The moisture requirement of potato varies at different growth stages. If the soil is dry, the field should be irrigated 3-5 days before planting. After planting, potato requires a relatively low moisture level to harden the plants and lessen the incidence of tuber rotting. After emergence, the amount of water needed increases as the crop grows. The crop should have adequate moisture at stolon formation, tuber set and tuber growth. A period of interrupted growth due to insufficient moisture during tuber growth will result to the production of undersized and malformed tubers.
Inspect the soil within the root zones from time to time to determine the availability of soil moisture. The simplest method to do this is to form a soil ball. Get a handful of soil from the root zone area. If a ball can be formed and crumbled when dropped to the ground, the moisture in the soil is just right. However, if the ball does not crumble, there is too much water in the soil.
Water is applied either by overhead irrigation with the use of watering cans or by means of furrow irrigation. However, to avoid occurrence of fungal disease, furrow irrigation is recommended.
Weeds compete for light, space, water and plant nutrients. Their roots absorb more water and nutrient than the shallow rooted potato. Weeds can reduce potato yield to as low as 38%. They serve as alternate hosts of virus diseases that cause tuber degeneration and low yield. Weeds cause faster spread of insect pests such as asphids and thrips, and diseases such as late blight, blackleg and rhizoctonia.
To reduce yield loss due to weeds, the field should be thoroughly prepared and relatively free from weeds during the early stage of growth (4-6 weeks after planting). A well-prepared land will have less weed problems and allows the crop to compete better with the weeds.
Control weeds by hand pulling, hoeing the field and hilling-up or by mulching (2-5 cm thick). Hill-up when the plants are 15-25 cm high or at 30 days after planting, after which additional cultivation should be avoided. Pre-emergence application of 0.25 kg a.i./ha of metribuzin (Sencor) and 0.5 kg a.i./ha of prometryn followed by hilling-up 30 days after planting is effective in controlling weeds.
Insect Pests and Their Control
A. Thrips (Thrips tabaci Lindeman and Thrips palmi Karny)
These are small soft bodied insects about 1-1.2 mm long. The larvae are dark yellow and slow moving while the adults are brown and fast moving.
The larvae (nymps) and adults damage the leaves by piercing the surface and sucking the plant juices, causing the bronzing and drying up of leaves.
Plowing and harrowing of the field after harvest reduce thrips population in the soil. Overhead and surface irrigation have similar effects. The seedling stage should not coincide with the peak population of thrips which occurs during the dry months of the year. They come in a variety of colors, ranging from yellowish green to grayish green and yellow to brown. The winged forms are usually dark in color. Both the nymphs and adults are destructive.
Remove and dispose weeds and other host plants. Other crops which serve as alternate hosts like onion, cabbage, cauliflower, cucurbits, and tomato should not be grown in rotation with potato in threatened areas.
When weather conditions favor the development of thrip population, resort to chemical pesticides. Spray chemicals from emergence up to 57 days.
B. Tuber moth (Phthorimaea operculella Zeller)
Small gray-colored moth with narrow wings fringed with hair and mottled with black and brown spots.
Newly hatched larvae or caterpillars are colorless to pale pink. When fully grown, they reach 9-11 mm in length. They are white to creamy with a dark brown head. The dorsal surface has a light green shade which changes to pale pink when ready to pupate.
Like most other moths, the larval or caterpillar stage is destructive. During the day they take refuge on the foliage and on the ground and become active at dusk.
The eggs hatched on the leaf surface. The larvae tunnel in the leaves and feed on the leaf tissues. They move down to the leaf veins, petioles and into the stems and tubers leading to the loss of leaf tissue, death of growing points and weakening or breaking of stems. Damage from tuber moth infestation may reach as high as 45% in the field.
The damage becomes worse when the larvae tunnel into the tubers by way of the eyes or sprouts. The larvae make black excreta-filled tunnels which extend inside the tuber. Even harvested potatoes are not safe from tuber moth attack if these have not been properly protected or treated before storing. One hundred percent loss in storage is common.
Even before actual infestation, remove and burn volunteer potatoes and plants (including crop residues) left in the field after harvest to prevent the build-up of the moth population. Cover developing tubers by hilling-up with at least 6 cm of soil.
Irrigate on schedule to help control the pest and to keep the soil from drying and forming cracks which serve as entry points for moths and larvae. Remove tubers from the field on the day of harvest to prevent oviposition on newly harvested potatoes by adult tuber moths.
Many chemical pesticides (Table 1) are available in the market if cultural measure prove inadequate. Avoid using restricted pesticides (Appendix A) which still find their way to third world countries.
Before storage, dip seed potato tubers in solution of either Decis 2.5 EC (at 2 tnsp/20 liters of water) or Kafil 10 EC (at 3 tbsp/20 liters of water) for 5-10 minutes. A 20 L solution can accommodate 20 kg of potatoes at a time. A total of 60 kg of seed tubers can be treated with the prepared 20 L solution. Prevent the entry of moths into the storage house by putting insect-proof nets or screens on doors and windows.
C. Aphids (Myzus persicae Sulzer; Macrosiphum euphorbiae Thomas; Aphids gossypii Glover)
Aphids or plant lice are minute, sluggish, soft-bodied insects. They gather in large numbers on the plant. Once they have established themselves, they hardly move. With a few exemptions, these insects are wingless.
Aphids pierce potato leaves and succulent stems and suck off the plant juices. This causes wilting, severe curling and yellowing of leaves and stunting of infected plants. Aside from direct damage due to sap withdrawal, aphids pose greater anger because of their ability to transit a variety of virus diseases such as potato leafroll virus (PLRV) and potato virus Y (PVY).
Rogue out all infected plants to prevent aphids from spreading viruses in the field. All volunteer plants should likewise be removed. Keep the field free from weeds which serve as alternate hosts for aphids to deny the pest access to other sources of food with or without a standing crop. Practice clean culture whenever possible. At times, although natural factors such as heavy rain keep aphid population down, infestation may still occur.
Common Cutworms (Spodoptera litura Fabr.) and Black Cutworms (Agrotis ipsilon Hufnagel). Larvae worms feed actively at night on the young and mature leaves, making small holes on leaf blades. The young stems often cut off at ground level resulting gaps.
Plowing the fields to expose worms to predators before planting to help continue the cutworms infection. Handpicking of worms and field sanitation are necessary.
E. Mole Crickets (Gryllotapha africana Pal de Beauvios)
The crickets feed on the roots of the plants and the base of the seedings. The insects chew the surfaces of tubers in the soil causing rounded excavations.
Adults can be light-topped to control infestation. Flooding of the field before planting is a preventive control. Tubers should also be harvested as soon as possible after maturity.
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