法国向日葵种子中向日葵黑茎病菌的首次截获与检测

从法国进境向日葵种子中分离到3株疑似向日葵黑茎病的菌株,对所有菌株进行形态学、致病性测定和分子序列比对分析。分离菌菌落乳白色或象牙色至灰白色,有大量黑褐色分生孢子器产生,分生孢子器球形至扁球形,内含无色单胞、卵圆形分生孢子,有明显或不明显油球;针刺接种4片真叶向日葵幼苗的下胚轴,7～9d后茎部产生典型黑茎病黑色椭圆形病斑,病斑上着生黑色分生孢子器;菌丝DNA用ActF1/R1和ITS1/ITS4扩增,序列与NCBI基因库中P.macdonaldii序列相似性为98%～100%。形态学、分子生物学及致病性检测结果显示,截获的3株菌均为向日葵黑茎病菌。     

波兰、英国油菜种子样本的健康测试及黑胫病病原菌感染分析

对来自波兰及英国的2个相同品种的油菜种子样品进行了健康测试，并重点观察了油菜黑胫病病原菌(Letosphoeria maculans/Phoma lingam)感染的程度及菌系类型。结果表明来自2个国家的种子都一定程度受微生物感染，品种间差异不大，所检测到的较多的菌属为Alternaria,Fusarium及Penicillium。黑胫病病原菌感染的百分率波兰种子样品平均为0．5％，英国种子样品平均为0.6％，菌系鉴定表明，7．1％的波兰株系及12．3％的英国株系为强致病的Tox 类型。此外，还发现健康测试前种子表面消毒可提高黑胫病病原菌的检出率。     

The susceptibility of potato cultivars to gangrene in laboratory tests in relation to origin of tubers,damage, method of inoculation and test centre

Summary Undamaged and damaged tubers of 10 cultivars were inoculated withPhoma foveata by two methods at five test centres over three years. Damaging tubers before inoculation increased the discrimination between cultivars. While this increase was small with tubers rolled in cornmealsand cultures, it was often substantial with those dipped in a suspension of comminuted agar cultures. Correlations between centres for the four tests were significant (P<0.05) and were highest for tubers inoculated with cornmeal-sand with or without damage. The gangrene scores of the cultivars in the four tests were correlated (P<0.05) with their field susceptibility ratings but there was a strong interaction between cultivar reaction and year of test. When tubers grown in Cumbria and Midlothian were tested at one centre over two years, the gangrene scores were also correlated (P<0.05).     

Assessing the resistance to gangrene of progenies of potato (Solanum tuberosum L.) from parents differing in susceptibility

Summary The resistance to gangrene (caused byPhoma exigua var.foveata) of progenies from two hybridisation schedules involving cultivars resistant, susceptible, or of intermediate reaction to gangrene was assessed. Two methods of sampling tubers from field-grown single-plant plots of 18 clones representing each progeny were used: one tuber was taken from each plant, and a bulk sample of 18 tubers was selected at random from the whole progeny. Gangrene resistance was assessed after incubating tubers which had been rolled in cornmeal-sand inoculum ofP. exigua var.foveata, and was expressed as a percentage of tubers infected. Results from the two methods of sampling correlated closely, and agreed well with tests in a subsequent year on 10-tuber samples of each individual clone. It is concluded that an 18-clone sample adequately represented each progeny, and that the mid-parent gangrene score satisfactorily predicted the resistance of the resulting progeny. There are indications that susceptibility tended to be dominant. The implication of these findings to breeding for resistance to gangrene is discussed.     

Damage to potato tubers caused by fumigation with 2-aminobutane

Summary Sunken, irregular shaped lesions on the surface of tubers of Pentland Squire and to a lesser extent Pentland Dell, were associated with damaged tubers and with very high residue levels of 2-aminobutane. Severely affected tubers did not grow when chitted at 20°C and a considerable loss in weight occurred.     

Identification of a single dominant allele for resistance to blackleg in Brassica napus'Surpass 400'

The inheritance of resistance to blackleg (caused by Leptosphaeria maculans) was examined in the F1 and F2 of a cross between highly resistant canola ‘Surpass 400’ and susceptible ‘Westar’ in the field. Blackleg‐infected canola straw was collected from the field and scattered among plants to increase disease development with the aid of natural rainfall. Disease severity on seedlings was assessed as the average number of lesions on leaves 1 and 2, and on adult plants as the percentage necrosis on a cross‐section of stems immediately above the crown. All ‘Westar’ plants were susceptible (S) and all ‘Surpass 400’ and F1 plants were resistant (R) at both growth stages. Disease severity on F2 plants segregated 3 : 1 (R : S) as expected for a single dominant resistance allele in both the seedling and adult plant stages. There was a high proportion (91.1%) of matching reactions (R‐R and S‐S) between seedling and adult plants. ‘Surpass 400’ is the source of a single dominant allele for blackleg resistance in Brassica napus that is expressed strongly in both seedlings and adult plants.     

Different Behavior of Brassica juncea and B. carinata as Sources of Phoma lingam Resistance in Experiments of Interspecific Transfer to B. napus

With the aim to transfer Phoma lingam resistance into rape, successful interspecific crosses were made between three oilseed rape varieties (Brassica napus) and the resistant species B. carinata and B. carinata. Although both hybrid types B. napus×B. juncea and B. napus×B. carinata showed the same high level of resistance as the respective resistant parent, the resistance could be only transferred from juncea crosses. After three backcross generations, lines morphologically undistinguishable from rape, fertile, and with a high degree of resistance were obtained. The resistance of B. carinata was practically lost in the first backcross. A possible explanation of this different behavior could be a higher recombination between the genomes B and C (juncea crosses) than between B and A (carinata crosses). The: applied embryo culture increased the yield of hybrids and first backcross plants and reduced considerably the generation time.     

Studies on Resistance to Phoma lingam in Brassies napus-Brassica nigra Addition Lines

B. nigra exhibits high levels of resistance to Phoma lingam. The genetic behaviour of this resistance was investigated using B. napus-B. nigra addition lines. At least 3 different B. nigra chromosomes were found to contribute to the blackleg resistance. Thus, this resistance was suggested to be polygenic. In addition, high levels of P. lingam resistance in euploid offspring led to the assumption that translocations have taken place in early generations after hybridization between B. napus and B. nigra.     

Comparison of the epidemiology of ascochyta blights on grain legumes

Asochyta blights of grain legumes are caused by fungal pathogens in the genus Ascochyta. Different species infect the different legume species, and in pea three species including Phoma medicaginis var. pinodella have been implicated in ascochyta blight. The impact of the diseases varies between crops, countries, seasons and cropping systems, and yield loss data collected under well-defined conditions is scarce. However, ascochyta blights are considered major diseases in many areas where legumes are grown. Symptoms appear on all aerial parts of the plant, and lesions are similar for most of the species, except for M. pinodes and P. medicaginis var. pinodella. Infected seed, stubble and/or air-borne ascospores are major sources of primary inoculum. Their importance varies between species and also between regions. All Ascochyta spp. produce rain-splashed conidia during the cropping season which are responsible for the spread of the disease within the crop canopy. Only in pea are ascospores involved in secondary disease spread. Limited data suggests that Ascochyta spp. may be hemibiotrophs; however, toxins characteristic for necrotrophs have been isolated from some of the species. Modelling of ascochyta blights is still in the developmental stage and implementation of such models for disease forecasting is the exception.     

A 10-year Survey of Populations of Leptosphaeria maculans in France Indicates a Rapid Adaptation Towards the Rlm1 Resistance Gene of Oilseed Rape

Leptosphaeria maculans, the cause of stem canker of oilseed rape (OSR), exhibits gene-for-gene interactions with its host plant. The race structure of L. maculans was assessed on the basis of the analysis of 1011 isolates collected in France between 1990 and 2000, with regards to three AVR genes, AvrLm1, AvrLm2 and AvrLm4. The effect of selection pressure, due to large-scale cropping of Rlm1 cultivars, on the evolution of races of the fungus was also evaluated. The results revealed a scarcity or complete absence of isolates harbouring AvrLm2, whereas isolates harbouring AvrLm4 were present at a variable level, that was as high as 17.2–31.2% depending on the sample year and location. When obtained from rlm1 cultivars, isolates harbouring AvrLm1 always represented more than 83% of the populations until the 1997–1998 growing season. As a consequence, the Rlm1 cultivars had been highly efficient at controlling the disease and were grown on an estimated 43.7% of the total French acreage in OSR in 1998–1999. However, the increased commercial success of Rlm1 cultivars was paralleled by a decrease in the proportion of isolates harbouring AvrLm1 in 1997–1998 and 1998–1999. This resulted in less than 13% of isolates harbouring AvrLm1 in populations being collected from rlm1 cultivars in 1999 and 2000, and contributed to the loss of efficiency of the Rlm1 resistance in the field. The present study is an illustration of one round of a `boom and bust' cycle that occurred for a pathosystem where it has never been reported before. These data and the high evolutionary potential of L. maculans are fully supportive of one pathogen species with a high risk of breaking down resistance genes in OSR and suggest that the development of integrated strategies aiming at maximising the durability of novel resistance is now a priority for this pathosystem.