Sources and seasonal dynamics of inoculum for brown spot disease of pear

The dynamics of the production of Stemphylium vesicarium conidia and Pleospora allii ascospores from different inoculum sources on the ground were compared in a model system of a wildflower meadow mainly composed of yellow foxtail, creeping cinquefoil and white clover. The meadow was either inoculated (each October) or not inoculated with a virulent strain of S. vesicarium, and either covered or not covered with a litter of inoculated pear leaves. Spore traps positioned a few centimetres above the ground were exposed for 170 7-day periods between October 2003 and December 2006. Ascospores and conidia were trapped in 46 and 25% of samples, respectively. Ascospore numbers trapped from the pear leaf litter were about five times higher than those from the meadow, while conidial numbers were similar from the different inoculum sources. The ascosporic season was very long, with two main trapping periods: December–April, and August–October; the former was most important for the leaf litter, the latter for the meadow. The conidial season lasted from April to November, with 92% of conidia caught between July and September. The fungus persistently colonized the meadow: the meadow inoculated in early October 2003 produced spores until autumn 2006. The present work demonstrates that orchard ground is an important source of inoculum for brown spot of pear. Thus, it is important to reduce inoculum by managing the orchard ground all year long.     

稻瘟病菌在培养基上有性世代的形成（英文）

选用国际水稻研究所遗传室从水稻上分离的4个稻瘟病菌株,在室温下的燕麦培养基上培养5天,又在20℃连续光照下培养16天,在相对的两个交配型菌株的菌丝联结处产生黑色成熟的子囊壳。稻瘟病菌在培养基上有性世代的形成,为进一步理解梨孢菌属和水稻之间的遗传学以及选育稳定持久的抗病品种提出了新的见识。     

Ascospores of the Japanese pear scab fungus (<Emphasis Type="Italic">Venturia nashicola</Emphasis> Tanaka &; Yamamoto) are discharged during the day

We investigated the diurnal pattern of ascospore discharge of the Japanese pear scab fungus (Venturia nashicola Tanaka & Yamamoto) in an orchard. Ascospores of V. nashicola were mainly discharged during the day. Most ascospores were discharged from 7:00 to 19:00: 99.6% in 2001, 99.3% in 2002, and 93.8% in 2005. Because the ascospores were discharged only when the fallen diseased leaves were wet from precipitation, the wetness of these leaves is probably imperative for spore discharge. Ascospore discharge began immediately after precipitation in the daytime. When it rained at night, however, ascospore discharge did not begin until the following morning and never began immediately after precipitation. We also investigated other meteorological factors. When fallen diseased leaves were wet, the percentage of ascospore discharge was positively correlated with the amount of solar radiation and atmospheric temperature and negatively correlated with relative humidity. Ascospore discharge was interrupted by a decrease in solar radiation and atmospheric temperature and by increased relative humidity at night. This report is the first that V. nashicola discharges ascospores primarily during the day.     

Control of apothecia of Sclerotinia sclerotiorum by soil amendment with S-H mixture or Perlka in bean, canola and wheat fields

Ascospores of Sclerotinia sclerotiorum produced from apothecia are the primary source of inoculum for causing diseases such as white mold of common bean, pod rot of pea, stem blight of canola and head rot of sunflower and safflower in the Canadian prairies. A field study was conducted for 4 years to determine efficacy of control of production of apothecia from carpogenically germinated sclerotia of S. sclerotiorum by soil amendment with Perlka^® (calcium cyanamide) and S-H mixture (a formulated compound). Results of the 4-year experiments showed that amendment of soil with Perlka^® at low (30 g/m²) or high (60 g/m²) rate was effective in reducing carpogenic germination of sclerotia and production of apothecia under the canopy of host crops (common bean and canola) and a non-host crop (wheat). In the experiments of 1988, for example, the numbers of apothecia produced in the treatments of Perlka^®-low rate (30 g/m²), Perlka^®-high rate (60 g/m²) and untreated control were 42, 46, and 182 apothecia/plot (m²), respectively, for bean; 89, 42, and 318 apothecia/plot (m²), respectively, for canola; and 146, 143, and 412 apothecia/plot (m²), respectively, for wheat. However, soil amendment of S-H mixture at low (30 g/m²) or high (60 g/m²) rate was ineffective in reducing carpogenic germination of sclerotia and production of apothecia for all the 4 years of testing in all three crops. The ineffectiveness of S-H mixture and the practicality of Perlka^® for control of Sclerotinia diseases of crops grown under Canadian prairie conditions are discussed.     

Assessment of airborne primary inoculum availability and modelling of disease onset of ascochyta blight in field peas

Ascochyta blight is a serious disease affecting field peas. In France, disease management relies mainly on scheduled chemical applications without taking into account the actual disease risk. A better understanding of the factors affecting disease onset would therefore help in the timing of the first application. Field experiments involving eight sowing dates between mid-September and mid-December were conducted for two consecutive years. The seasonal dynamics of airborne inoculum were investigated through trap plants. The weekly availability of airborne primary inoculum was extremely low during autumn and winter and was partially influenced by mesoclimatic conditions. Disease onset occurred between mid-October and early March depending on the sowing date. Generally, the later the sowing date, the longer the period between sowing and disease onset. This was due to an increase in the period between sowing and emergence. Disease onset was observed 14–35 days after emergence. A disease onset model based on the calculation of weather-dependent daily infection values (DIVs) was established, assuming that disease onset occurs once the temperature and moisture requirements for incubation are met. Cumulative daily infection values (cDIVs) were determined by sowing date and experiment through addition of consecutive DIVs between emergence and disease onset. A frequency analysis of cDIVs was performed to determine the 10th and 90th percentiles of the distribution. An analysis of the observed and predicted values showed that observed disease onset dates were almost always included in the forecast window defined by these two percentiles. This study is the first attempt to predict ascochyta blight onset in field peas and should contribute to development of a more rational fungicide application strategy.