Macadamia is an important nut crop in Kenya grown mainly for export. It is an attractive cash crop especially among small-scale farmers due to its low requirement for external inputs. With new superior varieties, farmers replace old plantations with new varieties. However, old plantations still contain untapped genetic potential and hence the need for ex situ conservation. Since it is highly out-crossing, true-to-type clones are propagated through vegetative means. A study was carried out to evaluate the effectiveness of cuttage, graftage and direct and indirect regeneration tissue culture techniques across 39 accessions covering two Macadamia spp. (M. integrifolia and M. tetraphylla) and the natural (M. integrifolia x M. tetraphylla) hybrids. Results indicated that cuttings could be used, but took 9 to 15 months before whole plants could be obtained and also the success rate varied with genotype. All genotypes were amenable to grafting, but with varying success and the highest (100%) bud break was obtained in accessions related to M. integrifolia. Direct regeneration from nodal segments was achieved with up to 85% bud break and shoot multiplication of up to 12 shoots per explant, while callus induction was possible from medium to mature cotyledonary tissue in some accessions. Although these studies are still ongoing, it can be recommended that grafting of seedlings with the accessions to be conserved and planting them closely in a small field is an effective and cost-effective method of conserving the material for several years.

Cutflower production and marketing is partly challenged by lack of new, exciting and improved varieties to satisfy customer needs of a different flower. Ranunculus (Ranunculus asiaticus L.) is a bulbous plant that originated in the Mediterranean basin region where growth and development take place optimally in cool, moist winter season. The extent of dormancy breaking and growth of this flower in tropical conditions is not known. Consequently, the present research studied the effects of shade and gibberellic acid (GA₃) in breaking dormancy and reducing high light intensity and high temperature so as to assure optimal growth of R. asiaticus under tropical conditions. The fixed factors were: four GA₃ levels (0, 100, 500 and 1500 mg/L) and three shade intensities (0%, 40% and 80% shade). The experiment was laid out in split plots arranged in randomised complete block design, replicated three times and repeated in two seasons. Tuberous roots were soaked in the respective GA₃ concentrations for 10 minutes followed by planting under shaded plots. Data were recorded from sprouting through tuberous root formation stages. Season 1 (June to November 2006) was characterised by cool temperatures, whereas season 2 (December 2006 to April 2007) was characterised by warm temperatures. The 80% shade promoted significantly longer stems. Nevertheless, early flowering, highest quality flower stem diameter, flower bud initiation, flower head diameter and tuberous root fresh weight resulted for 40% shade. These quality parameters were highest in tuberous roots that had been soaked in 100 mg/L GA₃ pre-plant. Therefore, growers in tropical high altitudes should use 80%, 40% and 0% shade in hot, warm and cool seasons, respectively. Pre-soaking of the tuberous roots in 100 to 500 mg/L GA₃ is optional, depending on the desired growth magnitude and speed desired.

Negative growth responses have commonly been associated with sole ammonium nutrition in most plant species. The results presented here suggest a lack of nitrate rather than sole ammonium nutrition as causal factor for inhibitory growth, and by extension relative growth rate (RGR). To test this hypothesis, tomato (Lycopersicon esculentum Mill.) variety Money Maker was cultured in Hoagland nutrient solution. Plants were grown under 24^oC/20^oC, 250 to 350 µEm^-2s^-1 light intensity, and 60% relative humidity. Nitrogen treatments consisted of ammonium/nitrate or nitrate/urea mixture. In either nitrate/urea or nitrate/ammonium treatments, nitrate concentrations were: 0, 10, 100 or 200 µM, 1.0, 1.5 or 2.0 mM and the other part was made of ammonium or urea as follows: 2 mM, 1990, 1900, 1800 µM, 1.0, 0.5, 0 mM. Prior to this, a minirhizotron experiment supplied with Arenosol with nitrogen supplied as nitrate or ammonium (stabilized with DIDIN as nitrification inhibitor) was conducted. In minirhizotrons, ammonium-treated plants had low rhizosphere pH with poor root elongation and poor plant growth, while nitrate treatment led to rhizosphere alkalinization and better plant growth. In the nutrient solution, the inhibitory effects observed with sole ammonium were alleviated by addition of nitrate. Similar trends were replicated with urea in terms of RGR and shoot growth. The SPAD (measure of chlorophyll content) increased with nitrate concentration in ammonium/nitrate mixture, but a reversal of this was observed with urea/nitrate mixture. Other than being a nutrient and osmotica, nitrate may be important in promoting growth as a signal molecule, through phytohormonal (cytokinin and abscissic acid) transduction cascade. In present experiment, nitrate concentration within 100-200 µM domain, although apparently low, could already illicit RGR and SPAD reading comparable to 2 mM. This has practical relevance in situation where ammonium is the main source of nitrogen in the market.

Citrus production in Kenya comprises sweet oranges, lemons, limes, tangerines, grapefruits and pummelo, occupying about 10,000 ha. Despite this large hectarage, Phytophthora foot rot (gummosis) occurs frequently leading to considerable losses in major citrus growing areas of the country. A countrywide survey to determine the incidence and distribution of citrus gummosis was conducted between October 2007 and February 2008 in major growing areas of Kenya. Ninety citrus orchards in nine agroecological zones (AEZs) covering nine districts were sampled for the presence of citrus gummosis. Diseased plants were sampled from each orchard and gummosis incidence and prevalence determined. Information on the varieties of citrus grown, farmers’ knowledge and perception of the disease, particularly its cause, spread, control, cropping system, husbandry practices, source of planting material was gathered using a semi-structured questionnaire. The disease was observed in 79% of the 90 orchards visited with high prevalence being observed in low altitude areas. The highest incidence was observed in Kilifi district, whose predominant AEZ was Coastal Lowland 3. There was a positive and significant correlation (r= +0.471) between disease incidence and temperature. Sixty-seven percent of the farmers interviewed did not recognize the disease and 77% did not know its cause. All citrus varieties grown by farmers were susceptible to gummosis. Thus, gummosis is widespread in Kenya and continues to gain importance as a constraint to citrus production. Concerted efforts should be directed towards educating farmers and extension personnel about the disease’s spread and management.

A field experiment was conducted over two cropping seasons to investigate the effectiveness of cereal border crops in managing potato aphids and the associated viruses during seed potato production. Potato plots were surrounded with maize, sorghum or wheat borders. Aphid population was monitored on leaves and on yellow sticky traps. Other data collected included viral disease incidence and tuber yield. The border crops reduced aphid population on leaves compared to non-bordered potato plots. More alate aphids were caught on yellow sticky traps placed inside potato plots than on traps placed inside the border crops. In addition, viral disease incidence was reduced in all plots surrounded by the border crops. However, plots surrounded by border crops had reduced tuber yield, although the yield of seed grade was increased. Thus use of border crops is beneficial in management of viral diseases during seed potato production.

Seven tomato (Lycopersicon sp.) accessions were evaluated for their resistance to the tomato red spider mite, Tetranychus evansi Baker and Pritchard, in the laboratory. Data recorded was: (i) trichome density and (ii) fecundity and longevity. Whole potted intact tomato plants were used for the olfactometer bioassays, while leaf disk sections were used for the glass bridge bioassays. There was a significant (P<0.05) negative correlation between fecundity and longevity with the density of trichome types I and IV. Significantly more females chose the direction with ‘Money Maker’ (susceptible accession) when compared to other accessions. Thus, trichome density and plant-emitted volatiles seem to play an important role in host-plant selection of spider mites.

The onion, Allium cepa L., is an important local market vegetable in Kenya. Production of the crop does not meet the demand due to several constraints including weeds. Hand-weeding, although strenuous and labour intensive, is the sole weed control method used by onion growers in Kenya. Two field trials were carried out at the Kenya Agricultural Research Institute, Thika to compare efficacy and profitability of weed management options in onion. The experiments were laid in randomized complete blocks with seven treatments replicated three times. The treatments included black polythene mulch during transplanting, grass mulch placed two weeks after transplanting, hand-weeding fortnightly, hand-weeding monthly, pre-emergent herbicide (Linuron) application, herbicide and hand-weeding after eight weeks and the control (un-weeded). Weeds significantly (P<0.05) reduced onion plant weight by 69.53% during the first trial and 86.74% in the second trial and final bulb yield by 89.45% and 95.13% in the first and second trials, respectively. The most common weeds were blue couch grass (Digitaria abyssinica), bristly foxtail (Setaria verticullata), goat weed (Ageratum conyzoides), black jack (Bidens pilosa) and the double thorn (Oxygonium sinuatum). Onion plants from the un-weeded plots grew thin and tall due to competition with weeds for nutrients and sunlight. Black polythene mulch resulted in the highest bulb yield (46,527 kg/ha) in the first season, while grass mulch resulted in the highest (65,700 kg/ha) in the second season. Black polythene mulch or grass mulch gave the best weed control, the highest plant weight and onion bulb yield, resulting in the highest economic benefits in the first and second seasons. Mulching is therefore considered as a cost-effective weed control alternative to hand-weeding of onion. Further investigations are recommended to establish the other benefits of mulching besides weed control.

Tomato is attacked by several plant parasitic nematodes, but root knot nematodes are the most devastating and cause considerable yield losses in Kenya. Studies were undertaken under greenhouse and field conditions to determine the suppressive effect of a wide range of plant species to root knot nematodes (Meloidogyne spp.). Potted plants were inoculated with 6000 eggs and/or juveniles, while the field experiments were conducted in nematode-infested fields. Among the plants tested, Tagetes patula, Tagetes minuta, Gossypium hirsutum, Desmodium uncinatum, Chloris gayana, Zea mays, Alstroemeria sp., Capsicum annum, Crotalaria juncea, Arachis hypogaea, Sorghum bicolor, Tithonia diversifolia and Pennisetum purpureum were rated as poor hosts with galling and egg mass indices ranging from 0 to 3. High galling and egg mass indices, ranging from 7-9, resulted on Lablab purpureus, Coriandum, sativum, Statice sp., Brassica oleracea var. capitata, Helianthus annuus, Vigna subterranean, while Muguna pruriens, Lactuca sativa, Allium ampeloprasum, Sesamum indicum, Allium cepa, Onnis sp., Brassica campestris spp. chinensis, Asparagus officinalis, Brassica oleracea var. botrytis, Ornithogolum arabicum, Tuberose sp. and Chrysanthemum indicum, with galling and egg mass indices ranging from 3 to 6, were rated moderately resistant. Damage by nematodes was significantly (P<0.05) reduced in tomato planted after sweet corn or in sweet corn interplanted with Tagetes patula, Crotalaria juncea, Sorghum bicolor and Asparagus sp. in the field. The present study shows that despite the fact that Meloidogyne spp. has a wide host range, there is a wide range of economically important plants from which suitable candidates can be selected by farmers for use as rotation or interplant crops to manage the root knot nematodes.

Effects of temperature stress on gas exchange, chlorophyll fluorescence and antioxidative enzyme activity in detached tomato (Lycopersicon esculentum Mill. cultivar 9021) leaves were studied. Detached intermediate leaves were subjected to 15°C, 25°C and 35°C for 5 days. High temperature stress significantly reduced net photosynthetic rate. Maximal photochemical efficiency (Fv/Fm), quantum yield of photosystem II (Ф_PSII) reaction centres and efficiency of excitation capture of open PSII centre (Fv’/Fm’) were also reduced. High temperature caused a significant (P>0.05) increase in activities of ascorbate peroxidase (APX), superoxide dismutase (SOD) and guaiacol peroxidase (G-POD) and malonaldehyde (MDA) content, whereas no significant (P>0.05) difference in activities of catalase (CAT) resulted. Thus, high temperature (35°C) destroyed photosynthetic apparatus in detached tomato leaves through increased activities of SOD, APX and G-POD. Photorespiration acted as a protective mechanism at high temperatures, whereas cyclic electron transport probably contributed as an alternative electron sink under low temperatures. Leaves suffered general metabolic alterations and over-production of ROS, which triggered oxidative damage.

Systematic studies on negative pesticide impacts are scarce in developing countries including Kenya. This paper reports the current pesticide use and associated impacts in Kenya. As a proxy for assessing the impact, the Environmental Impact Quotient (EIQ) model was applied. Results showed that in vegetable production 62 products, comprising of 36 active ingredients are in use. Approximately 263 tons of pesticides were applied at an average rate of 0.82 kg/ha. About 35% of the volumes belong to the organophosphates, 25% carbamates, 22% pyrethroids, 7% tetranortriterpenoids, and 7% inorganics. Of the pesticides, 8% were classified as highly hazardous compounds by the World Health Organization, 25% as carcinogens, while 43% are said to be possible carcinogens. Approximately 60% of the pesticides quantities were indicated to be bad actor chemicals, 64% to be ground water contaminants, and 47% very harmful to beneficial insects. Calculated mean EIQ-values were 22, 6 and 82 for farm workers, consumers and the environment, respectively, with an overall average of 37. The present results indicate that the sub-sector potentially has environmental pesticide negative impacts. The EIQ field use rating, which is based on the percent active ingredient and application rate clearly demonstrate that some pesticides that pose fairly low threat can be chosen to manage pests.

A trial was carried out at Kabaru to develop agronomically optimum and economically appropriate application rates for nitrogen and phosphorus fertilizer. Factorial combinations of 0, 130, 230 and 280 kg/ha P₂O₅ applied as triple superphosphate at planting, and 0, 70, 140, and 180 kg/ha N applied as calcium ammonium nitrate for topdressing were tested. The experimental design used was randomised complete block design. Soil was collected and chemically analysed. Results showed 7.9 pH, 0.24% N, and 6.2 ppm P. The treatment effects were not significantly different at 5% level of significance. The highest pod yield of 8.1 t/ha and net benefit of K. sh. 112,127/ha was obtained with the application of 130 kg P₂O₅ + 70 kg/ha N. The farmers’ acceptable minimum rate of return (AMRR) of over 100% was obtained using the same rate hence the recommendation for farmers at this site for snowpeas is 130 kg/ha P₂O₅ and 70 kg/ha N.

Endophytes which are mutualistic symbiots living asymptomatically within plant tissues have been reported to have beneficial effects on plant growth. In bananas a wide diversity of endophytic fungi have been isolated from healthy plants, where Fusarium oxysporum has been documented as the dominant species. The effects of three endophytic F. oxysporum isolates (V5W2, Emb2.4o and Eny7.11o) originating from Uganda on the growth of tissue-cultured banana plants was evaluated under Kenyan conditions in screenhouse trials. Dessert banana cv. Giant Cavendish and cv. Grand Naine were inoculated with the endophytic isolates two months after weaning and growth evaluated for a period of six months. Plant responses to endophyte treatment was assessed on plant height, girth, number of functional leaves, length and width of the youngest leaf, and shoot and root weights. Improved growth of all parameters was observed for plants inoculated with endophytes when compared to the control plants. Although the differences were not significant, up to 11.3% increase in plant height, 8.3% increase in girth, 7.5% increase in shoot weight and 11.5% increase in root weight resulted for endophyte-inoculated plants when compared to the control plants. The study shows that endophytes may have potential to enhance growth of tissue-cultured banana plants.

A formal citrus insect pest survey was conducted in two citrus growing districts of Kenya, Bungoma and Machakos, to determine important insect pests of citrus and evaluate the pest control practices used by small-scale farmers. Observations were made on the insect pests and their damage, methods of pest control practiced, pesticide products used, sources of farming knowledge and how decisions to control the insect pests. Farmers identified important pests as aphids (Toxoptera citricidus Kirkaldy), psyllids (Trioza erytreae Del Guercio), citrus black flies (Aleurocanthus woglumi Ashby), false codling moths (Cryptophlebia leucotreta), soft green scales (Coccus viridis Green), citrus woolly whiteflies (Aleurothrixus flocossus Maskell), mites (Phyllocoptruta oleivora McGregor), fruit flies (Ceratitis spp), leaf miners (Phyllocnitis citrella Stainton), and orange dogs (Papillio demodocus) in decreasing order of importance. Farmers’ management practices included indigenous traditional knowledge and mainly pesticides. Farmers mainly used their own experience and that of their neighbours to decide on what to use and when to deal with the insect pest situations. Current insect pest management practices by citrus farmers are inadequate to deal with insect pest and disease situations within farms. These findings have an implication in the spread and management of huanglongbing disease (HLB) and citrus tristeza vectored by psyllids and aphids, respectively.