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Pathogen Population Size PLP 6404 Epidemiology of Plant Diseases Spring 2015 Ariena van Bruggen Lecture 7: Influence of pathogen on disease development: vector-borne pathogens Various pathogens can be vector-borne, namely viruses, bacteria, mollicutes and some fungi. The vectors are primarily insects, but could sometimes be mites, nematodes, fungi or higher animals. The main insect vectors that transmit viruses, bacteria and mollicutes belong to the order Homoptera (aphids, whiteflies, psyllids, planthoppers, leafhoppers). While most viruses are transmitted by members of the Homoptera, some viruses are transmitted by thrips, mites, fungal- like organisms or nematodes. Spores of plant pathogenic fungi can be transmitted to flowers by thrips and various pollinators or to the bark and wood of trees by bark boring beetles. Pathogen characteristics that influence disease development: Pathogen population size (number of propagules) Vector relationships (specificity and persistence) Level of virulence, replication and movement inside plants Ecology (soilborne or foliar, survival) Pathogen population size Quantifying viruses and other pathogens in vectors For many plant virus diseases, the epidemiologically important propagule is often the vector. The same is true for phloem limited bacteria or mollicutes. Quantification of insect vectors: Landing and impaction traps Water pan trap Catch depends on trap size, color, height, and location Yellow sticky traps Rectangular plates or cylinders of variable size Better to catch Homoptera than water pans Can be positioned vertically Longer time intervals, greater catch Vertical net Good for live insects Not good for measuring insect density Labor intensive Best used in combination with another method 1 Suction traps Useful for measuring vector density Catch depends on both wind speed and vector behavior Requires a motor for fan - expensive Difficult to replicate due to expenses Determining proportion of infective insects Place insects on susceptible seedlings ELISA or PCR to test for pathogen Analyze insects in batches when the proportion of inoculative insects is low Identification and quantification of viruses in plants or vectors Real-time quantitative PCR Amplicon sequence non-specific methods: DNA-binding dye emits fluorescent light when bound to dsDNA fluorescence is proportional to the amount of total dsDNA in the reaction Example: SYBR Green I Amplicon sequence specific methods: based on use of oligonucleotide probes labeled with a donor fluorophore (reporter) at the 5' end and an acceptor dye (quencher) at the 3' end probe is cleaved during amplification, releasing the reporter, which gives off a fluorescent signal the increase in fluorescent signal is directly proportional to the number of amplicons generated Examples: TaqMan, Molecular beacons, and Scorpion PCR Vector relationships Specificity - specific transmission of a pathogen by a particular vector species - non-specific transmission of a pathogen by several vector species, like pollinators - non-specific transmission of more than one pathogen by a vector species like various viruses by some aphids Persistence - Non-persistent (stylet-borne pathogens, mostly viruses; can be acquired and inoculated after short feeding of seconds to minutes; for example viruses vectored by aphids or fungal spores vectored by pollinators) - Semi-persistent (acquisition of enough virus in minutes to days; persistence in the vector for 1-4 days; transmitted to plants in minutes to hours) 2 - Persistent and circulative (viruses, bacteria or mollicutes taken up through the mouth parts, moving through the intestinal wall, circulating in the vector’s body [insects or nematodes], and transmission to a plant host through the mouth parts or salivary gland; transmission can start only several hours after ingestion and can continue for many days) - Persistent and propagative (some circulative pathogens can multiply in the vector’s body, in the hemolymph of insect vectors, and can be transmitted to a plant via the salivary gland of the insect vector; transmission through the insect molts is common, and transovarial transmission to the next generation of insect vectors is sometimes possible; transmission through molts of nematode vectors is not possible) The type of vector relationship in terms of persistence in the vector has a profound effect on epidemic development over time. Virus disease dynamics has been modeled extensively. Plants change from being healthy to latently infected to symptomatic and infectious to being removed from the infectious category. Similarly, the vector goes from the healthy to latent and infective status (Chan and Jeger, 1994; Madden et al., 2000). State variables (boxes) and rate variables (arrows) for and rate variables (arrows) of a model for virus transmission by an insect vector (Madden et al., 2000). Modeled virus titer and symptom expression of cassava mosaic virus (persistently transmitted by whiteflies) during the latent period, the infectious period and the post-infectious period (‘removed’). Symptoms are visible long after the infectious period has started, suggesting that roguing might not be effective (Chan and Jeger, 1994). 3 Acquisition and inoculation of virus by an insect vector over time for non-persistent (NP), semi-persistent (SP), circulative persistent (CP) and propagative persistent (PP) transmission (Madden et al., 2000). Modeled disease incidence over time for a non-persistent (NP) semi-persistent (SP), circulative persistent (CP) and propagative persistent (PP) virus disease; the numbers refer to vector densities; the multiplication factor (R0) is the same (Madden et al., 2000). Level of virulence, replication and movement inside the vector and host plants There are five steps in the transmission of vector-borne pathogens: Acquisition of pathogen by vector Movement inside the vector (persistent pathogens) Multiplication in the vector (in case of propagative persistent transmission) Inoculation of pathogen into the host plant Movement and replication in the host plant Factors affecting transmission and epidemic development: Proportion of infected source plants Pathogen content per source plant Vector density Vector aggregation Vector movement (short- versus long-distance) - ‘crowd diseases’ like cocoa swollen shoot virus transmitted by mealy bugs - ‘vagile diseases’ like African cassava mosaic virus transmitted by whiteflies (Thresh, 1991) Presence of alternate hosts (e.g. weeds). 4 Once the pathogen is transmitted, epidemic development depends also on the multiplication rate of the pathogen in the host tissues and the movement of the pathogen inside the plant (local versus systemic). The multiplication rate of the vector itself can be affected by plant virus infection. Molting rate can be faster, vectors can be heavier and egg production greater on virus infected plants (Castle and Berger, 1993). Psyllids also develop faster and lay more eggs on HLB-infected citrus trees. Vectors can be more attracted to diseased than to healthy plants, indicating co-evolution (Daugherty et al., 2010). Proportion of aphids molted, adult weights, and nymphs per female on virus-free potato plants, plants infected with potato leaf roll virus , potato virus X and potato virus Y (Castle and Berger, 1993). Ecology Habitat - foliar vs. soilborne - affects movement of pathogen and likelihood of infection 5 - foliar pathogens are often distributed randomly in the field, if inoculative insects invade from outside the field; but patchy distribution can occur due to local secondary spread (for example by aphids) - soilborne pathogens are generally distributed in patches in the field but may be dispersed by redistribution of vectors throughout a field with irrigation water or equipment Survival, overwintering of vectors and infected plant (parts) - affects availability of initial inoculum for initiation of epidemics (Agrios, 2005) 6 Epidemic development Vector-borne diseases are generally polycyclic (several transmission cycles within one season), except for viruses that are transmitted by nematodes with long generation times such as Longidorus and Xiphinema, which have one or two generations per year (Pinkerton et al., 2008). The spatial spread depends on vector movement and aggregation. Aphids have two forms of adults: apterae (without wings local movement resulting in aggregation) and alatae (winged forms that can disperse over short and long distances).Vector movement is affected by wind direction. Aphids are carried further downwind than upwind from an in-field source. Vector-borne diseases often start at the edges of the field, especially if there are alternate hosts around the fields. Disease gradients are estimated from observed frequencies at different distance from the source plant. Because transmission and inoculation close to the source may not lead to new infections (in the case of systemic diseases), a transformation is often used to estimate the number of inoculations. This is called the ‘multiple infection transformation’; it results in higher numbers close to the source than the observed infections. Transformation: ln[1/(1-x)], where x is the proportion of diseased plants or diseased area. The effect of wind on the spatial spread of black currant reversion virus transmitted by aphids. Solid line: observed; dotted line: transformed by the multiple infection transformation (Thresh, 1976). Distribution of wheat streak mosaic virus transmitted by the wheat curl mite, as measured by a spectral reflectance
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