Effect Of Environmental Factors On Formation, Discharge And Germination Of Ascospores Of Venturia Nashicola

Pear scab, caused by Venturia nashicola Tanaka et Yamamoto, is an economically important disease on pear in China. Conidia are believed to be the main source of primary inoculum for pear scab. Environmental conditions which affected the formation of ascospore and role of ascospore in primary inoculum are unclear till now. Researches on role of ascospore in primary inoculum and effect of environmental factors on formation, discharge and germination of ascospore are important to improve veracity of forecasting and instruct control of pear scab. In 2003 to 2004, effects of environmental factors, includes temperature, moisture and light on formation, discharge and germination of ascospores of Venturia nashicola Tanaka et Yamamoto were studied, the results were reported in the paper.Leaves with pear scab lesions were collected from Laiyang and Zhaoyuan pear orchards before defoliation and were placed in different environments, such as in pear orchards, in polythene tunnel, beside of aqueduct el al, to induce the production of ascospore. The result showed that production of ascospores does not require special conditions and occur readily in common orchard of northern China in winter. The pear scab leaves collected from Laiyang, Zhaoyuan, Guan county orchards of Shandong province and Fufeng county orchard of Shanxi provice were examined. Results showed that more that 80% pear leaves with scab lesions over wintered in common orchard producted ascospore. Induced results in polythene suggested that rainfall during winter or spring was the key factor for pseudothecium production. Wetted environment, such as beside the aqueduct, accelerated leaf decay and the leaves producted only few pseudothecia. A plenty of pear leaves were observed with scab lesions that did not sporulated just before pear defoliateon, these lesions were called non-sporulated lesions. As the sporulated seabed lesions, non-sporulated lesions can also produced a large number of pseudothecia.Development progress of ascospores was monitored in pear leaves over wintered in the orchard of Laiyang agricultural college experimental station. Pseudothecia were initially observed in December, asciwere firstly examined in middle March. Ascospores began to mature in middle of April and large numbers of ascospores matured in the first ten days of May. Almost all ascospores had matured till late-May. Both ascospores and conidia of V. nashicola were caught with glass slide scrawled with Vaseline in the pear orchard. Ascospores began discharge from late-April when pear blossom. The discharge reached peak at mid-May. More than 70% ascospores discharged by late-May and diacharge lasted to late June. More ascospores were captured after raining.Effect of Nustar and urea on control of formation of ascospores of V. nashicola was studied. Nustar (8000 times) and urea (20 times) were sprayed to pear scab leaves before and after the leaves falling. The leaves were then collected and placed in orchard to intduce pseudothecia. Nustar and urea were all showed the significant control effect on formation of pseudothecia. Spraying urea after pear falling was the most effective treatment in the experiment and the control effect was 92%.Experiments were conducted under controlled environmental conditions to examine the effect of temperature, moisture and light on release and germination of ascospores of V. nashicola. The results revealed that a short time of wetness, 10 sec in present experiment, and 100% relative humidity stimulated the discharge of ascospores. Temperature didn't show the significant affection on release of ascospores. Only few ascospores discharged in dark and large number of ascospores discharged in light. Most pseudothicia started to release ascospores after 10 min being wetted. Ascospore discharge reached peak at 30 minutes and more than 80% ascospores were released within 60 minutes.Ascospore of V. nashicola germinated at temperatures ranging from 5 to 30°C with an optimum at about 21 °C. Germination dynamics of ascospores at each of six tested temperatures was satisfactorily described by a logistic model. Two polynomial models were developed to described the relationships between two (rate and elapse time to 50% of the maximum germination) of the three logistic model parameters and temperature.The morphology and the sizes of pseudothecia, asci and ascospores of V. nashicola on pear leaves sampled from Laiyang Chi-pear, Zhaoyuan Changba-pear and Guan county Ya-pear of Shandong province, and Fufeng Su-pear were examined under microscopes. The result revealed that the sizes of pseudothicia, asci and ascospore of V. nashicola from the four places were significantly different. V. nashicola on Laiyang Chi-pear and Zhaoyuan Changba-pear shared the similar size and the pathogen from the other place was signicicantly different from the above one. The inoculation with ascospore result showed thatincubation period of pear scab infected by ascospores was 20 days and the sympytom was more seriousthan that caused by conidia.