Examination of some morphologically unusual cultures of Phytophthora species using a mitochondrial DNA miniprep technique and a standardised sporangium caducity assessment

Using the mitochondrial DNA miniprep technique, the identity of sixteen morphologically unusual cultures allocated to Phytophthora nicotianae, Phytophthora mexicana or Phytophthora porri was determined by comparison with a library of mtDNA band patterns obtained from reference cultures. Seven cultures were identified as Phytophthora nicotianae (including those assigned to Phytophthora mexicana and Phytophthora porri), six as strains of Phytophthora palmivora with small, ovoid, weakly caducous sporangia, and one as Phytophthora citrophthora. Some cultures of P. nicotianae had a low percentage of caducous sporangia. Percentage sporangium caducity, but not sporangium L : B ratio, is considered a useful taxonomic criterion for separating species morphologically similar to Phytophthora nicotianae. One culture from tobacco in New Zealand had a highly unusual morphology and a unique DNA band pattern, but was not identifiable. One culture from Acacia mearnsii in South Africa had a unique DNA band pattern which was identical to that of an isolate from Annona squamosa from Australia previously identified as Phytophthora palmivora, the precise identity of which is still unclear. The identity of most isolates from diseased durian was found to be Phytophthora palmivora, confirming its role as the main pathogen, but P. nicotianae was also identified from this host. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crop connectivity under climate change: future environmental and geographic risks of potato late blight in Scotland

The impact of climate change on dispersal processes is largely ignored in risk assessments for crop diseases, as inoculum is generally assumed to be ubiquitous and nonlimiting. We suggest that consideration of the impact of climate change on the connectivity of crops for inoculum transmission may provide additional explanatory and predictive power in disease risk assessments, leading to improved recommendations for agricultural adaptation to climate change. In this study, a crop‐growth model was combined with aerobiological models and a newly developed infection risk model to provide a framework for quantifying the impact of future climates on the risk of disease occurrence and spread. The integrated model uses standard meteorological variables and can be easily adapted to various crop pathosystems characterized by airborne inoculum. In a case study, the framework was used with data defining the spatial distribution of potato crops in Scotland and spatially coherent, probabilistic climate change data to project the future connectivity of crop distributions for Phytophthora infestans (causal agent of potato late blight) inoculum and the subsequent risk of infection. Projections and control recommendations are provided for multiple combinations of potato cultivar and CO₂ emissions scenario, and temporal and spatial averaging schemes. Overall, we found that relative to current climatic conditions, the risk of late blight will increase in Scotland during the first half of the potato growing season and decrease during the second half. To guide adaptation strategies, we also investigated the potential impact of climate change‐driven shifts in the cropping season. Advancing the start of the potato growing season by 1 month proved to be an effective strategy from both an agronomic and late blight management perspective.     

PsHint1, associated with the G‐protein α subunit PsGPA1, is required for the chemotaxis and pathogenicity of Phytophthora sojae

Zoospore chemotaxis to soybean isoflavones is essential in the early stages of infection by the oomycete pathogen Phytophthora sojae. Previously, we have identified a G‐protein α subunit encoded by PsGPA1 which regulates the chemotaxis and pathogenicity of P. sojae. In the present study, we used affinity purification to identify PsGPA1‐interacting proteins, including PsHint1, a histidine triad (HIT) domain‐containing protein orthologous to human HIT nucleotide‐binding protein 1 (HINT1). PsHint1 interacted with both the guanosine triphosphate (GTP)‐ and guanosine diphosphate (GDP)‐bound forms of PsGPA1. An analysis of the gene‐silenced transformants revealed that PsHint1 was involved in the chemotropic response of zoospores to the isoflavone daidzein. During interaction with a susceptible soybean cultivar, PsHint1‐silenced transformants displayed significantly reduced infectious hyphal extension and caused a strong cell death in plants. In addition, the transformants displayed defective cyst germination, forming abnormal germ tubes that were highly branched and exhibited apical swelling. These results suggest that PsHint1 not only regulates chemotaxis by interacting with PsGPA1, but also participates in a Gα‐independent pathway involved in the pathogenicity of P. sojae.     

福建、浙江、江苏、上海疫霉种的研究

1985—1986年,从福建漳州,浙江杭州,上海,江苏南京,苏州,扬州,常州等地采集100多种植物进行分离,在雪松(Cedrus deodara),倒挂仙人鞭(Cereus flagelliformis),冬青卫茅(Euonymus japonieus),构树(Broussonetia papyrifera),蓖麻(Ricinus communis),柑桔(Citrus reticulata),带叶兜兰(Paphiopedilum hirsutissimum),芋(Colocasia esculenta),鳄梨(Persea americana),山茶(Camellia sp.),珊瑚樱(Solanum pseudo-capsicum),百合(Lilium brownii var.viridulum),蔓常春花(Vinca major),西瓜(Citrullus lanatus)等14种寄主植物上分离到51个疫霉菌株,鉴定为9个种:烟草疫霉(=寄生疫霉)Phytophthora nicotianae B.de Haan(=P.parasitica Dast.),掘氏疫霉P.drechsleri Tucker,樟疫霉 P.ciunamomi Rands,棕榈疫霉P.palmivora(Butl.)Butl.,柑桔褐腐疫霉P.citrophthora(R.et E.Smith) Leon.,苎麻疫霉P.bochmeriae Saw.,蜜色疫霉P.meadii McRae,簇囊疫霉P.botryosa Chee,隐地疫霉P.cryptogea Pethyb.et Laff.。其中,烟草疫霉,掘氏疫霉,樟疫霉出现频率较高,分布较广,初步认为是上述地区植物疫病的主要病原菌。簇囊疫霉、蜜色疫霉是国内新记录。     

Effect of cadmium and Phytophthora infestans on polyamine levels in potato leaves

Changes in putrescine, spermidine and spermine concentrations in potato ( Solanum tuberosum L.) leaves stressed either with cadmium, with the fungal pathogen Phytophthora infestans Mont. (de Bary) or with both simultaneously were investigated. The leaves of two cultivars, Bintje and Bzura, respectively susceptible and resistant to P. infestans were examined. The level of of putrescine did not change under any stress conditions. Cadmium stress caused at least a 2-fold increase in spermidine and spermine concentrations in susceptible leaves. In resistant leaves there was a 4-fold increase in spermidine and a decrease in spermine. The pathogenic stress produced similar changes in polyamine concentrations, i. e. with differences between the cultivars. The double stress induced antagonistic alterations in the concentrations of spermidine and spermine.     

Use of morphometric analysis for characterization of tobacco root growth in relation to infection byPhytophthora parasitica var.nicotianae

Development of tobacco root systems was characterized under controlled environmental conditions by use of morphometric root analysis. According to the classification scheme of this system, roots terminating in apical meristems are defined as first-order roots. Elements of second-order roots begin where two first-order roots merge, and so forth. Growth of root systems was similar for susceptible and resistant tobacco cultivars in nonautoclaved and autoclaved soils. During 15 days of growth subsequent to transplanting of 2-week-old plants, relative multiplication and extension rates of first-order and second-order roots were constant. Apparent unit extension rates of first-order and second-order root elements increased through 15 days of root system growth. Classification of tobacco root systems by the morphometric scheme provided a useful means of partitioning susceptibility of tissues to infection byPhytophthora parasitica var.nicotianae. Zoospores applied at the tips of first-order roots were most successful in causing infections; 73.3% of the roots inoculated with 16 zoospores per root tip became infected. Percentages of infections after inoculation of first-order root tissues 2 cm behind root tips or after inoculation of second-order roots were 10 and 4.3%, respectively.Florida Agricultural Experiment Station, Journal Series Paper 8106.     

Effect of solarization of soil within plastic bags on root rot of gerbera (Gerbera jamesonii L.)

Solarization of soil, (potting mix = coarse sand:Eucalyptus marginata fines = 1∶1) infested with 3 fungi pathogenic to gerbera (Phytophthora cryptogea, Fusarium oxysporum andRhizoctonia solani), for 3 to 4 weeks within transparent polyethylene bags controlled root rot of gerbera. Solarization for 2 weeks however, was less effective. All plants grown in the infested potting mix which had been kept in shade for 2, 3 or 4 weeks were severely attacked. Solarization of soil within plastic bags for 4 weeks also increased availability of nutrients such as NH₄ ⁺-N, PO₄ ⁻ and K⁺ in comparison to bagged soil kept in the shade for the same period.     

Encystment of zoospores of the fungus,Phytophthora cinnamomi,is induced by specific lectin and monoclonal antibody binding to the cell surface

Summary Only two of a number of macromolecules that bind to the surface of zoospores of the dieback fungus,Phytophthora cinnamomi, induce encystment when added to a suspension of actively swimming zoospores. One, the lectin Concanavalin A (ConA), binds to the entire surface of the zoospores including the surface of both flagella. Within 10 minutes more than 70% of the cells have encysted in the presence of 5 g/ml ConA. This encystment is inhibited by preincubation of the lectin with its hapten sugar, -methyl-D-mannoside. The other effective molecule, a monoclonal antibody designated Zf-1, is one of 35 that have been raised to components on the surface of zoospores and cysts ofP. cinnamomi. The antigen for Zf-1 occurs only on the surface of the two flagella. Purified Zf-1 at 15 g/ml causes encystment of 75% of the zoospores in 13minutes. To show that the induction of encystment by these two probes is not due simply to the presence of protein either in solution or bound to the zoospore a number of other proteins were tested, including other antibodies that bind to the zoospore surface. None of these other molecules caused encystment even at concentrations greater than 200 g/ml. The results are consistent with the surface components that bind ConA and Zf-1 being involved in the critical step of triggering encystment at the surface of a potential host during infection.