Deficiency in silicon uptake affects cytological, physiological, and biochemical events in the rice--Bipolaris oryzae interaction

This study investigated how a defect in the active uptake of silicon (Si) affects rice resistance to brown spot. Plants from a rice mutant (low silicon 1 [lsi1]) and its wild-type counterpart (cv. Oochikara), growing in hydroponic culture with (+Si; 2 mM) or without (-Si) Si, were inoculated with Bipolaris oryzae. Si concentration in leaf tissue of cv. Oochikara and the lsi1 mutant increased by 381 and 263%, respectively, for the +Si treatment compared with the -Si treatment. The incubation period was 6 h longer in the presence of Si. The area under brown spot progress curve for plants from cv. Oochikara and the lsi1 mutant was reduced 81 and 50%, respectively, in the presence of Si. The reduced number of brown epidermal cells on leaves from cv. Oochikara and the lsi1 mutant supplied with Si contributed to the lower lipid peroxidation and electrolyte leakage. The concentration of total soluble phenolics in cv. Oochikara supplied with Si (values of 4.2 to 15.4 μg g(-1) fresh weight) was greater compared with plants not supplied with Si (values of 1.9 to 11.5 μg g(-1) fresh weight). The concentration of lignin was also important to the resistance of cv. Oochikara and the lsi1 mutant. Polyphenoloxidase activity did not contribute to the resistance of cv. Oochikara and the lsi1 mutant to brown spot, regardless of Si supply. Peroxidase and chitinase activities were higher in cv. Oochikara and the lsi1 mutant supplied with Si. These results bring novel evidence of the involvement of Si in a more complex defense mechanism than simply the formation of a physical barrier to avoid or delay fungal penetration.     

The effect of silicon on antioxidant metabolism of wheat leaves infected by Pyricularia oryzae

This study investigated whether the increase in wheat resistance to blast, caused by Pyricularia oryzae, potentiated by silicon (Si) is linked to changes in the activity of antioxidative enzymes. Wheat plants (cv. BR 18) were grown in hydroponic culture with either 0 (–Si) or 2 mm (+Si) Si and half of the plants in each group were inoculated with P. oryzae. Blast severity in the +Si plants was 70% lower compared to the ?Si plants at 96 h after inoculation (hai). Superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX) and glutathione‐S‐transferase (GST) activities were higher in the leaves of the ?Si plants compared with the +Si plants at 96 hai. This indicates that other mechanisms may have limited P. oryzae infection in the +Si plants restricting the generation of reactive oxygen species, obviating the need for increased antioxidative enzyme activity. In contrast, glutathione reductase (GR) activity at 96 hai was higher in the +Si plants than in the ?Si plants. Although the inoculated plants showed significantly higher concentration of malondialdehyde (MDA) than the non‐inoculated plants, lower MDA concentrations were observed in the +Si plants compared with the ?Si plants. The lower MDA concentration associated with decreased activities of SOD, CAT, POX, APX and GST, suggest that the amount of reactive oxygen species was lower in the +Si plants. However, GR appears to play a pivotal role in limiting oxidative stress caused by P. oryzae infection in +Si plants.     

Biochemical responses of coffee resistance against Meloidogyne exigua mediated by silicon

Coffea arabica cultivars Catuaí 44 and IAPAR 59, susceptible and resistant, respectively, to the root knot nematode Meloidogyne exigua, were grown in pots containing Si‐deficient soil amended with either calcium silicate (+Si) or calcium carbonate (?Si). There was an increase of 152 and 100%, respectively, in Si content of root tissue of cvs Catuaí 44 and IAPAR 59 in the +Si compared to the ?Si treatment, but no significant difference between Si treatments for calcium content. Plants, assessed 150 days after inoculation (d.a.i.) showed that the number of galls (NG) and number of eggs (NE) significantly decreased by 16·8 and 28·1% respectively, for susceptible cv. Catuaí 44 in the presence of Si, whilst both NG and NE were significantly lower for cv. IAPAR 59 compared to the susceptible cultivar regardless of Si treatments. In a separate experiment, biochemical assays were carried out 5 and 10 d.a.i. There was no significant difference between Si treatments and cultivars for concentration of total soluble phenolics. The concentration of lignin‐thioglycolic acid (LTGA) derivatives significantly increased by 11·5% in roots of nematode‐inoculated plants of susceptible cv. Catuaí supplied with Si. In roots of inoculated plants of resistant cv. IAPAR 59, the increase was 23 and 10%, respectively, for treatments with and without silicon. Peroxidase (POX), polyphenoloxidase (PPO) and phenylalanine ammonia lyase (PAL) activities significantly increased in roots of inoculated plants compared with roots of non‐inoculated plants, regardless of cultivar or Si treatment. In +Si treatments at 10 d.a.i., POX activity in roots of nematode‐inoculated plants of cvs Catuaí 44 and IAPAR 59 increased by 39·9 and 31·3%, respectively; PPO increased by 54·9 and 56·1%; and PAL activity was also higher at 26·6 and 62·9%. It was concluded that supplying Si to coffee plants increases root resistance against M. exigua by decreasing its reproductive capacity.     

Studies into primary and secondary infection processes by Plasmodiophora brassicae on canola

The early stages of infection of canola roots by the clubroot pathogen Plasmodiophora brassicae were investigated. Inoculation with 1 × 10⁵ resting spores mL^?1 resulted in primary (root hair) infection at 12 h after inoculation (hai). Secondary (cortical) infection began to be observed at 72 hai. When inoculated onto plants at a concentration of 1 × 10⁴ mL^?1, secondary zoospores produced primary infections similar to those obtained with resting spores at a concentration of 1 × 10⁵ mL^?1. Secondary zoospores caused secondary infections earlier than resting spores. When the plants were inoculated with 1 × 10⁷ resting spores mL^?1, 2 days after being challenged with 1 × 10⁴ or 1 × 10⁵ resting spores mL^?1, secondary infections were observed on the very next day, which was earlier than the secondary infections resulting from inoculation with 1 × 10⁷ resting spores mL^?1 alone and more severe than those produced by inoculation with 1 × 10⁴ or 1 × 10⁵ resting spores mL^?1 alone. Compared with the single inoculations, secondary infections on plants that had received both inoculations remained at higher levels throughout a 7‐day time course. These data indicate that primary zoospores can directly cause secondary infection when the host is under primary infection, helping to understand the relationship and relative importance of the two infection stages of P. brassicae.     

Response of photosynthesis and chlorophyll a fluorescence in leaf scald‐infected rice under influence of rhizobacteria and silicon fertilizer

Leaf scald caused by Monographella albescens reduces the photosynthetic area, causing yield losses in rice. This study investigated the efficacy of the rhizobacteria Burkholderia pyrrocinia (BRM‐32113) and Pseudomonas fluorescens (BRM‐32111), combined with silicon (Si) fertilization, to reduce lesion size and the area under the disease progress curve (AUDPC), as well as to minimize the negative effects on gas exchange, chlorophyll a fluorescence, chlorophyll content and the activity of oxidative stress enzymes. The experiment used a completely randomized design with four replications and seven treatments. Compared with plants only fertilized with Si, plants fertilized with Si and treated with BRM‐32113 showed reductions of 22% in scald lesion expansion and 37% in AUDPC, a 27% increase in the rate of CO₂ assimilation (A), a 33% decrease in the internal CO₂ concentration (C_i), and a 40% increase in ascorbate peroxidase activity. It was therefore concluded that the combination of BRM‐32113 with Si fertilization reduces the severity of leaf scald, protecting the photosynthetic apparatus, thus representing a sustainable method of reducing the loss of income caused by leaf scald in rice.     

Defense strategies of pea embryo axes with different levels of sucrose to Fusarium oxysporum and Ascochyta pisi

The aim of this study was to compare the defense responses of embryo axes of Pisum sativum L. cv. Kwestor with different sucrose levels to pathogenic fungi, i.e. systemic acting Fusarium oxysporum f. sp. pisi and locally acting Ascochyta pisi. Embryo axes were cultured on Heller medium for 96 h. Four variants were compared: these included inoculated embryo axes cultured with or without 60 mM sucrose (+Si and −Si) and non-inoculated embryo axes cultured with or without 60 mM sucrose (+Sn and −Sn). After inoculation of the pea embryo axes with pathogenic fungi a generally higher concentration of free radicals was detected by electron paramagnetic resonance (EPR), in comparison to non-inoculated embryo axes. The inoculation with F. oxysporum caused stronger generation of free radicals in −Si than in +Si embryo axes. A different response was observed after inoculation with A. pisi; starting from 48 h, the concentration of free radicals in +Si axes was found to be 1.5 times higher than in −Si embryo axes. The values of spectroscopic splitting coefficients for these radicals suggest that they are semiquinone radicals. The EPR method also revealed Mn²⁺ ion accumulation after 24 h of culture. Over time, high levels of these ions were recorded in +Si embryo axes inoculated with F. oxysporum, while in +Si embryo axes inoculated with A. pisi they decreased. Up to 48 h after inoculation with the pathogenic fungi, Mn²⁺ ion levels were higher in +Si embryo axes than in +Sn axes. The activity of superoxide dismutase (SOD, EC 1.15.1.1) increased in +Si embryo axes up to 72 h after inoculation with pathogenic fungi; however, it was generally lower than in +Sn axes. Catalase activity (CAT, EC 1.11.1.6) increased up to 72 h after inoculation with F. oxysporum and the values were higher than in the non-inoculated tissue. Especially high activity of this enzyme was noted in −Si embryo axes after inoculation with either F. oxysporum or A. pisi. Peroxidase activity (POX, EC 1.11.1.7) towards pyrogallol in embryo axes increased during culture; however, it was lower or similar to that in non-inoculated embryo axes. SOD, CAT and POX zymograms showed that the synthesis of new isoforms was induced after inoculation with pathogenic fungi. Peroxidase isozymes detected by the reaction with diaminobenzidine in native PAGE were intensely stained in +Si embryo axes after inoculation with pathogenic fungi. Respiratory activity of the inoculated tissues was considerably higher than in non-inoculated tissues. The respiration rate was generally much higher in +Si than in −Si embryo axes. Growth of −Si embryo axes was more significantly retarded as a consequence of inoculation than that of +Si embryo axes.These results indicate that, depending on the manner of influence of a pathogenic fungus, both similar and differing defensive strategies may be initiated and a raised sugar levels in pea tissues limit the development of F. oxysporum and A. pisi.     

A Tobacco etch virus NW isolate that overcomes pvr1 and pvr12 resistance in Capsicum sp.

Two important sources of Capsicum annuum (bell pepper) resistance were evaluated for their response to inoculation with two isolates of Tobacco etch virus strain NW (TEV‐NW, genus Potyvirus). The resistant cultivars were CA4 and Dempsey, which contain the pvr1 and pvr1² resistance genes, respectively. TEV‐NW was maintained by mechanical passage in the susceptible pepper cultivar Early Calwonder and Nicotiana tabacum cv. Kentucky 14. In initial experiments, the TEV‐NW isolate maintained in Early Calwonder infected two of seven CA4 plants; however, none of the CA4 plants inoculated with the TEV‐NW isolate maintained in Kentucky 14 were infected. The infected CA4 plants had low virus titres in non‐inoculated leaves and did not develop visible symptoms. When the infected CA4 plants were used as inoculum of additional CA4 plants, all newly inoculated plants became infected, developed systemic symptoms and accumulated virus in non‐inoculated leaves more quickly than the originally infected CA4 plants. This new NW isolate, referred to as NW‐CA4, was shown to overcome the resistances expressed by both CA4 (pvr1) and Dempsey (pvr1²). The potyviral VPg is believed to be the determinant for pvr1 and pvr1² resistance genes, both of which are eIF4E‐encoding genes. The VPg amino acid sequence for NW‐CA4 was determined and compared with that of NW isolates and different TEV strains. No amino acid variation was identified that explained the infectivity of NW‐CA4 in CA4 and Dempsey plants.     

Comparison of root and foliar applications of potassium silicate in potentiating post‐infection defences of melon against powdery mildew

The application of silicon to the roots or leaves reduces the severity of powdery mildew (Podosphaera xanthii) in melon but the latter treatment is less effective. This study compared key biochemical defence responses of melon triggered by P. xanthii after root or foliar treatment with potassium silicate (PS). Treatments consisted of pathogen‐inoculated or mock‐inoculated plants supplied with PS via roots or foliarly, as well as a non‐treated control. The activity of defence enzymes and the concentration of phenolic compounds, lignin and malondialdehyde were determined from leaf samples at different time points after inoculation. Pathogen‐inoculated plants irrigated with PS showed both an accumulation of silicon and primed defence responses in leaves that were not observed in pathogen‐inoculated plants either sprayed with PS or not treated. These responses included the anticipated activity of peroxidase and accumulation of soluble phenols, the activation of chitinase and repression of catalase, and the stronger activation of superoxide dismutase, peroxidase and β‐1,3‐glucanase. Moreover, the lignin concentration increased in response to inoculation, whereas the malondialdehyde concentration decreased. For the foliar treatment, however, only an increase in lignin deposition was observed compared with the control plants. The results show that silicon strongly plays an active role in modulating the defence responses of melon against P. xanthii when supplied to the roots as opposed to the foliage.     

Programmed cell death pathways induced by early plant‐virus infection are determined by isolate virulence and stage of infection

Programmed cell death (PCD) pathways caused by Turnip mosaic virus (TuMV) infection before symptom appearance were studied by light microscopy and electrolyte leakage following sap inoculation of Brassica carinata (Ethiopian mustard) TZ‐SMN‐44‐6 plants. Leaf responses to inoculation with avirulent (TuMV‐avir) and virulent (TuMV‐vir) isolates, and mock‐inoculation, were compared at 2, 20 and 52 h after inoculation (hai). The phenotypes induced were localized resistance (TuMV‐avir) and systemic susceptibility (TuMV‐vir). No visible TuMV symptoms were recorded in any inoculated plants during the 2–52 hai sampling period, but appeared as chlorotic spots in inoculated leaves at 5 days after inoculation. With TuMV‐vir alone, they were followed by systemic infection (mosaic). Dead cell number, deformation, percentage area and percentage integrated intensity, and conductivity of electrolyte leakage data, were analysed to examine their possible roles in stimulating cell death pathways. At 2 hai, dead cell number and percentage area were significantly greater for TuMV‐avir than TuMV‐vir infection or mock‐inoculation. Overall, isolate TuMV‐vir caused significantly greater cell deformation than TuMV‐avir, whereas wounding by mock‐inoculation had negligible effects. By 52 hai, isolate TuMV‐avir caused significantly greater electrolyte leakage than isolate TuMV‐vir or mock‐inoculation. This suggests both isolates triggered morphological changes consistent with apoptotic‐like PCD and necrosis‐like PCD that depended upon isolate virulence and stage of infection, respectively. These findings highlight how quantification of dead cell deformation and electrolyte leakage offer a new understanding of compatible and incompatible plant responses to early virus infection in plants.     

Effects of plant age on disease development and virulence of Clavibacter michiganensis subsp. michiganensis on tomato

The effect of plant age at the time of inoculation on the severity of bacterial wilt and canker disease caused by Clavibacter michiganensis subsp. michiganensis (Cmm) was examined in six greenhouse experiments. The period during which inoculations led to wilt and death of tomato plants was defined. This period, designated ‘window of vulnerability’, ranged from transplanting to the 17‐ to 18‐leaf stage. Plants inoculated after this period expressed disease symptoms but did not wilt or die. No significant changes in disease incidence were observed when leaves of different ages were inoculated. Yield accumulation was significantly reduced in plants inoculated within the window of vulnerability compared with those inoculated after this period. Expression of virulence genes, viz. celA, encoding a secreted cellulase, and the serine protease‐encoding pat‐1, chpC and ppaA, was induced during the early stages after inoculation in plants inoculated within the window of vulnerability. Differences in Cmm population between plants inoculated within and outside of this period were insignificant after the first week post‐inoculation, indicating that differences in disease severity, yield loss and expression of virulence determinants are not correlated with Cmm population level. Results suggest that implementation of precautionary measures during the window of vulnerability to avoid secondary spread of Cmm will have a season‐long effect on plant mortality and may minimize, or even prevent, yield losses.     

Co‐infection by Botryosphaeriaceae and Ilyonectria spp. fungi during propagation causes decline of young grafted grapevines

Decline of newly planted, grafted grapevines is a serious viticultural problem worldwide. In the Riverina (New South Wales, Australia), characteristic symptoms include low fruit yields, very short shoots and severely stunted roots with black, sunken, necrotic lesions. To determine the cause, roots and wood tissue from affected plants in 20 vineyards (Vitis vinifera cv. Chardonnay grafted to V. champini cv. Ramsey rootstock) were assayed for microbial pathogens. Ilyonectria spp. (I. macrodidyma or I. liriodendra, producers of phytotoxin brefeldin A, BFA, and cause of black foot disease of grapevines) and Botryosphaeriaceae spp. (predominantly Diplodia seriata) were isolated from rootstocks of 100 and 95% of the plants, respectively. Togninia minima and Phaeomoniella chlamydospora (cause of grapevine Petri disease) were isolated from 13 and 7% of affected plants, respectively. All Ramsey rootstock stems of grafted plants sampled from a supplier nursery were infected with Ilyonectria spp. and D. seriata. Diplodia seriata, but not Ilyonectria spp., was also isolated from 25% of canes sampled from the rootstock source block. Root inoculation of potted, disease‐free Chardonnay plants with Ilyonectria isolates from diseased vineyards caused typical disease symptoms, while co‐inoculation with Botryosphaeriaceae spp. increased disease severity. This is the first study to show that a major cause of young grapevine decline can be sequential infection by Botryosphaeriaceae from rootstock cuttings and Ilyonectria spp. from nursery soil. Although the Petri disease fungi were less common in young declining grafted grapevines in the Riverina, they are likely to contribute to the decline of surviving plants as they mature.     

苹果炭疽叶枯病病原Glomerella cingulata及其侵染过程

为明确苹果炭疽叶枯病病原菌围小丛壳Glomerella cingulata的侵染致病特征,在分离获得该病原菌的基础上,采用形态学观察、ITS序列分析和致病性测定对其进行了鉴定,并利用光学和扫描电子显微镜对病原菌在嘎啦苹果叶片上的侵染过程进行了研究.结果表明,在陕西咸阳地区分离获得的9株病原菌均为围小丛壳G.cingulata.25 ℃下接种9 h后,分生孢子中间产生隔膜,双胞化,并萌发产生芽管和附着胞;24 h后分生孢子的2个细胞均可萌发并形成芽管及附着胞,部分芽管顶端可产生次级分生孢子;48 h后次级分生孢子萌发形成附着胞;72 h后,附着胞下形成的侵染钉可直接入侵寄主,在表皮细胞内形成初生菌丝和次生菌丝,此时叶片表面已出现褐色斑点.接种7 d后叶片病斑处出现分生孢子盘和子囊壳.表明陕西省近年出现的苹果炭疽叶枯病病原菌为围小丛壳G.cingulata,该病菌在嘎啦叶片上的一些特殊侵染行为可能是导致该病害易在短时间内暴发的重要原因.     

Susceptibility of wild carrot (Daucus carota ssp. carota) to Sclerotinia sclerotiorum

Sclerotinia soft rot, caused by Sclerotinia sclerotiorum, is a severe disease of cultivated carrots (Daucus carota ssp. sativus) in storage. It is not known whether Sclerotinia soft rot also affects wild carrots (D. carota ssp. carota), which hybridise and exchange genes, among them resistance genes, with the cultivated carrot. We investigated the susceptibility of wild carrots to S. sclerotiorum isolates from cultivated carrot under controlled and outdoor conditions. Inoculated roots from both wild and cultivated plants produced sclerotia and soft rot in a growth chamber test. Two isolates differed significantly in the ability to produce lesions and sclerotia on roots of both wild carrots and cv. Bolero. Flowering stems of wild carrots produced dry, pale lesions after inoculation with the pathogen, and above-ground plant weight was significantly reduced 4 weeks after inoculation in a greenhouse test. Wild and cultivar rosette plants died earlier and fewer plants survived when inoculated with the pathogen under outdoor test conditions. Cultivar plants died earlier than wild plants, but survived as frequently. Plants inoculated in the crown died earlier and at a lower frequency than plants inoculated on leaves. Wild carrots may thus serve as a host of S. sclerotiorum and thus eventually benefit from any uptake of resistance genes, among them transgenes, via introgression from cultivated carrots.     

Leaf gas exchange and oxidative stress in sorghum plants supplied with silicon and infected by Colletotrichum sublineolum

Considering the economic importance of anthracnose, caused by Colletotrichum sublineolum, and silicon (Si) to enhance sorghum resistance against this disease, this study aimed to investigate the effect of this element on leaf gas exchange and also the antioxidative system when infected by C. sublineolum. Plants from sorghum line CMSXS142 (BR 009 [Tx623] - Texas), growing in hydroponic culture with (+Si, 2 mM) or without (-Si) Si, were inoculated with C. sublineolum. Disease severity was assessed at 2, 4, 6, 8, and 10 days after inoculation (dai) and data were used to calculate the area under anthracnose progress curve (AUAPC). Further, the net carbon assimilation rate (A), stomatal conductance to water vapor (g(s)), internal-to-ambient CO? concentration ratio (C(i)/C(a)), and transpiration rate (E); the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR); the electrolyte leakage (EL), and the concentrations of hydrogen peroxide (H?O?) and malondialdehyde (MDA) were determined. The AUAPC was reduced by 86% for the +Si plants compared with the -Si plants. The values of A, g(s), and E were lower upon inoculation of -Si plants in contrast to inoculated +Si plants with decreases of 31 and 60% for A, 34 and 61% for g(s), and 27 and 57% for E, respectively, at 4 and 8 dai. For the noninoculated plants, there was no significant difference between the -Si and +Si treatments for the values of A, g(s), and E. The C(i)/C(a) ratio was similar between the -Si and +Si treatments, regardless of the pathogen inoculation. The activities of SOD, CAT, APX, and GR tended to be higher in the +Si plants compared with the -Si plants upon inoculation with C. sublineolum. The EL significantly increased for -Si plants compared with +Si plants. The MDA concentration significantly increased by 31 and 38% at 4 and 8 dai, respectively, for the -Si plants compared with the +Si plants. Based on these results, Si may have a positive effect on sorghum physiology when infected by C. sublineolum through the maintenance of carbon fixation and also by enhancing the antioxidant system, which resulted in an increase in reactive oxygen species scavenging and, ultimately, reduced damage to the cell membranes.     

Influence of magnesium on physiological responses of wheat infected by Pyricularia oryzae

Although magnesium (Mg) is considered an essential element for wheat growth, its importance for disease control has often been overlooked, and the physiological features of diseased plants mediated by Mg remain elusive. In this study, the effect of three Mg concentrations (0·25, 2·5 and 4 mm ) on wheat resistance to leaf blast (Pyricularia oryzae), leaf gas exchange, invertase activity, cellular damage and foliar concentration of photosynthetic pigments and nutrients was investigated. Foliar Mg increased from 1·9 to 3·9 g kg^?1, whereas calcium (Ca) decreased from 7·8 to 4·9 g kg^?1 as the applied Mg increased from 0·25 to 4 mm . Blast severity increased from 11·3 to 39·6% as the applied Mg increased from 0·25 to 4 mm . Photosynthesis, stomatal conductance, transpiration and photosynthetic pigment concentrations decreased in inoculated plants compared to non‐inoculated plants regardless of the Mg concentration; however, the reductions were more pronounced for plants grown with 4 mm Mg than those grown with 0·25 mm Mg. On the other hand, a higher internal CO₂ concentration, invertase activity and malondialdehyde concentration was recorded in inoculated plants grown with 4 mm Mg compared to those grown with 0·25 mm Mg. In conclusion, reduced Ca uptake may partially explain the increased susceptibility of wheat to leaf blast with the highest Mg concentration. Mg‐induced susceptibility to leaf blast appeared responsible for the photosynthetic impairments. These were most probably due to biochemical constraints because plants grown with the highest Mg concentration suffered extensive cellular damage and degradation of photosynthetic pigments as a result of high disease severity.     

Development of three fusarium crown rot causal agents and systemic translocation of deoxynivalenol following stem base infection of soft wheat

Fusarium pseudograminearum, F. culmorum and F. graminearum are the most important fusarium crown rot (FCR) causal agents. They have the common ability to biosynthesize deoxynivalenol (DON). To elucidate the behaviour of each of the three species, a comparative study was carried out to investigate symptom progression, fungal systemic growth and translocation of DON following stem base inoculation of soft wheat. FCR symptoms were mainly localized in the inoculated area, which extended up to the second node for all inoculated species. Only the most aggressive strains caused symptoms up to the third node. Real‐time quantitative PCR showed that fungal colonization reached the third node for all the tested species, but a low percentage of plants showed colonization above the third node following inoculation with the most aggressive strains. Fungal growth was detected in symptomless tissues but none of the three species was able to colonize as far as the head tissues. However, even if the pathogens were not detected in the heads, DON was detected in head tissues of the plants inoculated with the most aggressive strains. These results demonstrate that F. pseudograminearum, F. culmorum and F. graminearum, under the same experimental conditions, follow a similar pattern of symptom progression, fungal colonization and DON translocation after stem base infection. Differences in the extent of symptoms, fungal colonization and mycotoxin distribution were mainly attributable to strain aggressiveness. These findings provide comparative information on the events following infection of the stem base of wheat by three of the most important FCR casual agents.     

Phenylalanine ammonia lyase activity in chilli CM-334 infected by Phytophthora capsici and Nacobbus aberrans

We tested the hypothesis that PAL activity in chilli plants CM-334 inoculated with Nacobbus aberrans (Na) alone or in combination with Phytophthora capsici (Pc), is lower than in those inoculated only with Pc. At 21 days after nematode inoculation, inoculated plants showed a significant (P < 0.01) reduction of 48% in PAL activity compared to those non-inoculated in two separate experiments. In two other tests, where plants were inoculated with the oomycete 21 days after inoculation with the nematode, PAL activity at 2, 4, 6, 8 and 24 h after inoculation with Pc was significantly higher (Tukey, P < 0.01) in plants inoculated only with Pc than in plants inoculated only with Na or both pathogens (Na+Pc).     

Reducing the build‐up of Plasmodiophora brassicae inoculum by early management of oilseed rape volunteers

Clubroot of oilseed rape (OSR), caused by Plasmodiophora brassicae, is a disease of increasing economic importance worldwide. Previous studies indicated that OSR volunteers, Brassica crops and weeds play a critical role in the predisposition of the disease. To determine the effect of timing of foliar application of the herbicide glyphosate or mechanical destruction of OSR volunteers in reduction of clubroot severity and resting spore production, a series of studies was conducted under controlled conditions with a susceptible OSR cultivar and an isolate of P. brassicae. Plants were inoculated by injecting a spore suspension beside the root hairs at growth stage 11–12 (BBCH scale) and were terminated at 7 (early) or 21 (late) days post‐inoculation (dpi). Under controlled conditions, the first symptoms on roots were observed as early as 7 dpi. The early application of glyphosate as well as early mechanical destruction resulted in significant (P ≤ 0.05) reduction in the development of clubroot symptoms, root fresh weight and the number of resting spores?g root. Furthermore, the effect of volunteer management on clubroot severity in the succeeding OSR was studied by inoculating plants with the resting spores obtained from treated clubbed roots. Inoculated OSR exhibited root clubs similar to the initial symptoms after 35 dpi. Plants that were inoculated with spore suspension from early treated roots resulted in significant reductions in clubroot incidence and severity. Conversely, plants inoculated with the spore suspension from the late treated roots displayed levels of clubroot similar to the plants inoculated with the spore solutions of positive controls.     

Factors influencing Ralstonia pseudosolanacearum infection incidence and disease development in rose plants

Glasshouse experiments were conducted to study infection and disease development in rockwool-grown rose plants inoculated with Ralstonia pseudosolanacearum. A R. pseudosolanacearum strain isolated from rose plants was more aggressive than strains from anthurium or curcuma. The three rose cultivars tested, Avalanche, Red Naomi, and Armando, differed in susceptibility. At 20°C, the rose strain caused hardly any symptoms over a 6-week period, whereas at 28°C typical wilt symptoms were observed within 2 weeks after stem inoculation of Armando, the most susceptible cultivar. Inoculating roots with the rose strain resulted only in weak atypical symptoms. Nevertheless, inoculating roots of cv. Armando at a relatively low inoculum dose of 10⁴ cfu/ml led to high densities in the base of stems in one out of two experiments. R. pseudosolanacearum occasionally spread from stem inoculated plants with symptoms in rockwool slabs. This limited spread resulted in a low infection incidence, and only of plants directly adjacent to the plants with symptoms.     

Spread and interaction of Pepino mosaic virus (PepMV) and Pythium aphanidermatum in a closed nutrient solution recirculation system: effects on tomato growth and yield

Pepino mosaic virus (PepMV) was shown to be efficiently transmitted between tomato plants grown in a closed recirculating hydroponic system. PepMV was detected in all plant parts after transmission via contaminated nutrient solution using ELISA, immunocapture RT‐PCR, RT‐PCR, electron microscopy, and by inoculation to indicator plants. Detection of PepMV in nutrient solution was only possible after concentration by ultracentrifugation followed by RT‐PCR. Roots tested positive for PepMV 1–3 weeks after inoculation, and subsequently a rapid spread from the roots into the young leaves and developing fruits was found within 1 week. PepMV was only occasionally detected in the older leaves. None of the infected plants showed any symptoms on fruits, leaves or other organs. Pre‐infection of roots of tomato cv. Hildares with Pythium aphanidermatum significantly delayed PepMV root infections. When mechanically inoculated with PepMV at the 2–4 leaf stage, yield loss was observed in all plants. However, only plants of cv. Castle Rock recorded significant yield losses when infected via contaminated nutrient solution. Yield losses induced by infection with PepMV and/or P. aphanidermatum ranged from 0·4 up to 40% depending on experimental conditions.