Effect of alfalfa mosaic virus isolates on forage yield of lucerne (Medicago sativa) in Britain

The effect of alfalfa mosaic virus (A1MV) on lucerne forage yield was investigated in experimental plots in which all plants were indexed for freedom from seed-borne A1MV. Plants which were artificially inoculated with A1MV were tested for successful infection prior to transfer to the plots. Inoculated plots yielded 14-8-22-8% (fresh weight) and 15.0 18.1% (dry weight) less than uninoculated plots. Isolate A1MV-I reduced the yield of five lucerne cultivars and yield was decreased by several A1MV isolates in cv. Vertus. Yield decreases tended to be greatest at the first harvest and progressively smaller at the second and third. Despite aphicide treatments and a cereal barrier crop, ELISA indexing showed natural infection developed in control plots. Most known infected plants failed to produce symptoms and yield decreases were associated with predominantly symptomless infection.     

Histopathology of roots of three tomato cultivars infected with two separate isolates of the false root-knot nematode <Emphasis Type="Italic">Nacobbus aberrans</Emphasis>

The plant-parasitic nematode Nacobbus aberrans attacks weeds and cultivated plants, causing drastic crop yield losses. Several tomato cultivars, such as Superman and Mykonos, are produced by Seminis® as nematode resistant and are widely used in Argentina; their specific resistance response to different nematode species, however, is still unknown. In this study we explored the response of tomato cultivars to the infection of N. aberrans isolates, and determined the host status by performing histological analyses and estimating egg mass index (EMI). Two Argentina isolates (from Lules-Tucumán and Río Cuarto-Córdoba) were tested separately in plants of Superman, Mykonos and Platense cultivars. Plants were maintained in a greenhouse for 90 days; then EMI was estimated in root systems and the material was processed to prepare histological slides and for histochemical test. Infected roots exhibited galls with females and a syncytium (feeding site) developed inside them. The vascular tissues were disorganized and displaced to the periphery; xylem percentage was lower than that in the control plants. All the cultivars were susceptible and developed a close plant-parasite relationship, with Mykonos and Platense cultivars infected with the Lules isolate being the most highly affected, as indicated by their highest EMI values. Superman was the least susceptible cultivar, as evidenced by its lowest EMI values, the amount of starch observed, the presence of thickened cell walls around the nematode and the egg mass, and the low percentage of gall occupied by syncytium.     

Evaluation of Olive Cultivars for Resistance to Verticillium dahliae

Resistance of 23 important olive cultivars to Verticillium dahliae has been evaluated in four experiments under controlled conditions. Nine-month-old nursery olive plants were inoculated with a cotton non-defoliating (ND) (V4) or a cotton defoliating (D) (V117) isolate of V. dahliae. Resistance was evaluated by assessing symptom severity using a 0–4 rating scale and estimating the area under disease progress curves. The percentage of plants killed and of those which recovered from the disease were used as additional parameters for including a particular cultivar into a defined category. Most of the evaluated cultivars were susceptible, although at different levels, to both isolates of V. dahliae. All cultivars were more susceptible to the D pathotype than to the ND one. A group of 11 cultivars, including several important Spanish cultivars, were susceptible or extremely susceptible to both pathotypes of V. dahliae. A second group showed differences of resistance depending on the pathotype used. They were susceptible or extremely susceptible to the D pathotype but resistant or moderately susceptible to the ND one. Finally, 'Frantoio', 'Oblonga' and 'Empeltre' were moderately susceptible to the D isolate of V. dahliae and resistant to the ND one. The resistance of 'Empeltre' was evident by the plant ability to recover from infection with either isolates. 'Empeltre' is considered to be a valuable cultivar for inclusion in breeding programmes for resistance to Verticillium wilt.     

Assessment of Host-Induced Selection on Three Geographic Isolates of Heterodera schachtii Using RAPD and AFLP Markers

ABSTRACT The hypothesis that host plants exert selection pressure on Heterodera schachtii populations was tested. Host selection of genotypes from three genetically distinct isolates of H. schachtii was assessed using cabbage, sugar beet, oilseed radish (Raphanus sativus), and white mustard (Sinapis alba). The plants represent a range of susceptibility to H. schachtii and included R. sativus and S. alba, because cultivars of those species have been used as trap crops for H. schachtii in Europe. Genotypic differences in amplified fragment length polymorphism (AFLP) and random amplified polymorphic DNA (RAPD) markers were detected among the isolates after they reproduced on the different hosts. The poorest host plant, R. sativus, resulted in the greatest number of changes in both AFLP and RAPD markers. Oilseed radish selected nematode genotypes in less than four nematode generations. The nematode population genotypes detected by RAPD analyses after selection on oilseed radish were observed even after nematode populations were transferred back to the other three hosts. The genetic markers that were detected after selection were influenced by the genotypes of the original nematode isolates. The results indicate the utility of RAPDs and AFLPs for identifying and monitoring intraspecific genetic variability in nematodes and for understanding nematode population responses to host plants. Nematode management practices such as using resistant cultivars may alter gene frequencies, thereby reducing the efficacy of the tactic and exacerbating the nematode's potential to damage subsequent crops.     

The reproductive potential of the root-knot nematode <Emphasis Type="Italic">Meloidogyne incognita</Emphasis> is affected by selection for virulence against major resistance genes from tomato and pepper

The emergence of virulent root-knot nematode populations, able to overcome the resistance conferred by some of the resistance genes (R-genes) in Solanaceous crops, i.e., Mi(s) in tomato, Me(s) in pepper, may constitute a severe limitation to their use in the field. Research has been conducted to evaluate the durability of these R-genes, by comparing the reproduction of several laboratory-selected and wild virulent Meloidogyne incognita isolates, on both susceptible and resistant tomatoes and peppers. We first show that the Me1 R-gene in pepper behaves as a robust R-gene controlling avirulent and virulent Me3, Me7 or Mi-1 isolates. Although the reproductive potential of the virulent isolates was highly variable on susceptible and resistant plants, we also confirm that virulence is highly specific to a determined R-gene on which selection has occurred. Another significant experimental result is the observation that a reproductive fitness cost is associated with nematode virulence against Mi-1 in tomato and Me3 and Me7 in pepper. The adaptative significance of trade-offs between selected characters and fitness-related traits, suggests that, although the resistance can be broken, it may be preserved in some conditions if the virulent nematodes are counter-selected in susceptible plants. All these results have important consequences for the management of plant resistance in the field.     

A new aggressive strain of Verticillium albo-atrum in Verticillium resistant cultivars of tomato in the Netherlands

In 1991 serious losses caused byVerticillium wilt were found on two holdings in the Westland glasshouse district in the Netherlands in which theVerticillium resistant tomato cultivars Calypso and Criterium were grown in soilless systems. Isolates from diseased plants were identified asVerticillium albo-atrum.In inoculation experimentsVerticillium resistant tomato cultivars were seriously affected by the new isolates but not by a control isolate. Moneydor, a susceptible cultivar without the Ve gene, was the most seriously diseased by all isolates. The isolates from theVerticillium resistant tomato cultivars were less virulent on the susceptible cultivar than the control isolate.     

Variation in resistance to Verticillium wilt within seedling populations of some varieties of lucerne (Medicago sativa)

Verticillium wilt was assessed in seedling populations of five lucerne varieties: Euver, Maris Kabul, Sabilt, Verneuil and Vertus. Visual assessments of disease severity in Vertus were compared with estimates of fungal biomass obtained by a maceration-dilution plate method and glucosamine assays. Symptom severity correlated with the amounts of fungus present only at the lower disease ratings. In contrast, direct correlations were obtained between visually assessed disease severities and the chlorophyll contents of plant shoots. Wide variation in disease reaction type were found within populations of all five varieties and disease development was found to differ in two environments with differing temperatures. The significance of these variations is discussed in relation to the genetic background of the host and studies on the biochemical basis of resistance to Verticillium wilt.     

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.     

Evaluation of antisera raised against Phytophthora fragariae for detecting the red core disease of strawberries by enzyme-linked immunosorbent assay (ELISA)

Antiserum was raised against pooled mycelial suspensions from five isolates (designated Pf 1, Pf 2, Pf 3, Pf 10 and Pf 11) representing five physiologic races of Phytophthora fragariae. In enzyme-linked immunosorbent assay (ELISA), this antiserum detected homologous soluble antigens at protein concentrations as low as 2 ng/ml.
Fungal antigens could also be detected in extracts of strawberry plants infected with P. fragariae. Root extracts prepared from the alpine strawberry Fragaria vesca and F. ananassa cv. Cambridge Favourite infected with any of the five isolates studied produced strong reactions in ELISA. In F. vesca , ELISA-positive material was detectable 6-8 days after inoculation before macroscopic symptoms appeared. The cultivar Red Gauntlet, which is resistant to Pf 1, 2 and 3 but susceptible to Pf 10 and 11, reflected this differential response in ELISA; the absorbance produced by extracts of plants infected with virulent isolates was significantly higher than that obtained with the corresponding extracts of plants inoculated with a virulent isolates. The recently introduced cultivars Hapil, Ostara and Providence were found to be susceptible to all isolates in this study: the corresponding root extracts were also positive in ELISA.
The antiserum also detected P cactorum infections. Nevertheless, the ELISA test described should prove valuable in screening certified strawberry stocks.     

Histochemical and Cytochemical Investigations of Phenols in Roots of Banana Infected by the Burrowing Nematode Radopholus similis

ABSTRACT The burrowing nematode Radopholus similis is one of the most damaging pathogens on banana plantations. The role of phenolics in plant defense responses to the nematode was histochemically and ultrastructurally investigated in susceptible and partially resistant cultivars. Histochemical observations of healthy roots revealed that high levels of lignin, flavonoids, dopamine, cafeic esters, and ferulic acids were associated with a very low rate of nematode root penetration in the resistant cultivar. The presence of lignified and suberized layers in endodermal cells contributed to limit invasion of the vascular bundle by the pathogen. After infection, flavonoids were seen to accumulate early in walls of cells close to the nematode-migrating channel in both cultivars and in all tissues of the infected resistant roots including the vascular tissues. The labeling pattern obtained with the gold-complexed laccase and with anti-pectin monoclonal antibodies showed that phenolics were distributed in a loosened pectin-rich material surrounding the nematode. This study provides indications that constitutive phenolics in banana roots are associated with the limitation of host penetration and colonization by R. similis. Accumulation of flavonoids in response to infection was detected in the vascular tissues of susceptible plants and in all root tissues in the partially resistant plants.     

Responses of cucumber cultivars to induction of systemic resistance against anthracnose by plant growth promoting fungi

Initial experiment on the reactions of five Japanese cultivars of cucumber toColletotrichum orbiculare infection in the greenhouse revealed that cv Suyo and Gibai were susceptible and moderately susceptible, respectively, while cv Shogoin fushinari and Sagami hanjiro were resistant to infection byC. orbiculare; cv Ochiai fushinari was moderately resistant. The ability of 16 plant growth promoting fungi (some isolates belonged to species ofPhoma and some non-sporulating isolates) isolated from zoysiagrass rhizospheres to induce systemic resistance in the above five cucumber cultivars was tested by growing plants in potting medium infested with barley grain inocula of PGPF in the greenhouse. The second true leaves of 21-day-old plants were challenge inoculated withC. orbiculare and disease assessed. Nine, out of 16 isolates, caused significant reduction of disease caused byC. orbiculare in at least two cultivars.Phoma isolates (GS8-1 and GS8-2) and non-sporulating isolates (GU21-2, GU23-3, and GU24-3) significantly reduced the disease in all the five cultivars. The disease suppression in cucumber was due to the induction of systemic resistance, since the inducer(s) and the pathogen were separated spatially and that the inducer did not colonize aerial portions. The resistance induced by certain isolates in a susceptible cultivar was less than that in a resistant cultivar. Disease suppression caused by isolate GU21-2 was similar to theC. orbiculare induced control in certain cultivars. The average rate of expansion of lesion diameter on leaves due toC. orbiculare was slower due to induction with the selected plant growth promoting fungi compared to the uninduced control plants. Roots of four cultivars were colonized by only three isolates, however, roots of one cultivar (Suyo) was colonized by five isolates suggesting the cultivar-specific root colonization ability.Abbreviations cv cultivar(s) - PGPF plant growth promoting fungal isolates - PGPR plant growth promoting rhizobacteria     

A high-throughput method for early screening of coffee (<Emphasis Type="Italic">Coffea</Emphasis> spp.) genotypes for resistance to root-knot nematodes (<Emphasis Type="Italic">Meloidogyne</Emphasis> spp.)

Root-knot nematodes (Meloidogyne spp.) threaten the livelihood of millions of farmers producing coffee worldwide. The use of resistant plants either as cultivars or rootstocks appears to be the single most effective method of control. A screening method was developed to evaluate large populations of plants for resistance to root-knot nematodes. Two coffee cultivars, one susceptible and the other resistant to Meloidogyne paranaensis, were grown under controlled conditions in two substrates: a commercial sieved potting compost and an inert substrate containing sand with a water-absorbent synthetic polymer. Plant growth and development and nematode multiplication were compared for two inoculation dates (2 and 8 weeks after planting) and two evaluation dates (eight and 13 weeks after inoculation). Root growth, but not nematode multiplication, was influenced by the choice of substrate. Evaluation of the differences in root weight and nematode numbers between the different cultivars, substrates and dates of inoculation suggested that an optimal condition could be defined. The best discrimination between susceptible and resistant plants was found in the experiment where inoculation occurred at 2 weeks after planting and evaluation occurred at 8 weeks after inoculation. Because the total duration of this experiment was only 3 months, high-throughput evaluation was possible, opening up new possibilities for screening large germplasm collections and studying the genetic control of root-knot nematode resistance in coffee.     

The development of juveniles of Heterodera schachtii in roots of resistant and susceptible genotypes of Sinapis alba,Brassica napus,Raphanus sativus and hybrids

The development ofHeterodera schachtii Schm. (beet cyst nematode, BCN) juveniles in roots of resistant and susceptible genotypes belonging to cruciferous crop species and hybrids was studied from 4 to 28 days after inoculation. No difference in root penetration by larvae was observed between resistant and susceptible plants.The development of nematodes in roots from resistant plants ofRaphanus sativus L., resistant xBrassicoraphanus Sageret and a resistant hybrid xBrassicoraphanus x Brassica napus L. was similar. BCN resistance in these three sources of plant material appeared to be related to an increased male:female nematode ratio as compared to the ratio found in susceptibleR. sativus plants.Also in resistant plants ofSinapis alba L. and a resistant intergeneric hybridS. alba x B. napus the increase in male:female nematode ratio, as compared to the ratio found for susceptibleS. alba cultivars and a susceptible intergeneric hybridS. alba x B. napus, seemed to be related with the observed resistance. In roots of the resistantS. alba and of a resistant hybridS. alba x B. napus, however, BCN resistance might also be due to a slower development of larvae and increased necrosis of root cells at the site of larval penetration.     

Viruses transmitted by nematodes1

The transmission of plant viruses by nematodes is remarkable in involving only two distinct groups of viruses, nepo viruses and tobraviruses, and being limited to longidorid, Paratrichodorus and Trichodorus nematodes respectively. Tobraviruses and their associated vector nematodes are not discussed here. Only 11 of the 36 described nepoviruses are transmitted by nematodes, and 6 of these 11 viruses are present in Europe naturally associated with 8 virus-vector longidorids. Specific relationships exist between the serologically distinct viruses and their vector nematode species. Specificity is largely determined by the virus coat protein and by an inherited ability of the nematode to retain virus particles at specific sites within its oesophagus. This specific relationship can be quite subtle, extending to populations of vector nematodes and also to virus isolates which apparently are serologically indistinguishable. Several serological and/or symptomatological variants of nepoviruses may be present at a field site in association with one or more vector nematode species. The exposure of different crops and new cultivars to these virus and vector combinations will probably result in the occurrence of further nematode-transmitted virus diseases. New methods for suppressing damage to crops caused by these diseases are required including the likely use of transgenic resistant cultivars.     

Effectiveness and profitability of the <Emphasis Type="Italic">Mi</Emphasis>-resistant tomatoes to control root-knot nematodes

Experiments were conducted to determine the effectiveness and profitability of the Mi-resistance gene in tomato in suppressing populations of Meloidogyne javanica in a plastic-house with a natural infestation of the nematode. Experiments were also conducted to test for virulence and durability of the resistance. Monika (Mi-gene resistant) and Durinta (susceptible) tomato cultivars were cropped for three consecutive seasons in non-fumigated or in soil fumigated with methyl bromide at 75 g m^–2 and at a cost of 2.44 euros m^–2. Nematode densities were determined at the beginning and end of each crop. Yield was assessed in eight plants per plot weekly for 6 weeks. The P_f/P_i values were 0.28 and 21.6 after three crops of resistant or susceptible cultivars, respectively. Growth of resistant as opposed to susceptible tomato cultivars in non-fumigated soil increased profits by 30,000 euros ha^–1. The resistant Monika in non-fumigated soil yielded similarly (P > 0.05) to the susceptible Durinta in methyl bromide fumigated soil but the resistant tomato provided a benefit of 8800 euros ha^–1 over the susceptible one because of the cost of fumigation. Selection for virulence did not occur, although the nematode population subjected to the resistant cultivar for three consecutive seasons produced four times more eggs than the population on the susceptible one. Such a difference was also shown when the resistant cultivar was subjected to high continuous inoculum pressure for 14 weeks. The Mi-resistance gene can be an effective and economic alternative to methyl bromide in plastic-houses infested with root-knot nematodes, but should be used in an integrated management context to preserve its durability and prevent the selection of virulent populations due to variability in isolate reproduction and environmental conditions.     

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.     

The role of aggressiveness and competition in the selection of Phytophthora infestans populations

Selection within populations of Phytophthora infestans was investigated by comparing the aggressiveness of single‐lesion isolates on detached leaflets of four potato cultivars with differing levels of race‐nonspecific resistance to P. infestans. The isolates included 23 representative of Northern Ireland genotypes from the early 2000s, used to inoculate previously reported field trials on competitive selection (2003–2005), plus 12 isolates recovered from the 2003 trial. The cultivars were those planted in the previous trials: Atlantic (blight‐susceptible) and Santé, Milagro and Stirling (partially resistant). Very highly significant variation for latent period, infection frequency and lesion area was found between genotypes and cultivars; differences between genotypes were more marked on the more resistant cultivars, but no one genotype was the most aggressive across all. Detached leaflets were also inoculated with mixtures of isolates from each genotype group at three sporangial concentrations: differences in aggressiveness between genotypes were more apparent at lower concentrations and on the more resistant cultivars. Genotype groups that were the most aggressive on the more resistant cultivars tended to be those selected by the same cultivars in the field. A mixture of all isolates of all genotypes was used to inoculate detached leaflets of the same cultivars. With one exception, single spore isolates recovered from any one leaflet belonged to a single genotype, but different genotypes were recovered from different cultivars. Phytophthora infestans isolates from Northern Ireland showed significant variation for foliar aggressiveness, and pathogen genotypes exhibited differential aggressiveness to partially resistant cultivars and interacted competitively in genotype selection.     

Histopathological aspects of mango resistance to the infection process of Ceratocystis fimbriata

Mango wilt, caused by Ceratocystis fimbriata, is one of the most important diseases affecting mango yields in Brazil. Information regarding the infection process of C. fimbriata in the stem tissues of mango from different cultivars and the basis of host resistance to the pathogen is rare in the literature. Thus, the objective of the study was to investigate how infection by two isolates of C. fimbriata can be affected by mango cultivar‐specific mechanisms of resistance. Disease progress on the inoculated stem tissues of the mango cultivars was evaluated and stem sections were obtained from the site of inoculation and prepared for histopathological observations using light microscopy. The factors mango cultivars and C. fimbriata isolates and their interaction were significant for all measures of disease development. Plants from the cultivars Espada, Haden and Palmer inoculated with isolates of C. fimbriata were more susceptible, whereas plants from the cultivars Tommy and Ubá were moderately resistant and resistant, respectively. Histopathologically, fungal isolates apparently massively colonized the stem tissues of plants from the susceptible cultivars Espada, Haden and Palmer, starting from the collenchyma and moving in the direction of the cortical parenchyma, xylem vessels and pith parenchyma. By contrast, on stem tissues of plants from the resistant cultivars Tommy Atkins and Ubá, most of the cells reacted to C. fimbriata infection by accumulating amorphous material. The results from the present study strongly indicated the importance of phenolic compounds for mango cultivar resistance against infection by Brazilian C. fimbriata isolates.     

Impact of <Emphasis Type="Italic">Sugarcane yellow leaf virus</Emphasis> on growth and sugar yield of sugarcane

A survey revealed that Sugarcane yellow leaf virus (SCYLV) is found on all Hawaiian sugarcane plantations including those where no yellow leaf symptoms were observed. In a comparison of growth and yield between SCYLV-infected and SCYLV-free plants of the cultivar H87-4094, germination and early shoot growth of infected plants were retarded. The number of stalks per stool was reduced by 30%, biomass was reduced by 29%, and sugar yield by 26% when plants were harvested after 11 months. Yields did not decrease when plants were harvested after 2 years. Thus, SCYLV could reduce yield, even when the plants were asymptomatic. In a field test of SCYLV-susceptible (infected) and -resistant cultivars to compare growth and yield, 10 commercial cultivars (six susceptible and four resistant to SCYLV) were grown in eight fields with different climates and soils. Primary stalk length, biomass and sugar yield did not differ between susceptible and resistant cultivars under any field conditions. Thus, harmful effects of SCYLV on yield cannot be deduced by comparing different cultivars.