How do I Select Healthy Cassava Stem?

Author :  Wendy       Date Time : 2016-05-03
The most common sources of cassava stem planting material are farmers own farms. Occasionally, cassava stem cuttings are sold at village and town markets. Researchers and extension agents sometimes provide their contact farmers with cassava stem cuttings. Many cassava pests and diseases are stem-borne and spread by distribution, sale, and planting of infested or diseased stem cuttings. By planting healthy stem cuttings, you can greatly reduce the spread and damage caused by these cassava pests and diseases. The following guidelines will assist you to avoid unhealthy stem cuttings and to select healthy planting material for a healthy crop of cassava.

Look for healthy cassava plants: Select healthy cassava plants in the farm. Healthy cassava plants have robust stems and branches, lush foliage, and minimal stem and leaf damage by pests and diseases. From each plant select the middle brown-skinned portions of stems as stem cuttings. These parts sprout and ensure plant vigor better than the top green stem portions. Stem cuttings taken from the top green portions of stems or extreme top and bottom of stored stems are unsuitable. They will dehydrate quickly, produce unhealthy sprouts, and are easily damaged by pests and diseases.

Avoid plants with pests and diseases: In selecting cassava plants as sources of stem cuttings, you should avoid those infected with these pests and diseases. The common stemborne cassava pests and diseases are cassava mealybug, cassava green mite, spiraling whitefly, white scale insect, cassava mosaic disease, cassava bacterial blight, cassava anthracnose disease, and cassava bud necrosis.

The cassava mealybug, Phenacoccus manihoti, occurs on cassava leaves, shoot tips, petioles, and stems. The mealybugs are covered with white waxy secretions. Cassava mealybug damage symptoms include shortened internode lengths, compression of terminal leaves together into bunchy tops, distortion of stem portions, defoliation, and candlestick appearance of shoot tip. The insects survive on cassava stems and leaves and are easily carried to new fields in this way.

The cassava green mite, Mononychellus tanajoa, occurs on the undersurfaces of young leaves, green stems, and axilliary buds of cassava. The mites appear as yellowish green specks to the naked eye. Mites survive on cassava stems and leaves and are easily carried to new fields in this way. Cassava green mite damage symptoms include yellow chlorotic leaf spots (like pin pricks) on the upper leaf surfaces, narrowed and smaller leaves , candlestick appearance of the shoot tip, and stunted cassava plants.

The spiraling whitefly, Aleurodicus dispersus, damages cassava by sucking sap from the leaves. Colonies of the insect occur on the undersurfaces of cassava leaves and are covered with white waxy secretions similar to those of the cassava mealybug. Spiraling whitefly eggs occur in spiral patterns of wax tracks, mostly on the undersurfaces of leaves. Symptoms of whitefly damage are black sooty mold on the upper leaf surfaces, petioles, and stems, and premature leaf fall of older leaves. The insects survive on cassava leaves and stems and are easily carried to new fields in this way.

The white scale, Aonidomytilus albus, covers cassava stem surfaces with conspicuous white waxy secretions . The insect sucks sap from the stem and dehydrates it. Stem cuttings derived from affected stem portions normally do not sprout. The insects survive on cassava stems and leaf petioles and are easily carried to new fields in this way.

Cassava mosaic disease is caused by a virus which occurs inside cassava stems. Symptoms of cassava mosaic disease damage are patches of normal green leaf color mixed with light green and yellow chlorotic areas in a mosaic pattern (Figure 13). Generally, plants with these symptoms should be avoided as sources of stem planting material. However, the disease is very common in Africa and it is sometimes difficult to find cassava plants that are completely free from the disease. You can, however, reduce cassava mosaic disease problems by selecting stem cuttings from cassava stem branches and not from the main stems. Stem cuttings from the branches are more likely to sprout into disease-free plants than stem cuttings from the main stems.

Cassava bacterial blight is caused by a bacterium which occurs inside cassava stems. The disease damage symptoms are angular leaf spots on the under leaf surfaces, leaf blighting and wilting , gum exudate on the stems, and shoot tip die-back. Avoid selecting stem cuttings from plants with these symptoms.

Cassava anthracnose disease is caused by a fungus which occurs on the surface of cassava stems. The disease damage symptoms are cankers on the stem and bases of leaf petioles . The disease reduces the sprouting ability of stem cuttings.
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