Fungicidal control of metalaxyl-insensitive strains of Bremia lactucae on lettuce

Strains of B. lactucae insensitive to 0.01 g a.i. l⁻¹ metalaxyl have been detected in all the lettuce-growing districts of South Australia. Despite this, applications of a formulated mixture of 0.25 g a.i. l⁻¹ metalaxyl and 2 g a.i. l⁻¹ mancozeb every 7–10 days controlled downy mildew in field experiments, where similar applications of either metalaxyl or mancozeb alone were ineffective. Protectant schedules of either 0.3 g a.i. l⁻¹ dimethomorph or 2–2.4 g a.i. l⁻¹ phosphonic acid also controlled downy mildew, but were less effective than the metalaxyl/mancozeb formulation. Mixtures of either dimethomorph or phosphonic acid with mancozeb enabled the time between the sprays to be increased to 14 days without significantly reducing control. Fungicide applications increased marketable yield by 60% compared with that of the unsprayed plants. Drenching seedlings with 4 g a.i. l⁻¹ phosphonic acid controlled downy mildew for at least 14 days and should prevent the spread of the disease from plant production areas.     

Application of an emulsifiable mixture of 1,3-dichloropropene and chloropicrin against root knot nematodes and soilborne fungi for greenhouse tomatoes in Italy

A mixture of 1,3-dicloropropene 60.5% w/w and chloropicrin 33.3% w/w (Telone C35 EC) may be registered in Italy for soil drip fumigation. Five experiments on greenhouse tomatoes in Northern, Central and Southern Italy compared the effectiveness of this mixture in comparison with methyl bromide to find the optimum application rate in soils infested by Fusarium oxysporum f.sp. lycopersici, F. oxysporum f.sp. radicis lycopersici, Sclerotium rolfsii, Meloidogyne javanica and M. incognita. Its efficacy against F. oxysporum f.sp. radicis lycopersici and M. incognita was confirmed when applied to soils at 100, 200, 300 and 400 l ha⁻¹ (132.4, 268.4, 402.6 and 536.8 kg ha⁻¹) under gas-tight films with 15–45 mm of application water (900–1200 mg Telone C35 EC l⁻¹). In sandy soils, with slight F. radicis lycopersici infections and with heavy nematode (M. incognita) attacks, the mixture, drip applied at 900 mg l⁻¹ during late summer (fumigation: late summer; transplant: late-summer/autumn; last harvest: early spring), performed well up to 132.4 kg ha⁻¹ (100 l ha⁻¹). In sandy loam soils with slight F. radicis lycopersici infections and severe infections of F. lycopersici and galling nematodes (M. javanica), 268.4 kg ha⁻¹ (200 l ha⁻¹) of the mixture applied at 900 mg l⁻¹ as a drip provided yields similar to those of methyl bromide treated plots both in spring and summer cycles. In sandy loam soils, the diseases (F. lycopersici, F. radicis lycopersici) were controlled at rates 268.4 kg ha⁻¹ (containing 90 kg ha⁻¹ of chloropicrin), but the mixture was ineffective against Sclerotium rolfsii occasionally observed in sandy loam soils. In both sandy and sandy loam soils, no significant relationships were found between the rates of mixture applied (132.4, 268.4, 402.6 and 536.8 kg ha⁻¹) and the degree of nematode infestation.     

Fumigation of stonefruit with acetic acid to control postharvest decay

Acetic acid was an effective postharvest fumigant to destroy fungal spores on peaches, nectarines, apricots, and cherries. Decay by Monilinia fructicola and Rhizopus stolonifer on Harbrite peaches was prevented by as little as 1.4 or 2.7 mg l⁻¹ acetic acid, respectively. Harbrite peaches fumigated with 2.7 mg l⁻¹ acetic acid were slightly injured, the phytotoxicity indicated by light brown streaks. Higher concentrations of acetic acid increased injury; the streaks darkened and became much more pronounced. Glohaven peaches treated in the orchard with captan at 5% bloom, full bloom, ripening fruit, and 2 days before harvest then fumigated with 2.7 mg l⁻¹ acetic acid after harvest had significantly less postharvest brown rot (12.5%) than fruit treated with captan alone (25.0%). Decay of Lambert cherries, primarily due to Alternaria spp., was reduced from 38.9 to 10.0% by fumigation with 2.7 mg l⁻¹ acetic acid. Unfortunately, small pits developed in the fruit surface during storage at 1 °C. Brown rot (M. fructicola) of Tilton apricots was reduced from 100 to 25% by fumigation with 2.0 mg l⁻¹ acetic acid without signs of severe phytotoxicity.     

Cynodon dactylon control with tillage and glyphosate

Field experiments were conducted during two cropping seasons at two sites in Botswana to determine the efficacy of combining glyphosate and tillage in controlling C. dactylon. Treatments consisted of no-tillage, single and double ploughing during winter and spring plus the application of glyphosate at 0, 1.08, and 2.16 kg ha⁻¹ 3–4 weeks after the last ploughing. Visual assessment indicated higher levels of grass control from combining glyphosate and tillage than when these elements were applied separately. However, biomass data indicated that either double ploughing or glyphosate application controlled C. dactylon. A single ploughing followed by glyphosate at 2.16 kg ha⁻¹ after a regrowth of C. dactylon may provide an effective and affordable control method to small-scale farmers.     

Improving spring and summer fungicide treatments for winter barley

The response of winter barley to broad-spectrum fungicides applied at first node stage [growth stage (GS) 31] or as two-spray programmes [first node stage and flag leaf to ear emergence (GS 39–59)] was examined at 22 sites over five years during 1986–1990. Single sprays were also applied at an intermediate timing [third node to flag leaf emerging (GS 33–37)] at all sites in 1989 and 1990. The mean response to the single spray at GS 30–31 was 0.86 t ha⁻¹ but ranged from 0.25 to 3.03 t ha⁻¹ (4–80%). With the addition of a late spray (GS 39–59) of propiconazole plus tridemorph to the spring treatment, mean response was 1.25 t ha⁻¹ [range 0.35–3.32 t ha⁻¹ (5–87%)]. There were some differences between fungicides and fungicide mixtures applied at GS 31 and a prochloraz component was needed for effective control of eyespot. Seasonal differences in the mean response to the GS 39–59 treatment reflected differences in disease pressure after GS 59. The GS 30–31 treatment failed to maintain disease control up to the milky ripe stage (GS 75) in years when disease pressure was high. Yield responses were correlated with disease severity on leaf 2 at the time of the spring (GS 31) application and with the degree of control of eyespot on the stem base and of foliar diseases on the flag leaf at GS 75. A single spray applied at GS 33–37 gave comparable yields to the two-spray programmes and has potential to improve profitability where disease levels are low at the first node stage.     

Fungicidal disease suppression and yield losses associated with sunflower rust in Israel

The effects of rust on the yield components and harvested achene yield of a non-oilseed sunflower cultivar were examined in two field trials in Israel. Head area and the number of achenes per head were decreased during severe rust epidemics, but were unaffected by moderate or mild epidemics. Individual achene weight and the frequency of fertile achenes were influenced by the disease, in relation to its intensity. The fungicide tebuconazole, applied at a rate of 0.125 kg a.i. ha⁻¹, arrested rust development for at least 14 days. Highly significant effects were obtained by spraying when anthesis had occurred only in the outer quarter of the inflorescence radius, whereas the effects of spraying when anthesis was completed were negligible. Differences in yield (0.86–1.15 t ha⁻¹) and in net profit (US$696–1153 ha⁻¹) between sprayed and unsprayed plots were significant (p<0.05).     

Allelopathic potential of native plants against water hyacinth

Classical biocontrol of water hyacinth using insects in India is constrained by seasonal occurrence of water flow and interrupted host range. Use of fungal pathogens is also difficult due to lack of shelf-life and virulence under severe climatic fluctuations and a lack of knowledge of spray techniques. Hence, the allelopathic potential of native plants is reviewed for use as an alternative bio-control tactic. Dried powder of the leaves of Omavalli Coleus amboinicus L. at 40 g l⁻¹ as a water suspension killed water hyacinth within 24 h reducing the fresh weight by 80.72% and the dry weight by 75.63% within one week. The lowest dose required to kill the whole plant was 10 g l⁻¹. Coleus powder was injurious to cut leaves of water hyacinth even at 0.1 g l⁻¹ dose as it was absorbed directly by the cut leaf, indicating that the dose required to kill the whole plant could still be reduced, if either the natural product or the active principle is formulated for absorption through foliage.     

Possible alternative utilization of Cynara spp.: I. Biomass, grain yield and chemical composition of grain

Globe artichoke (Cynara scolymus L.) and cultivated cardoon (C. cardunculus L. var. altilis DC.) are horticulturally important crop plants. These species have potential as biomass and oilseed crops. We field tested, for 3 years, two artichoke and two cardoon cultivars and one wild cardoon (C. cardunculus L. var. sylvestris Lam.) population on the Sicilian plain of Catania (37°27′ N, 15°04′ E, 10 m a.s.l.). On a 3-year average, the dry aboveground biomass resulted about 31 t ha⁻¹ in both cultivated cardoons, 18.8 t ha⁻¹ in wild cardoon, 13.7 t ha⁻¹ in globe artichoke ‘3/10 V.S.’ and 9.9 t ha⁻¹ in globe artichoke ‘374’ F₁. The caloric values of aboveground biomass (except for seeds), which was not significantly different among genotypes, ranged between 16 005 and 17 028 KJ kg⁻¹ of dry matter. The cultivated cardoon ‘Gigante di Lucca’ had the greatest grain yield (on 3-year average, 2.6 t ha⁻¹), whereas the two globe artichokes had the lowest yield (on 3-year average, 0.5 t ha⁻¹). Regardless of genotypes and years, the grains contain 20.1% crude protein, 24.4% oil, 18.5% crude fiber and 4.1% ash (dry weight basis). The grains of globe artichokes showed the highest crude protein content (21.6%), whereas those of cardoons the highest oil content (25.2%).     

Responses of wheat and barley to liming on a sandy soil with subsoil acidity

A 3 years field trial examined the effect of newly and previously applied lime on the growth and yield of two near-isogenic wheat genotypes differing only in aluminium (Al) tolerance (Triticum aestivum L. Al-sensitive line ES8 and Al-tolerant line ET8), and barley (Hordeum vulgare cv. Mundak) on an acid soil (pH_CaCl2 4.6 in 0–10 cm and pH 4.1–4.3 in 10–40 cm) in the medium rainfall region of Western Australia. The trial consisted of four lime treatments: (i) no lime control; (ii) surface liming at 1.5 t ha⁻¹ in 1999; (iii) surface liming at 2.5 t ha⁻¹ in 1984; (iv) liming in 1984 and re-liming in 1999. Wheat crops were grown in 1999 and 2001, and barley was grown in 2000.

Liming in 1984 increased the pH in both topsoil and subsoil and decreased toxic Al in the subsoil. Liming in 1999 largely increased soil pH in the 0–10 cm in previously unlimed and limed plots, but only slightly increased the pH in 10–20 cm 2 years after application. In 1999, there was an overall 14% grain yield increase by growing ET8, mostly due to much better performance (41%) of ET8 over ES8 in the treatment with surface liming in 1999. In 2001, ET8 had yield 24% higher in the no lime control and 14% higher in the treatment with liming in 1999 compared with ES8. While both genotypes had similar root length density in the topsoil, root length density in acid subsoil was 22–160% higher for ET8 than for ES8. Wheat genotypes produced 23–24% higher yield due to the liming in 1984 compared to the no lime control. In 2000 season, shoot biomass of barley increased by 45–70% in the limed treatments compared with the no lime control. Liming at 2.5 t ha⁻¹ in 1984 or liming at 1.5 t ha⁻¹ in 1999 increased yield by 25%. Liming in 1984 and re-liming in 1999 increased the yield by over 50%. The results suggest that surface liming can ameliorate subsoil acidity as measured 15–17 years after application, and that growing an Al-tolerant crop in combination with surface liming provides a good strategy to combat subsoil acidity. The genotypic variation in response to liming appears to result from the difference in the sensitivity of root proliferation to low pH and high Al.     

Effects of saline irrigation water and heat waves on potato production in an arid environment

Production of spring potato (Solanum tuberosum L. cv. Désirée) on a deep sandy soil in the central highland of the Negev desert of Israel under drip irrigation with saline water (up to 6.2 dS m⁻¹) was studied in the years 1992–1997. The objective of the study was to determine the effects of saline water irrigation on potato production in an arid environment with special focus on the interactions with weather conditions. Although yields were often high, salinity effects were evident in some years. Thus 1992 and 1996 yields were 6–7 kg m⁻² and showed no significant effect of salinity, while a pronounced drop in yield with increasing salinity was observed in 1993 and 1994. Analysis of weather data for 1993–1994 suggests that the decline in yield was due to interactions between saline irrigation and prolonged heat wave events occurring during crop development. Further experimental work (1997) revealed that tuber yield was most sensitive to combined salt and heat stress when heat waves occurred at 40–60 days after emergence. The combined stress apparently leads to the collapse of mechanisms for avoiding salt accumulation in young expanding leaves, resulting in failure of vegetative growth recovery and a consequent reduction in the leaf area index and canopy functioning. The relationship between tuber sink demand and available photoassimilate supply at certain stages of plant development is discussed with reference to the ability of the potato plant to recover from the combined stress.     

Relationship between leaf injury and colonization of cabbage by Sclerotinia sclerotiorum

Mechanical and insect feeding injuries of cabbage provided sites for ingress by Sclerotinia sclerotiorum. In field studies, injuries sustained by hitting cabbage heads with a blunt object penetrated several leaf layers and were readily infected by S. sclerotiorum. Incidence of Sclerotinia rot ranged from 15 to 36% infected plants in the hitting treatment. Disease incidence was low (0–5%) when wounds were created by allowing lepidopterous larvae (the imported cabbageworm, Pieris rapae L., the diamondback moth, Plutella xylostella L., and the cabbage looper, Trichoplusia ni Hubner) to feed on cabbage leaves. The incidence of colonization of lepidopterous larvae feeding sites by S. sclerotiorum was greater in greenhouse studies (5–60% infected feeding sites) than in field studies. In greenhouse studies, disease incidence was significantly greater in plants receiving hitting injuries than tearing injuries. There was little evidence of inoculation day or injury day effects. Colonization frequency of tissue injured mechanically was similar whether wounds were inoculated immediately or 4 days later. Plants that were incubated in a dry environment developed fewer infections than plants held in a moist environment. Inoculated plants that were not injured did not develop disease.     

Leaf gas exchange, chloroplastic pigments and dry matter accumulation in castor bean (Ricinus communis L) seedlings subjected to salt stress conditions

Brazilian castor bean (Ricinus communis L) crops have expanded towards semiarid lands, in which soil salinity is an important factor limiting plant development. In order to evaluate the effects of salt stress on leaf gas exchange, pigments, and dry matter accumulation of seedlings, seeds of castor bean var. BRS Nordestina were planted in 15-L pots filled with 13 kg of soil (control) or with soil plus 2 g NaCl kg⁻¹ soil corresponding to 30 mM NaCl (salt treatment). Seedlings were grown under greenhouse conditions and the effects of salinity were assessed by measuring physiological parameters at 38 and 59 days after germination. Salt treatment induced decreases in leaf water potential at pre-dawn (ψ_pd, 42%), stomatal conductance to water vapor (g_s, 36%), and net carbon assimilation rate (A, 24%) only at 38 days after germination. At the same time, the values for transpiration rate were unchanged and the hydraulic conductance was increased (34%). After 59 days under hypersalinity, leaf ψ_pd increased to −0.44 MPa, although g_s, A, transpiration rate, and hydraulic conductance were not different between treatments. Salt stress effects on chlorophyll a, chlorophyll b and chlorophyll a + b contents were evident only on experimental day 59. The dry matter accumulation of leaves, roots and stems, as well as the total dry matter, and the root to above ground ratio increased with plant growth in both treatments, however, these parameters were lowered in salt-stressed seedlings. In short, all physiological variables were more drastically affected after 38 days under high salinity, and despite acclimatization of A after 59 days exposure to salt, no recovery of the dry matter accumulation was observed in the seedlings, suggesting that this species does not tolerate salt stress at its initial growth stages.     

Increased frequency of phytosanitary pod removal in cacao (Theobroma cacao) increases yield economically in eastern Peru

Diseases caused by Moniliophthora roreri (moniliasis), Crinipellis perniciosa (witches’ broom) and Phytophthora palmivora (black pod) are the most important factors limiting cocoa production in Peru and cultural management is considered to be the only practical means of control for the smallholder. The objective of this study was to develop an epidemiologically and economically sound control recommendation based on the frequency of phytosanitary pod removal. Weekly removal of diseased pods reduced the incidence of diseases significantly in comparison with fortnightly removal. In three fields, moniliasis was decreased by 26–41%, black pod by 35–66% and witches’ broom on pods by 14–57%. The cumulative effect was a consistent yield increase from an average of 504–660 kg ha⁻¹ yr⁻¹. Returns compensated for increased labour costs. Weekly pod removal was 32% more profitable. Time-course analyses indicated that weekly removal should be practised throughout the year under field conditions of eastern Peru.     

Performance of prolific and nonprolific maize hybrids under reduced-input and high-input cropping systems

Previous research indicated that prolific (multi-ear) maize (Zea mays L.) hybrids might perform better than nonprolific (single-ear) hybrids under lower-yielding environments. Field experiments were conducted during 1996–1999 to evaluate the agronomic responses of 10 maize hybrids differing in ear prolificacy under reduced-input and high-input cropping systems. Hybrids were of similar maturity (FAO 400) and divided into two prolificacy groups (prolific versus nonprolific), each consisting of five hybrids. The reduced-input system consisted of plowing at 20–22 cm; fertilization at 105, 104, and 104 kg ha⁻¹ N, P₂O₅, and K₂O; 37–38 000 plants ha⁻¹; and low input of herbicide. The high-input system involved plowing at 30–32 cm; fertilization at 213, 130, and 130 kg ha⁻¹ N, P₂O₅, and K₂O; 60–65 000 plants ha⁻¹; and high input of herbicides. Grain yields significantly decreased under reduced-input compared to high-input cropping system by an average of 26.1%. Significant cropping system×prolificacy group interactions were found for most yield components but not for grain yields. This indicated that both prolificacy groups exhibited a similar yield decrease under the reduced-input system even though prolific hybrids had 1.33 ears per plant compared to only 1.01 ears per plant of nonprolific types. All prolific hybrids responded similarly to various cropping input levels principally by means of changes in kernels per plant, whereas some nonprolific hybrids had greater response through 1000-kernel weight then kernels per plant. Prolific hybrids tended to achieve higher grain yields which averaged 10 414 kg ha⁻¹ compared to 9383 kg ha⁻¹ for nonprolific types partly due to less barren plants per hectare and primarily because of a higher grain weight per plant. Larger grain weights per plant of prolific hybrids were primary due to more kernels per plant in the reduced-input system, and a combined effect of more kernels and heavier 1000-kernel weight per plant in the high-input system. Improved kernel number per plant for prolific hybrids was associated with kernels from secondary ears. Although prolific hybrids outyielded nonprolific types, our findings failed to indicate that the prolificacy trait per se had any important effect on hybrid performance when grown under reduced-input compared to high-input cropping system.     

四倍体白掌新品种‘绿萌'的选育

白掌又名白鹤芋,原产于美洲和亚洲的热带地区,为天南星科白鹤芋属多年生草本植物,是重要的观花观叶盆栽植物,当前市场主栽品种基本源于国外,多为人工杂交产生的后代。利用人工诱变技术对植物体细胞染色体组进行加倍的多倍体育种方法已应用于观赏花卉新品种的选育,多倍体植株具有叶片增大、叶色加深、花朵变大、花色艳丽等优良性状,与二倍体比较,目标性状的观赏价值更高。利用化学诱变产生多倍体是目前人工多倍体育种中常用的诱导方法,其具有操作简易、突变率高、专一性强和适用性广等特点。秋水仙素是目前主要采用的多倍体化学诱变剂,其诱导的效果与诱导时间、使用的浓度、处理的植物种类和器官等因素有关。本研究以白掌‘美酒'的组培苗愈伤组织为材料,优化四倍体白掌诱导方法,经秋水仙素浸泡处理后,开展试管苗形态学初筛、田间种植优选及株系比较试验,同时进行染色体计数、气孔大小和基因组大小比较、四倍体白掌新品种选育。结果表明：最佳诱导四倍体白掌的方式为0.10%秋水仙素浸泡处理7 d,其四倍体诱导率最高;选育出的四倍体白掌新品种‘绿萌'在设施大棚种植时自然花期为4—6月,与对照‘美酒'相比,叶片长和宽、叶柄长和直径、叶鞘长、佛焰苞苞片长和宽、花梗长和直径、肉穗花序长和直径、气孔长度和宽度、气孔密度均差异极显著,根尖染色体数为2n=4X=60,是对照（2n=2X=30）的2倍。流式细胞仪测定DNA显示,‘绿萌'基因组大小为10.87 Gb,对照‘美酒'基因组大小为5.73 Gb,鉴定为四倍体白掌新品种。本研究利用人工化学诱变方法选育的四倍体白掌新品种‘绿萌',具有叶片增厚、叶色加深、茎杆变粗、叶柄变粗、花梗变粗、肉穗花序变粗等特点,大大提高了白掌的观赏价值。     

Comparing weed and crop seedling response to pre-emergence pendimethalin application in Corchorus olitorius and Abelmoschus esculentus

Field experiments and bioassay tests were carried out to evaluate the relative response of the crops, tossa jute (Corchorus olitorius L.) and okra [Abelmoschus esculentus (L.) Moench], and their associated weeds to pre-emergence application of pendimethalin at 0.33, 0.66, 0.99, 1.32 kg ai ha⁻¹ and a pendimethalin+atrazine tank mixture at 1.32+2.05 kg ai ha⁻¹. Bioassay tests were carried out using herbicide solutions of corresponding concentrations and herbicide-treated soil. Field application of pendimethalin at 0.66 kg ai ha⁻¹ and the herbicide mixture in both crops effectively controlled most seedling weeds including Rottboellia cochinchinensis. Euphorbia heterophylla and Calopogonium mucunoides, which persisted from 2 weeks after treatment (WAT) were not controlled. Herbicide application during crop establishment markedly inhibited the growth of both seedling weeds and crops. The mixture caused the highest weed and crop injury. Pendimethalin at 0.33 kg ai ha⁻¹ had minimal effect on these crops. Weed growth, weed tolerance of herbicide treatment and crop seedling injury were higher in tossa jute than that in okra, under the conditions of this study. The use of low pendimethalin doses in an integrated weed management system will ensure effective control of seedling weeds, and prevent crop injury and residue accumulation in edible plant produce.     

Effects of alfalfa leaf extracts and phenolic allelochemicals on early seedling growth and root morphology of alfalfa and barnyard grass

Alfalfa (Medicago sativa L.) plants contain water-soluble substances that are autotoxic as well as allelopathic to other plants. Laboratory experimentation through a petri dish assay with imbibed seeds was conducted to evaluate the phytotoxic effects of alfalfa cv. “Vernal” leaf extracts, coumarin, trans-cinnamic acid, o-coumaric acid, and hydro-cinnamic acid on the seedling growth and anatomical responses of 6 day-old alfalfa and barnyard grass. With increasing concentration, the alfalfa leaf extracts significantly reduced root lengths of alfalfa and barnyard grass (Echinochloa crus-galli, Beauv. var. oryzicola Ohwi). Coumarin, trans-cinnamic acid, o-coumaric acid, and hydro-cinnamic acid at 10⁻⁴ M significantly reduced root lengths of alfalfa and barnyard grass. The root systems, especially root tips of alfalfa, were stunted and swollen by the aqueous alfalfa leaf extracts at 30 g l⁻¹ and coumarin at 10⁻³ M. This research suggests that alfalfa plant extracts significantly affected root growth and morphological differentiation of susceptible plants, resulting in reduction of their biomass in the presence of either autotoxic or allelopathic compounds. The results may have value in enabling weed control based on natural plant extracts.     

Amelioration of dense sodic subsoil using organic amendments increases wheat yield more than using gypsum in a high rainfall zone of southern Australia

Subsoil constraints are major limiting factors in crop production in many soils of southern Australia. A field study examined effects of deep incorporation of organic and inorganic amendments in 30–40 cm on soil properties, plant growth and grain yield of wheat (Triticum aestivum var. Ambrook) on a Sodosol with dense sodic subsoil with or without lucerne history in a high rainfall region (long-term average annual rainfall 576 mm) of Victoria. Amendments were applied at a rate of 10–20 t ha⁻¹. Deep ripping alone and deep ripping with gypsum did not significantly affect grain yields. In comparison, application of organic materials doubled biomass production and increased grain yield by 1.7 times. Organic amendment-treated plots produced 60% more grains per area than the untreated control. The crop extracted over 50 mm extra water from below 40 cm soil in organic amendment-treated plots than the untreated control. Nitrogen uptake was almost doubled (403 kg ha⁻¹) in the organic amendment-treated plots than the untreated control (165 kg ha⁻¹). The improved yield with amendments was related to an increase in plant available water in the hostile subsoil, and prolonged greenness of leaves and supply of nitrogen and other nutrients.     

Comparative toxicity of chlorfluazuron (IKI-7899) and cypermethrin to Spodoptera littoralis, Lobesia botrana and Drosophila melanogaster

Chlorfluazuron sprayed on cotton seedlings was more efficient than cypermethrin against third-instar larvae of the Egyptian cotton leafworm, Spodoptera littoralis. At a concentration of 3·2 x 10⁻⁵% a.i., chlorfluazuron caused 98 ± 2% larval mortality and cypermethrin 35 ± 9%. All larvae in chlorfluazuron assays died during moulting; hence the leaf area consumed during the test instar was not appreciably affected. On the other hand, in cypermethrin assays, larvae died during the intermoult stage. Chlorfluazuron exhibited potency similar to that of cypermethrin on third-instar larvae of the grape berry moth Lobesia botrana, resulting in 90–95% and 33–34% mortality in treatment concentrations of 5 x 10⁻³% and 2 x 10⁻⁴% a.i., respectively. In another grapevine pest, Drosophila melanogaster, cypermethrin suppressed larval and adult formation to a greater extent than chlorfluazuron. The high potency of chlorfluazuron on various insects, together with its low toxicity to man and the environment, renders this compound a potential agent for controlling important agricultural pests.     

Rice yield and productivity gaps in irrigated systems of the forest zone of Côte d'Ivoire

Much of the rapidly growing demand for rice in West Africa will be met from increased production in irrigated lowlands, which cover about 12% of the regional rice-growing area. A large potential for expansion of irrigated areas exists particularly in the inland valleys of the humid forest zone. Current production is characterized by large variability in productivity, management practices and production constraints. Quantifying the variability in rice yield and identifying the determining factors are prerequisites to the development of site-specific recommendations and to improved targeting of technologies. Diagnostic on-farm trials were conducted on 64 irrigated lowland fields in the humid forest zone of southern Côte d'Ivoire, in 1995–1996. This was a part of the regional gradient study of irrigated systems from the desert margin to the humid forest zone. Cropping calendars, field operations and input use were monitored. Weed biomass, rice N uptake, and grain yield were determined in farmers' fields as well as in super-imposed, researcher-managed subplots (clean weeding, no N control, and mineral fertilizer N application). Rice yield potential was simulated by using the Oryza-S crop growth model. Yield losses were attributed to management factors based on performance of rice in researcher-managed subplots (management-related yield gap) and by multiple regression with management options. Grain yields varied between 0.2 and 7.3 Mg ha⁻¹ with mean yields of 3.2 in partially and 4.2 Mg ha⁻¹ in fully irrigated systems, 44% and 57% of the potential yield of 7.3 Mg ha⁻¹, respectively. Age of seedlings at transplanting, timeliness of operations and application of P fertilizer were correlated to yield and explained 60% of the observed variability. Grain yield was correlated with N uptake (r² = 0.93^***) but not with N application rate. Split application of mineral fertilizer N was associated with a 0.48 Mg ha⁻¹ yield increase (p = 0.002), regardless of the quantity applied. Additional weeding increased yield only in systems with imperfect irrigation. Weed biomass was reduced with improved water control and it increased with age of seedlings at transplanting, and was higher in direct-seeded than in transplanted rice. Echinochloa spp. were the most common weeds in fully irrigated systems and Panicum laxum was more common in the imperfectly irrigated fields. While improved water management was associated with substantial rice yield increases (1.16 Mg ha⁻¹), the timeliness of transplanting, weeding and N fertilization appears to be the key to increased rice yields in the forest zone of West Africa.