The African Journal of Horticultural Sciences (AJHS) Vol. 1 ABSTRACTS

The first Issue of the journal contains 11 papers. The abstracts of the papers are presented below:

You can obtain a copy of the journal from the Secretariat. Send an email to hak@agr.jkuat.ac.ke or hak.2006@yahoo.com

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1. Effect of Soil pH on Postharvest Pathological Deterioration of Sweet Potato Storage Roots [Kihurani, A.W., Narla, R.D., Shibairo, S., Imungi, J. and Carey, E.]

2. An Outbreak of Banana Xanthomonas Wilt in Banana Orchards in Western Kenya [Mbaka, J.N., Maina, M., Nakato, V.G. and Auma, J.]

3. Effects of Phytohormones and Dark Storage on Postharvest Quality of Pelargonium Cuttings [Mutui, T.M., Mibus, H. and Serek, M.]

Maintenance of the leaf green colour, capacity to initiate roots and continued growth of initiated roots are important quality attributes in Pelargonium cuttings. Leaf senescence is a common problem in Pelargonium and leads to high postharvest losses. Ethylene, abscisic acid (ABA) and darkness have been implicated in promoting senescence, while cytokinins retard it. The effects of post-harvest treatments with ethylene, Thidiazuron (TDZ), ABA or dark storage were investigated to determine a potential commercial approach for improving postharvest quality of Pelargonium cuttings. Experiments were conducted in a completely randomized design, with two replications. The data obtained were subjected to a single factor analysis of variance using Statistical Analysis System programme. Storing cuttings in the dark for 4 days and/or treating them with 2 µL/L ethylene or 100 µM ABA hastened the onset of visible leaf yellowing in ‘Fire’, ‘Ganymed’ and ‘Katinka’. Conversely, 5 µM TDZ markedly delayed the onset of leaf senescence in all the cultivars. Thidiazuron increased leaf hue values (greenness), while ethylene, ABA or dark storage decreased leaf hue (hastened yellowing). Exposing Pelargonium cuttings to ethylene reduced root fresh weight in ‘Katinka’, but had no effect on root dry weight and root water content. Storing Pelargonium cuttings for 4 days in the dark reduced root fresh weight, root dry weight and water content in ‘Katinka’, but this was not apparent in other cultivars. Applying, 100 µM ABA reduced root fresh weight, root dry weight and water content in all cultivars. Thidiazuron severely inhibited adventitious root formation in all cultivars. The inhibitory effect was overcome by inclusion of indole-3-butyric acid (IBA) in the nutrient solutions. Thidiazuron counteracted the deleterious effects of dark storage, ethylene and ABA by delaying the onset of leaf yellowing in Pelargonium cuttings during storage and/or shipment. Thidiazuron should be applied to Pelargonium cuttings as a postharvest treatment to prevent subsequent leaf yellowing. Cuttings should not be stored under darkness or water stress to prevent accumulation of ethylene and ABA. After TDZ treatment, transportation and storage, Pelargonium cuttings should be dipped in 4 µL/L IBA to induce root development and enhance quality.

4. Effects of Near Infra-Red Reflection Greenhouse Cooling on Blossom-End Rot and Fruit Cracking in Tomato (Solanum lycopersicum L.) [Mutwiwa, U.N., Tantau, H.J., von Elsner, B. and Max, J.]

Blocking the near infra-red (NIR) portion of the global radiation from entering a greenhouse is one of the emerging environmentally friendly and economical ways of reducing greenhouse temperature without increasing the humidity. In this study, the effect of cooling greenhouses by NIR-reflection on the microclimate, fruit quality and growth of tomato crop was investigated. Two greenhouses measuring 10 m long by 20 m wide by 6 m high were constructed at the Asian Institute of Technology, Klong Luang, Thailand. Both greenhouses were naturally ventilated and were clad using insect proof screens (78-mesh) on the sidewalls and ventilation openings, while the roof was covered with an ultra violet (UV) absorbing plastic film. Reduheat®, a NIR-reflecting pigment was applied on the roof of one of the greenhouses treated (Trt). Tomato plants were grown in the greenhouses in 10 L white plastic pots at a density of 1.5 plants/m² and grown following commercial recommendations using the high wire system. Results show that the quantities (both weight and count) of tomato fruits affected by blossom-end rot (BER) as well as undersized fruits (weighing less than 50 g) during both dry and rainy seasons were lower for the plants grown inside the greenhouse with the NIR-reflecting pigment on the roof. On the other hand, in this greenhouse the number of cracked fruits was slightly higher. Additionally, NIR-reflecting pigment reduced electricity consumption by the ventilation fans (used during emergency). Application of NIR-reflecting pigments on greenhouse covers for cooling, reduction of water requirement and fruit quality improvement is recommended for crops grown inside greenhouses when solar radiation is in abundance.

5. Effects of Organic Soil Amendments in Nutrition of Banana Plants [Njeru, E.S., Muthamia, J.G. and Amboga, S.C.]

Soil degradation is among the most common causes of low productivity of bananas in Kenya. Four organic soil amendments were evaluated at the Regional Research Centre at Embu during the 2006 long rainy season for their relative contribution to soil fertility in bananas. The amendments tested were: farmyard manure, poultry litter, Tagetes minuta and Tithonia diversifolia. A control treatment was also included in which no amendment was used. The treatments were arranged in a Randomized Complete Block Design replicated ten times. Ten tissue-cultured banana plants of the cultivar Grand Nain grown in polyethylene sleeves containing sterilized forest soil were used for each treatment. The amendments were incorporated in the soil at the rate of 5% (w/w) per plant. The study was conducted for a period of 90 days. Plant growth and soil fertility data were collected at the termination of the study. Plant growth data included plant height, pseudostem girth, number of leaves and plant biomass, which were analyzed using SAS. Residual soil fertility was determined through fertility analysis conducted at the National Agricultural Research Laboratories. Significant (P<0.05) improvements were observed in plant growth as well as residual levels of major soil nutrients in all amended soils. Poultry manure had the strongest influence on both parameters, showing increases of 54% and 190% in residual levels of nitrogen and phosphorous, respectively, compared to the control. Plant height, girth and total plant biomass increased by 26%, 42% and 46%, respectively over the control due to the application of this amendment. The results indicate that the organic soil amendments are suitable alternatives for restoration of soil fertility and have potential to improve banana productivity in small-holder farms when applied periodically.

6. Effects of Integrated Nutrient Sources on Growth and Yield of Strawberry Grown under Tropical High Altitude Conditions [Odongo, T., Isutsa, D.K. and Aguyoh, J.N.]

Strawberries are popular both in fresh and processed forms because of their enormous values. Low soil fertility is among major factors constraining strawberry productivity in Kenya. Consequently, effects of cattle farmyard manure (FYM) (0, 18, 36, and 54t/ha) in combination with triple super phosphate (TSP), equivalent to 0, 17, 34 and 68kg/ha phosphorus on strawberry productivity, were tested in split plots embedded in randomized complete block design, replicated three times. Farmyard manure formed main plots, whereas TSP formed sub-plots. Each treatment had 10 plants spaced at 0.3m´0.45m in double rows per plot, measuring 0.6m´1.5m. The plots were mulched with black polyfilm, irrigated with 2.5.cm diameter drip lines, and separated with a 0.5m trench. The study was done in three seasons (August 2003-July 2004, Feb. 2004-Jan. 2005, and July 2005-June 2006). Each season was conducted in new plots. Growth and fruit yield were assessed from 3 to 6 and 6 to 12 month after planting, respectively. Since seasons 1 and 2 data were not significantly different they were pooled and reported as season 1, whereas the different season 3 data were reported separately as season 2. Generally, the 54t/ha FYM promoted the highest growth, although not significantly different. Phosphorus (P) significantly (P<0.05) increased root biomass, and root: shoot biomass ratio. The effects of FYM, P and interaction on fruit yield were not consistent. The 36 and 54t/ha FYM resulted in the highest fruit weight in seasons 1 and 2, respectively. Phosphorus did not significantly (P>0.05) influence fruit weight, although 17 and 34kg/ha P yielded highest in seasons 1 and 2, respectively. The effect of interaction on cumulative yield was not significant in both seasons. Thus the observed variation was also attributed to soil and seasonal effects. Based on the current results, 36 t/ha to 54t/ha FYM and 17 kg/ha to 34kg/ha P are recommended for enhancing strawberry productivity under conditions similar to the ones for the present study. Phosphorus should only be applied where soil tests show deficiency.

7. Effects of Amending Compost and Green Manure With Phosphate Rock on Quality of Amaranth [Minja, R.R., Maerere, A.P., Rweyemamu, C.L .and Kimbi, G.G.]

A study was conducted in Iringa, Tanzania to evaluate the effects of crotalaria green manure and compost applied alone or amended with Minjingu Phosphate Rock (MPR) as sources of N and P and their effects on vegetable quality attributes. The field experiment was conducted using a split plot design with three replications. Crop species formed main plots, while fertilizer treatments constituted the subplots. Two amaranth species (Amaranthus cruentus and Amaranthus hypochondriacus) were used. The treatments included crotalaria (Crotalaria ochroleuca), crotalaria + MPR, compost, compost + MPR and NPK, which was used as a standard. Results indicated that amendment of compost or crotalaria with MPR enhanced P supply by 22% and 100%, respectively. The tested materials significantly increased vitamin C, vitamin A and crude protein content of amaranth. Plants from plots treated with crotalaria had significantly higher levels of protein, calcium, vitamins C and A contents, compared to those fertilized with compost. There were significant and positive correlations between soil available N, P, and uptake with protein, calcium, iron, vitamin C and A content of amaranth. Due to the role of Ca in cell wall synthesis, it probably influenced the keeping quality of amaranth. The high Ca content in MPR amended treatments increased the evaluated keeping quality attributes of amaranth. It is therefore recommended that the tested organic materials be adopted as sources of N and P in the organic production of amaranth. Application of crotalaria and compost should be supplemented with P sources such as MPR in order to increase their P contents.

8. Effects of Weather on Population Dynamics and its Implications on the Thrips Pest Management [Waiganjo, M.M., Gitonga, L.M. and Mueke, J.M.]

Onion thrips, Thrips tabaci Lindeman, is the most destructive insect pest of the onion crop, Allium cepa L. worldwide. Both larvae and adults attack the onion crop at all stages of its growth, resulting in reduction of yield and quality. Despite the heavy losses incurred as a result of damage by the onion thrips, very little efforts have been made to develop an integrated pest management strategy for the control of the pest in many African countries. Field trials were conducted in four seasons at the Kenya Agricultural Research Institute (KARI) Mwea-Tebere farm in Kirinyaga District. Onion plants were planted periodically and thrips populations monitored weekly from onion plant samples and blue sticky traps for a full year under natural field conditions. The purpose of the trials was to assess the effect of weather on thrips population in onions with the aim of predicting thrips control requirements for a given climatic trend. Weather variables monitored included: rainfall, temperature, relative humidity and wind. Thrips occurred in the onion field and infested onions in all the crop seasons. However, there was significant (P=0.05) variation in thrips numbers between the crop seasons. Dry weather (30.3 mm rainfall) with moderately high temperatures (15.6-28.2oC) increased seasonal thrips numbers, while wet season (391mm rainfall) with moderately high relative humidity was negatively correlated with thrips numbers. Regression analysis (step-wise selection model) showed that minimum relative humidity was the only significant weather factor for predicting thrips infestation in the onion crop (R2=0.15; y = 60.342-0.1022x). The results suggest that climatic trends can be used to determine the potential thrips control needs in onion production.

9. Evaluation of Legumes as Components of Integrated Soil Nutrient Management for Kale Production [Wambani, H., Nyambati, E.M. and Kamidi, M.]

Declining soil fertility as a result of nutrient depletion is a major constraint to crop production in smallholder farms in northwestern Kenya. The use of legumes could play an important role in improving soil productivity of these production systems. Sunhemp (Crotolaria juncea L.) locally known as mito and mucuna [Mucuna pruriens var. Utilis (L) DC] were evaluated for soil fertility improvements as preceding rotation crops for subsequent kales (Brassica oleracea var. acephala) when herbage above 10 cm was removed as vegetable or feed. Eight treatments were evaluated using a randomized complete block design replicated five times in two seasons. The treatments were: (1) defoliated sunhemp, (2) defoliated mucuna, (3) defoliated sunhemp + 5 t/ha farm yard manure (FYM), (4) defoliated sunhemp + 30 kg/ha P₂O₅ + 30 kg/ha N, (5) defoliated mucuna + 5 t/ha FYM, (6) defoliated mucuna + 30 kg/ha P₂O₅ + 30 kg/ha N ha, (7) 60 kg/ha P₂O₅ + 60 kg/ha N and (8) No fertilizer (control). Legume dry matter yields and fresh kale leaf yields were measured. Sunhemp yielded greater (P<0.01) lower canopy stem stubble than mucuna. There were no significant differences (P>0.05) in herbage removed as food or feed, which ranged from 2.2 to 3.4 t/ha. The combination of stubble legume residue and either FYM or inorganic fertilizer yielded greater (P<0.05) kale leaf than no fertilizer control. The yields under the combination of stubble legume residue and half the recommended inorganic fertilizer were not significantly different (P>0.50) from those for recommended inorganic fertilizer. The remaining stubble legume residue after defoliating top canopy herbage as food or fodder can substitute for 30 kg/ha P₂O₅ + 30 kg/ha N. Despite the greater stubble residue incorporated under the sunhemp treatments, kale yields under mucuna treatments were similar to those from sunhemp treatments, suggesting that nutrient use by succeeding kale was enhanced under mucuna treatments. Farmers preferred the combination of legume stubble with FYM because it was associated with larger kale leaf, low cost of production and good persistence of the kales.

10. Interaction of Farmyard Manure and Plant Population Density Effects on Soil Characteristics and Productivity of Mulched Strawberry in a Tropical Climate [Ogendo, R.O., Isutsa, D.K. and Sigunga, D.O.]

The cultivated strawberry (Fragaria ananassa) is a perennial herbaceous plant, whose berries are principally consumed fresh or after processing into preserves, jam, jelly, juice, or flavours. Information on the influence of farmyard manure (FYM) and plant population density (PPD) on strawberry performance in the tropics is meager. The present study determined the effects of FYM and PPD on soil characteristics, growth, fruit yield and quality of mulched strawberry under tropical conditions. The experiment was laid in a split block design, replicated three times and conducted in two crop years. Farmyard manure (0, 10, 30 and 60 t/ha) formed main plots, while PPD (71,111, 53,333 and 40,000 plants/ha) formed strip plots. Parameters measured were: soil pH, cation exchange capacity (CEC), nutrient content and temperature. Plant parameters were: leaf tissue nutrient content, chlorophyll content, number of leaves, number of runners, canopy diameter, root and shoot biomass, number of berries, weight of berries per plant and total soluble solids (TSS) per plot. The data were subjected to analysis of variance. Manure significantly (P<0.05) increased soil CEC. The 40,000 plants/ha PPD significantly raised soil temperature at 10 cm depth. The effect of interaction between FYM and PPD was significant on leaf tissue Mn content. The 60 t/ha FYM significantly (P<0.05) increased number of leaves. Plant population density significantly (P<0.05) decreased number of runners, but increased number and weight of berries. The interaction between FYM and PPD significantly (P<0.05) affected TSS. It is recommended that 53,333 to 71,111 plants/ha be adopted to realise 47% to 92% higher strawberry fruit weight. Further research is recommended to determine the FYM rate that significantly increases fruit yield.

11. Postharvest Evaluation of Introduced French Bean (Phaseolus vulgaris L.) Varieties [Muchui, M.N., Ndegwa, A., Wachiuri, S., Muthama, B. and Kimamira, J]

French beans (Phaseolus vulgaris L.) are major export vegetables in Kenya. New varieties introduced from temperate countries need to be tested for adaptability to local conditions as well as their postharvest characteristics to ascertain that they meet the export standards. Three introduced varieties(R-1516, R-1515 and Lexus) were evaluated for their performance at KARI Thika. The snap beans were grown and harvested in long rains season of 2001. The plants were grown in rows following agronomic recommendations. The pods were harvested when mature at a frequency of 3 times per week, graded and weighed in a general laboratory. Random samples of 2 kg per grade from each variety were taken on the same day and taken to the postharvest laboratory for evaluation. Parameters recorded included number of pods per plant, pod length, pod diameter, seed size, colour of pods, pod appearance, shape and curvature, incidence of fibre, and yield (kg per ha). Variety R-1516 had the highest number of pods followed by R-1515 and Lexus in that order. There was no significant difference in yield, although all the varieties gave an acceptable higher yield compared to the local check (Amy). All the snap beans evaluated had acceptable green pod colour, pod length, pod diameter, pod appearance, seed size, and fibre content. All the introduced varieties can therefore be recommended for production since the pods met the export standards.