Graduate Thesis Or Dissertation

The Primary Disease Gradient of Wheat Stripe Rust (Puccinia striiformis f. sp. tritici) Across Spatial Scales

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  • Wheat stripe rust (WSR), also called yellow rust of wheat (Tricitum spp.), causal agent Puccinia striiformis f.sp. tritici (Pst), is a foliar disease of major economic importance on wheat, especially grown in temperate locations. WSR causes major losses of wheat yield, estimated at nearly $ 1 billion per year, and has been found in upwards of sixty countries throughout the world, particularly in temperate regions. WSR, like many diseases of widely planted agricultural crops, has been managed largely by breeding resistant cultivars of wheat and by applying fungicide, but these strategies have resulted in periodic intensifications of WSR epidemics due to Pst mutations overcoming resistance or becoming resistant to fungicides. However, these strategies can be augmented to control WSR and other plant diseases by understanding the epidemiology, including the dispersal patterns of their causal pathogens, factors affecting the pathogen’s infection efficiency and reproduction rate, allowing managers to make better informed decisions. In this dissertation, I have examined the spread of WSR, bridging theoretical dispersal and disease ecology with plant epidemiology to provide analyses that have direct applicability to managing the spread of diseases caused by foliar wind-dispersed plant pathogens, as well as increase our understanding of basic biological processes. To investigate the dispersal gradient of Pst, I inoculated single wheat leaves with Pst urediniospores, and sampled all wheat leaves within two intersecting 0.3 m × 3.0 m transects in eight replicates over three years. The lesions observed on each of the top three leaves on plants within 1.5 m from the source lesion were three-dimensionally mapped. The total number of lesions within a 1.5 m radius was estimated by dividing the number of lesions observed within each 0.025 m-wide annulus by the fraction of the annulus sampled. The estimated total number of lesions produced within 1.5 m of a single source lesion ranged from 27 to 776 with a mean of 288 lesions. Eighty percent of the lesions were recorded within 0.69 m of the source infection. The proportion of total lesions observed at a given distance from the source was fit well by the Lomax and Weibull distributions, reflecting the large proportion of lesions arising close to the source, and when fit to a modified inverse-power distribution had a slope (b) of 2.5. There were more lesions produced on leaves higher in the canopy than on lower leaves, with more lesions being detected above than below the point of inoculation. Simultaneous measurement of lesion gradients and spore dispersal in the final year of the study suggests that this pattern is owing to greater susceptibility of upper leaves, rather than increased dispersal to upper leaves. In addition to dispersal of pathogen propagules, disease spread requires successful infection of host tissue. In plant disease epidemiology, susceptibility of host tissue is often assumed to be constant. This assumption ignores changes in host phenology due to developmental stage. To examine relative susceptibility of wheat leaves of different ages and leaf positions, 3-, 4-, and 5- wk old wheat plants were inoculated with equal quantities of urediniospores of Puccinia striiformis f.sp. tritici, the causal agent of wheat stripe rust (WSR). Disease severity on each leaf was assessed and fit by mixed effect linear model as function of leaf position and plant age. Younger plants had significantly greater disease severity than older plants, with mean severities of 50.4%, 30.1%, and 12.9% on plants that were 3 wk, 4 wk, and 5 wk old at time of inoculation, respectively. This effect was greater on leaves higher on the plant. Within same-aged plants, younger leaves had significantly greater disease severity than older leaves, with mean severities of 40.2%, 34.8%, and 17.7% on the uppermost, second, and third leaf, respectively. These results corroborate field data suggesting the vertical distribution of lesions was due more to differences in host susceptibility than to propagule dispersal. The results of the primary dispersal gradient of Pst study were contextualized with previous studies of dispersal of cereal rusts by investigating their dispersal across spatial scales. This was accomplished by combining the local dispersal dataset with a previous Pst dispersal dataset across agricultural fields out to 91.4 m. These datasets were normalized by the mean number of infections observed per plant at 0.914 m from the center of the disease focus after a single generation of spread. These datasets were well-fit by a single modified inverse power function, 𝑦=1.17∗(𝑥+0.10 𝑚)⁻²·²⁴. This function was then used to combine the local and fieldwide datasets with a primary dispersal dataset of Puccinia graminis f.sp. tritici (Pgt) out to 10.62 km, by predicting the number of lesions per plant at 2730 m from the disease source, to which all observations in the Pgt regional dataset were normalized, which was well fit by 𝑦=1.26∗(𝑥+0.14 𝑚)⁻²·³⁹. I created a susceptible-latent-infectious-removed (SLIR) compartmental time-step model to assess disease spread over time across spatial scales. The modified inverse-power function fit to the combined dispersal data across all three spatial scales was used to seed the dispersal kernel, comparing epidemics stemming from initial foci of 1.52 m with compartment sizes of 1.52 m and 0.025 m, and comparing epidemics stemming from initial foci of 152 m with compartments size of 152 m and 1.52 m. The resulting models produced very similar disease levels, as quantified by taking the area under the dispersal curve after seven generations of spread. In this dissertation, I have examined the spread Pst, as a model system to better understand the spread of diseases caused by aerially-dispersed pathogens. This research will do much to fill in gaps in the literature on empirical measures of pathogen dispersal and infection, and the theoretical ramifications of modelling disease spread incorporating multiple spatial scales. It is the aim of this research to be applicable to a wide range of systems exhibiting similar attributes, particularly in regards to dispersal, in addition to aiding our understanding of Pst and other cereal rusts.
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