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Doctoral Degrees (Horticultural Science)

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Browsing Doctoral Degrees (Horticultural Science) by Author "Askew, Derek James."

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    A multi-factor study of cabbage production in the Umlaas River valley.
    (1995) Askew, Derek James.; Smith, Irwin Elcoate.; Le Roux, Fanie.; Wolstenholme, B. Nigel.
    A research and extension programme monitored 59 cabbage crops and many factors associated with cabbage production in the Umlaas River Valley of KwaZulu-Natal, from July 1991 - June 1993. A large data bank was developed, accumulating all information regarding soils, weather patterns, farming practices, management, crop water stress, yields, plant populations, weeds, pests, diseases, soil fertility, leaf nutrient concentrations and the presence of hormone herbicides in the rain. Analysis of this large data bank of information enabled the identification of limiting factors and optimized production practices. Excessively high or low levels of soil nutrients and soil acidity problems, reduced cabbage yields in the Umlaas River Valley. Identification of critical values and an ideal cation ratio in soils, and recommendations to rectify limiting factors, could drastically improve yields. Wide variation and excesses of soil nutrients were seen from soil analysis of 59 cabbage crops in the Umlaas River Valley. Soil P > 155, K > 486 and Mg > 568 mg 1(-1), exchangeable acidity> 0.11 cmol(c) 1(-1), acid saturation> 2 % and pH < 4.51 resulted in lower yields. Top yields were found between a certain range of soil nutrient levels and critical values lay within this range. Optimal yields resulted with an ideal ratio of Ca, Mg and K (65 :25: 10) in plots monitored. A soil acidity complex affected cabbage yields in the Umlaas River Valley. High exchangeable acidity, acid saturation and low pH increased Mn and Zn availability in the soil. Both Mn and Zn were absorbed in large quantities by the plant and maximum levels of 406 mg kg(-1) and 114 mg kg(-1) respectively were recorded in most recently mature (MRM) cabbage leaves at headform. Mn concentrations > 100 mg kg(-1) at headform and 50 mg kg(-1) at harvest reduced yields. Nutrient analysis results of a MRM leaf at headform, and a 1/8 head slice of a mature head at harvest, generated leaf nutrient norms and crop removal figures for cabbage. Adequate nutrient levels of MRM leaf at head form were: N = 3.3 - 4.8 %, P = 0.32 - 0.55 %, K = 2.1 - 4.2 %, Ca = 1.3 - 2.5 %, Mg = 0.25 - 0.65 %, Mn = 15 - 100 mg kg(-1), Zn = 18 - 60 mg kg(-1) and Cu = 4 - 100 mg kg(-1). Nutrient concentrations above or below adequate ranges resulted in lower yields. Nutrient removal figures were: 1.9 kg N, 0.3 kg P, 2.03 kg K, 0.43 kg Ca, 0.19 kg Mg, 2.8 g Mn, 2.6 g Zn and 0.4 g Cu t(-1) fresh material. These leaf nutrient norms were developed for reliable identification of soil fertility problems and the crop removals for improved fertilizer recommendations. A study of fertilization practices showed that most farmers supplied enough (+ 200kg) N to cabbage crops but used incorrect pre- and postplant proportions (1/4 and 3/4) for top yields. For maximum yields, 2/3 of the total N should be applied preplant and 1/3 at 4-6 weeks. Topdressings of N should be applied once only and not split as is common practise in the area. Application of P on cabbage crops was proportional to soil requirements but was generally excessive. Sufficient quantities of K were applied but scant attention was given to soil analysis results for K, Ca and Mg requirements. Input costs could be decreased and high yields maintained if soils were fertilized up to critical values and no further nutrients added when soil test values exceeded critical values. Fertilization was not generally based on results of soil analysis and few farmers limed their soils in spite of soil acidity problems. The more acidifying NH4 containing fertilisers were mostly used in preplant applications. The study recommended using more N0(3) based fertilisers to slow down acidification or, at least, corrective applications of lime. Good pest and disease control is essential for maximum yields in the Umlaas River Valley. This study identified optimum infection periods and maximum and minimum temperatures for infection by blackrot, club root , ringspot, damping-off, sclerotinia, downy mildew and alternaria leaf spot; and evaluated disease effects on yields. Clubroot, blackrot and damping-off significantly reduced cabbage yields. Clubroot incidence was generally associated with soil acidity problems, waterlogged lands or sandy soils subjected to slight over-irrigation. Blackrot was reduced by adequate N and K fertilization, increased with higher concentrations of Mn in most recently mature leaves (MRM) at head form (also associated with soil acidity) and always occurred with the warm, wet conditions of summer. Aphids and thrips occurred throughout the year; cutworm, bollworm, greater cabbage moth, leafminer and diamond-back moth, occurred mostly during the warmer season; and webworm over the cooler period in autumn. Aphids, thrips and webworm were the most important cabbage pests, causing serious losses especially during the first six weeks after transplanting. Factors such as amounts of fertilizer applied, soil acidity problems, slope of the land, infiltration rate, soil nutrient status, water stress, leaf concentrations of each element, weeds and disease were all associated with the incidence of various pests. This study highlighted the importance of good management practices, which affect all these aspects of cabbage production, and therefore pest incidence. An awareness of these factors and recommendations made in this study, could improve cabbage management practices in the Umlaas River Valley and therefore, reduce pest infestation. The study also evaluated relationships between weather patterns, hormone herbicide-like symptoms which occurred simultaneously on various garden trees and shrubs and vegetable seedlings, and pest and disease outbreaks or hormone herbicide symptoms on field crops of cabbage. Hot days with temperatures> 30°C, high VPD and high solar radiation followed by a rapid drop in temperature, high relative humidity and low solar radiation (stress weather cycles) were characteristic of weather conditions with an approaching frontal low in the Umlaas River Valley. All the dates of hormone herbicide-like symptoms on some garden shrubs and trees and vegetable seedlings in the nursery (indicator plants) were correlated with stress weather cycles, however not all stress weather cycles were correlated to these symptoms on indicator plants. It appeared that an unknown factor (Factor X) occurred with these weather cycles, and together with the stress induced by the harsh weather conditions, caused these symptoms on the indicator plants. A Canary creeper (Senecio tamoides DC.), fig tree (Ficus natalensis Hochst.) and an indigenous Cape chestnut (Calodendrum capense (L.F.) Thunb.) showed symptoms of leaf bubbling, twisting, burning, deformities and leaf drop. Lettuce and cabbage seedlings were twisted, etiolated and suffered from downy mildew outbreaks. All trees, shrubs and nursery crops experienced these symptoms 1 - 4 days after the stress weather cycles and Factor X. It is possible that low levels of hormone herbicide < 25 ng 1(-1)) deposited during dynamic fumigations (associated with approaching frontal lows), were Factor X. However, when examining the relationship between field cabbages and all related factors, no clear correlation could be established between weather patterns, Factor X, cabbage plant health, and pest and disease incidence. An overall study examined the effects of a range of factors on cabbage yield including: soil analysis data; management and crop water stress ratings; total nutrients supplied; weed, pest and disease indices; tillage operations; soil characteristics; plant populations; percentage marketable yield and headmass. Poor water management, high incidence of weeds, pest and disease, excessive nitrogen and phosphorus applied as fertilizer, and too many tillage operations, were associated with low yields. Conversely, good water management, good liming and soil fertility practices, higher levels of soil potassium and better weed, pest and disease control, resulted in higher yields. A greater percentage of marketable cabbage heads was more important than head mass for higher cabbage yields. The methodology used in this study provided an efficient, reliable method of identifying factors which limited and maximized vegetable crop production and provided data for the compilation of production guidelines for cabbage. The research and extension aspects of this study also provided an opportunity for the rapid transfer of technology to the farmer and excellent farmer participation.