Genetic diversity and pathotype determination of Colletotrichum sublineolum isolates causing anthracnose in sorghum

Amplified fragment length polymorphism (AFLP) based genetic diversity was analyzed for 232 Colletotrichum sublineolum isolates collected between 2002 and 2004 from three geographically distinct regions of Texas, and from Arkansas, Georgia, and Puerto Rico. Results revealed significant levels of polymorphism (59%) among the isolates. Even so, genetic similarity between isolates was high, ranging from 0.78 to 1.00. Clustering of similar isolates did not correlate with either geographic origin or year of collection. Pathotypes of 20 of the isolates were determined using 14 sorghum lines previously used in Brazil and the United States and 4 from Sudan. Seventeen new pathotypes were established from the 18 isolates that gave uniform and consistent reactions on all host differentials over 2?years of greenhouse testing. Differentials BTx378 and QL3 were resistant to all isolates while BTx623 and TAM428 were universally susceptible both years. Each of these lines had shown differential responses in prior studies indicating that the pathogen population has sufficient diversity to adapt rapidly to changes in resistant host lines deployed. When the 2-step pathotype classification scheme was used, the 18 isolates examined in this study were placed in four pathotype groups (A, C, D and G), which would further then be separated into ten distinct pathotypes. Common sets of differentials and a standardized nomenclature will allow for comparison to be made among pathotypes of C. sublineolum detected from different regions and also could help direct planting of appropriate sorghum lines and aid in the development of more durable forms of resistance.     

Genetic differentiation and gene flow in Colletotrichum sublineolum in Ethiopia,the centre of origin and diversity of sorghum,as revealed by AFLP analysis

Isolates of Colletotrichum sublineolum were collected from different sorghum‐producing regions of Ethiopia and divided into five groups based on their geographic origin. The growth rate of 50 isolates showed considerable variation: 1·7–5·8 mm day^?1, mean 3·3 mm day^?1. However, the isolates displayed little variation in colony colour and colony margin, except for isolates from the north, which were different from the others. Amplified fragment length polymorphism analysis of 102 isolates revealed much greater variations among the different groups. Dice similarity coefficients ranged from 0·32 to 0·96 (mean 0·78). Cluster analysis and principal coordinate analysis revealed a differentiation of the isolates according to their geographic origin, and both methods clearly indicated a genetic separation between the southern, the eastern and the other isolates. Analysis of molecular variance (amova ) indicated a high level of genetic variation both among (42%) and within (58%) the C. sublineolum sampling sites in Ethiopia. The amova also indicated a high level of genetic differentiation (F_ST = 0·42) and limited gene flow (Nm = 0·343). The results of this study confirmed the presence of a highly diverse pathogen, which is in agreement with the existence of diverse host genotypes and widely ranging environmental conditions in sorghum‐producing regions of the country. Such diversity should be taken into account in future breeding programmes to achieve an effective and sustainable disease management strategy.     

Carbon dioxide assimilation by virus-free sugarcane plants and by plants which were infected by Sugarcane Yellow Leaf Virus

Infection of sugarcane by Sugarcane Yellow Leaf Virus (SCYLV) remains mostly asymptomatic. The assimilation rates and the water relations parameters were measured to test whether the plants are already inflicted by the infection. The assimilation rate and the stomatal conductance were 10–30% higher in virus-free plants than in infected plants. The observed differences were significant in young potted plants, not in older field plants. Virus-free leaves had a higher bundle sheath leakiness and a lower ¹³C discrimination rate than infected leaves. The water relations parameters of SCYLV-infected asymptomatic plants showed resemblance to those of salinity- and drought-stressed plants.     

Carbon dioxide assimilation by virus-free sugarcane plants and by plants which were infected by Sugarcane Yellow Leaf Virus

Infection of sugarcane by Sugarcane Yellow Leaf Virus (SCYLV) remains mostly asymptomatic. The assimilation rates and the water relations parameters were measured to test whether the plants are already inflicted by the infection. The assimilation rate and the stomatal conductance were 10–30% higher in virus-free plants than in infected plants. The observed differences were significant in young potted plants, not in older field plants. Virus-free leaves had a higher bundle sheath leakiness and a lower ¹³C discrimination rate than infected leaves. The water relations parameters of SCYLV-infected asymptomatic plants showed resemblance to those of salinity- and drought-stressed plants.     

Nitric oxide reduces oxidative stress generated by lactofen in soybean plants

Nitric oxide (NO) is a molecule able to directly scavenge ROS and end chain reactions, which can be generated by some herbicides. This study aimed to evaluate whether the pretreatment of soybean plants with sodium nitroprusside (SNP) solution, a NO-donor substance, provides protection against oxidative stress generated by lactofen. Soybean plants were pretreated with SNP before lactofen application. The levels of lipoperoxides and photosynthetic pigments were quantified, and the activity of the antioxidant enzymes glutathione S-transferase (GST), superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) was assessed. Although lipid peroxidation was not completely prevented, NO was able to scavenge ROS generated by the lactofen action, avoiding the photosynthetic pigment breakdown. Consequently, ROS scavenging by NO leads to a decrease in the available substrate for the antioxidant enzymes SOD, CAT, and POD, which are essential to protect plants under oxidative stress situations such as absence of GST induction by H₂O₂.     

Photosynthesis and sugar concentration are impaired by the defective active silicon uptake in rice plants infected with Bipolaris oryzae

The effect of soluble silicon (Si) on photosynthetic parameters and soluble sugar concentrations was determined in leaves of rice cv. Oochikara and mutant plants of Oochikara defective in active Si uptake [low silicon 1 (lsi1)]. Plants were grown in hydroponic culture amended with 0 (?Si) or 2 mm Si (+Si), under either low or high photon flux density (PFD) and with or without inoculation with Bipolaris oryzae, the causal agent of brown spot of rice. Leaf Si concentration increased by 141 and 435% in +Si cv. Oochikara and by 119 and 251% in +Si lsi1 mutant plants under high and low PFD, respectively, compared with ?Si plants. Plant biomass accumulation was improved by Si regardless of PFD, especially plants for cv. Oochikara. Brown spot severity was highest in ?Si plants for cv. Oochikara and lsi1 mutant plants under low PFD. In the presence of Si, disease severity in plants grown under both low and high PFD was reduced, except for lsi1 mutant plants under high PFD. Plant inoculation reduced the photosynthetic parameters measured regardless of plant material or Si supply. A decrease of net carbon assimilation rate (A) of inoculated plants under low PFD compared with non‐inoculated plants was associated with damage in the photosynthetic apparatus, except for +Si cv. Oochikara in which stomatal restriction [low water vapour conductance (g_s)] contributed to A reduction. Under high PFD, damage to the photosynthetic apparatus of inoculated plants was the main reason for the reduction in A for +Si and ?Si lsi1 mutant plants. In addition, for ?Si cv. Oochikara, a reduction in g_s contributed to reduced A. However, for +Si cv. Oochikara, g_s was the limiting factor for A. Inoculated plants of +Si cv. Oochikara had higher A values than +Si lsi1 mutant plants, regardless of environmental conditions. Soluble sugars were not detected in leaf tissues of plants under low PFD. For high PFD, Si improved the hexose concentration in non‐inoculated plants at 144 h after inoculation (hai) for lsi1 mutant plants and from 96 hai onwards for cv. Oochikara compared with ?Si plants. However, plant inoculation reduced hexose concentration compared with non‐inoculated plants, mainly in +Si plants, regardless of plant material. Sucrose concentration increased in leaves of cv. Oochikara in the presence of Si whether inoculated or not. For +Si lsi1 mutant plants, sucrose concentration increased only at 48 hai compared with ?Si plants, whether inoculated or not. The results of this study show that a minimum Si concentration is needed in leaf tissues of rice plants to avoid the negative impact of B. oryzae infection on photosynthesis and sugar concentration. High leaf Si concentration resulted in an increased soluble sugar concentration and together, but in independent ways, soluble sugar and Si reduced brown spot severity of rice.     

Strobilurin fungicides induce changes in photosynthetic gas exchange that do not improve water use efficiency of plants grown under conditions of water stress

The effects of five strobilurin (beta-methoxyacrylate) fungicides and one triazole fungicide on the physiological parameters of well-watered or water-stressed wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and soya (Glycine max Merr.) plants were compared. Water use efficiency (WUE) (the ratio of rate of transpiration, E, to net rate of photosynthesis, A(n)) of well-watered wheat plants was improved slightly by strobilurin fungicides, but was reduced in water-stressed plants, so there is limited scope for using strobilurins to improve the water status of crops grown under conditions of drought. The different strobilurin fungicides had similar effects on plant physiology but differed in persistence and potency. When applied to whole plants using a spray gun, they reduced the conductance of water through the epidermis (stomatal and cuticular transpiration), g(sw), of leaves. Concomitantly, leaves of treated plants had a lower rate of transpiration, E, a lower intercellular carbon dioxide concentration, c(i), and a lower net rate of photosynthesis, A(n), compared with leaves of control plants or plants treated with the triazole. The mechanism for the photosynthetic effects is not known, but it is hypothesised that they are caused either by strobilurin fungicides acting directly on ATP production in guard cell mitochondria or by stomata responding to strobilurin-induced changes in mesophyll photosynthesis. The latter may be important since, for leaves of soya plants, the chlorophyll fluorescence parameter F(v)/F(m) (an indication of the potential quantum efficiency of PSII photochemistry) was reduced by strobilurin fungicides. It is likely that the response of stomata to strobilurin fungicides is complex, and further research is required to elucidate the different biochemical pathways involved.     

The role of silicon fertilization in the synthesis of phenolic compounds on chestnut plants infected with P. cinnamomi and C. parasitica

Journal of Plant Diseases and Protection - Silicon is considered a beneficial nutrient for agricultural crops by conferred protection against diseases. The antifungal effect of the phenolic...     

Visualization of localized pathogen-Induced pH modulation in almond tissues infected by Colletotrichum acutatum using confocal scanning laser microscopy

Modulation of pH within the host during infection of almond by the anthracnose pathogen Colletotrichum acutatum was studied using confocal scanning laser microscopy and the dual emission fluorescence indicator SNARF-1. This highly sensitive method allowed visualization of the spatial distribution of localized pathogen-induced pH modulation within and in proximity to fungal infection structures in host tissue at the cellular level. Ratiometric measurement of fluorescence at two emission wavelengths and in situ calibration allowed the quantification of pH ranges. After incubation of leaf epidermal tissue with SNARF-1, distinct alkaline (pH 8 to > or =9), red-spectrum (650 nm wave length) fluorescent zones developed as partial or complete halos around many fungal appressoria and in infection vesicles at 24 to 36 h after inoculation. In samples taken after 48 to 72 h, colonizing hyphae in the biotrophic phase and subsequently in the necrotrophic phase were also emitting the red fluorescence that extended into the surrounding host tissue, as also verified by depth analyses. Host epidermal cells were intact and apparently alive during the fungal alkalization process, with no visible disruption of cell structure. Generally, the pH of epidermal cells in noninoculated samples or in areas away from the infection in inoculated samples was lower than pH 7 with green (i.e., 500 to 550 nm wave length) fluorescence detected. Using standard electrodes, a significant increase in pH and ammonia concentration in leaf and fruit tissue was also measured but only at advanced stages of disease. In contrast, hyphae of the pathogen Alternaria alternata were mostly acidic and no change in fluorescence was found inside invaded host cells. The sequence of events in the C. acutatum-almond interaction includes penetration, production of ammonia by C. acutatum, and subsequent pH modulation within almond epidermal tissue to an alkaline environment that leads to further colonization of the host.     

The influence of nitrogen on growth and photosynthesis of sorghum infected with Striga hermonthica from different provenances

The influence of Striga hermonthica on sorghum growth and photosynthesis is dependent on the provenance of the parasite seed. Inter-prove-nance differences are correlated with emergence time of the parasite above ground, as well as with the mass (but not number) of Striga her-monthica plants supported by the host. The greater effects of early attachment on host growth and photosynthesis were demonstrated by inoculating S. hermonthica plants to both 3-day-old and 19-day-old hosts. There were statistically significant interactions between ni-trogen supply and provenance on the relationship between (1) host biomass and parasite biomass and (2) photosynthetic impairment and parasite biomass. There was no statistically significant interaction between nitrogen supply and provenance on the relationship between host biomass and parasite emergence time. Thus, ni-trogen may influence provenances through processes which occur post-germination. Poten-tial explanations for inter-provenance difference on these later processes are discussed. Influence de l'azote sur la croissance et la photo-synthèse du sorgho infecté par Striga hermonthica de différentes provenances L'influence de Striga hermonthica sur la croissance et la photosynthèse du sorgho est dépendante de la provenance du parasite. Les différences entre provenances sont corrélées avec la date de levée du parasite ainsi que par la masse (mais pas le nombre) de plantes de Striga supportees par la plante hôte. L'influence prépondérante d'une fixation précoce sur la croissance et la photosynthèse de la plante hôte a été montrée en inoculant S. hermonthicaà des plantes hôtes âgées de 3 et 19 jours. On observait des interactions significatives entre la fourniture d'azote et l'origine du parasite en ce qui concerne la relation entre (1) la biomasse de l'hôte et la biomasse du parasite et (2) l'effet sur la photo-synthfese et la biomasse du parasie. L'interaction ne s'observait pas en ce qui concerne la relation entre la biomasse de l'hote et la date de levee du parasite. L'azote pourrait done interagir avec l'orgine du parasite par le biais de processus qui surviennent apres germination. Les explications possibles des différences entre provenances en ce qui concerne ces derniers processus sont discutées. Einfluß der Stickstoffdüngung auf Wachshim und Photosynthese der Sorghum-Hirse hei Befall mit Striga hermonthica verschiedener Herktinft Die Beeinflussung des Wachstums und der Photosynthese der Sorghum-Hirse durch Striga hermonthica hängt von der Herkunft des Parasiten ab. Der unterschiedliche Einfluß ist sowohl mit dem Zeitpunkt des Austreibens des Parasiten als auch mit der Masse, jedoch nicht der Dichte der Striga-hermonthica-Pflanzen korreliert, Der stärkere Einfluß eines frühen Befalls der Wirtspflanze auf Wachstutn und Photosynthese wurde dadurch erfaßt, daß der Parasit sowohl auf 3 Tage als auch auf 19 Tage alte Wirtspflanzen inokuliert wurde. Die Wechselwirkung zwischen der Stickstoffdüngung und der Herkunft sowohl auf die Beziehung zwischen der Biomasse der Wirtspflanze und der des Parasiten als auch auf die Beziehung zwischen der Photosyntheseleistung und der Biomasse des Parasiten war statistisch signifikant. Die Beziehung zwischen der Biomasse der Wirtspflanze und dem Zeitpunkt des Austreibens des Parasiten wurde durch die Stickstoffdüngung und die Herkunft nicht signifikant beeinflußt. Die Stickstoffdüngung beeinflußt also die Parasitierung mehr durch Prozesse, die nach dem Austreiben ablaufen, als die Keimung und die fruhen Entwick-lungsstadien. Mogliche Erklarungen für die Wirkungsunterschiede je nach Herkunft auf die letztgenannten Prozesse werden diskutiert.     

Spatiotemporal distribution of tomato plants naturally infected with bacterial canker in greenhouses

The distribution of plants with tomato bacterial canker within a greenhouse was analyzed using Morisita’s binomial index of dispersion, I _B , to assess spatial distribution patterns. The distribution patterns of diseased plants were similar in four commercial greenhouses. The degree of clustering of added together diseased plants based on the I _B index did not increase with time, but the statistical significance of the cluster distribution did increase, suggesting that new independent cluster points had formed during the investigation. Therefore, a scattered pattern of potentially or apparently diseased plants caused by primary inoculum from residual plants in the soil emerged with time.     

Inclusion bodies in plants infected with sharka (plum pox) virus

Sharka virus was found to give rise to the formation of inclusion bodies in nucleus and cytoplasm of host cells, as is known for several other viruses of the potato virus Y group. In inoculatedNicotiana clevelandii needle-shaped inclusion bodies were found loosely distributed in the nucleus 10 days after the first external symptoms appeared. In the cytoplasm, bundles of needles and granular inclusions arose 14 and 18 days, respectively, after external symptoms became visible. The intranuclear needles disappeared shortly before or after the first appearance of granular cytoplasmic inclusions. Inclusion bodies abound in parenchyma cells of fruits from sharka-diseased plum trees, but they did not occur in fruits from sharka-free trees, with or without pseudo-pox symptoms. Thus, inclusion bodies can be of value in the diagnosis of sharka and be of great help in differentiating between plum pox and pseudo-pox.     

干旱胁迫下高粱叶温与叶片水分状况的关系

根据叶片温度可以反映植物体内水分状况及气孔运动等一系列生理变化的特征,研究干旱胁迫下高粱叶片温度与水分状况变化间的关系,分析高粱抗旱反应机制。采用4个不同抗旱性高粱品种,在盆栽条件下,分别在拔节期、开花期和灌浆期进行干旱胁迫,以正常灌水为对照,胁迫7 d后测定高粱叶片自由水含量、离体叶片失水速率、相对含水量等水分状况,利用远红外热成像仪测定叶片温度变化。结果表明,干旱胁迫下,高粱叶片自由水含量、束缚水含量、相对含水量和离体叶片失水速率等水分状况在不同生育时期均呈下降趋势,吉杂305（高度抗旱品种）的下降幅度显著小于吉杂127（高度干旱敏感品种）（P＜0.05）,锦杂106（中等抗旱品种）和锦杂103（干旱敏感品种）介于二者之间。叶温差与自由水含量和相对含水量显著相关（P＜0.05）,与束缚水含量极显著相关（P＜0.01）。叶片温度变化可以反映高粱叶片的水分状况,叶温差可以作为高粱抗旱性筛选的一个重要指标,应用远红外热成像技术在高粱抗旱性的鉴定上具有可行性。     

Species from the Colletotrichum acutatum,Colletotrichum boninense and Colletotrichum gloeosporioides species complexes associated with tree tomato and mango crops in Colombia

Colletotrichum species cause typical anthracnose symptoms in tree tomato and mango. To characterize species of Colletotrichum in these two crops in Colombia, 91 isolates were collected from several localities. Phylogenetic analyses using nuclear gene sequencing of the ITS region and the glyceraldehyde 3‐phosphate dehydrogenase (GAPDH) gene allowed the identification of three groups: acutatum, gloeosporioides and boninense. These three groups were further confirmed using two additional genomic regions (chitin synthase 1 and actin) for 30 isolates representative of the three previously identified complexes and one genomic region (ApMat) for the Colletotrichum gloeosporioides complex strains. The entire approach permitted a robust strain identification that allowed phylogenetic species recognition (PSR) based on the identification of well‐supported monophyletic clades and concordance between individual and multilocus phylogenies. Morphological and physiological assays were also conducted. Isolates that were morphologically identified as C. gloeosporioides showed high phenotypic diversity. Pathogenicity data revealed a considerable degree of host preference.     

Synthesis and functioning of membrane lipids in fungi and infected plants

The synthesis and functioning of membrane lipids in saprotrophic fungi and in the physiological relationships between pathogenic fungi and host plants are reviewed briefly. The roles of lipids in membranes of infected plant tissue are discussed in more detail, with particular reference to the host—pathogen interface in biotrophic infections.     

Spatiotemporal distribution of tomato plants naturally infected with leaf mold in commercial greenhouses

The distribution of tomato plants infected by Passalora fulva, causal agent of leaf mold, was analyzed by using Taylor’s model and Iwao’s model to assess the patterns of spatial distribution within the greenhouses. In Taylor’s model, the sample variance (s ²) of diseased plants newly recognized at each observation increased only slightly with mean density (m). In Iwao’s model, the mean crowding (m*) of newly recognized diseased plants at each observation increased with m of diseased plants. The statistical analysis in this study suggests that new infections in greenhouses observed during this investigation tended to cluster around a diseased plant and that secondary infections occurred as independent cluster points.     

First report of anthracnose on Spigelia anthelmia caused by Colletotrichum karstii and Colletotrichum siamense in Brazil

Journal of Plant Diseases and Protection - In Brazil, Spigelia anthelmia is an herbaceous weedy plant normally used for the treatment of helminths in humans and animals. Leaves and stems of S....     

Influence of silicon on some components of resistance to anthracnose in susceptible and resistant sorghum lines

This study aimed to evaluate the effect of silicon (Si) rates on some components of sorghum resistance to anthracnose. Two 2×5 factorial experiments, consisting of two sorghum lines (BR005 and BR009, resistant and susceptible, respectively) and five Si application rates (0, 0.06, 0.12, 0.24 and 0.30 g Si kg⁻¹ of soil) were arranged in a completely randomised design with three replications. Plants from both lines were inoculated with a conidial suspension of Colletotrichum sublineolum (1×10⁶ conidia ml⁻¹) 30 days after emergence. The incubation period (IP), latent period (LP₆₀), area under relative infection efficiency progress curve (AURIEPC), area under anthracnose index progress curve (AUAIPC), final disease severity (FDS), percentage of pigmented leaf area (PLA), and percentage of necrotic leaf area (NLA) were evaluated. Silicon and calcium (Ca) content in leaf tissue of both lines was also determined. The content of Si in leaf tissue increased relative to the control by 55 and 58%, respectively, for the susceptible and resistant lines. There was no significant change in Ca content in leaf tissue for either of the lines; therefore the variations in Si accounted for differences in the level of disease response. The IP for the resistant line was not affected by Si application rates. The LP₆₀ was not evaluated in the resistant line due to the absence of acervuli. For the resistant line, Si application rates had no significant effect on AUAIPC, FDS, percentage of PLA, and percentage of NLA. On the susceptible line, a quadratic regression model best described the effect of Si application rates on IP, LP₆₀, AURIEPC, AUAIPC, FDS, percentage of PLA, and percentage of NLA. The correlation between Si content in leaf tissue of the susceptible line and the AURIEPC, AUAIPC, FDS, PLA, and NLA was negatively significant (r = −0.57, −0.37, −0.40, −0.67, and −0.77, respectively). There was no correlation between Si content and IP or LP₆₀. The correlation between the percentage of PLA with the percentage of NLA was negatively significant (r = −0.74). In conclusion, the results from this study underscore the importance of Si in sorghum resistance to anthracnose particularly for the susceptible line.