Studies on the use of vesicular-arbuscular mycorrhizal fungi for fruit production in Kenya.

Abstract

A country-wide survey to establish mycorrhizal development in tropical and sub-tropical fruit orchards was carried out in 103 fruit orchards that were distributed in 25 locations, 12 soil types and 4 ecological regions of Kenya. The survey was carried out about a month before expected commencement of a long rainy season following a long dry period during the months of February and March 1996. Data that was obtained from the survey indicated that root infection by fungal symbionts was very low and the number of spores of vesiculararbuscular mycorrhizal (V AM) fungi in the rhizosphere of fruit trees was low. Data of the soil analysis showed that the P-content was about 20 ppm in most of the orchards and the N-content was similarly low. When the types of VAM fungi were determined, the spores of the genus Glomus were the most dominant in all the soil samples that were examined. Other data obtained from the survey also showed that:

1. Fruit orchards usually received no or little attention to fertilizer applications

2. Intercropping with beans, maize and even vegetable crops was a normal practice

3. During a dry period, the soil cover in most orchards was bare following harvest of intercrops. As a result of the aforementioned conditions, the soil became somewhat hardened and it had visible signs of topsoil erosion. Where intercropping was not practiced, however, many kinds of weeds were found to have grown. In view of the above observations, a series of experiments were established with the aim of seeking ways to use mycorrhizal fungi for the benefit of fruit production. The study had the following objectives: Overall objective: Utilization of mycorrhizal fungi ill a low chemical fertilizer input for sustainable fruit production.

Specific objectives:

1. Isolation, identification and multiplication of an effective indigenous vesicular-arbuscular mycorrhizal fungi for use in fruit nursery inoculations to help alleviate nutritional problems especially phosphorus

2. Determine a suitable nurse plant and media for the propagation of mycorrhizal inoculant

3. Seek ways of sustaining a high population of mycorrhizal inoculum in orchards

4. Determine ways of promoting spore germination soil and in laboratory as a means of enhancing root colonization by mycorrhizal fungi.

Spores of a VAM fungus Glomus caledonium isolated in October 1994 from an orchard of citrus at Jomo Kenyatta University of Agriculture and Technology was used in experiments for VAM studies. Also, Gigaspora margarita, a VAM fungus obtained from Japan, was used in some experiments for comparison with Glomus caledonium.

The results obtained from experiments showed that:

I. A medium containing 4 parts soil, 2 parts sand and 1 part charcoal particles of size s; 3cm3 by volume basis significantly stimulated spore propagation of Glomus caledonium when millet plants were used as nurse plants

2. Leaf phosphorus content of rootstocks of citrus, macadamia, mango, papaya and purple passion fruit raised in the charcoal incorporated medium that was inoculated with Glomus caledonium was significantly increased

3. The charcoal incorporated soil significantly increased growth of the vine, fruit number, yield of fruit and fruit quality of field grown passion fruit.

Extract from millet root was found to stimulate hyphal growth of Glomus caledonium under axenic condition. When the extract was applied to soil, it increased seedling growth of papaya and passion fruit. Stimulatory effects for V AM fungi were similarly noted from extracts of weeds, citrus sediments, and green and red algae. The extracts were found to increase hyphal growth of Glomus caledonium and Gigaspora margarita and to increase the growth of papaya seedlings and passion fruit cuttings. Further research is needed to establish expedient utilization of mycorrhizal fungi for fruit production along with low fertilizer inputs such as rock phosphates for sustainability of fruit yields of good quality. Another area that needs further attention is the possible utilization of cheap sources of VAM stimulatory extracts to be applied to orchards in order to enhance root infection by mycorrhizal fungi. In this connection, possible sources of V AM stimulatory substances such water hyacinth weed that has overgrown lake Victoria are recommended for future investigations.