Differential Activity of Carboxylic Acid Amide Fungicides Against Various Developmental Stages of Phytophthora infestans

ABSTRACT Three carboxylic acid amide (CAA) fungicides, mandipropamid (MPD), dimethomorph (DMM) and iprovalicarb (IPRO) were examined for their effects on various asexual developmental stages of Phytophthora infestans in vitro and in planta. Germination of cystospores and direct germination of sporangia were inhibited with nanomole concentrations of MPD (0.005 mug/ml) and micromole concentrations of DMM (0.05 mug/ml) or IPRO (0.5 mug/ml). A temporary exposure of 1 h to CAAs was not detrimental to germination and infectivity of sporangia or cystospores. CAAs applied to cystospores at 1 h after the onset of germination did not prevent the emergence of germ tubes, but inhibited their further growth and deformed their shape. None of the fungicides affected discharge of zoospores from sporangia or the encystment (cell wall formation/assembly) of the zoospores. Mycelium growth in solid or liquid media was inhibited with micromole concentrations. CAAs mixed with sporangia and drop inoculated onto detached leaves strongly suppressed infection. Curative application at 1 day postinoculation (dpi) required higher concentrations of CAAs than preventive application to inhibit infection and lost its effectiveness at 2 dpi. When sprayed on established late blight lesions 4 days after inoculation, CAAs reduced sporangial production in a dose-dependent manner. Trans-laminar protection of potato or tomato leaves, although achieved with higher doses, was more effective with MPD than with DMM or IPRO. Shade house studies demonstrated superior control of late blight epidemics by MPD compared with the other molecules. The data suggest that germ tube formation by cystospores or sporangia is the most sensitive stage in the life cycle of P. infestans to CAAs. Of the three CAAs, MPD had the highest intrinsic activity against spore germination. This property, together with its better trans-laminar activity, makes MPD more effective than DMM or IPRO in controlling epidemics caused by P. infestans.     

Activity of the New Fungicide Benthiavalicarb Against Plasmopara viticola and its Efficacy in Controlling Downy Mildew in Grapevines

Benthiavalicarb is a new fungicide active against Oomycetes fungal plant pathogens. The present study shows that benthiavalicarb is effective for controlling the Oomycete fungal pathogen Plasmopara viticola, which causes downy mildew in grapevines. The fungicide did not affect zoospore discharge from sporangia of P. viticola, but strongly inhibited zoospore encystment, cystospore germination in vitro and mycelial growth, together with sporangial production in vivo. Benthiavalicarb showed strong prophylactic and local activity in intact plants or detached leaves and low translaminar activity. The compound was not translocated from leaf to leaf in either a acropetal or basipetal direction. Benthiavalicarb applied at 1, 3 and 6 days post-inoculation protected grapevine plants against downy mildew and inhibited sporulation of the pathogen. Similar results were obtained on leaf disks if benthiavalicarb was applied up to 96 h post-inoculation. Benthiavalicarb diminished the sporulation of P. viticola when applied to established disease in the tissue. Benthiavalicarb remained active on leaves for a period up to 28 days. Two foliar applications of benthiavalicarb, 2 weeks apart, to field-grown grapevines inhibited downy mildew development and were as effective as the standard metalaxyl-Cu treatment in controlling the disease. A formulated mixture of benthiavalicarb + Folpet was similar or superior in performance to metalaxyl-Cu and the new strobilurin trifloxystrobin in controlling downy mildew. The effectiveness of benthiavalicarb makes it well suited for integration into a control programme against downy mildew disease in vineyards, and as a component to delay resistance buildup.     

Influence of nitric oxide and reactive oxygen species on development of lettuce downy mildew in <Emphasis Type="Italic">Lactuca</Emphasis> spp.

The role of nitric oxide and reactive oxygen species, molecules indispensable for plant-pathogen signalling, was studied in the Lactuca spp.-Bremia lactucae pathosystem. Using a leaf disc model the translaminar effect of various compounds affecting their metabolism was studied by light microscopy. Time course studies revealed a slowdown in the development of B. lactucae (race BL16) infection structures by rutin (scavenger of reactive nitrogen and oxygen species) and SNP (NO donor) within 48 h post inoculation, followed by a retardation of sporulation. Application of the specific NO scavenger, PTIO, accelerated penetration of B. lactucae but had no further effects on the plant-pathogen interaction. Inhibitors of NO synthase (L-NAME) and nitrate reductase (sodium tungstate) did not influence pathogen development. Our results suggest that drastic change in the NO: hydrogen peroxide ratio seems to determine the pathogen’s fate. NO synthase-like activity significantly increased early after B. lactucae challenge in resistant L. virosa. Confocal laser scanning microscopy revealed the accumulation of nitric oxide in the penetrated cells, pointing to a role in the initiation of the hypersensitive reaction. The tips of germ tubes and appressoria of B. lactucae also accumulated NO, suggesting an essential role for this molecule in penetration of the biotrophic pathogen. Additionally, temporal changes in endogenous levels of rutin and quercetin in extracts from Lactuca spp. leaves will be discussed in connection to their role as part of the antioxidative machinery that influences the plants’ susceptibility/resistance to lettuce downy mildew.     

EMS and UV irradiation induce unstable resistance against CAA fungicides in <Emphasis Type="Italic">Bremia lactucae</Emphasis>

Wild type (WT) field isolates of Bremia lactucae failed to germinate in vitro or infect lettuce leaves in the presence of CAA (carboxylic acid amide) fungicides. Minimal inhibitory concentrations (MIC) for mandipropamid, dimethomorph, benthiavalicarb and iprovalicarb were 0.005, 0.5, 0.5 and 5 μg ml⁻¹, respectively. Mutagenesis experiments showed that spores exposed to EMS (ethyl methane sulphonate) or UV irradiation (254 nm) could infect lettuce leaves in the presence of up to 100 μg ml⁻¹ CAA. The proportion of infected leaves relative to the number of spores inoculated (infection frequency) was inversely related to the concentration of CAA used, ranging between 0 and 160 per 1 × 10⁶ spores. Resistant mutants (RM) lost their resistance within 1–14 reproduction cycles on CAA-treated plants. Crosses were made between RMxWT isolates and RMxRM isolates with an attempt to obtain stable homozygous resistant off-springs. Such crosses yielded few resistant but unstable progeny isolates. Mutagenic treatments given to hybrid isolates also failed to produce stable resistance. Previous gene sequencing data showed that stable resistance to CAAs is based on a single SNP in the cellulose synthase 3 (CesA3) gene of Plasmopara viticola. Therefore, we sequenced a 582 bp DNA fragment of Ces3A of WT, RM and hybrid isolates of B.lactucae. No mutation in this gene fragment was found. We conclude that mutagenic agents like EMS or UV may induce resistance to CAA in Bremia lactucae but this resistance is not stable and not linked to mutations in CesA3 gene.     

In vitro andin vivo evaluation of fungicides for control ofStemphylium botryosum F.lactucae on lettuce leaves

Maneb was the most efficient chemical in inhibiting germination ofStemphylium botryosum f. lactucae sporesin vitro. At 0.1 ppm it inhibited germination of all spores in the original population. Growth on PDA was markedly inhibited by 100 ppm daconil or maneb, withca. 90-95% spore mortality. The inhibitory effects of chemicals at sublethal doses on the rate of colony growth resulted both in retarded radial growth and in prolonged incubation period in culture. Maneb at 100 ppm and daconil at 10⁴ ppm had an inhibitory effect on inoculated leaf discs only when applied immediately after inoculation, whereas sodium dimethyldithiocarbamate (SDMC) inhibited fungal development at 10⁴ ppm both immediately after inoculation and 24 h later. SDMC kept the tissue very fresh during the experimental period. Fungal development in inoculated lettuce heads was prevented by 10³ ppm maneb or SDMC when applied before inoculation, but not when applied 24 h after inoculation, although inin vitro tests germinating spores were more sensitive to maneb than non-germinating ones.     

Virulence patterns of<Emphasis Type="Italic">Bremia lactucae</Emphasis> in Israel

The variation and distribution of virulent phenotypes ofBremia lactucae Regel, the causal agent of lettuce downy mildew, were studied during 2002–2003 in lettuce fields (Lactuca sativa) in Israel. A total of 21 isolates ofB. lactucae were collected from nine locations in three regions of Israel: Galilee, the Coastal Plain, and the Shefela. The isolates were examined for the presence of 21 virulence factors (v-factors) and their combinations with differential sets of lettuce lines/varieties. There were clear differences in v-factors, and a broad diversity of v-phenotypes among the isolates was found. Although 17 different v-phenotypes and 20 v-factors were detected, a composite of similar v-phenotypes generally occurred between isolates within the three regions. They differed mostly in the presence or absence of only a few v-factors. The Coastal Plain region averaged the highest virulence complexity (0.63), significantly different from that of the Shefela (0.45) and of Galilee (0.4). Comparison of the IsraeliB. lactucae isolates that were tested in this study with data of other countries showed that factor v18, which did not occur in the Israeli populations, was detected only in Czech and German pathogen populations. http://www.phytoparasitica.org posting Dec. 21, 2006.     

Silicon and increased electrical conductivity reduce downy mildew of soilless grown lettuce

Downy mildew of lettuce, caused by Bremia lactucae, is difficult to control in soilless systems by using conventional methods of disease management because few chemicals are registered, while resistant cultivars face the problem of resistance break down; therefore other methods for disease control need to be investigated. The effect of silicon salt as well as increased electrical conductivities against downy mildew was evaluated in four experiments carried out in hydroponically systems, using the cultivar of lettuce “Cobham Green”, known for its susceptibility to the pathogen. Silicon, as potassium silicate, was added at 100 mg l⁻¹ of nutrient solution at three levels of electrical conductivity: 1.5–1.6 mS cm⁻¹ (EC1), 3.0–3.5 mScm⁻¹ (EC2, 0.70 g l⁻¹ NaCl) and 4.0–4.5 mS cm⁻¹ (EC3, 0.95 g l⁻¹ NaCl) respectively. Lettuce plants, grown for 14–20 (trials 1 and 2) and 36–45 (trials 3 and 4) days in the different nutrient solutions tested, were inoculated with B. lactucae conidia with a maximum of two inoculations before final disease assessment carried out 14–21 days after the inoculation able to give symptoms. EC and potassium silicate significantly influenced downy mildew incidence and severity, while their interaction was not a significant factor. The addition to the standard nutrient solution (EC1) of potassium silicate resulted in a significant reduction of downy mildew severity in trials 1 and 2 where plants were artificially inoculated 15 and 20 days after transplanting. This efficacy was slight on plants grown for 36 and 45 days before inoculation in a soil drenched with EC1 amended with potassium silicate. EC2 gave a significantly similar downy mildew reduction than EC2 added with potassium silicate in trial 3. Plants grown for 36 and 45 days at the highest electrical conductivity (EC3) showed a significant reduction in severity of downy mildew compared with that observed at EC2 level. The best results, in terms of disease control, were given by the addition of potassium silicate to the EC3 solution. This combination also led to a significantly increased plant biomass. The possibility and benefits of applying potassium silicate and increased EC amendments in practice is discussed.     

Phytogard® and DL-β-amino Butyric Acid (BABA) Induce Resistance to Downy Mildew (Bremia Lactucae) in Lettuce (Lactuca sativa L)

Downy mildew of lettuce (Bremia lactucae) is a serious disease. An alternative to chemicals is the application of disease resistance inducers. The aim of this study was to test whether DL--amino butyric acid (BABA) and Phytogard® (K₂HPO₃) could induce resistance in downy mildew susceptible plants. Aqueous solutions of BABA (0, 10, 20, 30, 50, 80, 100mM) and Phytogard® (0.0, 5.8, 29.0, 40.6, 58.0 and 87.0ppm) were sprayed on seven-day-old seedlings 0, 3, 7 and 15 days before or 1–3 days after inoculation with B. lactucae. Results obtained showed that Phytogard®- and BABA-induced resistance was dose-dependent. At 40.6ppm for Phytogard® and 10mM for BABA, complete protection was obtained. Both compounds had a curative effect and the induced resistance lasted for at least 15 days. It was also shown that both compounds induced systemic resistance in lettuce against downy mildew. Phytogard® at 40.6ppm completely inhibited spore germination while BABA at 20mM did not. Pathogenesis related (PR) protein analysis showed that BABA induced weak accumulation of PR-2, but not PR-1, PR-5 and PR-9. Phytogard® induced none of these proteins. The use of these two compounds to protect lettuce from B. lactucae is discussed.     

Monitoring virulence and sexual compatibility in Brazilian Bremia lactucae populations

During the winter, there is a high occurrence of downy mildew on lettuce caused by Bremia lactucae. This oomycete shows variability in virulence, so understanding the genetic structure of the pathogen population becomes essential for obtaining resistant cultivars. Thus, the objective of this study was to determine sexual compatibility in Brazilian populations of B. lactucae and investigate the occurrence of sexual reproduction of the pathogen on lettuce (Lactuca sativa) and prickly lettuce (Lactuca serriola). Leaf samples were collected in 33 municipalities in seven Brazilian states. The virulence structure of the populations was monitored using the EU-C sextet code. B. lactucae populations from the states of São Paulo, Paraná, Rio de Janeiro, and Rio Grande do Sul shared six of the 15 virulence factors evaluated. Twenty-five virulence phenotypes (v-phenotypes) were found, with the sextet codes 31-00-02, 31-16-02, 31-24-02, and 31-01-02 being more frequent. The predominance of some v- phenotypes indicates that clonal reproduction is still the main form of B. lactucae propagation. The genes and resistance factors of the cultivars Argelès (Dm38), Balesta, and Bartoli are recommended as suitable sources of lettuce resistance in Brazil. Natural occurrence of oospores was detected in most sampled locations, in lettuce and prickly lettuce plants. Virulence variability of Brazilian isolates is the result of the pathogen's ability to reproduce both sexually and asexually, with a prevalence of homothallic isolates; although the majority were of the predominant B₂ mating type, there was a high incidence of predominant B₁ in addition to B₁ = B₂.     

Epidemiology,prediction and control of onion downy mildew caused byPeronospora destructor

The effect of environmental factors on the development of each stage ofPeronospora destructor (Berk.) Caspary on onions is reviewed. For sporulation to take place, a period of light must precede the period of darkness and high humidity in which spores are formed. Spores are discharged when the relative humidity (RH) is increasing or decreasing, and over a wide range of temperatures. Their discharge is triggered by exposure to red-infrared radiation and by vibration of the leaf. Dissemination of spores follows a daily periodic cycle and spores can be blown by wind over long distances. Duration of spore survival depends on temperature, RH and, especially, the absence of strong radiation. The rate of spore germination is highest at 10°C and declines with the rise in temperature. Germ tubes develop in liquid water, and a continuous period of wetness is required for infection to be completed. Systemic infection is common in cooler climates, where necks of onion bulbs are slow to dry. The principal sources of downy mildew infection by wind-borne spores are systemically infected propagation material, onion volunteer plants, and neighboring older crops.     

Diversity of defence mechanisms in plant–oomycete interactions: a case study of <Emphasis Type="Italic">Lactuca</Emphasis> spp. and <Emphasis Type="Italic">Bremia lactucae</Emphasis>

Plant pathogenic oomycetes, including biotrophic downy mildews and hemibiotrophs/necrotrophs such as Phytophthora and Pythium, cause enormous economic losses on cultivated crops. Lettuce breeders and growers face the threat of Bremia lactucae, the causal agent of lettuce downy mildew. This pathogen damages leaf tissues and lettuce heads and is also frequent on wild Asteraceae plants. The interactions of Lactuca spp. with B. lactucae (abbr. lettuce–Bremia) display extreme variability, due to a long co-evolutionary history. For this reason, during the last 30 years, the lettuce–Bremia pathosystem has been used as a model for many studies at the population, individual, organ, tissue, cellular, physiological and molecular levels, as well as on genetic variability and the genetics of host–parasite interactions. The first part of this review summarizes recent data on host–parasite specificity, host variability, resistance mechanisms and genetics of lettuce–Bremia interactions. The second part focuses on the development infection structures. Phenotypic expression of infection, behaviour of B. lactucae on leaf surfaces, the process of penetration, development of primary infection structures, hyphae and haustoria are discussed in relation to different resistance mechanisms. In the third part, the components of host resistance and the variability of defence responses are analysed. The role of reactive oxygen species (ROS), antioxidant enzymes, nitric oxide (NO), phenolic compounds, reorganization of cytoskeleton, electrolyte leakage, membrane damage, cell wall disruption, hypersensitive reaction and plant energetics are discussed in relation to defence responses. In general, the extreme variability of interactions between lettuce and Bremia, and their phenotypic expression, results from diversity of the genetic background. Different mechanisms of resistance are conditioned by an orchestra of defence responses at the tissue, cell, and molecular levels. The various events responsible for defence involve a complex interaction of the processes and reactions mentioned above. This review also provides an overview on the timing of pathogen development, host pathological anatomy, cytology and physiology of lettuce–Bremia associations. The significance of these factors on the expression of different resistance mechanisms (non-host and host resistance, race-specific and race non-specific resistance, field resistance) is discussed.     

成都蒿笋霜霉病菌初侵染及寄主范围研究初报

成都萵笋霜霉病菌孢子囊寿命很短,生活力只能維持7—10天。在病叶、茎及种子中,均未发現卵孢子。其初侵染来源可能是种子或病組織中潛藏的菌絲。調查結果表明,萵笋霜霉病菌可以藉气流或水流終年侵染,不在活体外休眠。 萵笋霜霉病菌寄主范围小,在6种菊科植物上接种未获成功,表現出明显的专化現象,品种間抗病性亦有显著的差異。     

Post infection application of DL-3-amino-butyric acid (BABA) induces multiple forms of resistance against <Emphasis Type="Italic">Bremia lactucae</Emphasis> in lettuce

DL-3-amino-butyric acid (BABA) induces local and systemic resistance against disease in numerous plant species. In a recent study we showed that preventive application of BABA to lettuce (Lactuca sativa) plants induced resistance against downy mildew caused by the oomycete Bremia lactucae by callose encasement of the primary infection structures of the pathogen. Now we show that post-infection application of BABA to the foliage or the roots, even at progressive stages of disease development, is highly protective against B. lactucae. Resistance induced by BABA is manifested in multiple microscopic forms, depending on the time of its application. When applied at 1 day post inoculation (dpi) BABA induced HR in penetrated epidermal cells; at 2 dpi it caused massive encasement with callose of the primary haustoria; and, at 3 or 4 dpi it enhanced the accumulation of H₂O₂ in the developing mycelia runners and altered their colour to red. The pronounced change in the colour of the mycelium was visually apparent to the naked eye. In all cases the pathogen failed to sporulate on the treated plants. This is the first indication that an immunizing compound may be protective at advanced stages of disease development.     

Classical and molecular genetics of <Emphasis Type="Italic">Bremia lactucae</Emphasis>, cause of lettuce downy mildew

Lettuce downy mildew caused by Bremia lactucae has long been a model for understanding biotrophic oomycete–plant interactions. Initial research involved physiological and cytological studies that have been reviewed earlier. This review provides an overview of the genetic and molecular analyses that have occurred in the past 25 years as well as perspectives on future directions. The interaction between B. lactucae and lettuce (Lactuca sativa) is determined by an extensively characterized gene-for-gene relationship. Resistance genes have been cloned from L. sativa that encode proteins similar to resistance proteins isolated from other plant species. Avirulence genes have yet to be cloned from B. lactucae, although candidate sequences have been identified on the basis of motifs present in secreted avirulence proteins characterized from other oomycetes. Bremia lactucae has a minimum of 7 or 8 chromosome pairs ranging in size from 3 to at least 8 Mb and a set of linear polymorphic molecules that range in size between 0.3 and 1.6 Mb and are inherited in a non-Mendelian manner. Several methods indicated the genome size of B. lactucae to be ca. 50 Mb, although this is probably an underestimate, comprising approximately equal fractions of highly repeated sequences, intermediate repeats, and low-copy sequences. The genome of B. lactucae still awaits sequencing. To date, several EST libraries have been sequenced to provide an incomplete view of the gene space. Bremia lactucae has yet to be transformed, but regulatory sequences from it form components of transformation vectors used for other oomycetes. Molecular technology has now advanced to the point where rapid progress is likely in determining the molecular basis of specificity, mating type, and fungicide insensitivity.     

Chitosan enhances disease resistance in pearl millet against downy mildew caused by Sclerospora graminicola and defence-related enzyme activation

BACKGROUND: The present study investigated the effect of chitosan seed priming on the induction of disease resistance in pearl millet against downy mildew disease caused by Sclerospora graminicola (Sacc.) Schroet. RESULTS: Pearl millet seeds were primed with chitosan at different concentrations: 0.5, 1.5, 2.5 and 3 g kg^?1 seed. Of the different concentrations, 2.5 g kg^?1 was found to be optimum, with enhanced seed germination of 99% and seedling vigour of 1782, whereas the untreated control recorded values of 87% and 1465 respectively. At optimum concentration, chitosan did not inhibit sporulation and release of zoospores from sporangia. Furthermore, pearl millet seedlings raised after seed treatment with chitosan showed an increased level of the defence‐related enzymes chitosanase and peroxidase as compared with the untreated pearl millet seedlings on downy mildew pathogen inoculation. The effect of chitosan in reducing downy mildew incidence was evaluated in both greenhouse and field conditions, in which respectively 79.08 and 75.8% disease protection was obtained. CONCLUSION: Chitosan was effective in protecting pearl millet plants against downy mildew under both greenhouse and field conditions by inducing resistance against the pathogen. Thus, chitosan formulation can be recommended for seed treatment in the management of downy mildew disease. Copyright © 2008 Society of Chemical Industry     

Structure and variation in the wild-plant pathosystem: <Emphasis Type="Italic">Lactuca serriola–Bremia lactucae</Emphasis>

Over the past decade, extensive research on the wild-plant pathosystem, Lactuca serriola (prickly lettuce)–Bremia lactucae (lettuce downy mildew), has been conducted in the Czech Republic. Studies focused on pathogen occurrence and distribution, host range, variation in symptom expression and disease severity, interactions of B. lactucae with another fungal species (Golovinomyces cichoracearum) on L. serriola, variation in resistance within natural populations of L. serriola, the structure and dynamics of virulence within populations of B. lactucae, sexual reproduction of B. lactucae, and a comparison of virulence structure and changes in B. lactucae populations occurring in wild (L. serriola) and crop (L. sativa) pathosystems. The incidence of B. lactucae on naturally growing L. serriola and other Asteraceae was recorded. Lactuca serriola was the most commonly occurring host species, followed by Sonchus oleraceus. Over the duration of these studies, the incidence of B. lactucae in L. serriola populations varied between 45–87%. Disease incidence and disease prevalence were partly related to the size, density and different habitats of L. serriola populations. In addition to B. lactucae infection, infection by the lettuce powdery mildew fungus (Golovinomyces cichoracearum) was quite common, including co-infection. Variation in resistance to B. lactucae was studied by using ten isolates (NL and BL races with known virulence patterns) at a metapopulation level, i.e. 250 L. serriola samples representing 16 populations from the Czech Republic (CZ). Our comparisons revealed broad variation in host resistance among host populations and also intrapopulation variability. In the CZ populations, 45 resistance phenotypes were recorded, the most frequent were race-specific reaction patterns. Structural and temporal changes in virulence variation of B. lactucae populations on L. serriola were studied during 1998–2005. Altogether, 313 isolates of B. lactucae originating from the Czech Republic were examined for the presence of 32 virulence factors (v-factors), and 93 different virulence phenotypes (v-phenotypes) were recorded. A study of v-factor frequency showed that common v-factors in B. lactucae populations match some of the race-specific resistance genes/factors (Dm genes or R-factors) originating from L. serriola. The highest frequency was recorded by v-factors v7, v11, v15–17, and v24–30. In contrast, v-factors (e.g. v1–4, 6, and 10) matching Dm genes originating from L. sativa were very rare. This demonstrates the close adaptation of B. lactucae virulence to the host (L. serriola) genetic background. Temporal changes in virulence frequencies over the period were recorded. In many v-factors (v11, v14, v16, and v25–28), fluctuations were observed, some (v14 and v17) shifting to higher frequencies, and others (v5/8 and v23) decreasing. The occurrence of mating types was studied (1997–1999) in a set of 59 B. lactucae isolates. Both compatibility types (B1 and B2) were recorded; however the majority of the isolates (96%) were type B2. A comparative study of B. lactucae virulence variation between the wild (L. serriola) and crop (L. sativa) pathosystems showed major differences. Migration and gene flow between both pathosystems and the potential danger of wild B. lactucae populations for cultivated lettuce are discussed. This paper summarizes comprehensive and unique research on an oomycete pathogen (B. lactucae) that is shared between a crop (lettuce, L. sativa) and its close wild relative (prickly lettuce, L. serriola). The data demonstrate clear evidence about race-specific interactions, variation and changes in virulence, and coevolutionary relationships in the wild pathosystem L. serriola–B. lactucae. Conclusions contribute to the broadening and better understanding of gene-for-gene systems in natural host–pathogen populations and their relationships to crop pathosystems.     

一体化智能孢子捕捉系统在黄瓜霜霉病和黄瓜白粉病预测上的应用

为探索国内研制的新型一体化智能孢子捕捉系统在黄瓜霜霉病和黄瓜白粉病预测预报上的应用,在田间自然发病情况下,通过对捕捉孢子的形态进行识别,优化一体化智能孢子捕捉系统主要工作参数如有/无空气切割头、空气采集口高度和空气采集时间;通过病害及孢子的动态监测分析大棚黄瓜霜霉病和黄瓜白粉病病情指数与孢子捕捉量的关系。结果表明,当不加装空气切割头、空气采集口高度为70 cm、孢子捕捉时间在10:00—10:30时段有利于孢子的捕捉。黄瓜霜霉病和黄瓜白粉病病情指数与连续7 d孢子捕捉总量具有强正相关性。连续多日监测到黄瓜霜霉病菌孢子囊且数量快速增加是黄瓜霜霉病发生或快速上升的一个预测指标。黄瓜白粉病发病之前没有监测到黄瓜白粉病菌分生孢子,且在病害盛发期分生孢子捕捉量仍较少。研究表明,一体化智能孢子捕捉系统适用于黄瓜霜霉病的预测,但在黄瓜白粉病的预测上尚存在一定问题。     

Evidence for an Osmotic Mechanism in the Control of Powdery Mildew Disease of Wheat by Foliar-applied Potassium Chloride

The mechanism by which foliar application of potassium chloride solution reduces symptoms of powdery mildew disease (Erysiphe graminis f.sp. tritici) of wheat was investigated. The hypothesis that potassium chloride reduces mildew by an osmotic effect on spore germination was tested in three glasshouse experiments. Either potassium chloride solution or the osmoticum polyethylene glycol 200 was sprayed on wheat at the three- or four-leaf stage. The plants were inoculated with spores and spore germination and leaf area affected by mildew were assessed. Leaf water potential was determined as a measure of the osmotic effect of the treatments. Spore germination and leaf area affected by mildew were related to leaf water potential using regression analysis in groups on the data averaged over the three experiments. Both spore germination on the leaf and leaf area affected by mildew were reduced as leaf water potential decreased. There was no difference between potassium chloride or polyethylene glycol in the relationship between spore germination and leaf water potential, but polyethylene glycol was slightly less effective at reducing mildew symptoms at any given leaf water potential. The results are compatible with the hypothesis that potassium chloride reduces symptoms of powdery mildew by an osmotic effect on spore germination.     

Phylogenetic investigations in the downy mildew genus <Emphasis Type="Italic">Bremia</Emphasis> reveal several distinct lineages and a species with a presumably exceptional wide host range

Bremia lactucae is one of the most devastating and widespread pathogens in lettuce production worldwide. Despite its economical importance, uncertainty prevails about the species delimitation in the genus Bremia. Commonly, Bremia is considered to be monotypic, containing only Bremia lactucae, while taxonomists have described additional species, and molecular phylogenetic studies have shown significant sequence divergence between accessions from different hosts. Here, we report that several previously described species are genetically highly distinct from Bremia lactucae parasitic to Lactuca sativa. These include Bremia lapsanae, Bremia sonchicola, and Bremia taraxaci. In addition to these host-specific species, a plurivorous species is revealed, which infects hosts from three different tribes in the Asteraceae subfamilies Asteroideae and Carduoideae. The broad host range of clade 1 is exceptional for downy mildews and only paralleled by Pseudoperonospora cubensis, which infects a broad range of Cucurbitaceae. The taxonomic status of Bremia cirsii and of Bremia centaureae remains unresolved, as the accessions from Cirsium and Centaurea, respectively, did not form a monophylum but were partly contained in the plurivorous clade 1. Bremia lactucae was found to be restricted to Lactuca sativa and Lactuca serriola. Thus, it can be assumed that Bremia infections on weeds apart from Lactuca species do not pose a significant risk for lettuce production. However, it is unlikely that breeding resistance genes from Lactuca serriola into Lactuca sativa will result in durable resistance of lettuce to downy mildew disease, because the current study provides additional evidence that Bremia accessions from both hosts form a population continuum.     

Application of fuzzy logic for the simulation of Plasmopara viticola using agrometeorological variables*

A mechanistic model called PLASMO was developed earlier to simulate grapevine downy mildew (Plasmopara viticola) and has been applied in several viticultural areas of Italy since 1988 by the collaboration of several research institutions of Firenze. In this study, a new simulation model based on fuzzy logic has been developed for the same structure (biological cycle of P. viticola). This approach allows classical quantitative information to be used together with qualitative information. Vague concepts can also be handled. Agrometeorological data is used, with an hourly time step, starting from budbreak to the end of the growing season. Air temperature, relative humidity, rainfall and leaf wetness are required. The simulated processes are the growth of grapevine leaf area and the main phases of the biological cycle of the pathogen: incubation, sporulation, germination, spore survival and inoculation. The main epidemiological outputs are timing of infection events and disease intensity. The performance of the model is evaluated and the mechanistic and fuzzy logic approaches are compared.