ALBERT

Notes: The root of the persistent weed, dandelion (Taraxacum officinale Weber), contains a predominant 18 kDa protein which undergoes small seasonal fluctuations in amount, increasing in the late autumn months, and declining in the spring. This protein has been purified and found to consist of two major isoforms of pI 5·56 and pI 5·49. A full-length cDNA has been obtained, coding for the pI 5·56 isoform, and the 156-amino-acid sequence deduced. The protein shows homologies in amino acid composition to several allergen and intracellular pathogenesis-related proteins. The deduced protein does not contain a signal peptide nor any known organelle-targeting sequences, and thus is likely to be cytosolic. Expression of the 18 kDa protein gene is exclusive to the roots and stem tissues; transcripts accumulate during the late autumn months, and decline in the early spring. Changes in the amount of protein in the root are much less. The mRNA for the 18 kDa protein is not present in the dry seed, but appears in the roots within 16–18 h from the start of imbibition, and is expressed constitutively thereafter. Although it is the predominant protein in dandelion roots, its properties are different from those commonly associated with vegetative storage proteins.

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: Published research concerning disease development often shows that the response of fungal development to temperature is non-linear. Unfortunately, this non-linear response has often been ignored when predicting fungal development under varying temperatures using non-linear models and when deriving fungal development models from data collected under fluctuating temperatures. In this paper, the magnitude of non-linear effects on fungal development is shown to depend on the types of non-linear models and on the extent of temperature fluctuations. A method is described, which has been used in other disciplines to fit non-linear models directly to varying temperatures. Hypothetical data were generated to demonstrate the usefulness of this method. With the underlying rate equation being non-linear, models derived from average temperatures underestimate the rates at intermediate temperatures; the greater the temperature fluctuation, the greater this underestimation.

Notes: Experiments were conducted to determine: (i) the effects of temperature and duration of continuous wet periods on the infection of pear seedlings by conidia of Venturia nashicola, the causal agent of pear scab; and (ii) the effects of the length and temperature of dry interrupting periods on the mortality of infecting conidia. Average number of scab lesions per leaf increased with increasing duration of wetness. Logistic models adequately described the change in the average number of scab lesions per leaf at 5, 10, 15, 20 and 25°C over the wetness duration. At 30°C, only a few lesions developed. Simple polynomial models satisfactorily described the relationship of the three logistic model parameters (maximum number of lesions, rate of appearance and the time to 50% of the maximum number of lesions) with temperature. The optimum temperature for infection was found to be approximately 20°C. The relationship between mortality and the length of a dry period interrupting an infection process can be satisfactorily described by an exponential model. The rate of mortality at 10, 16 and 22°C did not differ significantly, but was significantly less than that at 28°C.

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: Infected barks of chestnut blight cankers, caused by Cryphonectria parasitica, were collected from a naturally infected orchard and incubated at different temperatures. Cankers started to discharge ascospores about a week after incubation at 15–25°C; most ascospores were collected at 20 and 25°C. When incubated at 5, 10 or 30°C, only a few cankers released a small number of ascospores and only during the later stages of incubation. However, the rate of formation of perithecia was not affected by the incubation temperature. The number of airborne ascospores was monitored using a volumetric spore trap in a chestnut orchard during 1996 and 1997. In both years, the number of ascospores trapped daily varied greatly, but in general it increased sharply from March onwards, reached a peak in May, and then declined steeply. There was a significant correlation between daily counts of ascospores and air temperature. Time-series transfer function (TF) analysis showed a positive association of the daily number of ascospores with increasing temperature, rain events and wet/humid conditions. In general, values predicted by the TF model agreed well with the observed pattern. However, a multiple regression equation based on TF analysis failed to provide a satisfactory prediction of the daily number of ascospores.

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: Experiments were conducted to investigate the effects of temperature and relative humidity (RH) on the in vitro germination and viability of conidia of the apple brown rot fungus (Monilinia fructigena), and on colonization and sporulation on detached fruits by M. fructigena. Conidia only germinated under near-saturation humidity (≥ 97% RH) and the rate of germination initially increased with temperature to a maximum at ≈ 23–25°C and then decreased. Conidia germinated rapidly – more than 70% of viable conidia had germinated within 2 h at 20 and 25°C. The rate of colonization on detached fruits increased log-linearly with increasing temperature. Sporulation on detached fruits was not observed at 5 or 25°C; sporulation appeared to be unaffected by either temperature (10–20°C) or RH (45–98%) once infection was established. Detached conidia remained viable for a long period of time, up to 20 days, the longest assessment time in this study, depending on storage temperature (10 or 20°C) and RH (45 or 85%). Temperature appeared to be more important than RH in affecting conidial viability. Low temperature and high RH resulted in reduced loss of conidial viability. Storage at 10°C and 85% RH for up to 20 days appeared not to affect conidial viability. These results indicate that environmental conditions during the main UK growing seasons are unlikely to be limiting factors for the development of brown rot on apple.

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.