Relative importance of root rots of subterranean clover caused by Aphanomyces euteiches and Phytophthora clandestina

In a growth-cabinet experiment, Aphanomyces euteiches caused more severe root disease and greater reductions in root and shoot weights of subterranean clover cv. Mount Barker than Phytophthora clandestina in pasteurized sandy loam flooded with water for a 24-h period each week. A. euteiches also reduced plant growth more than P. clandestina in untreated sandy loam. In a similar experiment, both fungi caused the same amount of disease and reduction in growth of cv. Yarloop in pasteurized clay loam flooded for 24 h each week, and P. clandestina caused more disease and a greater decrease in plant growth than A. euteiches when the soil was flooded for 4 h each week. The pathogens did not interact positively in either soil. In an irrigated pasture, soil drenches with the fungicides metalaxyl and fenaminosulf together reduced both root rots to low levels and increased subterranean clover dry matter by 1 96, 0.50 and 1 20 t/ha in the autumn, winter and spring of 1985, respectively, and by 0.59 t/ha in the autumn of 1986. Results suggest that P. clandestina caused most of these losses in yield.     

A light and scanning-electron microscopic study of infection of tomato plants by virulent and avirulent races of Cladosporium fulvum

Infection of tomato plants byCladosporium fulvum Cooke was studied using light and scanning-electron microscopy. Races 1.2.3 and 4 ofCladosporium fulvum were used, whereas tomato cultivars, carrying the Cf2 gene (susceptible to race 1.2.3 and immune to race 4) and the Cf4 gene (immune to race 1.2.3 and susceptible to race 4) served as differentials. No differences were observed in growth between compatible and incompatible combinations during germination, subsequent formation of runner hyphae and stomatal penetration. Runner hyphae did not show directional growth towards stomata. Penetration usually occurred on the third or fourth day after inoculation. In compatible combinations the fungus grew intercellularly, often in close contact with spongy mesophyll cells. Under optimal conditions it did not cause visible damage to plant cells during early stages of infection. Under suboptimal conditions in winter, the host cells often reacted with callose deposition, but growth of the fungus did not appear to be inhibited. Ten to twelve days after inoculation conidiophores emerged through the stomata and produced conidia. In incompatible combinations fungal growth was arrested one to two days after penetration and confined to stomata and surrounding cells. Very soon the host cells, in contact with the fungus, deposited extensive amounts of callose. Later these cells turned brown and collapsed. At the surface of the host cells, contacted by fungal hyphae, abundant extracellular material could be observed by scanning-electron microscopy. Removing the epidermis of leaves before inoculation delayed the resistant response. On stripped leaves the rate of fungal growth was equal for both interactions up to ten days after inoculation, but the incompatible combination lacked sporulation.     

Control of Phytophthora root rot of irrigated subterranean clover with potassium phosphonate in Victoria, Australia

Phosphonic acid was more effective in inhibiting the production of sporangia of Phytophthora clandestina in sterile pond water (ED₅₀= 1·4 p.p.m.) than it was in inhibiting the growth of mycelium of the fungus on either corn meal agar (ED₅₀= 13·8p.p.m.) or lima bean agar (ED₅₀= 236 p.p.m.). Experiments under controlled environmental conditions showed that better control of tap root rot of subterranean clover caused by P. clandestina was achieved by application of potassium phosphonate to a pasteurized soil mixture than by a spray on the foliage alone. However, in a soil from a pasture, foliar sprays were more effective than soil treatments. Application of superphosphate at a rate of 250 kg/ha or higher to the soil reduced the effectiveness of sprays. In three experiments in irrigated pastures a spray of potassium phosphonate at 300 or 313 ml/ha, applied to cotyledons of subterranean clover and to soil, reduced severity of Phytophthora root rot and increased the annual production of dry matter of the legume by 1·96 to 5·11 t/ha in comparison with untreated controls.     

Expression of the PR-b1' gene in roots of two Nicotiana species and their amphidiploid hybrid infected with virulent and avirulent races of Chalara elegans

The expression levels of PR-b_1' gene coding for PR-b_1' protein (a PR-1 group protein) were investigated in the roots of two Nicotiana species and their amphidiploid hybrid in response to infection by virulent (84–1) or avirulent (85–26) races of black root rot fungus Chalara elegans. Nicotiana glutinosa is susceptible to C. elegans , while Nicotiana debneyi and the amphidiploid hybrid N. glutinosa × N. debneyi are highly resistant. The results showed that irrespective of whether the plant is susceptible or resistant, and the fungal race virulent or avirulent, low infection rates induce low levels of PR-b_1' mRNA, and high infection rates result in high production of PR-b_1' mRNA. The amphidiploid hybrid, which expresses the protein PR-b_1', constitutively, showed similar PR-b_1' mRNA levels with both races of C. elegans. At maximal levels of accumulation, PR-b_1' mRNA is quantitatively similar in the two Nicotiana species, and the amphidiploid hybrid, whatever the nature of the inoculum.     

Carbohydrate composition of apoplastic fluids isolated from tomato leaves inoculated with virulent or avirulent races of Cladosporium fulvum (syn. Fulvia fulva)

Inoculation of tomato (Lycopersicon esculentum) with virulent races ofCladosporium fulvum (compatible interactions), resulted in substantial changes of the carbohydrate composition of apoplastic fluids isolated from the leaves, during the course of the infection process. In addition to a decrease in the concentration of the translocation sugar sucrose, a transient accumulation of the hexoses glucose and fructose and an accumulation of the polyol mannitol were observed. The latter coincided with a rising level of mannitol dehydrogenase, an enzyme that reduces fructose to mannitol. Only minor changes were detected in the carbohydrate composition of apoplastic fluids isolated from leaves of uninoculated control plants or plants inoculated with avirulent races ofC. fulvum (incompatible interactions). The fungal metabolite mannitol was not detected in apoplastic fluids isolated from the latter plants.These results suggest that, upon colonization of the intercellular spaces by virulent races ofC. fulvum, apoplastic sucrose is hydrolyzed by a host and/or fungal invertase and the resulting hexoses, glucose and fructose, are converted into mannitol by the fungus. In incompatible tomato-C. fulvum interactions a functional nutritional relationship between plant and fungus is prevented by plant defense responses, which might explain why in these interactions the carbohydrate composition of apoplastic fluids is similar to that of uninoculated control plants.Samenvatting Inoculatie van tomaat (Lycopersicon esculentum) met virulente fysio's vanC. fulvum (compatibele interacties), leidde tot aanzienlijke veranderingen in de koolhydraatsamenstelling van apoplastische vloeistoffen die uit de bladeren werden geïsoleerd in de loop van het infectieproces. Naast een sterke daling van de concentratie van de transportsuiker saccharose, vond er ophoping van de hexoses glucose en fructose en de polyol mannitol plaats. De accumulatie van mannitol ging gepaard met een toename in de activiteit van mannitol dehydrogenase, een enzym dat fructose reduceert tot mannitol. In de koolhydraatsamenstelling van apoplastische vloeistoffen geïsoleerd uit bladeren van niet geïnoculeerde controleplanten, of planten geïnoculeerd met avirulente fysio's vanC. fulvum (incompatibele interacties), werden slechts kleine veranderingen waargenomen. De schimmelmetaboliet mannitol kon niet worden aangetoond in de apoplastische vloeistoffen die uit deze planten werden geïsoleerd.Deze resultaten suggereren dat bij de kolonisatie van de intercellulaire ruimtes door virulente fysio's vanC. fulvum, saccharose uit de apoplast wordt gehydrolyseerd door invertase afkomstig van de plant of de schimmel waarna de ontstane hexoses, glucose en fructose, door de schimmel worden omgezet in mannitol. Bij incompatibele tomaatC. fulvum interacties wordt een functionele voedingsrelatie tussen plant en schimmel voorkomen door het optreden van afweerreacties van de plant, hetgeen kan verklaren waarom in deze interacties de koolhydraatsamenstelling van apoplastische vloeistoffen vergelijkbaar is met die van niet geïnoculeerde controleplanten.     

小麦品种与条锈菌小种间的相互作用机制

为了在小麦多品种、条锈菌多小种体系中研究小种间的相互关系,1995年9月至1997年5月在田间设置方块圃,以全感、水平抗性和垂直抗性三种不同抗病类型的6个品种及条中29、水源11致病类型I与混合小种(条中29:水源11致病类型I=1:1)三类小种组合为试材,对各品种-小种组合的流行速率进行方差分析。结果表明:在多数品种上,条中29与水源11生理小种间存在互作关系,表现为协同作用,且混小种的流行速率明显高于单小种的流行速率。     

Histopathology of Infection and Colonization of Susceptible and Resistant Port-Orford-Cedar by Phytophthora lateralis

ABSTRACT Port-Orford-cedar (POC) root disease, caused by Phytophthora lateralis, continues to kill POC in landscape plantings and natural forests in western North America. POC trees resistant to P. lateralis have been identified and propagated. Cytological observations of P. lateralis in susceptible and resistant roots and stems were made with light and transmission electron microscopy to identify resistance mechanisms. No differences in infection pathway and initial colonization were observed between susceptible and resistant roots, although there were differences in the rate and extent of development. Germ tubes formed appressoria, and penetration hyphae grew either between or directly through epidermal cell walls; inter- and intracellular hyphae colonized the root cortex. In susceptible roots, hyphae penetrated into the vascular system within 48 h of inoculation. In contrast, hyphae in roots of resistant seedlings grew more slowly in cortical cells and were not observed to penetrate to the vascular tissues. In resistant roots, infection was marked by general thickening of cortical cell walls, wall appositions around penetrating hyphae, collapse of cortical cells, and accumulation of osmophillic granules around hyphae. In susceptible stems, hyphae grew inter- and intracellularly in all cells of the secondary phloem except fiber cells, but were concentrated in sieve and parenchyma cells in the functional phloem. The pattern of penetration and colonization of hyphae was similar in the resistant stems, except that hyphae were found in the fiber cells of the xylem. In resistant stems, there were fewer hyphae in the functional phloem, and cytological changes such as damaged nuclei and disintegrated cytoplasm were evident. Structural changes in resistant stems included collapsed cells, wall thickening, secretory bodies, apposition of electron dense materials, and crystals in cell walls.     

Compartment-specific antioxidative defense in Arabidopsis against virulent and avirulent Pseudomonas syringae

The accumulation of reactive oxygen species (ROS) during biotic stress is either part of a hypersensitive response of the plant or induced directly by the pathogen. Antioxidants such as ascorbate and glutathione counteract the accumulation of ROS and are part of the defense reaction. The aim of the present study was to investigate the compartment-specific importance of ascorbate and glutathione during a virulent and avirulent Pseudomonas syringae infection in Arabidopsis thaliana. Peroxisomes were found to be the hotspot for glutathione accumulation reaching 452% and 258% of control levels 24 h postinoculation during the virulent and avirulent infection, respectively. An accumulation of ascorbate could also be observed in vacuoles during Pseudomonas syringae infection, whereas glutathione remained absent in this cell compartment. Neither glutathione nor ascorbate accumulated in the apoplast during pathogen infection demonstrating an only negligible role of these antioxidants in the apoplast during pathogen infection. Compartment-specific changes followed a recently proposed stress model with an increase of ascorbate and glutathione in most cell compartments at the early stages of infection and a strong drop at the later stage of infection when a strong accumulation of ROS and symptoms occurred in the leaves. This study highlights the importance of certain cell compartments and antioxidants in general for the protection of pathogen-induced ROS accumulation.     

Leaf area dynamics of potato cultivars infected by Phytophthora infestans

The effect ofPhytophthora infestans on foliage growth and senescence of three potato cultivars was studied in two field experiments. Inoculum or fungicide was applied in different frequencies to establish a range of levels of disease. At weekly intervals leaf numbers were determined as well as vertical canopy profiles of senescent and lesion covered leaf and stem area. P. infestans reduced appearance of new leaves on the main stem only at the highest level of disease. The cultivars differed more in rate of primary infection of healthy leaves than in the subsequent increase in percentage lesion coverage of the infected leaves. Differences between cultivars in stem lesion coverage resembled the differences for leaf lesions, but in every cultivar stem lesions were most prominent in the top of the canopy, contrary to leaf lesions.P. infestans stimulated leaf senescence similarly in the different cultivars.     

Colonization of roots, stolons, tubers and stems of various potato (Solanum tuberosum) cultivars by the black-dot fungus Colletotrichum coccodes

In an attempt to better understand the importance of tuber-borne inoculum in black dot development, several potato cultivars were inoculated with various Colletotrichum coccodes isolates. Symptoms developed first on underground organs (starting 2 weeks after inoculation on roots, and later on stolons and tubers) of inoculated plants; stem infections developed only after 7–10 weeks, depending on the cultivar. Infection with C. coccodes resulted in a reduction in numbers of stolons and tubers in cv. Bintje, but not in the later maturing cv. Roseval. Significant isolate by cultivar interactions were detected from the analysis of root symptoms after inoculation of three potato cultivars (Bintje, Spunta and Desiree) with five C. coccodes isolates. Such an interaction was also detected for stolon/tuber symptoms at the latest scoring date (98 days after inoculation), but not at earlier dates (58, 70 and 84 days after inoculation). These results suggest that protocols based on root colonization might be used for investigating cultivar response to black dot and pathogenicity of C. coccodes isolates, and that some specificity exists in the reaction of potato genotypes to this pathogenic fungus.     

Survey of fungi associated with subterranean clover leaves and petioles in Western Australia

A wide range of fungi was recorded from leaves and petioles of subterranean clover ( Trifolium subterraneum L.), with and without symptoms of disease, in Western Australia. Leptosphaerulina trifolii (Rostrup) Petrak and Phoma medicaginis Malb. & Roum. were the two most frequently isolated fungi. Ascochyta caulicola Lamb., Colletotrichum truncatum (Schw.) Andus & Moore, Curvularia trifolii (Kauffm.) Boedijn., Fusarium avenaceum (FR.) Sacc, Myrothecium verrucaria (Fr.) Sacc, Phoma medicaginis , and Stemphylium globuliferum (Vestergr.) Simmons were new records on subterranean clover foliage in Western Australia. Phoma medicaginis was demonstrated to be pathogenic and this is the first record as a pathogen on subterranean clover foliage in Western Australia, M. verrucaria and F. avenaceum caused leaf spotting or more extensive lesions as a consequence of undetermined phytotoxic products produced on agar plate cultures.     

Comparison of resistance-inducing abilities of virulent and avirulent races of Erysiphe graminis f.sp. hordei and a race of Erysiphe graminis f.sp. tritici in barley

Resistance to powdery mildew was induced in barley by preinoculation with virulent and avirulent races of barley powdery mildew ( Erysiphe graminis f.sp. hordei ), and with a race of wheat powdery mildew ( E. graminis f.sp. tritici ). Four inducer densities were tested in 13 different induction periods between 1 and 24 h. Generally, the resistance induced by barley powdery mildew increased up to 10-12 h of induction and was maintained in longer induction periods. The inducing abilities of virulent and avirulent races could not be distinguished up to 10-12 h of induction, after which the inducing ability of avirulent races increased significantly in relation to virulent races. Wheat powdery mildew was able to induce more resistance than barley powdery mildew in induction periods up to 8 h. In a single inoculation procedure the number of haustoria developing from virulent barley powdery mildew decreased as inoculum density increased. The effect was ascribed to induction of resistance. This reduction of infection efficiency in the compatible interaction was compared to induced resistance. However, the inoculum density needed for 50% resistance induction in the double inoculation procedure was approximately 40 times higher than the inoculum density needed for 50% reduction in infection efficiency in the single inoculation procedure.     

Histopathology of infection and colonization of Quercus ilex fine roots by Phytophthora cinnamomi

Quercus ilex is one of the European forest species most susceptible to root rot caused by the oomycete Phytophthora cinnamomi. This disease contributes to holm oak decline, a particularly serious problem in the ‘dehesas’ ecosystem of the southwestern Iberian Peninsula. This work describes the host–pathogen interaction of Q. ilex and P. cinnamomi, using new infection indices at the tissue level. Fine roots of 6‐month‐old saplings inoculated with P. cinnamomi were examined by light microscopy and a random pool of images was analysed in order to calculate different indices based on the measured area of pathogen structures. In the early stages of invasion, P. cinnamomi colonizes the apoplast and penetrates cortical cells with somatic structures. On reaching the parenchymatous tissues of the central cylinder, the pathogen develops different reproductive and survival structures inside the cells and then expands through the vascular system of the root. Some host responses were identified, such as cell wall thickening, accumulation of phenolic compounds in the middle lamella of sclerenchyma tissues, and mucilage secretion blocking vascular cells. New insights into the behaviour of P. cinnamomi inside fine roots are described. Host responses fail due to rapid expansion of the pathogen and a change in its behaviour from biotrophic to necrotrophic.     

Colonization and sporulation of Peronospora viciae on cultivars of Pisum sativum

The behaviour of Peronospora viciae was examined on four pea ( Pisum sativum ) cultivars (Maro, Superb, Victory Freezer and Surprise) which appeared to exhibit varying degrees of resistance to this pathogen. Penetration into all cultivars was similar and mainly by direct entry through the cuticle. The pathogen grew in a similar manner and at a similar rate in all the cultivars during the early stages (<96 h) of tissue colonization. As it approached the onset of sporulation its growth slowed down in the more resistant cultivars but in no instance were any hypersensitive or other host cell reactions observed. Greater differences between the cultivars were seen in the extent of sporulation, with the least resistant cultivar (Superb) bearing the largest number of sporangiospores. There was much less sporulation on the most resistant cultivar (Maro) and the process was more dependent on temperature than it was on Superb. The development of mature plant resistance occurred in a similar way and on a similar time-scale in all cultivars. The youngest leaflets on plants which were less than 8 weeks old at inoculation showed the least resistance to infection and colonization.     

青枯雷尔氏菌Tn5转座子无致病力突变株在番茄根部的定殖特性

为筛选防治番茄青枯病的优良生防菌株,本研究以弱化指数、胞外多糖含量和盆栽苗番茄发病率为指标确定20株经形态初步判定为无致病力的青枯雷尔氏菌Ralstonia solanacearum Tn5突变菌株的致病性,测定其在番茄根部的定殖数量,并于显微镜下观察其定殖特性。结果表明,供试的20株青枯雷尔氏菌无致病力突变菌株的弱化指数均大于0.75,胞外多糖含量介于1.59～16.68μg/mL之间,显著低于强致病力菌株FJAT-91,接种40 d番茄植株未出现青枯病症状;20株青枯雷尔氏菌无致病力突变菌株均能在番茄根部定殖,定殖数量呈先上升后下降的趋势,其中菌株T659的定殖数量最大,定殖时间最长,分别为2.86×10⁶CFU/g和35 d;透射电镜观察发现,青枯雷尔氏菌无致病力突变菌株T659从番茄植株根部表皮细胞中侵入,然后进入维管束厚壁细胞,并在维管束细胞中大量繁殖和定殖,但未引起番茄根部细胞结构病理变化。表明供试的青枯雷尔氏菌无致病力突变菌株T659的定殖能力最强,具有良好的生防潜力。     

Colonization of resistant and susceptible lettuce cultivars by a green fluorescent protein-tagged isolate of Verticillium dahliae

Interactions between lettuce and a green fluorescent protein (GFP)-expressing, race 1 isolate of Verticillium dahliae, were studied to determine infection and colonization of lettuce cultivars resistant and susceptible to Verticillium wilt. The roots of lettuce seedlings were inoculated with a conidial suspension of the GFP-expressing isolate. Colonization was studied with the aid of laser scanning confocal and epi-fluorescence microscopes. Few differences in the initial infection and colonization of lateral roots were observed between resistant and susceptible cultivars. Hyphal colonies formed on root tips and within the root elongation zones by 5 days, leading to the colonization of cortical tissues and penetration of vascular elements regardless of the lettuce cultivar by 2 weeks. By 8 to 10 weeks after inoculation, vascular discoloration developed within the taproot and crown regions of susceptible cultivars well in advance of V. dahliae colonization. Actual foliar wilt coincided with the colonization of the taproot and crown areas and the eruption of mycelia into surrounding cortical tissues. Advance colonization of stems, pedicels, and inflorescence, including developing capitula and mature achenes was observed. Seedborne infection was limited to the maternal tissues of the achene, including the pappus, pericarp, integument, and endosperm; but the embryo was never compromised. Resistant lettuce cultivars remained free of disease symptoms. Furthermore, V. dahliae colonization never progressed beyond infected lateral roots of resistant cultivars. Results indicated that resistance in lettuce may lie with the plant's ability to shed infected lateral roots or to inhibit the systemic progress of the fungus through vascular tissues into the taproot.     

Microscopy of invasion of red clover roots by Trichocladium basicola, and effects of benomyl and prochloraz

Roots of red clover seedlings grown on plates of water agar, or water agar containing benomyl or prochloraz, were inoculated with conidia of Trichocladium basicola and examined by light and transmission electron microscopy. Penetration of host epidermal cells occurred from about 16 h after inoculation of untreated or fungicide-treated seedlings. Intracellular hyphae were constricted at septa and had a beaded appearance. They invaginated the host plasmalemma, but had no obvious deleterious effect on the cytoplasm until they had grown to fill much of the lumen, when host cells degenerated and died. As colonization of the cortex progressed, straight, unconstricted hyphae were formed and from these reproductive hyphae developed, which produced endoconidiophores and chlamydospores on the root surface. Penetration of host cell walls appeared to involve localized action of fungal enzymes. Papillae were often found at sites of penetration, but these rarely obstructed fungal development. Seedlings treated with prochloraz had fewer sites of fungal penetration, and fewer cells in the beaded hyphae than untreated seedlings or those treated with benomyl. Both fungicides caused abnormalities in fungal ultrastructure. Hyphae treated with benomyl were often found to contain lomasomes, while those treated with prochloraz had thickened, fragmented walls, and disorganized cell contents.     

年际间干旱对晚疫病菌生理小种复杂性的影响

由致病疫霉Phytophthora infestans引起的晚疫病是马铃薯生产中最具毁灭性的病害,生理小种是晚疫病菌最重要的生物学特性之一,其组成直接关系到晚疫病的发生与流行。本研究以2007年黑龙江省干旱为契机,对2004-2010年采自黑龙江省的晚疫病菌菌株进行生理小种鉴定,结果表明,382个晚疫病菌菌株中共鉴定出91个生理小种类型,2004-2006年的优势小种为3.4.6.7.8.9.10.11,2007-2009年的优势小种为3.4.7.11,2010年的优势小种为1.3.4.7.8.10.11。2007年生理小种的复杂程度为历年最低,并影响了2008年与2009年的生理小种组成,2010年的生理小种复杂程度则出现明显上升。     

小麦条锈病流行小种对小麦新品系产量的影响

研究了小麦条锈病流行小种条中30、31、32对不同小麦品种(系)的影响.结果表明:目前小麦条锈病流行小种因品种(系)不同可平均减产1 206kg/hm2,减幅15.1%,最大减产3076.50kg/hm2,减幅45.9%;千粒重平均下降5.52g,降幅14%,最大下降14.0g,降幅40%;穗粒数的差异较小,平均下降0.01粒,降幅0.13%.相关分析表明,条锈病的反应型和严重度是影响小麦产量和千粒重的主要因素,相关系数均达极显著水平;普遍率也具有一定的影响,但未达到显著水平.根据减产程度将材料分为三种类型,即按接种与药剂防治的差异分为极显著、显著和不显著.我们认为:小麦条锈病对产量影响较大,生产中应注意利用接种与药剂防治差异不显著的抗锈材料;在缺乏抗锈材料的地区,如果材料不抗条锈病,但产量等其他性状突出,则可以考虑采用药剂防治.