem borer damage causes grain yield losses ranging from 10-80% (Kfir et al., 2002).

 

Sorghum midge (Stenodiplosis sorghicola):

Stenodiplosis sorghicola occurs in almost all regions of the world where sorghum is grown (Boyd and Bailey, 2000).

Damage symptoms

The feeding damage by sorghum midge larvae prevents the normal grain development with total destruction of the grain.

Economic impact

The damage caused by sorghum midge could be as high as 30% loss, estimated at US $28 million (Americo, 2005).

Shoot fly (Antherigona soccata)

Sorghum shoot fly is one of the most destructive insect pests of sorghum which causes significant losses at the seedling stage.

Damage symptoms

Shoot fly feeds on the plants at the seedling stage at the central growing points, causing dead heart symptoms.

Economic impact

The feeding damage due to shoot fly causes 12% loss in sorghum production (Ali, 2017).

Parasitic weed (Striga hermonthica):

Striga hermonthica, also known as Witch-weed is characterized by bright-green stems and leaves and small, brightly colored and attractive flowers. It is an obligate hemi-parasite of roots and require a living host for germination and initial development, though it can still survive on their own.

Damage symptoms

Host plant symptoms include stunting, wilting, and chlorosis, similar to those seen from severe drought damage, nutrient deficiency, and vascular disease (Agrios, 2005). 

Economic impact

Witch-weed is one of the most destructive pests of sorghum in the whole world. In the United States alone, grain yield losses due to Striga are estimated at the value well over $20 billion annually. Witch-weed affects 40% of Africa's arable savanna region, resulting in up to $13 billion lost every year (Samarrai, 2010).

 

Key diseases of sorghum

 

Anthracnose (Colletotrichum graminicola):

Anthracnose is one of the most common foliar diseases of sorghum caused by a fungal disease, Colletotrichum graminicola.

Damage symptoms

It infects the aerial tissues of plants. Infection is primarily seed-borne, but infected plants rapidly produce secondary inoculum, which can be spread to crops by wind and rain splash. Symptoms include small, yellowish watery spots that become brownish, leading to plant death.

Economic impact

The damage due to anthracnose can cause seed yield losses of up to 50% (Thakur and Mathur, 2000).

Leaf blight (Exserohilum turcicum):

 

Leaf blight of sorghum is a fungal disease caused by Exserohilum turcicum.

Damage symptoms

The disease causes large cigar-shaped lesions on the leaf with reddish or purple margins. Within the lesions, there often black conidia formed by sporulation of the fungus, giving an ashy gray to dark olive appearance. This reduces the photosynthetic activities of the plant, leading to yield loss.

Economic impact

Grain yield loss due to leaf blight of sorghum is up 50% or more under severe epidemics (Goergen, 2017).

 

Management options

Monitoring

Monitoring is intended to actively track the presence, population, and movement of FAW within a specified geography. In both cases, monitoring typically relies on scouting and pheromone traps. The recommended pheromone traps include; Universal Bucket Trap for smallholder farmers and Heliothis-style Pheromone Trap for regional monitoring.

 

Mechanical control

Destruction of alternative host plants and deep ploughing of crop residues can assist with control as adults experience difficulty in emerging from puparia buried deeply in the soil. In South Africa, slashing sorghum stubble was found to result in the destruction of 70% of the C. partellus population. Subsequent ploughing and disking was found to destroy a further 24% of the population (Kfir et al., 2002).

 

Cultural control

Field sanitation

Use of tolerant varieties

Plant early maturing varieties.

Rotate sorghum with a non-host, e.g. beans.

Push-Pull approach reduces FAW infestation and crop damage by up to 86%, with a 2.7-fold increase in yield (Goergen, 2017).

Adjustment of the planting date

For the case of Striga, use uncontaminated seeds, resistant varieties (N-13, Framida and Serena), hand weeding and quarantine.

Biological control

Parasitic wasps, natural predators, and pathogens help to control the population of fall armyworms.The egg parasitoid Telenomus remus is frequently introduced to effectively control fall armyworm and other Spodoptera species. Also the use of entomopathogens, like viruses, bacteria, fungi have been reported to be effective against lepidopteran pests (Goergen, 2017).

Chemical control:

Chemical control can be difficult to achieve because larvae are exposed for very short periods of time and they cannot be killed after tunneling into the plant. In order to be effective, insecticide application should commence before larvae burrow into the whorls or ears and insecticide spray should penetrate the crop canopy. Insecticides recommended for control of Spodoptera species include various pyrethroids, carbamates, neonicotinoids and organophosphates. Use of herbicide application and seed dressing with Imazapyr is recommended for Striga control (Goergen, 2017).

Host resistance

Screening and breeding for resistance are ones of the options for finding the resistant or tolerant varieties against insect pests, diseases and Striga damage. Certain varieties of Bt sorghum provide adequate control of pests and diseases (FAO, 2017).

IPM

All the suitable combined techniques are employed in a compatible manner so that the pest damage is kept below the economic injury level.

REFERENCES:

Andrews KL, 1998. Latin American Research on Spodeoptera frugiperda (Lepidoptera: Noctuidae). Florida Entomologist, 71, 630–653.

Barfield CS, Mitchell ER and Poeb SL, 1978. A temperature-dependent model for fall armyworm development 1, 2. Annals of the Entomological Society of America, 71, 70–74. https://doi.org/10.1093/aesa/71.1.70

CABI, (2017). Spodoptera frugiperda (fall armyworm). Cabi Crop Protection Compendium Datasheet, CABI Wallingford, http://www.cabi.org/cpc/datasheet/5731 Last modified 2nd May, 2017; [Accessed: 23rd May 2017].

Cruz I. and Turpin FT, 1983. Yield impact of larval infestations of fall armyworm (Lepidoptera: Noctuidae) to mid-whorl growth stage of corn. Journal of Economic Entomology, 76, 1052–1054.

FAO, 2017. FAO Advisory Note on Fall Armyworm (FAW) in Africa. http://www.fao.org/3/a-bs914e.pdf

Goergen G, Kumar PL, Sankung SB, Togola A, Tamò M (2016) First report of outbreaks of the fall armyworm Spodoptera frugiperda (JE Smith) (Lepidoptera, Noctuidae), a new alien invasive pest in west and central Africa. PLoS ONE, DOI:10.137/journal.pone.0165632.

ICRISAT, 1992. The medium term plan. Annual progress report, Volume II. International Crop Research Institute for the Semi-Arid Tropics. Patancheru, 502 324, Andhra Pradesh, India. pp. 312.

Kfir R, Overholt WA, Khan ZR, Polaszek A (2002). Biology and management of economically important lepidopteran cereal stem borers in Africa. Ann. Rev. Entomol. 47:701-731.