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Pisum Sativum L.) to Erysiphe Pisi DC

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Pisum Sativum L.) to Erysiphe Pisi DC a b c Field expression of quantitative resistance in pea (Pisum sativum L.) to Erysiphe pisi DC. a) cultivar Quantum in the foreground, susceptible cultivars Pania and Bolero in the background b) resistant Trounce (left) and susceptible Pania c) Quantum (left) and Trounce Tell me, and I'll forget. Show me, and I may not remember. Involve me, and I'll understand. Native American saying Expression and detection of quantitative resistance to Erysiphe pisi DC. in pea (Pisum sativum L.) A thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University S.L.H. Viljanen-Rollinson Lincoln University 1996 1 Abstract of a thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy Expression and detection of quantitative resistance to Erysiphe pisi DC. in pea (Pisum sativum L.) by S.L.H. Viljanen-Rollinson Characteristics of quantitative resistance in pea (Pisum sativum L.) to Erysiphe pisi DC, the pathogen causing powdery mildew, were investigated. Cultivars and seedlines of pea expressing quantitative resistance to E. pisi were identified and evaluated, by measuring the amounts of pathogen present on plant surfaces in field and glasshouse experiments. Disease severity on cv. Quantum was intermediate when compared with that on cv. Bolero (susceptible) and cv. Resal (resistant) in a field experiment. In glasshouse experiments, two groups of cultivars, one with a high degree ofresistance and the other with nil to low degrees of resistance to E. pisi, were identified. This indicated either that a different mechanism of resistance applied in the two groups, or that there has been no previous selection for intermediate resistance. Several other cultivars expressing quantitative resistance were identified in a field experiment. Quantitative resistance in Quantum did not affect germination of E. pisi conidia, but reduced infection efficiency of conidia on this cultivar compared with cv. Pania (susceptible). Other . epidemiological characteristics of quantitative resistance expression in Quantum relative to Pania were a 33% reduction in total conidium production and a 16% increase in time to maximum daily conidium production, both expressed on a colony area basis. In Bolero, the total conidium production was reduced relative to Pania, but the time to maximum spore production on a colony area basis was shorter. There were no differences between·the cultivars in pathogen colony size or numbers of haustoria produced by the pathogen. Electron microscope studies suggested that haustoria in Quantum plants were smaller and less lobed than those in Pania plants, and the surface area to volume ratios of the lobes and haustorial bodies were larger in Pania than in Quantum. The progress in time and spread in space of E. pisi was measured in field plots of cultivars Quantum, Pania and Bolero as disease severity (proportion of leaf area infected). Division of leaves (nodes) into three different age groups (young, medium, old) was necessary because of large variability in disease severity within plants. Disease severity on leaves at young nodes was less than 4% until the final assessment at 35 days after inoculation (dai). Exponential disease progress curves were fitted for leaves at medium nodes. Mean disease severity on medium nodes 12 dai was greatest (P<O.OOl) on ii Bolero and Pania (9.3 and 6.8% of leaf area infected respectively), and least on Quantum (1.6%). The mean disease relative growth rate was greatest (P<O.OOl) for Quantum, but was delayed compared to Pania and Bolero. Gompertz growth curves were fitted to disease progress data for leaves at old nodes. The asymptote was 78.2% of leaf area infected on Quantum, significantly lower (P<O.OOl) than on Bolero or Pania, which reached 100%. The point of inflection on Quantum occurred 22.8 dai, later (P<O.OOl) than on Pania (18.8 dai) and Bolero (18.3 dai), and the mean disease severity at the point of inflection was 28.8% for Quantum, less (P<0.00l) than on Pania (38.9%) or Bolero (38.5%). The average daily rates of increase in disease severity did not differ between the cultivars. Disease progress on Quantum was delayed compared with Pania and Bolero. Disease gradients from inoculum foci to 12 m were detected at early stages of the epidemic but the effects of background inoculum and the rate of disease progress were greater than the focus effect. Gradients flattened with time as the disease epidemic intensified, which was evident from the large isopathic rates 1 (between 2.2 and 4.0 m d- ). Some epidemiological variables expressed in controlled environments (low infection efficiency, low maximum daily spore production and long time to maximum spore production) that characterised quantitative resistance in Quantum were correlated with disease progress and spread in the field. These findings could be utilised in pea breeding programmes to identify parent lines from which quantitatively resistant progeny could be selected. Keywords: Colony size, conidium germination, conidium production, epidemiology, Erysiphe pisi DC., haustorial efficiency, image analysis, image processing, infection efficiency, pea, Pisum sativum L., powdery mildewiquantitative resistance, serial sections, spatial and temporal spread, transmission electron microscopy. iii Contents Abstract . .............................................................................. i Contents . .. iii List of Tables ........................................................................ yii List of Figures. .. viii List of Appendices .................................................................. xi Chapter 1 Introduction and literature review 1 1.1. General introduction ....................................................... 1 1.2. The host .................................................................. 2 1.3. The pathogen .............................................................. 2 1.3.1. Taxonomy ........................................................ 2 1.3.2. Host range ........................................................ 3 1.3.3. Disease impact on peas ............................................. 4 1.3.4. Life-cycle . .. 5 1.4. The infection process ....................................................... 1.5. Genetics ofhost-:pathogen interaction ........................................ 1.5.1. Fungal genetics .................................................... 1.5.2. Host genetics and inheritance of resistance in pea to E. pisi ............. 1.5.3. Genetics of host-pathogen interaction ............................... 1.6. Quantitative resistance ................... .' ................................ 1.6.1. Assessment of and selection for quantitative resistance ............... 1.6.2. Effects of plant and leaf age on quantitative resistance ................ 1.6.3. The effect of inoculum density on quantitative resistance ............. 1.6.4. The effects of light and temperature on quantitative resistance ........ 1.6.5. Structural aspects of quantitative resistance ......................... 1.7. Aims and objectives of the study ............................................ iv Chapter 2 Identification of quantitative resistance to Erysiphe pisi in cultivars and seedlines of peas .............................................................................. 20 2.1. Introduction .............................................................. 20 2.2. Materials and methods ..................................................... 21 2.2.1. Field experiment to confirm quantitative resistance in Quantum and the effect of inoculum pressure on disease development (Experiment 1) ......... 21 2.2.2. Glasshouse assessments to identify quantitative resistance in different seedlines (Experiments 2 and 3) . .. 22 2.2.3. Field experiment to characterise quantitative resistance in seedlines (Experiment 4) ............... .'................................... 24 2.3. Results.................................................................... 24 2.3.1. Experiment 1 ...................................................... 24 2.3.2. Experiment 2 . .. 25 2.3.3. Experiment 3 ...................................................... 25 2.3.4. Experiment 4 ..................................................... 25 2.4. Discussion ................................................................ 31 Chapter 3 Epidemiological basis of quantitative resistance in pea plants to Erysiphe pisi 33 3.1. Introduction ............................................................... 33 3.2. Materials and methods ..................................................... 36 3.2.1. Generai methods .................................................. 36 3.2.2. Germination of conidia (Experiments 1-6) ............................ 36 3.2.3. Infection efficiency (Experiments 7 -13) .............................. 37 3.2.4. Conidium production and latent period (Experiments 14 - 16) .......... 41 3.3. Results.................................................................... 43 3.3.1. Germination of conidia ............................................. 43 3.3.2. Infection efficiency ................................................. 44 3.3.3. Latent period and conidium production .............................. 45 3.4. Discussion ................................................................ 57 3.4.1. Conidium germination ............................................. 57 3.4.2. Infection efficiency. 58 3.4.3. Latent period ..................................................... 58 3.4.4. Conidium production
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