Influence of weather conditions on infection of peach fruit by Taphrina deformans

The effect of environment on the infection of peach fruit by Taphrina deformans was investigated using, orchard observations under natural de conditions (in 2001 to 2004) or in trees managed in such a way to exclude rainfall. These conditions were then validated using pot-grown peach plants exposed to single infection events and independent orchard observations. Leaf Curl incidence was related to rainfall, length of wet periods, and the temperature during wetness and during the incubation period, as well as to the developmental stage of flowers and fruit. Weather conditions before petal fall did not influence fruit infection. After petal fall, rainfall and the duration of the wet period triggered by rainfall played a key role in infection occurrence. The minimum rainfall required for infection was 12 mm, with at least 24 h of wetness interrupted by no more than 4 h. No infection Occurred when temperature was >= 17 degrees C during the wet period or >19 degrees C during incubation. Disease symptoms appeared on fruit, after approximately 3 weeks of incubation, which is equivalent to 240- to 290-degree-days (base 0 degrees C). The period for fruit infection was relatively short being from petal fall until air temperature remained greater than 16 degrees C. During this period, the incidence of fruit that developed symptoms was closely related to the number of favorable events and the total wetness duration during such events

Temperature and humidity requirements for germination and infection by ascospores of Pleospora alii, the teleomorph of Stemphylium vesicarium

Pleospora allii is the teleomorph of Stemphylium vesicarium, the causal agent of the brown spot disease on pear. The ability of P. allii ascospores to cause infection has not yet been demonstrated, and no information is available on environmental conditions favouring ascospore germination and infection. These ecological aspects were investigated by environment-controlled experiments. Dynamics of ascospore germination were observed between 0.5 and 48 hours of incubation at different temperatures (T 5 to 35\ub0C), in water or in dry conditions, with relative humidity (RH) between 100 and 67%. Maximum germination occurred after 48 hours of incubation in water at 21-23\ub0C; few ascospores germinated below 15\ub0C and at 30-35\ub0C. At 100% RH germination decreased by about one third and no germination was observed below 80%. Ascospores were inoculated on leaves of three pear varieties showing different susceptibility under orchard conditions (\u2018Abate F\ue9tel\u2019, \u2018Conference\u2019, and \u2018William\u2019 in decreasing order of susceptibility). Leaves were incubated at different T (5 to 35\ub0C), 100% RH, and observed daily for the appearance of necrotic spots. Ascospores caused higher infection on \u2018Abate F\ue8tel\u2019 than on \u2018Conference\u2019, while sporadic symptoms were observed on \u2018William\u2019. Highest disease incidence occurred at 25\ub0C

Equations for the distribution of Venturia inaequalis ascospores versus time during infection periods

Distribution of Venturia inaequalis ascospores versus time during an infection event was investigated by integrating in a dynamic simulation model the available knowledge on the biology of infection processes and the effect of environmental conditions. Processes of spore immigration on leaf surface, germination, appressorium formation, and successful infection establishment were incorporated into the model by elaborating mathematical equations depending on air temperature and length of the wet period. Survival of spores belonging to different development stages (ungerminated, germinated, with appressorium) was also included in the model as a function of temperature, relative humidity, and duration of wetness interruption. Based on comparison with previously published data the architecture of the model and its algorithms can be considered accurate and robust. Nevertheless, validation of model simulations under orchard conditions will be necessary before its use in management decisions

Influence of air temperature on the release of ascospores of Venturia inaequalis

The influence of air temperature on the release pattern of Venturia inaequalis ascospores was studied by volumetric spore samplers in two spore sampling periods. In the first period (1991-1996; Passo Segni, Ferrara), 15 ascospore dispersal events were considered occurring in daylight, with high spore counts (168-5892 ascospores per m3 air per event), at an average temperature between 8.4 and 20.3\ub0C. Both the length of the ascospore release period and distribution of airborne spores over time were significantly influenced by temperature. A logistic regression model was used to fit the proportion of ascospores trapped from the orchard air as a function of time after the beginning of the discharge event and air temperature. The accuracy of this equation was tested against data collected in the second spore sampling period (1997-2000; Sala Bolognese, Bologna, and Castelfranco, Modena); 16 dispersal events were considered, triggered by rainfall that occurred both in the dark and in daylight, with low to high spore counts (29-458 ascospores per m3 air per event), at an average temperature between 2.8 and 14.3\ub0C. There was a general agreement between the proportion of ascospores trapped from the orchard air during these events and that estimated by using the logistic equation - in most cases, actual and estimated values showed a high coincidence. Statistical comparison showed a significant correlation (r = 0.93, P < 0.01) between observed and estimated data

Weather conditions triggering ascosporedischarge in Venturia pirina

A 5-year study (2002-2006) was carried out in two pear orchards in northern Italy, by trapping air-borne ascospores of Venturia pirina. Characteristics of 155 ascospore discharge events (hour of the day, duration, ascospore number) and weather conditions of the hours preceding the beginning of the discharge (WD, wetness duration; R, rainfall; Tw, temperature during wetness) were analysed with the aim of better defining environmental conditions favouring ascospore discharge. Ascospore discharge showed a diurnal periodicity, with the 92% of total spores trapped in daylight. Thirty-seven percent of ascospore discharge events were triggered by rainfall, 55% by leaf wetness, while 8% occurred under dry conditions. The probability of ascospore discharge to occur was calculated using a logistic regression procedure with a stepwise selection of the independent variables. The variable \u2018WD 7 Tw\u2019 was selected as the most influential, while Tw, WD and R were not. The logistic equation provides the probability of an ascospore discharge to occur based on the combination of wetness duration and average temperature during the wet period preceding the beginning of the ascospore discharge. Probability was higher than 0.5 when \u2018WD 7 Tw\u2019 was higher than 197.5\ub0C 7h, while it was 0.9 when \u2018WD 7 Tw\u2019 was 585\ub0C 7h. This result showed that ascospore discharge in V. pirina is mainly influenced by wetness and temperature, while in V. inaequalis it depends mainly on rainfall. Calculation of the infection periods for controlling pear scab should take account of this difference

Virulence of Stemphylium vesicarium isolates from pear and other host species

Brown spot, caused by Stemphylium vesicarium, is one the most important pear disease in Europe. The disease is caused by fungal strains producing host-specific toxins which are responsible for the disease symptoms on some pear varieties. It is known that there is a high degree of differentiation in host specificity among the different isolates of S. vesicarium. Pathogenicity and virulence of 78 S. vesicarium strains obtained from pear and other host species were studied by a leaf necrosis assay on 3 pear varieties showing different susceptibility to natural brown spot epidemics. The bioassay was performed using conidial suspensions and autoclaved fungal culture filtrates. Strains of S. vesicarium showed high variability for both progress of necrotic spot appearance and final disease incidence. Four virulence groups were defined using a multivariate data analysis. Group I included 49 strains from pear, which caused severe necrosis on all the varieties. Group II included only 5 strains isolated from pear which caused severe necrosis on \u2018Abate F\ue9tel\u2019 and \u2018Conference\u2019, as the strains of group I did, but symptoms on \u2018William\u2019 were very light. In group III there were 19 strains from pear which showed less severe symptoms on all the varieties. Finally, group IV was formed by the S. vesicarium strains isolated from asparagus, pea, and onion, as well as the un-inoculated test. These fungal strains showed only small sporadic necrosis at the end of incubation

Prediction of Xanthomonas harboricola pv. pruni infection on peaches

X. arboricola pv. pruni (Xap) is present on Prunus spp. in some European countries, and it is listed as an A2 quarantine pest by EPPO; its importance in Northern Italy has increased in the last decade. An empiric model predicting Xap infection has been developed in late \u201890s. Occurrence of the first seasonal infection was monitored in peach orchards of Romagna, in 1992 to 2008, and compared to model predictions: an infection was predicted when there were at least 3 successive rainy days, with temperature between 14 and 19\ub0C; symptom\u2019s onset was expected after one to four weeks of incubation. Xap symptoms appeared in 10 out of 17 years; first seasonal symptoms become visible between 19 May and 12 July. These infections were always correctly predicted by the model, with an average incubation period of three weeks. Five infection periods were predicted by the model that did not result in actual infection. In five years the disease did not appear at all. In four of these years the model did not predict infection all season long, while in one year it wrongly predicted two possible infection periods. Sensitivity, specificity and accuracy of the model showed that one would have somewhat more confidence in predictions of non-infections than in predictions of infections. In a practical use of the model, this would lead to some unjustified alarms