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• W2031246101 abstract "SEEM, R. C., and J. D. GILPATRICK. 1980. Incidence and severity relationships of secondary infections of powdery mildew on apple. Phytopathology 70:851-854. Severity (amount of leaf tissue affected by disease) can be determined relationship remained constant for data from different locations or from incidence (proportion of leaves diseased) for secondary infections of cultivars, but it varied according to different seasons. The relationship apple powdery mildew (caused by Podosphaera leucotricha) on apple remained relatively constant for leaves of different age although younger (Maluspumila) leaves. A simple relationship was derived in which /severity leaves developed the most disease. Mildew severity can be derived by was proportional to incidence. The equation was developed iteratively measuring incidence on the five to 10 youngest terminal leaves. For although a similar solution was arrived at by making the variance of quantitative severity assessment, adjustment must be made for differing incidence and severity data independent of their respective means. The apple cultivars and/or growing seasons. Additional key words: epidemiology, survey. The relationship between proportion of plant units diseased Idared cultivars on MM.106 rootstock and Cortland on M.7 (incidence) and the amount of plant tissue affected by disease rootstock; an orchard located at Geneva containing 50-yr-old (severity) is a valuable tool for disease assessment and McIntosh and Cortland cultivars on seedling rootstock, and an management. In most cases it is considerably easier to determine orchard located at Sodus, NY 10 km south of Lake Ontario the incidence of a disease rather than its severity; nevertheless, containing 50-yr-old Cortland on seedling rootstock. Mildew disease severity often is the preferred measurement because it evaluations were made in August of each year by collecting 15-25 describes the relative area of plant tissue infected. Although the vegetative terminals from each tree. Individual leaves represented area of infected tissue is the most commonly used measurement of the sampling unit and they were rated for the absence or presence of severity, other measurements, such as number of lesions, can be mildew and the number of secondary lesions when mildew was used, especially when lesions are relatively uniform in size. present. A maximum lesion count of 10 was given to heavily Attempts have been made to relate incidence to severity for coffee infected leaves. The data were reduced prior to analysis by rust (13) as well as powdery mildew and leaf rust of wheat (11). In summing the lesion counts for each leaf position on all terminals of both instances severity was considered to increase logarithmically a tree and dividing by the total number of leaves observed on the with arithmetically increasing incidence. James and Shih (11) were tree. Thus, the final data represented average number of lesions able to show that the relationships for wheat powdery mildew and (severity) and the proportion of infected leaves (incidence) for each leaf rust were constant over a large geographical area, but varied tree. A total of 319 trees were analyzed according to year, location, with differing seasons or leaf positions. Little is known about the and cultivar. incidence/severity (I/S) relationship in other diseases. The data also were analyzed according to leaf position on the Field assessment of secondary infection of apple (Maluspumila terminal. Because of varying numbers of leaves per terminal and to Miller) by powdery mildew (Podosphaera leucotricha [Elli. and simplify data analysis, the leaf positions were classified into five Ev.] Salm.) is difficult under New York growing conditions. In regions. Region I consisted of the five oldest leaves (positions 1-5); contrast with distinct lesion formation under prolonged humid region II to IV were progressively younger blocks of five leaves conditions in the orchard or greenhouse, secondary powdery each, while region V consisted of the youngest leaves on terminals mildew in the field typically occurs as an expansive and rather that had 2 1-28 positions (leaves). diffuse lesion that is often more apparent on the abaxial leaf Two methods of data analysis were utilized. Data were surface. A recommended method of assessment for fungicide empirically transformed to achieve the best linearity between efficacy involves determining both incidence and severity (10) incidence and severity as measured by ordinary least squares or although other methods are used (2,6,8). Barlow (3) and Butt and weighted least squares regression analyses. Tests for equal Souter (5) have suggested the use of incidence to predict severity, intercepts and parallel slopes were used to compare variation and a simplified procedure for field assessment on which to base within years, locations, cultivars, and leaf positions (12), and when management decisions has been developed (4). significantly different (P =0.01) the intercepts or slopes were The initial intent of this study was to determine a simplified separated according to a multiple comparison test for regression assessment procedure by which apple powdery mildew severity coefficients (P=0.01) (7). The second procedure transformed both could be predicted from incidence data. Further elucidation of the incidence and severity values to achieve independence between the I/S relationship became possible as the data were analyzed. data means and variance prior to regression analysis. This was done Preliminary results of this study have been reported previously because natural populations (ie, lesions or infected leaves) often are (14). not randomly distributed, and a method of transforming data to MATERIALS AND METHODS account for non-random cases was used (15). Data were collected from mildewicide evaluation orchards for RESULTS the years 1969 through 1975. There were three locations: an orchard located at Geneva, NY containing 10-yr-old Mclntosh and Severity, as measured in this study, was the average number of lesions per leaf per tree rather than the actual area of diseased 0031 -949X/80/090851 05/$03.00/0 tissue. Incidence was determined by the proportion of infected ©The American Phytopathological Society leaves per tree. Considering all years, locations, cultivars, and leaf Vol. 70, No. 9, 1980 851 positions, the I/S relationship was curvilinear and had a greater coefficients were highly significant. Similar equations were derived variance as incidence increased (Fig. I A). for data within each parameter set; year, location, cultivar, and leaf Empirical transformation. The simplest type transformation to position (Table 1). linearize the relationship was a power function: The I/S relationship varied according to the particular year (Table 1). The intercepts for 1969 and 1973 were significantly lower S' = b + cI (1) than those for 1970-1972 and 1975 while 1971 had a significantly in which S is severity, I is incidence, k is a real number, b is the lower slope compared to all other years except 1970. ThI intercept coefficient, and c is the slope coefficient. A best fit k of 0.49 relationship was not affected by orchard location or cultivar. Leaf was calculated by simple iteration based on test statistics of position on the terminal had a significant, though random, effect maximum coefficient 1-ratios, minimum standard deviation about on both the slope and Intercept coefficients. Due to the va the regression, and randomly distributed residuals. However, we sample size and coefficient variance, the intercept and slope of leaf chose to use k = 0.50 (ie, 5/) to simplify the relationship (Fig. I B). positions 1-5 was significantly different from positions 6-10 and We allowed an intercept term (b) in Eq. 1 because we could not 11-15, ahgh i ws t dffert from position s 16-2 and justify its exclusion on inductive grounds as did James and Shih 21-28. Fig. 2se and leaf poito on ( 11). The I/S relationship still had an increasing variance with incidnce ig. 2A) and severity increasing incidence. Because further transformation of the data Variance independence. Neither the incidence nor severity data did not make the variance more uniform, a weighting procedure (Fig. 3) were normally distributed. The incidence data tended to be was used to achieve the best regression equation (12). Residual uniformly distributed, but the severity data tended to be skewed to analyses indicated an appropriate weight was 1 / S; ie, the inverse of the left. Taylor (15) has suggested transforming data to a new the predicted severity value when using ordinary least squares. All further computation utilized the weighted least squares method. f(m) = m -(b/2) (2) The general regression equation based on all data was S50° 0.25 + 2.341 where b is a population statistic indicating the ramdomness of the data. The b is derived from the mean and variance according to: It has a standard deviation of S about the regression line of 0.224, the coefficient of determination (R ) was 92.3% and both (var) -am (3) or ln(var) = In a + b In m (4) 10 ' I I I e-in which a is a population constant. In this study m was the incidence or severity value for each tree and var was the variance 8 . * among leaf positions on the tree. According to Eq. 4 for incidence, 8-." @default.
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• W2031246101 title "Incidence and Severity Relationships of Secondary Infections of Powdery Mildew on Apple" @default.
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