Crop Systems for Sweet Potatoes

Sweet potatoes (Ipomoea batatas) are an extremely important crop in world agriculture. Sweet potatoes are a staple crop in many sub-tropical countries, especially those that are still developing countries (Crop Trust 2017). Sweet potatoes are grown widely across the globe, and because of this, methods of cultivation differ greatly. Factors such as farm size vary largely form farm to farm because of the fact that operations in developing countries can be large or small, depending on the size of the market and the amount of land that is available to use (Crop Trust 2017). It is common for the size of sweet potato growing operations to range from a few square meters to hundreds of acres (USDA NASS 2015). This essay will focus on the methods in place on a mid-scale sweet potato operation, catering to the world market, located in central America and the Caribbean.

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The sweet potato originated in Central America, and was brought back to Europe by Christopher Columbus. Sweet potatoes were introduced to Africa, the Caribbean islands and Latin America in the 17^th century and became a widely grown vegetable due to its nutritional value and crop hardiness and resilience (Crop Trust 2017). There are 7593 known varieties of sweet potato (MSU 2017) many of which are heirloom cultivars. Sweet potatoes prove challenging to breed due to their complex hexaploid (6x=90) ploidy (MSU 2017). This makes it very difficult to determine gene sequences and isolate desirable traits, especially in a small scale, low technology operation. Sweet potatoes are a perennial and also a short day plant, requiring less than 12 hours of irradiance in order to induce flowering (FAO 1992). The morphology of the sweet potato plant is quite unique (Figure 6). The plant can grow in an erect manner (straight up) or in a spreading fashion (along the ground) (Figure 1). The plant exhibits two types of roots (Figure 2). The first are fibrous roots that’s purpose are to absorb nutrients and water and to anchor the plant within the soil (Huaman 1992). The other class of roots are the storage roots (tubers), which are the harvestable sweet potato. Under ideal growing conditions, some cultivars of sweet potatoes do not flower, some have a few flowers and some flower plentifully (Huaman 1992). Of the cultivars that do display flowers, they are perfect flowers, having both stamens and carpels present (Huaman 1992). The flowers have 5 fused petals arranged in a funnel shape, and the colour is typically white or purple depending on the variety (Huaman 1992). The fruit or seed pod of the sweet potato plant is a spherical capsule-like structure (Huaman 1992). It holds 1-4 seeds and turns brown when mature (Figure 3). The storage root (or sweet potato) is the most valuable part of the plant (figure 4). The tuber consists of 4 layers respectively: a protective periderm skin on the outside, the cortex, the camblum ring and the medulla or middle (Huaman 1992) (figure 5). Sweet potatoes can enter a dormancy phase normally when triggered by cold weather, but can also become dormant when atmospheric concentrations of O₂ or CO₂ become too high or low (FAO 1992). In this dormancy phase, the plant will enter senescence, a phase where the foliage canopy will die off and the tuber will cease growth and remain dormant until suitable growing conditions return (FAO 1992). Different cultivars of sweet potatoes are suitable for different climate and soil types. An example of this can be found in table 1, where 3 separate varieties were grown in different regions of Cuba, all with significantly different yield and days to harvest periods (table 1).

Depending on the region, sweet potatoes can be grown year round. In the cropping system in question in this essay, the optimal growth period is between April and October, where the life cycle is 150-200 days on a 2-year rotation (Ramirez 1990). The field should be top dressed with a granular 7.5-6-18 NPK fertilizer and harrowed 10 days prior to planting (Ramirez 1990). The soil should be irrigated to moisten the soil and irrigation should continue every 7-10 days throughout the growing season at a rate of 200-250m³/ha (DAFF, Republic of South Africa, 2011). Planting should take place within 4 days of tuber stems being cut, at a distance of 22.5-30 cm, 7-10 cm deep and stems should be immediately hilled (Ramirez 1990). After planting, the plant will enter the establishment phase (40-60 days), in which roots will establish quickly and stems will grow slowly (Ramirez 1990). At day 10, insecticide and fungicide should be applied as a preventative measure, either as a foliar spray or as a granular soil application (Ramirez 1990). At day 60, tuber initiation will begin, where storage roots become present, the vine grows quickly and leaf area increases largely (Ramirez 1990). At this point, hand weeding, fertilization and re-hilling should occur (DAFF, Republic of South Africa, 2011). Top dress fertilization should occur every 40 days using a 2% foliar urea spray at a rate and density of 20g/L and 500L/ha (Ramirez 1990). At this point, the plant is in the intermediate phase, lasting 60-120 days until the plant reaches maximum leaf formation. At this point, hand weeding, fertilizing and re-hilling need to be completed again (DAFF, Republic of South Africa, 2011). In this final phase, lasting 45-90 days, vine growth slows and eventually stops, tubers begin bulking (Ramirez 1990). Eventually the nutrients from the leaves will be stored in the tuber and the plant will enter senescence (when the canopy dies, and the plant goes dormant) (Ramirez 1990). At this point, the tubers are totally developed and ready for harvest (Ramirez 1990). Irrigation should stop 10 days prior to harvest to ease in the removal of tubers form the soil (Nelson and Elevitch, 2011). A major insect pest of sweet potatoes is the Sweet Potato Weevil (Cylas formicarius elegantus) (Capinera, 1998). Adult females are 5.5-8 mm in length and feed on tubers in storage and also on the leaves (Capinera, 1998). Females deposit eggs in cavity they create with their mouths and seal it with fecal matter, usually at the juncture of the stem and root (Capinera, 1998). Eggs hatch 5-12 days later and enter the larvae stage (Capinera, 1998). The larvae are white in colour and have 3 separate instars, the first lasting 8-16 days, the second 12-21 days and the third 35-56 days (temperature being the limiting factor in development) (Capinera, 1998). The larvae develop into pupae for 7-10 days, and create a pupa chamber within the tuber (figure 7), before exiting the tuber as adults by chewing through the outer skin (Capinera, 1998). A major fungal pest of sweet potatoes is Black Rot (Ceratocystis fimbriata) (CABI Crop Protection Compendium, 2004). The fungus can remain dormant year round, for several years in soil, water and organic matter until interrupted by freezing temperatures, in which case it will die within 3 months (CABI Crop Protection Compendium, 2004). The maximum infection stage is May-August, where humidity is high, drought is common and temperatures are at an optimal range of 18-28ºC (CABI Crop Protection Compendium, 2004). It takes one week from a spore entering the outer epidermis to create a colony of mycelium and damage the tuber (CABI Crop Protection Compendium, 2004). Unchecked tubers will enter storage and the fungus will spread to infect all sweet potatoes in the storage container (CABI Crop Protection Compendium, 2004). The most effective way to prevent these pests is to apply insecticide and fungicide in the field and to inspect tubers before entering them into storage to prevent the spread of the pests (Nelson and Elevitch, 2011).

Sweet potatoes can be dug using commercial digging machines, however in the region in question, they are usually dug by hand (Nelson and Elevitch, 2011). The sweet potatoes are dug, being careful not to damage the outer skin, sorted and washed (Nelson and Elevitch, 2011). Tubers that show signs of fungus or insect or mechanical damage are removed before being stored (Nelson and Elevitch, 2011). Sweet potatoes are stored off the ground in 45Kg bags (no more than 10 bags high) on a wooden palate with space around each bag for circulation in a location with good circulation and low humidity. (Ramirez 1990). Bags should be transported within 24 hours unless they are being refrigerated, in which case they can be stored for 4-6 months at 13-15ºC and a relative humidity of 80% (Ramirez 1990). Yield of sweet potatoes range from 3-45 ton/ha depending on climate, soil type and nutrient availability, with the average yield at approximately 25 ton/ha and a revenue of $23.6 per hundredweight (FAO 1992). World consumption of sweet potatoes increased by 80% between 2004 and 2014 (USDA NASS 2015). Medium-large volume producers usually sell to the world market, while small producers sell directly to the consumer/restaurant/local market (USDA NASS 2015).

Works Cited

Crop Trust (2017). Sweet Potato. Web. Retrieved from croptrust.org/crop/sweet-potato
USDA NASS (2015). Vegetables, 2014 Summary.
Huaman Z. (1992). Systematic Botany and Morphology of the Sweet Potato Plant. Technical Information Bulletin 25. International Potato Center.
Michigan State University (2017). Sweet Potato Genomics. Web. Retrieved from sweetpotato.plantbiology.msu.edu
FAO (1992). Roots, tubers, plantains and bananas in animal feeding. Web. Retrieved from http://www.fao.org/docrep/003/T0554E/T0554E00.htm#TOC

Ramirez G. (1990) Cultivation harvesting and storage of sweet potato products. Roots, tubers, plantains and bananas in animal feeding pg. 203-215.

Department of Agriculture, Forestry and Fisheries Republic of South Africa, (201. Sweet Potato Production. Pg. 1-20.

CABI Crop Protection Compendium, (2004). Ceratocystis fimbriata. Web. Retrieved from: http://www.public.iastate.edu/~tcharrin/CABIinfo.html#anchor240700