Process For The Post Propagation Activities

Propagating plants requires the understanding of different physiological stages that the plant faces before it goes into production. The propagator must be able to match the growth stage to the appropriate environment of propagation to achieve the best results.

Readiness for Transfer to Next Phase

There are different stages at which propagated plants can be transferred or transplanted as seedlings. The population density, per surface area, decreases from the seed germination stage to polythene bags in shade-houses, including the seedling tray stage.

Transplant readiness in woody perennial plants is determined by the seedling development phase, seedling age, seedling size and climatic conditions. Transplant readiness differs between crops and between propagation methods. In citrus production, for example, propagators will transplant only when the weather is favourable i.e. warmer season after winter. In citrus production, the first transplant is done only according to the development stage while climatic conditions are overlooked as the first transplants are kept in a greenhouse.

Transplants from the greenhouse to the field takes place at stages as determined by the requirements of the specific crop. Transplants into the field are crop-specific and have to be made to crop-specific procedures. In addition, each type of propagation product is treated differently, and these should also be done by following specific procedures.

Pests and Diseases

Pest- and disease control follows the same procedures as for mature crops. Although some major pests and diseases are mentioned below, specific pest and disease management practices are implemented for specific crops and production areas that should be followed.

Foliar Damaging Pests

Foliage damaging pests, including those that cause encrustations on foliage and leaf eaters, are our major pest problems. A reduction in the leaf effective area will reduce the potential of the transplants for development.

Some important crop pests include:

• Red scale
• Mealybug
• Aphids
• Citrus psylla
• Mites
• Thrips
• Loopers
• Slugs

Red scale: Red scale discolours the leaf and can turn it yellow, thus reducing the vitality of the plant. The surface area for photosynthesis is affected as little chlorophyll is synthesised. Severe infestations lead to leaf drop and twig die-back.

Mealybug: Mealybug is an oval, pale yellow insect covered in a powdery white wax. Adults are slow-moving and secrete honeydew and black sooty mould. Heavy infestations cause leaf drop and reduce the photosynthesis rate.

Aphids: Aphids are either winged\wingless black and brown insects that secrete honeydew. Infected leaves are covered with sooty mould which in turn reduces the transpiration and photosynthesis rate. Leaf malformation is also been associated with aphid damage. In most cases,, the damage associated with aphids is not the direct damage but that associated with the viruses carried and transmitted by the aphids.

Mites: Red mites are oval-shaped. They feed on the leaves and green bark of citrus plants. They prefer the upper surface of leaves which end up grey, silver or yellow. This affects photosynthesis and transpiration rates. Silver mites also have an oval shape but are flat, straw-coloured and translucent. They damage the leaves which become crinkled and may produce corky brown patches on the undersurface.

Thrips: Thrips are translucent orange-yellow in colour and wingless at immature stages, with adults having two pairs of narrow wings. The damage caused to citrus trees is found on young tender shoots and leaves that are malformed and result in stunted growth. Thrips extract chlorophyll from the infested plant parts, which then become pale.

Loopers: Loopers feed on leaves, starting at the margins of leaves. Both young and mature leaves are consumed by looper worms. Immature larvae have a different feeding pattern in that they first feed on the upper and lower epidermis and only then eat holes in the leaves.

Slugs: Slugs mainly eat holes in the crop’s leaves.

Wood Damaging Pests

Rodents feed on the bark of young trees and shrubs in the nursery. Severe infestations may lead to the trees drying down. When feeding on smaller seedlings, rodents can cut off the stem.

Root Damaging Pests and Diseases

Nematodes feed on roots which then appear darker in colour. Rootlets become stunted, swollen and irregular in shape and appearance. Infested trees cannot tolerate stress or drought conditions.

Common root diseases of concern include root and stem rot caused by Phytophthora and damping-off caused by fungi including Rhizoctonia.

Root and stem rot, as well as dumping off start in the nursery and are generally expressed in the nursery before trees are planted in the fields. If the plant is infected at the end of the nursery cycle, the disease might not be detected in the nursery and only be expressed in the field.

Damping-off is a seedling disease. It only occurs in the seedling stage of propagation; from the germination until after the first transplanting.

Phases of Propagation

In most cases propagation occurs under controlled environments. At a predetermined development stage, the plants are transplanted either to containers or planted in the field.

Each development phase requires specific environmental conditions for optimal growth. During the germination phase, light intensity, light quality, moisture, aeration, temperature and humidity are the most crucial factors. Once the seed has germinated, the physiological needs of the seedling require enough humidity to perform optimum transpiration.

Respiration and transpiration rates of the seedling determine the level of carbohydrates in the plant that can be used for growth. The more carbohydrates are used in the transpiration process, the less there is available to the plant for growth. Roots develop to ensure that the plant can sustain itself, absorb water and nutrients as well as anchor the seedling in the medium. During the initial growing phase, humidity and temperature are controlled and humidity is kept high. Should the plant then be transferred to a shade house, both humidity and temperature are reduced. The plant is thus subjected to lower humidity and light intensity. The thermal sheets or shade-cloth protect the plant from harsher environmental conditions of the immediate outdoor.

From one phase to the next the propagator must ensure that the transfer is done under optimum environmental conditions to avoid transfer shock.

Hardening-Off

Before the seedlings are transferred to shade-houses from greenhouses, seedlings are subjected to an acclimatisation process to minimise the transplant shock impact.

The seedlings are kept in a facility, for a minimum of a week, where the humidity is lower than in the greenhouse and light intensity is adjusted to resemble those of the ambient surroundings more closely. This leads to an accumulation of carbohydrates, increasing the ability of the plant to withstand its new environment and prevailing conditions.

Where cuttings are used, hardening off begins in the propagation area. The humidity is gradually reduced, and the rooted cuttings become more hardened to function in physiological activities such as water and nutrient absorption, photosynthesis, and leaf development.

Possible Problems with Hardening-Off

Disease infested plants will not survive the hardening off period because of their reduced ability to survive.

Plants with poorly developed root systems will display an inability to survive under harsher conditions during the hardening off process.

Reducing light intensity may lead to the development of spindly and elongated plants. In some cases, hardening off is done in structures covered in specialised netting such as aluminium coated netting, which diffuses light evenly in the structure. This aids in preventing the development of spindly plants.

Reduced humidity may lead to excessive water loss by the plant. If this problem is not corrected, plants will starve a pant (breathe with difficulty) death. It is thus crucial that irrigation programmes are adapted to suit the environment under which the plants are grown. In most cases, a plant will require a 14-day period to adapt to an altered environment. This should be kept in mind when moving a plant from one environment to the next.