Effect of host factors on the susceptibility of Rhododendron to Phytophthora ramorum

Phytophthora ramorum causes sudden oak death (SOD) in western coastal forests of the USA. In Europe, the pathogen is mainly present in the nursery industry, particularly on Rhododendron. Because of the primary role of Rhododendron as a host and potentially as a vector, the effect of Rhododendron host factors on P. ramorum susceptibility and sporulation was investigated. Inoculation methods using either wounded or non‐wounded detached leaves were applied to 59 Rhododendron cultivars and 22 botanical species, replicated in three separate years. All Rhododendron species and cultivars were susceptible when using wounded leaves, but not when using non‐wounded leaves, suggesting a resistance mechanism operating at the level of leaf penetration. Using a regression tree analysis, the cultivars and species were split into four susceptibility classes. Young leaves were more susceptible than mature leaves when wounded, but less susceptible when non‐wounded. This effect was not correlated with leaf hydrophobicity or the number of leaf hairs. The presence or the type of rootstock did not affect the cultivar susceptibility level. Sporangia and chlamydospore production in the leaf lesions varied widely among Rhododendron cultivars and was not correlated with the susceptibility level. The susceptibility to P. ramorum correlated well with the susceptibility to P. citricola and P. hedraiandra × cactorum, suggesting that the resistance mechanisms against these species are non‐specific. Susceptibility to P. kernoviae was low for most cultivars. These findings have implications for detection, spread and disease control, and are therefore important in pest risk assessment.     

Potential of Eurasian poplar rust to overcome a major quantitative resistance factor

The effect of R_US, a major quantitative resistance (QR) factor inherited from Populus trichocarpa, was assessed in two Populus deltoides × P. trichocarpa F₁ progenies against eight isolates of Melampsora larici‐populina, the causal agent of Eurasian poplar rust. Six isolates were identified on which R_US had no significant effect. The first R_US‐defeating isolate identified suggested a pre‐existing potential to overcome R_US in the pathogen’s populations. The P. deltoides genetic background made no difference either to the ability of a given isolate to overcome R_US or to the relative and absolute aggressiveness of the isolates. This study illustrates how extreme the isolate‐specificity of QR can be. It also yields insights into the relationship between size of uredinia and sporulation rate, discussing the epidemiological significance of spore production per mm² uredinium.     

环境因素对芸苔链格孢生长发育的影响

研究结果表明,芸苔链格孢(Alternaria brassicae)在0—35℃下均可萌发,最适温度15—20℃。菌丝在0—30℃下均能生长,最适温度20—25℃;孢子及菌丝的致死温度为50℃,10分钟。该菌对湿度要求较严格,相对湿度在90％以上时孢子才能萌发,最适相对湿度为98％;病菌在相对湿度达93％时可以侵染,98％以上最适。脱落后孢子曝露在室内(相对湿度约63％)经1小时萌发率下降32.4％,4小时下降80％。耐旱能力较差。一般情况下紫外光能促进该菌产孢,但是对某些菌系的促进作用不明显。过长的紫外光照射对该菌产孢不利,照射160分钟孢子萌发率下降近80％。麦芽糖、蔗糖为该菌的最佳碳源;硝态氮、有机氮是该菌的良好氮源。pH值为4—6时有利于分生孢子萌发,强酸、强碱对分生孢子萌发有抑制作用。     

Biotin Induces Sporulation of Mulberry Anthracnose Fungus, Colletotrichum dematium

Compounds in mulberry leaves inducing sporulation of C. dematium, mulberry anthracnose fungus, were detected. Dissolved or suspended aqueous solutions (1%) of 16 amino acids and 10 vitamins occurring in mulberry leaves were applied individually at the margin of the fungal colony growing on PSA plate. Sporulation was induced only where a biotin solution was applied on the mycelium at a concentration of at least 0.01 ppm. This result suggests that biotin, which occurs in mulberry leaves (ca. 0.6 mg/kg of dried leaves), has a role in inducing sporulation of C. dematium. Received 20 August 1999/ Accepted in revised form 16 November 1999     

Biocontrol of the pathogen Phytophthora parasitica by arbuscular mycorrhizal fungi is a consequence of effects on infection loci

The impact of colonization by the arbuscular mycorrhizal fungus (AMF) Glomus mosseae on tomato root necrosis caused by the soil-borne pathogen Phytophthora parasitica was investigated. Studies in situ permitted infection loci to be identified and the effects of the AMF on numbers to be elucidated. Effects were significant and, 7 and 16 days after inoculation with zoospores of the pathogen, roots of plants colonized by the AMF had 39% and 30%, respectively, fewer infection loci than those that were not. Concurrent studies of the rate of spread of necrosis within roots showed no changes caused by the AMF. At harvest, 26 days following inoculation with the pathogen, 61% of roots of noncolonized plants were necrotic compared with only 31% in AMF-colonized plants. It is concluded that effects on numbers of infection loci are one mechanism via which AMF achieve biocontrol of this pathogen in tomato. Measures of the effects of the AMF on root system architecture suggest that no significant changes occur and are thus not the reasons for the reduction in infection loci. The implications of these data for agricultural practice and biocontrol research are discussed.     

UV-B辐射增强对小麦条锈病影响的初步探讨

在温室条件下用铭贤169、吨半麦、咸农4号3个小麦品种,研究了接种条锈菌条中32号小种前后UV-B辐射增强对小麦条锈病的影响。结果表明,接种前对小麦照射UV-B增强了品种铭贤169和吨半麦的病情指数,但对咸农4号的病情指数没有影响。接种后照射UV-B对品种咸农4号、铭贤169和吨半麦的病情指数均无显著影响,但却减少了条锈菌在品种铭贤169和吨半麦上的产孢期与产孢量。     

小菜蛾高致病力绿僵菌的筛选、鉴定及培养特性研究

为开发蔬菜害虫小菜蛾Plutella xylostella的高致病力真菌生物农药,以3龄小菜蛾幼虫为供试虫源,测定了25株虫生真菌菌株对小菜蛾的致病力,从中筛选出1株对小菜蛾幼虫有较高致病力的菌株CQM125。生测结果显示,在20℃、1.0×108孢子mL 1菌株CQM125菌液处理小菜蛾幼虫的LT50为3.97 d;25℃、5.0×107孢子mL 1菌液的LT50为2.44 d;在25℃下,第7 d的LC50为2.31×104孢子mL 1;接种处理后,20℃、8 d的小菜蛾死亡率为84.22%,对小菜蛾表现出良好的控制效果。依据菌株的形态特征、培养性状和rDNA ITS序列分析将菌株CQM125鉴定为金龟子绿僵菌Metarhizium anisopliae。对该菌株的培养基和培养条件进行了优化,优化后的产孢培养基为PPDA,其中葡萄糖2%,蛋白胨0.5%;最适产孢温度30℃,最适产孢pH值为6.0;最适光照条件为前6 d黑暗,后8 d光照。     

苹果腐烂病菌诱导产孢方法

为了研究一种能诱导苹果腐烂病菌快速大量产孢的方法,测试了6种培养基对苹果腐烂病菌的诱导产孢效果。结果发现,苹果腐烂病菌在加蜂蜜水和蛋白胨的带壳大麦上能大量产孢。接种苹果腐烂病菌菌丝的带壳大麦,在黑暗中培养15天后可产生大量黑色子实体。已形成子实体的大麦粒在黑光灯下诱导25天后开始产生分生孢子。每颗大麦粒平均产生11个子实体,其中15%的子实体能溢出分生孢子角。70 g大麦粒上所产生的分生孢子可配制浓度为106个孢子/mL的孢子悬浮液1500~2000 mL。试验还发现,不同的腐烂病菌菌株在同一条件下产孢量存在很大差异。     

Components of resistance to late blight (Phytophthora infestans) in eight South AmericanSolanum species

Four components of partial resistance toPhytophthora infestans were measured after inoculation in the greenhouse and in the field ofSolanum arnezii x hondelmannii, S. berthaultii, S. circaeifolium, S. leptophyes, S. microdontum, S. sparsipilum, S. sucrense andS. vernei, and four hybrid progenies ofS. microdontum withS. tuberosum. The four components were infection efficiency, lesion growth rate, generation time and sporulation capacity. The results were compared with resistance ratings derived from field experiments, and with observations made on the potato cultivars Bintje, Bildstar, Libertas and Pimpernel. Genetic variation for all components was found, while the relative importance of the components of partial resistance appeared to vary between the species. InS. microdontum, generation time, infection efficiency and lesion growth rate, and inS. tuberosum infection efficiency, lesion growth rate and sporulation capacity appeared positively associated, but in other species no such association was found. A strong hypersensitive reaction, the expression of which appeared to depend on environmental conditions, was found inS. microdontum. ForS. berthaultii, infection efficiency appeared to be the main resistance component.Abbreviations ADPC area under the disease progress curve - IE infection efficiency - LGR lesion growth rate - GT generation time - SC sporulation capacity