PLP 6404 Epidemiology of Plant Diseases Spring 2015

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1 PLP 6404 Epidemiology of Plant Diseases Spring 2015 Ariena van Bruggen, modified from Katherine Stevenson Lecture 8: Influence of host on disease development - plant growth For researchers to communicate information on plant disease epidemics, they must be able to describe the details about the host. The primary source of reference for the influence of host growth is Campbell & Madden (1990), Chapter 3. Plant growth characteristics or processes of individual host plants or host populations that influence epidemic development: growth or loss of foliage plant reproduction plant age and/or tissue age stage of development annual versus perennial Importance and use of host information If we assess the intensity of disease at two different times and the proportion of attack is the same, the conclusion would be that the intensity of disease has not changed. However, if the host plant has doubled in size between the two assessments, than the amount of disease in absolute units has also doubled. Thus, the quality of this host information enables us to differentiate among epidemics and to interpret the responses. 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. Assessing host growth stage Since some pathogens attack only young tissue and other pathogens attack mature or senescing tissue, then the record of host growth or phenological stages are of great importance. Host growth stages = key times in the growth and reproduction of plants that have meaning in relation to host physiology or yield 1

2 The study of the relationship between the environment and periodic biological events such as growth stages is phenology (e.g. bud swell, flowering) Host growth assessment aids: keys or diagrams (can be pictorial) scales o For cereal grains: the Feekes Scale (range ); later expanded by Zadoks et al. in the Decimal Code Scale (range 0-100). o For soybeans: scale by Fehr and Caviness. o For peanuts: scale by Boote. o For tree fruits: bud, leaf, flower, and fruit stages. o For peas: decimal code scale by Knott. o For corn: 1-10 scale by P.T. Walker. The limitations of these methods (keys, diagrams, and scales) are that they are qualitative. 2

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4 Measuring host growth May be important to know exactly how much growth (or loss) has occurred in terms of changes in: 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) Techniques for measuring leaf area There are three (3) general approaches to measure leaf areas: visual estimates; e.g. diagrammatic keys based on photographs and dot-count methods electronic techniques related measures methods; e.g. regression models of area vs other (easier) measures Electronic techniques - measures leaf area present (not defoliation) leaf area meters Electronic leaf area meters are relatively expensive but very accurate (within 1 mm 2 ). The leaf samples are usually removed from the plants and passed through the leaf area meter in the laboratory digital image analysis systems With the use of a black and white video camera, the image of a leaf (or root) is analyzed with a digitizing board and software in a computer; the number of pixels covered by the leaf (or roots) are counted in relation to the total number of pixels in the field of view. spectral radiometers The leaf area of crop canopies in the field is estimated by the spectral reflectance taken with a multi-spectral scanner that is positioned several meters above the 4

5 Related measures methods canopy; this method depends on the contrast of the crop canopy to the background of soil. area as a function of wet or dry weights; however, ratio of leaf area to weight depends on species, growth in shady vs sunny environments etc. leaf area as a function of plant height leaf area calculated from leaf length, maximum leaf width, length width Examples: Winter wheat: significant relationship between leaf area and dry matter LA = x, where LA is the leaf area in cm 2 and x is dry matter in g. Leaf area may be estimated from leaf length and width (non-destructive). Leaf area index (LAI) = amount of leaf area/unit of soil area Direct calculation of total leaf area divided by soil surface area Indirect 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 Costs and comparisons of leaf area measurements Most appropriate technique will depend on: characteristics of the particular host plant experimental objectives It may be more efficient to take more samples using an indirect estimate than to determine leaf area precisely on fewer samples. Measuring root growth usually requires destructive sampling usually involves measuring root length per volume of soil or root area as a function of root volume (measured by displaced water in a container) 1. Direct measurement - accurate, but time-consuming 5

6 2. 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 The length of the root system is also inversely proportional to the total length of straight line used to determine the intersections. 6

7 Choice of time scale To characterize the growth of a crop over time, to relate the development of an epidemic to host growth, or to develop a plant growth model, frequent measurements of the crop need to be taken. 1. Calendar time does not take into account variable environmental conditions that may affect plant growth and development. 2. 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 curves and models if a detailed description of plant growth and development is required you can model any measure of plant growth as a function of time i.e., leaf area or total biomass Two categories of plant growth models: 1. Empirical models involves statistical fitting of a relatively simple model (2 or 3parameters) similar to modeling disease progress curves requires frequent measures of plant growth 2. 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 Literature cited: Campbell L. and Madden, L Introduction to Plant Disease Epidemiology. 7