Metabolic profiling in the roots of coffee plants exposed to the coffee root-knot nematode, Meloidogyne exigua

To understand the effect of nematode Meloidogyne exigua infestation on coffee plants, resistant and susceptible coffee seedlings were inoculated with second-stage juveniles of M. exigua, and root metabolites were studied for four time intervals at 0, 24, 48 and 96 h. During this important period for parasite establishment, the concentrations of phenols, carbohydrates, amino acids and alkaloids in the roots were measured, and hydrogen nuclear magnetic resonance spectra of the root extracts were used to identify and quantify other metabolites. One of the most striking changes was the concentration of fumaric acid on resistant plants, which varied from 59 μg g(of root)^?1 to 138 μg g(of root)^?1 during the first 24 h of the nematode inoculation. The same level of variation was observed much later (96 h) in susceptible plants. Similarly, formic and quinic acid concentrations increased more rapidly in the resistant plants compared to the susceptibles. Sucrose concentrations increased to 370 % in the first 48 h in the resistant plants but showed no significant variation in the susceptible plant. Besides, the concentration of alkaloids was much higher at 24 and 48 h in the susceptibles compared to the resistant plants. These results suggest that the higher production of sucrose as well as formic, fumaric and quinic acids, and the lower production of alkaloids by the resistant cultivar in the first 48 h after the nematode inoculation are associated with the resistance of coffee plants to M. exigua.     

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.     

The yam nematode (Scutellonema bradys), a potential threat to potato (Solanum tuberosum) production in West Africa

Potato (Solanum tuberosum) production in Africa is rapidly expanding and becoming increasingly important. As its geographical production range broadens, so does its potential to host new pests and diseases. Following the discovery that potato can be affected by Scutellonema bradys, further studies were undertaken to assess its potential pathogenicity on potato under screenhouse and field conditions, and on marketed tubers. Potato plants inoculated with S. bradys produced tubers with substantial cracking and evident tuber rot, compared with tubers from uninoculated plants. Symptoms of nematode infection on tubers included a scaly appearance, surface cracking as well as deeper tissue cracks, distortions, and darkened surface patches. In most cases these patches were related to sub‐surface rot. Nematodes were recovered from the soil, roots and tubers of inoculated plants. Eight weeks after inoculation, the reproduction factor of the nematode was greatest (2·0) at the lowest inoculation rate assessed (1000 nematodes per 2·5‐L pot) and least (0·4) at the highest inoculation rate (5000 nematodes per pot). In the screenhouse, potato tuber weights were low and mostly unaffected by nematode inoculation rate, except at 5000 nematodes per pot. In the field, non‐inoculated plants yielded over nine times more tubers than plants inoculated with 2000 S. bradys. Low densities of S. bradys were also recovered from 10 of 15 (67%) samples collected from market stalls, indicating field infection. This study confirms that potato can host and be damaged by S. bradys, raising its prospect as a likely significant biotic constraint to the crop.     

Thermal requirements for the embryonic development and life cycle of Meloidogyne hispanica

The life cycle of a Portuguese Meloidogyne hispanica isolate on susceptible cv. Easypeel and resistant (Mi‐1.2 gene) cv. Rossol tomato plants was studied in growth chambers at constant temperatures (10–35°C). The development within the egg and hatching were compared to those of a Portuguese M. arenaria isolate. The base temperature was 10·11 and 8·31°C with 179·5 and 235·3 thermal units for M. hispanica and M. arenaria, respectively, suggesting better potential adaptation to low temperatures by M. arenaria than M. hispanica. No egg development occurred at 10 or 35°C. An increase in invasion of tomato roots by M. hispanica second‐stage juveniles (J2s) was correlated with an increase in temperature on both tomato cultivars. Tomato cv. Rossol limited M. hispanica development at 20, 25 and 30°C, but not at 35°C, indicating that these high temperatures blocked the resistance mechanism provided by the Mi‐1.2 gene. At 15°C, J2s penetrated tomato cv. Rossol roots, but failed to develop and establish feeding sites. On tomato cv. Easypeel, nematode development and reproduction occurred at 20, 25 and 30°C, but at 20°C the life cycle was 1·5 and 2·0 times longer than at 25 and 30°C, respectively. No egg production was observed at 15°C. The results of this study showed that M. hispanica is most suited to soil temperatures around 25°C. Predicted climate change might favour the spread of this nematode species into southern Europe and northwards. The thermal requirements for M. hispanica development are analysed and compared with those of M. arenaria, M. hapla, M. incognita and M. javanica.     

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.     

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.     

Genetic variability of Meloidogyne paranaensis populations and their aggressiveness to susceptible coffee genotypes

Meloidogyne paranaensis is one of the most destructive root‐knot nematode (RKN) species parasitizing coffee in Brazil and in the Americas generally. The objectives of this study were to assess the genetic variability, aggressiveness and virulence of seven different M. paranaensis populations on susceptible and resistant Coffea spp. All seven RKN populations were identified by biochemical and molecular methods. Coffee seedlings were inoculated in the greenhouse, and the nematode reproduction factor was used to infer their reproduction on coffee genotypes. Phylogenetic studies showed a low genetic variability in M. paranaensis populations, regardless of the existence of three esterase phenotypes (Est P1, P2 and P2a), except for the population Est P2a from Guatemala, which is genetically different from other M. paranaensis populations from Brazil. The Est P2a and Est P2 (Herculândia, SP, Brazil) populations were the most aggressive on two susceptible C. arabica cultivars under greenhouse conditions. None of the M. paranaensis populations were virulent on resistant coffee genotypes, confirming their resistance to the seven M. paranaensis populations tested. The resistant coffee cultivars, namely Clone 14 INCAPER, Catuaí Vermelho × Amphillo MR2161 (E1 16‐5 III), Apoatã IAC 2258, Timor Hybrid UFV 408‐01 (E1 6‐6 II) and IPR 100, exhibited segregation for resistance in the ratio of 0%, 2.4%, 12%, 26% and 29%, respectively. These are promising results, because they validate resistance against several M. paranaensis populations in different Coffea spp. genetic resources, which can be used in breeding programmes or as rootstocks, such as Apoatã IAC 2258 and Clone 14 INCAPER.     

Post‐infection development and histopathology of Meloidogyne arenaria race 1 on Arachis spp.

The reproductive behaviour of the root‐knot nematode Meloidogyne arenaria race 1 was compared on two wild species of Arachis (A. duranensis and A. stenosperma) and cultivated peanut (A. hypogaea cv. IAC‐Tatu‐ST). The three species were considered moderately susceptible, resistant, and susceptible, respectively. Penetration and development of the root‐knot nematode in the resistant species was reduced in comparison with that occurring in susceptible plants. Several cell features, including dark blue cytoplasm and altered organelle structure were observed in the central cylinder of A. stenosperma, indicating a hypersensitive‐like response (HR) of infested host cells. Neither giant cells, nor nematodes developed beyond the second stage, were found on A. stenosperma. Arachis duranensis showed a delay in the development of nematodes in the roots compared to A. hypogaea. The two wild peanut species were chosen to be the contrasting parents of a segregating population for mapping and further investigation of resistance genes.     

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.     

Responses of avocado to laurel wilt,caused by Raffaelea lauricola

Laurel wilt, caused by Raffaelea lauricola, threatens native and non‐native species in the Lauraceae in the south‐eastern USA. Avocado, Persea americana, is the most important agricultural suscept of laurel wilt. Grafted plants (clonal scions on seedling rootstocks) of 24 cultivars were screened against the disease in the field from 2008 to 2010. Disease was induced with either mycelial plugs or conidial suspensions of R. lauricola. There were significant differences in the severity of disease that developed on different cultivars, and West Indian cultivars were most susceptible (P < 0·05). Simmonds, a West Indian cultivar that comprises 35% of the commercial production in Florida, was consistently susceptible and was used as a standard genotype in different studies. Disease severity increased significantly on cv. Simmonds as plant size (stem diameter) increased (P < 0·0042). In greenhouse studies, internal (sapwood) and external disease severities on cv. Simmonds were correlated (P < 0·0001), and a threshold was evident, in that external symptoms developed only after moderately severe symptoms had developed internally. Latent infection was uncommon; R. lauricola was usually isolated on a semiselective medium or detected via qPCR only from discoloured xylem of inoculated cv. Simmonds. As basipetal movement of the pathogen was common, its movement among trees via root grafts is probable. Greater understanding is needed of the movement of R. lauricola in naturally and artificially infected trees, and whether sufficient tolerance exists in avocado to assist in the management of this important new disease.     

Nematicidal activity of the leaf powder and extracts of Myrtus communis against the root‐knot nematode Meloidogyne javanica

Nematicidal activity of the leaf powder, leaf extracts and formulated leaf extracts of Myrtus communis, an evergreen shrub that is widely distributed in Israel and other Mediterranean countries, was evaluated using the root‐knot nematode Meloidogyne javanica in in vitro and pot experiments. Leaf powder added to sand at 0·1% (w/w) reduced the number of juveniles recovered from the sand by more than 50%. Reduction in galling index and number of nematode eggs on tomato roots was also observed by incorporating the leaf powder at 0·1–0·4% (w/w) in the soil in pot experiments. Leaf powder extracts with methanol or ethanol showed the highest nematicidal activity among all extracts tested. Emulsifiable concentrates of leaf‐paste extract at a concentration as low as 0·005% (a.i., w/w) reduced the number of juveniles recovered from treated sand and the gall index of cucumber seedlings. The extract paste at 26 g m^?2 was also effective in reducing the gall index of tomato plants in field‐plot experiments. The leaf powder at 0·2% and the formulated leaf‐paste extract at 0·02% were also nematicidal to Tylenchulus semipenetrans and Ditylenchus dipsaci, but not to Pratylenchus penetrans or Steinernema feltiae. At least three nematicidal compounds were found in the leaf extract upon fractionation by thin‐layer chromatography. The results suggest that the leaf powder and paste extract of M. communis are potential nematicides against root‐knot nematodes.     

Host and virus effects on reversion in cassava affected by cassava brown streak disease

The phenomenon of virus‐infected plants naturally recovering health is known as ‘reversion’, and is a type of resistance mechanism exploited in some crop plants for disease control. Various parameters were investigated that affect reversion from cassava brown streak disease (CBSD) in three cassava varieties (Albert, Kaleso and Kiroba) that differ in levels of resistance to the disease. Cassava plants were inoculated by grafting with two virus species (Ugandan cassava brown streak virus, UCBSV and Cassava brown streak virus, CBSV) that cause CBSD, and the plants grown from them were subsequently assessed for reversion. The rate of reversion depended on the cassava variety, virus species, and the length and position of the stem cuttings used. A significantly high proportion of progenies were virus‐free (reverted) for the resistant variety Kaleso (64·1% for UCBSV and 54·9% of CBSV), compared to the tolerant variety Kiroba (56·7 and 45·5%) and the susceptible control Albert (38·9 and 35·1%). The highest number of virus‐free plants was generated from short 10 cm long cuttings (e.g. 60·1% for Kaleso for CBSV) compared to 20 cm long stem cuttings (e.g. 21·4% for Albert). Cuttings taken from upper stems of diseased plants produced most virus‐free progenies compared to middle and lower parts. More than 50% virus‐free plants were obtained in the resistant and tolerant varieties. This is a highly valuable finding and could be exploited for developing strategies to control the current CBSD epidemic in eastern and central Africa.     

Pathogenicity and host‐parasite relationships of the root‐knot nematode Meloidogyne incognita on celery

Pathogenicity and host‐parasite relationships in root‐knot disease of celery (Apium graveolens ) caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Naturally and artificially infected celery cv. D’elne plants showed severe yellowing and stunting, with heavily deformed and damaged root systems. Nematode‐induced mature galls were spherical and/or ellipsoidal and commonly contained more than one female, males and egg masses with eggs. Feeding sites were characterized by the development of giant cells that contained granular cytoplasm and many hypertrophied nuclei. The cytoplasm of giant cells was aggregated along their thickened cell walls and consequently the vascular tissues within galls appeared disrupted and disorganized. The relationship between initial nematode population density (Pi) and growth of celery plants was tested in glasshouse experiments with inoculum levels that varied from 0 to 512 eggs and second‐stage juveniles (J₂) mL^?1 soil. Seinhorst's model y = m + (1 – m)z^P–T was fitted to height and top fresh weight data of the inoculated and control plants. The tolerance limit with respect to plant height and fresh top weight of celery to M. incognita race 1 was estimated as 0·15 eggs and J₂ mL^?1 soil. The minimum relative values (m) for plant height and top fresh weight were 0·37 and 0·35, respectively, at Pi ≥ 16 eggs and J₂ mL^?1 soil. The maximum nematode reproduction rate (Pf/Pi) was 407·6 at an initial population density (Pi) of 4 eggs and J₂ mL^?1 soil.     

Hypersensitive-like reaction conferred by the Mex-1 resistance gene against Meloidogyne exigua in coffee

The response of a susceptible coffee cultivar (Caturra) to infection by the root-knot nematode Meloidogyne exigua was compared histologically with that of cv. Iapar 59 possessing the recently identified Mex-1 resistance gene. The reproductive behaviour of the nematode was also compared in the two cultivars. Penetration and development in resistant plants were reduced in comparison with susceptible plants. Several cell features, including dark-stained cytoplasm and altered organelle structure, were observed in the resistant cultivar, indicating a hypersensitive-like (HR) response of the infested host cells. Features of giant cells were sometimes found beside necrotic-like areas, but the corresponding feeding sites were frequently associated with nematodes displaying abnormal shape. Six weeks after inoculation, root systems of cv. Caturra contained significantly more nematodes than those of cv. Iapar 59 (mean values 1574 and 41, respectively). The susceptible cultivar presented a minimum of 11 galls per plant, compared with only one or two galls per plant in the resistant cultivar. The findings are discussed in the context of plant–pathogen interactions.     

Present status of verticillium wilt of olive in Andalucía (southern Spain)1

Verticillium wilt caused by Verticillium dahliae is a disease highly prevalent in newly established olive orchards in Andalucía, southern Spain. Two syndromes of the disease occur in Andalucia, namely apoplexy and slow decline. Apoplexy is characterized by quick dieback of twigs and branches while slow decline consists of rapid drying out of inflorescences together with leaf chlorosis and necrosis. Systematic disease observations carried out in two experimental orchards planted with susceptible cv. Picual indicated that natural recovery of diseased trees occurred over time. Infection and vascular colonization of olive plants by V. dahliae were studied in susceptible (Picual) and resistant (Oblonga) cultivars inoculated with a mildly virulent or a highly virulent cotton-defoliating isolate of V. dahliae. Disease symptoms developed 24–32 days after inoculation in cv. Picual, but at that time plants of cv. Oblonga remained free from symptoms. However anatomical observations and isolations indicated that systemic infections by the two isolates had occurred to a large extent in both cultivars.     

Host status effect of cowpea and sunflower on the populations ofMeloidogyne javanica andRotylenchulus reniformis

A highly susceptible cowpea,Vigna sinensis cv. Baladi plants were tested as trap plants for eitherMeloidogyne javanica orRotylenchulus reniformis under greenhouse conditions. The plants were gathered by cutting them above the surface of the soil or by uprooting them, 1/2, 1, 3, 6, 12, 24, 36 and 48 days after nematode inoculation. Both of the mentioned nematodes began to mature and lay eggs after the 12th day from their inoculation. Hence, it is advised to pull up cowpea plants from 3–12 days after nematode inoculation. After planting sunflower,Helianthus annus cv. Miak replacing cowpea, the nematode populations were higher, in most cases, on sunflower plants replacing cutting cowpea than those on sunflower replacing uprooted cowpea. The highest percentages of nematode reduction were 98.55 and 99.57 forM. javanica and 95.09 and 92.90% forR. reniformis on sunflower plants replacing cutting and uprooted cowpea plants after 12 days from nematode inoculation, respectively.M. javanica andR. reniformis decreased the length and weight of sunflower plants as affected by planting time and method of cowpea harvest. This method of nematode control is cheaper, easy and pollution free.     

Infection of canola by secondary zoospores of Plasmodiophora brassicae produced on a nonhost

Clubroot, caused by Plasmodiophora brassicae, has two infection stages (primary and secondary). Although primary infection occurs in many plant species, secondary infection only continues to completion in susceptible hosts. As part of a larger study of clubroot pathogenesis, secondary zoospores collected from infected root hairs of canola and ryegrass were inoculated onto healthy roots of both plant species. The treatments consisted of all possible combinations of the two plant species and the two sources of inoculum. At 5 days after inoculation, levels of root hair infection were similar and in a range of 50–68% on roots in all of the treatments. Secondary infection was also observed from all of the treatments, with approximately 50% on canola and 40% on ryegrass. The proportion of secondary infection and the number of secondary plasmodia were higher in canola inoculated with zoospores from canola than in ryegrass inoculated with zoospores from ryegrass, with the other combinations intermediate. At 35 days after inoculation, typical clubs developed on 14% of the canola plants inoculated with secondary zoospores from canola, and tiny clubs developed on 16% of the canola plants inoculated with zoospores from ryegrass. Secondary infection occurred in about one-third of ryegrass plants but no clubs developed, regardless of inoculum source. These results indicate that resistance to secondary infection in ryegrass is induced during primary infection. This is the first report that secondary zoospores produced on a nonhost can infect a host and reconfirms that secondary infection can occur in a nonhost.