1/30/2015. Overview. Measuring host growth

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1 PLP 6404 Epidemiology of Plant Diseases Spring 2015 Lecture 8: Influence of Host Plant on Disease Development plant growth and Prof. Dr. Ariena van Bruggen Emerging Pathogens Institute and Plant Pathology Department, IFAS University of Florida at Gainesville Overview Plant growth characteristics that influence epidemic development Assessing host growth stage Measuring host growth Techniques for measuring leaf area Techniques for measuring root growth Time scale and plant growth models Types of host Effects of on epidemic development Measurement of host and components of Summary Why measure plant growth? Percent disease is relative to plant size No increase in % does not mean no absolute increase Changes to the host during the course of an epidemic: Production of new tissue Loss of host tissue May be especially important to consider host growth if: significant new growth occurs during the epidemic the ability of the pathogen to infect depends upon the type of host tissue or the age of the tissue. Plant growth characteristics that influence epidemic development growth or loss of foliage plant reproduction plant age and/or tissue age stage of development phenological stages annual versus perennial Assessing host growth stages - scales Measuring host growth Diagrams and scales, e.g. Zadoks scale for cereals plant height surface area of leaves or roots mass of fruits, or other organs, above-ground parts, or roots volume of fruits, organs, or roots, canopy volume leaf area index (LAI), percentage ground cover; the LAI is the amount of leaf area per unit of ground area. Therefore, a LAI of 4.0, would mean that for 1 m 2 of ground area, there would be 4 layers of leaves (with no vacant spaces) 1

2 Techniques for measuring leaf area Visual estimates; e.g. Diagrammatic keys based on photographs Dot-count methods Electronic techniques Leaf area meters Digital image analysis systems Spectral radiometers Related measures methods; e.g. regression models of area vs other (easier) measures Area as a function of wet or dry weights leaf area as a function of plant height leaf area calculated from leaf length, maximum leaf width, length width Leaf area index Leaf area index (LAI) = amount of leaf area/unit of soil area Direct measurement calculation of total leaf area divided by soil surface area Indirect measurement estimated from measured canopy light absorption α = 1 e -k (LAI) where α = proportion of incident light absorbed by canopy k = crop-specific growth coefficient Effects of LAI on epidemic development: high LAI humid microclimate, low LAI drier microclimate, more extreme temps Measuring root growth Measuring root growth Direct measurement accurate, but time-consuming Intersection method based on idea that the length of a set of any curved lines (or a root system) is directly proportional to the number of intersections between the roots and a random-direction straight line and to the surface area of the bottom of the container confining the root system Digital imaging methods Two line intersect methods Time scale Calendar time does not take into account variable environmental conditions that may affect plant growth and development Physiological time degree-days: DD = (T mean -T base ) Base temperature is temperature below which no growth occurs Example: let T base = 0 o C Day T mean DD Cum DD Plant growth models Two categories of plant growth models: Empirical models involves statistical fitting of a relatively simple model (2 or 3 parameters) similar to modeling disease progress curves requires frequent measures of plant growth Mechanistic models (physiological models) usually more complex models, based on physiological processes of plant growth usually developed as a computer simulation model, with linked submodels that describe individual processes 2

3 Types of host Types of host Convenient to think of host as belonging in one of two distinct categories: Qualitative Quantitative VanderPlank s categories of vertical and horizontal led to extensive discussion How to determine if there is vertical or horizontal? Test several cultivars and pathogen isolates; if there is differential interaction in ANOVA, then there is vertical If the isolates have a constant ranking on all cultivars, is horizontal Determine number of genes involved Many pathosystems have gene-for-gene interactions Found for: fungi, bacteria, viruses, insects and nematodes Even polygenic may be based on gene-for-gene interactions. Advantages and disadvantages of mono- or oligogenic Advantages easy to detect; easy to incorporate into a new variety; conditions a high degree of ; effects usually unaffected by environment. Disadvantages: New pathogen races may arise to attack this gene; The is effective against only one disease. Advantages and disadvantages of polygenic Advantages It is effective against all pathogen races It is sometimes effective against other diseases. Disadvantages Resistance is generally at a low level; Frequently it is controlled by many genes, making it difficult to incorporate (complicated inheritance); Resistance is frequently affected by environment; Resistance is sometimes difficult to detect. Effects of on epidemic development Host response based on disease or epidemic intensity measures: incidence at one or more times severity at one or more times epidemic development rate r, area-under-the-diseaseprogress-curve AUDPC, final disease severity Ymax Effects on epidemic: Differential reduces Y0, delays epidemic Quantitative reduces r Infection efficiency Varies with cultivars Negatively correlated with number of quantitative trait loci (QTL) Example: barley stripe rust Richardson et al.,

4 Latent period Incubation period Depends on cultivar Plant age and leaf position (from top) Example: early blight of potato, caused by Alternaria solani where pi=proportion of uredinia that appear on the ith day; ti=ith day after inoculation, n=number of days after inoculation when all uredinia have appeared Example: leaf rust Pelletier and Fry, 1989 Lehman and Shaner, 1996 Lesion size Spore production Depends on lesion density Some cultivars have low spore production even at low lesion density Example: leaf rust Varies with cultivars Negatively correlated with number of quantitative trait loci (QTL) Example: barley stripe rust Rimé, D. et al Richardson, K. L., et al Rate of lesion expansion Sporulation Many components are correlated Example: rice blast (Magnaporthe grisea) Infectious period Length of time spores are produced by one lesion or: Length of time spores are produced per leaf Example: grape powdery mildew (U. necator) 19C 26C 22C 30C Seebold et al.,

5 Summary Summary Why is it important to measure plant growth? How do you assess or measure plant growth? Foliage Roots How do you relate plant size to time? What is the effect of on epidemic development? And why? Vertical or differential Horizontal or quantitative How can you determine if is vertical or horizontal? Which components of can you distinguish? How do these components affect epidemic development? Do components of affect only horizontal? 5