OBSERVATIONS ON VEGETATIVE PROPAGATION OF AVOCADOS (PERSEA AMERICANA) AT THIKA

The avocado is relatively new and unknown as a commercial crop in Kenya although it has been grown in home gardens from ungrafted seedlings of unknown cultivars for many years. However, in the last 10 years there has been a keen interest in the planting of this crop, especially for the export market. This interest is reflected in the fact that in 1970, 20 065 kg of avocados were exported from Kenya to various European countries. In 1971, 34 020 kg were exported to the same destinations.

Seedling avocado trees do not produce fruits which are true to type, and they often vary in their habit of growth and the time taken to come into bearing. It is, therefore, necessary to graft or bud known cultivars onto seedling rootstocks in order to produce trees with desirable fruit characteristics and reduce the time taken to the first crop. It is also of prime importance to have a budding or grafting technique which will quickly multiply the required material to meet the needs of the nurseryman or the farmer.

The generally accepted rootstocks are seedlings of the Mexican race. This race is the hardiest of all and it is considered that trees budded onto such a stock are also less susceptible to frost than trees on stocks of the other races. However, Malan and Meulen (1954) claim that no proof for this assumption has been recorded. According to Platt and Frolich (1965), Mexican seedlings are more resistant to lime induced chlorosis, Dothiorella canker and Verticillium wilt than are Guatemalan seedlings. Chalker and Robinson (1969) working in New South Wales, reported that seeds from trees of Mexican or Guatemalan origin proved quite satisfactory for raising seedling rootstocks.

Seeds for rootstocks are normally obtained from mature fruits which should be harvested and allowed to ripen. Seed removed from decayed fruit on the ground must not be used as it may carry diseases which will later attack the growing plant. Parent trees in good health should be selected for seed purposes. Average size to large size seeds are generally considered to produce vigorous seedlings for rootstocks, since the cotyledons contain a large reserve of food to supply the growing plant.

After the desired seed has been selected, it should be placed under the soil with the apex uppermost. The apex may protrude through the soil without detriment to the germinating seed. According to Malan and Meulen (1954) the brown seed coats should be completely removed before planting as even small adhering pieces of this seed coat may delay germination. Johnson and Frolich (1957) observed that germination could be hastened by cutting a thin slice off the tip and base of the seed. They were of the opinion that the excercise is not necessary when seed is