<p>The epidemiology and damage effects of beet yellows virus (BYV) and beet mild yellowing virus (BMYV) were studied.<br/><em>Chapter 2</em> . The incubation period (time between infection and symptom expression) was determined so that progress curves of the disease (symptoms) could be translated into progress curves of the infection. The incubation period increased during the season from 3 (BYV) or 4 to 5 weeks (BMYV) in June to two months when plants were infected with either virus in August. The incubation period increased with plant development stage and lower temperature.<br/><em>Chapter</em> 3. Symptoms of systemic virus infection developed on the leaves that appeared after the inoculation. Older leaves (except those inoculated) remained healthy and green. In field experiments the infection date was retrospectively determined by calculating the appearance date of the oldest systemically-infected leaf.<br/><em>Chapter</em> 4. Theoretical analyses show that high infection percentages must be avoided in bait plant test for the determination of infection pressure. Otherwise the number of viruliferous vectors cannot be estimated accurately. Confidence intervals for the number of vectors are given as well as lower bounds when all plants have become infected.<br/><em>Chapter</em> 5. The extent of secondary spread of yellowing viruses was strongly affected by the date of primary infection. Inoculations before 15 June resulted in extensive secondary spread while negligible spread occurred in plots inoculated after this date. In early-inoculated plots spread started around 15 June when adjacent plants made leaf contact, so that the vector, <em>Myzus persicae</em> , <em></em> could disperse more readily. Little spread occurred in plots in which the number of <em>M. persicae</em> was reduced by coccinellids.<br/><em>Chapter</em> 6. Inoculations at the end of June in late-sown crops resulted in more extensive spread than inoculations in early-sown crops. The higher rate of spread in young crops was correlated with (1) a higher multiplication rate of <em>M. persicae</em> on young plants, (2) a better acceptance of young plants by <em>M.</em><em>persicae,</em> promoting virus transmission and (3) a shorter latency period (time between infection and possibility of virus acquisition).<br/><em>Chapter</em> 7. Damage by BYV resulted from (1) a smaller size of infected leaves. (2) reduced light absorption by yellow leaves, (3) reduced photosynthesis in yellow leaves and (4) increased respiration in infected leaves. Reduced photosynthesis was the most important damage component. Photosynthesis was almost completely inhibited in bright yellow, infected leaves while healthy leaves on infected plants or infected leaves without symptoms photosynthesized at normal rates. Yield loss decreases with later infection as the proportion of yellow leaves on the plants decreases.<br/>The results demonstrate that plant development stage plays a key role in vector population dynamics, virus spread, symptom development and damage. Thus, the benefit from pesticide applications for the control of virus spread depends on crop development stage. Therefore the development stage of the crop should be considered before control measures are taken.
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