ALBERT

Notes: The effects of temperature on the length of the incubation and latent periods of hawthorn powdery mildew, caused by Podosphaera clandestina, were studied. At constant temperatures over the range 10–28°C, the incubation period ranged from 5 to 14 days and the latent period from 5 to 16 days; no visible colonies had developed at 30°C after 15 days. The relationships between temperature and the rates of fungus development within the incubation and latent periods were well described by a nonlinear model. The resulting curves were asymmetrically bell-shaped with an optimum temperature of approximately 23°C. The lengths of the incubation and latent periods under fluctuating temperatures were also determined, and were used to evaluate the models developed from constant temperature experiments for their accuracy of prediction. The incubation and latent periods under fluctuating temperature regimes were predicted using a rate-summation scheme with a time step of 24 min, by integrating the respective incubation and latent rate functions obtained under constant temperatures. The predicted incubation or latent periods agreed well with the observed values. Under constant temperature the interval between the times when symptoms and sporulation on the same leaflet were first observed was very short, on average 〈1 day, and was not significantly correlated with temperature. However, this interval was negatively correlated with mean temperature under fluctuating regimes.

Notes: The spread of race-specific and -nonspecific fungal pathogens in cultivar mixtures over space and time was simulated using an individual-based, spatially explicit stochastic model. The spatial spread of disease was simulated using a half-Cauchy distribution. The effects of five simulation variables on the effectiveness of cultivar mixtures in reducing disease development were investigated. These simulation variables were the sporulation rate, the median spore dispersal distance, the probabilities of cross-infection among hosts and pathogen races, the proportion of host plants that were completely susceptible (or, in the case of race-specific pathogens, the numbers of mixture components) and the spatial arrangement of the mixture components. Disease dynamics were summarized by the rate parameters of logistic equations and by the area under the disease progress curve (AUDPC) of incidence and severity. The potential reduction in disease development in cultivar mixtures, compared with pure cultures, was considerable. Mixtures were more effective in reducing race-specific pathogens than race-nonspecific pathogens. For both types of pathogen, most variation in logit-transformed mixture efficacy was due to the main effects of the simulation variables. For race-nonspecific pathogens, the performance of mixtures was influenced mainly by the proportion of plants that were susceptible and by the spatial arrangement of the two mixture components. For race-specific pathogens, the performance of mixtures was determined mainly by the number and the spatial arrangement of mixture components. The smaller the homogeneous genotype area, the greater the mixture efficacy. Higher sporulation rate decreased mixture efficacy. Planting the mixture components in square blocks was more effective in reducing disease than planting in strips. For race-nonspecific pathogens, increasing the proportion of susceptible plants decreased the mixture efficacy. For race-specific pathogens, disease in mixtures decreased with increasing numbers of mixture components. The effect on the mixture efficacy of increasing cross-infection probability from 0 to 0.25 was generally small. For the AUDPC-based efficacy of disease severity, the effects of median spore dispersal distance were also very large: the shorter the median spore dispersal distance, the less effective the mixture.

Notes: The effects of constant and fluctuating temperatures on the incubation period of apple powdery mildew, caused by Podosphaera leucotricha, were studied. At constant temperatures, incubation periods ranged from 3 to 12 days over temperatures 8°C–30°C, and no visible lesions developed at 32°C. A nonlinear model was developed to describe the relationship between temperature and the rate of mildew colony development. The resulting curve is bell-shaped with an optimum temperature at about 23°C. When this model was used to predict mildew development under fluctuating temperatures at an integration step of 48 min, however, it consistently overestimated development rate for fluctuating periods with average temperatures higher than 20°C. A nonlinear model was also fitted directly to the fluctuating temperature data, thus taking into account the nonlinear effect. The overestimation of development rate by the constant model for high temperatures was confirmed when the two models were compared. This overestimation probably resulted from differences in the levels of relative humidity between constant and fluctuating temperature regimes. Possible practical use of the model is discussed.

Notes: Using a previously developed stochastic simulation model for plant disease epidemics, the relationship of the SADIE aggregation statistic Ia with initial epidemic conditions, spore dispersal distance, sampling quadrat size and other spatial statistics was investigated. Most variation in Ia was attributable to the initial spatial pattern of infected plants and sampling quadrat size. The importance of initial spatial pattern on SADIE clustering indices (for patches and gaps) was also demonstrated using a number of selected data sets. Correlation of Ia with clustering indices was close to 1·0. Epidemics arising from the regular and random initial patterns resulted in the smallest and greatest Ia values, respectively, at sampling times after disease spread had occurred. Furthermore, the variability in Ia between simulation runs also varied greatly with initial patterns, being lowest and greatest for the clumped and random initial patterns, respectively. Ia increased initially and then decreased with increasing incidence, especially for the clumped and random initial patterns. Overall, the effect of median spore dispersal distance on Ia was very small, especially for the random initial pattern. The correlation between Ia and intraclass correlation was generally small and varied greatly between initial patterns. However, there was a high positive correlation between Ia and a parameter describing the rate of decline of autocorrelation over spatial lags, indicating that Ia, clustering indices and autocorrelations measure some common properties of patterns.

Notes: Experiments were conducted to determine the effects of temperature, relative humidity (RH) and duration of wetness period on in vitro germination of conidia and infection of detached pear leaves by Venturia nashicola, the causal agent of pear scab. Conidia germinated only in near-saturation humidity (RH 〉 97%). The final percentage germination (24 h after inoculation) at 100% RH without free water was less than half that in free water. Conidia germinated over the range of temperatures tested (5–30°C); the optimum temperature for germination was ≈21°C. Changes in percentage germination of conidia over time were fitted by logistic models at each individual temperature. Polynomial models satisfactorily described the relationships between two (rate and time to 50% of maximum germination) of the three logistic model parameters and temperature. The minimum length of the wetness period for successful infection of detached pear leaves by conidia was observed at several temperatures. The shortest length of wetness period required for infection was 7 h at 22°C. Two polynomial models fitted well the relationship between the minimum wetness duration required for infection, and temperature.

Notes: The effects were investigated of fruit maturity and duration of wetness on infection of apple fruits by Venturia inaequalis, and subsequent scab development. Incubation rate (inverse of median incubation period) increased linearly with increasing temperature (5–20°C) on detached 5-week-old fruits of cv. Royal Gala. Fruits were highly susceptible in the early stages of development, but became increasingly resistant as they matured. Inoculation of attached 12-week-old and detached near-mature fruits did not result in any lesions, while inoculation of attached 4-, 5-, 7- and 9-week-old fruits resulted in various levels of infection. Fruits of cv. Mondial Gala were more susceptible than those of cv. Cox's Orange Pippin. On cv. Mondial Gala, a wet period of 9 h resulted in ≈ 90% infection of 4-week-old fruits, but only 9% infection of 9-week-old fruits. Numbers of scab lesions on an apple generally followed a Neyman type A rather than a Poisson distribution, indicating a certain degree of aggregation of lesions on a fruit. A two-parameter generalization of the Poisson model described the observed incidence–density relationship well. A longer duration of wetness was required to result in a similar level of scab infection on old fruits to that on young fruits. On cv. Mondial Gala, wet periods of 9 and 32 h were required for ≈ 90% incidence of fruit scab on 4- and 7-week-old fruits, respectively. A mathematical model was developed to relate the incidence of fruit scab to duration of wetness and fruit maturity. The potential use of these results in practical disease management is discussed.

Notes: Spatio-temporal development of brown rot (Monilinia fructigena) on apple and pear was monitored in an apple (cv. Cox) orchard and a pear orchard of several cultivars over several years. Disease on individual trees was recorded weekly from July to harvest, individual fruits with brown rot were tagged but not removed and rot-origin identified. On apple cv. Cox and pear (cvs Conference and Comice), all primary rot arose from infection via wounds caused by insects, birds and growth cracks. Birds were the most important wounding agents on pear in the field. Secondary (fruit-to-fruit contact) rot was considerably less than primary rot, especially for pear. Incidence of disease (percentage of fruits with brown rot) increased gradually from late July up to harvest; the final disease incidence varied with seasons and cultivars, ranging from 1 to 11%. For pear, Comice had greater incidence than Conference. Significant aggregation of diseased fruits among trees was detected for assessment dates when the overall incidence of disease was greater than 0·5%. On Cox and Conference, significant correlation of disease incidence between adjacent trees or trees separated by one or more trees (i.e. spatial lag measured as units of distance between adjacent trees) was detected, but there was no clear relationship between the correlation, the distance or time. For Comice, there was consistent and significant positive correlation of brown rot incidence over 3 years. It is speculated that behavioural characteristics of wounding agents may have played an important role in influencing the spatio-temporal dynamics of brown rot on apple and pear.

Notes: In each of the five years 1969 and 1971-1974 inclusive a volumetric spore trap was used in an apple orchard to monitor changes in the number of airborne conidia of Podosphaera leucotricha, the causal agent of apple powdery mildew. The number of trapped conidia varied greatly between years. Time-series analyses, using autoregressive integrated moving average (ARIMA) models, revealed that the temporal pattern of the number of airborne conidia was similar in all years, generally following a diurnal pattern with an afternoon peak. A strong correlation between consecutive hourly counts indicated that the number of trapped conidia depended on the strength of sporulating sources. Using the time-series transfer function (TF) method, it was shown that in each year the most important weather variables influencing the number of airborne conidia were vapour pressure deficit (VPD) and rainfall. Variation between years in the dynamic effects of these variables on conidium numbers was detected, and may reflect weather differences between years. Stepwise regression analysis was applied to the combined daily data for 1973 and 1974 using a subset of weather variables as independent variables, chosen on the basis of TF analysis. A resulting regression model accurately predicted the temporal pattern of conidium numbers (expressed as a percentage of the maximum daily number trapped in the same year) in both years. When this model was applied to the other three years there was good agreement between predicted and observed temporal patterns. Application of this regression model for practical disease control is discussed.

Notes: A new dynamic model of the infection of apple leaves by Venturia inaequalis is described. The model begins with the release of spores by rain and incorporates the effect of light on the discharge of ascospores from pseudothecia. The model then simulates infection through the sub-processes of germination, appressorium formation and penetration, separately for ascospores and conidia landed concurrently on wet leaves. The rate of the infection process is determined using different equations for ascospores and conidia. Spore mortality when leaves dry is determined by the stage of infection and RH in the dry period. The infection process is driven by surface wetness, temperature and RH. The progress of each infection period is measured as infection efficiency (IE), namely the percentage of landed spores which have penetrated and thereby infected leaves. The final IE quantifies the favourability of weather in each infection period. In orchard tests in each of three years, the new model detected crucial infection periods in spring and early summer which accounted for outbreaks of leaf scab. These periods were not detected by a static model based on Mills’criteria. The models performed similarly in detecting infection periods later in summer.