A basal application of Compound S at a rate of 1300 – 2100 kg/ha or Compound C at a rate of 1300 – 1500 kg/ha is recommended. Top dressing of Ammonium Nitrate (AN) at a rate of 290 kg/ha should be applied once, 3 weeks after emergence. In addition, top dressing of 400 – 500 kg/ha of Sulphate of Potash, splitting it into two equal applications, one at flowering and the other two weeks after flowering. The top dressing of Ammonium Nitrate (AN) should be applied between rows or per plant before the final earthing up. A table below shows nutrient requirement as recommended by (AGRITEX).
NB: All P and K must be applied at planting. P increases yield by increasing the number of medium sized tubers.
K increases the number of large sized tubers. 30 t/ha of organic manure supplies adequate P and K needed by the crop.
Half to two thirds N is applied at planting and the remainder, 2-3 weeks after emergence. Fertilizer must be banded slightly below and at the side of the seed to avoid contact with the seed.
Herbicides and Pesticides
Potato Pests
Nematodes
There are three important nematodes species that affects potatoes. These species are: Meloidogyne spp, Pratylenchus spp and Radopholus simils. Control is by fumigation where possible. Rotation with grass species that is resistant to nematodes eg Katambora, Rhodes, Sabi and Panucum is a recommended control method.
Potato tuber moth
The tuber moth larvae make tunnels in plant tissue, there by cutting off nutrients flow and causing the plant to wilt and dry off. Control is by cultural methods which includes ridging the plants up to 250 mm. chemical methods involves Azodrin 40, at 15 ml/ 10 litres of water, when symptoms apper. Or navacron 40 at 50 ml per 10 litres of water.
Aphids
Aphids are small greenish sucking pests which causes symptoms of wilting and curling of leaves. Control is by spraying Azodrin 40 at 15 ml/10 litres of water. Other chemicals such as Thionex, and Malathion may also be used.
Cutworms (Agrotis spp)
The pest is very serious at the beginning of the emergence of young stems. The larvae chew the plant at surface level, and they appear as plump darkish greasy grey caterpillars that normally feed at night. Chemical control is by use of Karate, Carbaryl 85%, Thiodan 50 WP.
Disease control
Late Blight (Phytophtora infestans)
Occurs when the relative humidity is more than 70 % and temperature is around 22-25 degrees. The disease spreads very quickly during the wet season. The symptoms include brown patches at the end of leaves with white mycelium on under side and brown spots on stems. Control is by spraying with Dithane M45 and Ridomil MZ 72 combined. Two sprays of Dithane alternated with two sprays of Ridomil MZ 72 at 50g / 10 litres of water is recommended.
Early Blight
The disease slow spreading at temperature around 25 0C. Bottom leaves show dark brown to black spots with typical concentric rings. Control is by using the same chemicals that are used in controlling late blight.
Common Scab (Streptomyces scabies)
Signs and symptoms appear as rough circular black scabby lesions which can enlarge and cover considerable part of the tuber. It is associated with soils that have high degree of aeration caused by underploughing of high undecomposed organic matter and high temperatures. No chemical control is available. Avoid liming the field during the period when the crop is to be grown.
Growing Guidelines
Most common varieties grown in Zimbabwe include BPI, Amethyst, Mont Claire, Opal, Emerald, Jasper and Jacaranda. The yield of these varieties in winter and summer are evaluated below.
Planting times
a. Summer Crops
The potato summer crop is planted in November and harvested before the end of the rainy season. Crops suffer from disease pressure, but germination is good. Prolonged rains may pose problems at harvesting due to increased sprouting.
b. First Winter Crop
The first winter crop is planted between February and April so that it matures before the frost period. Later planting in lowveld is recommended to take advantage of cooler weather. Both crops are affected by late blight, therefore growing resistant varieties is recommended.
c. Second Winter Crop
The second winter crop is planted between late July and early August after the risk of frost has passed. The crop is usually free from late blight.
Most soils are suitable for potato production, but medium textured loamy soils with good organic matter are best. Deep ploughing (600 mm depth) is recommended, with discing and harrowing done to create a fine tilth. The soils should be fine, loose and without compacted layers that hinder root penetration.
Clods and stones should be avoided as they reduce root contact with soils and cause tuber deformation. Heavy clays and some micaceous soils should be avoided especially when they dry off because they produce misshapen tubers. Soils must be well drained, especially for the summer crop when rainfall is high. Well aerated soils ensure sufficient oxygen for root, stolon and tuber growth. Optimum pH is 5.0 – 5.5. Liming should not be done immediately before planting because the resultant high pH levels can predispose the crop to common scab.
Sprouting Potatoes
Sprouting is the development of shoots in potatoes. Each tuber has from two to as many as 10 buds (or “eyes”), arranged in a spiral pattern around its surface. The buds generate shoots that grow into new plants when conditions are favourable. Pre-sprouting of tubers helps to increase the number of main stem, and consequently the crop’s final yield. Sprouting also ensures quick, uniform and full germination.
Sprouting is done by storing the tubers in diffuse light. Tubers smaller than 25 mm should not be used for seed. Farmers may sprout the tubers by chitting trays or by force sprouting method.
Chitting Trays
Potato tubers are exposed to sunlight but protected from strong sun. Apply suitable pesticides to the tubers to protect them from tuber moth, and newly developing sprouts from aphids. If cut tubers are used, cure them for 10-14 days at 7-8 0c and 100 % relative humidity. Dipping of tubers in fungicide drench is recommended.
Forced sprouting
This can be done when dealing with large potato quantities. The method involves use of heat or acetylene.
· Heat: cover potatoes in tarpaulin in moderate sunshine. Temperature of 30-35 0c will initiate sprouting.
· Acetyline: 0.1 % of acetylene gas in an airtight room between 21-27 degrees celsius will initiate sprouting. 30g of calcium carbide will generate sufficient gas for 2m3.
· Immerse the tubers in an acetylene solution for 4-6 hours. The mixture should be 45 liters of water to 230 g of calcium carbide added slowly.
· Gibberellic acid can also be applied to stimulate sprouting in potatoes. The mixture is 16ml/100litres of water. The seed potatoes should be dipped in the mixture for 3 – 4 minutes and then sun-dried before storing for the sprouting process.
Planting your crop
Newly-sprouted seed between 5 and 15 mm long is suitable for planting. During planting, potato tubers are mechanically or manually placed in the rows, 20 cm to 30 cm apart, with a row to row spacing maintained at between 60 to 120 cm. Spacing is influenced by seed size and soil fertility. Tubers can be planted 7 cm-10cm deep under irrigation farming and can be slightly deeper up to 15 cm when dryland farming is employed. Soon after planting, a ridger is run to cover the potato tubers by throwing the soil from both the sides and ridges pressed. The first irrigation must be applied to a depth of 600mm.
Storing your harvest
The purpose of potato storage is to maintain tuber quality and provide a uniform flow of tubers to fresh market and processing plants throughout the year. Good storage should prevent excessive dehydration, decay and sprouting. It should also prevent high sugar concentrations which result in dark colored fried products. A potato storage structure should have adequate insulation, outside waterproofing, inside vapor proofing, ventilation, air distribution, adequate humidification, and properly designed controls for precisely maintaining the storage atmosphere.
Temperature, humidity, and air movement are the most important environmental factors affecting storability. Temperature requirements are determined by the intended use of the potatoes. Tubers should always be kept in the dark since very small amounts of light will gradually cause greening. Lights should not be used more than absolutely necessary. Surface greening is due to chlorophyll formation and is harmless. However, its presence in potatoes is undesirable because of marketing restrictions and the fact that at times an alkaloid called solanine increases with the chlorophyll. Solanine and other
glycoalkaloids cause potatoes to have a bitter, undesirable flavor. Greening develops slowly in the light at 4 degrees or below but develops rapidly at 20 degrees.
Potatoes are usually held in bulk piles 2.4 m to 6 m deep. Some are stored in pallet boxes for short periods. Pressure bruise and internal black spot are substantially lower with pallet storage but decay is often increased because of poor air circulation within boxes. Long-term pallet storage is not recommended. Because of the large number of cultivars grown, only general storage recommendations can be given here. Growing and harvesting conditions influence the behavior of potatoes in storage.
Early harvested potatoes are usually stored only briefly if at all. Such tubers are quite perishable and damage easily because of immature skins. Early potatoes free from serious bruising and decay can sometimes be held 4 to 5 months at 4 0C for table use if they are cured 4 or 5 days at 12 to 18 degrees to heal wounds before storage. However, early crops should usually be sold immediately because of poor storability and typically high early season prices.
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