Translocation of fungicides and their efficacy in controlling Phellinus noxius, the cause of brown root rot disease

Brown root rot disease (BRRD), caused by Phellinus noxius, is an important tree disease in tropical/subtropical areas. To improve chemical control of BRRD and deter emergence of fungicide resistance in P. noxius, this study investigated control efficacies and systemic activities of fungicides with different modes of action. Fourteen fungicides with 11 different modes of action were tested for inhibitory effects in vitro on 39 P. noxius isolates from Taiwan, Hong Kong, Malaysia, Australia, and Pacific Islands. Cyproconazole, epoxiconazole, and tebuconazole (FRAC 3, target-site G1) inhibited colony growth of P. noxius by 99.9 to 100% at 10 ppm and 97.7 to 99.8% at 1 ppm. The other effective fungicide was cyprodinil + fludioxonil (FRAC 9 + 12, target-site D1 + E2), which showed growth inhibition of 96.9% at 10 ppm and 88.6% at 1 ppm. Acropetal translocation of six selected fungicides was evaluated in bishop wood (Bischofia javanica) seedlings by immersion of the root tips in 100 ppm of each fungicide, followed by liquid or gas chromatography tandem-mass spectrometry analyses of consecutive segments of root, stem, and leaf tissues at 7- and 21-days post-treatment. Bi-directional translocation of the fungicides was also evaluated by stem injection of fungicide solutions. Cyproconazole and tebuconazole were the most readily absorbed by roots and efficiently transported acropetally. Greenhouse experiments suggested that cyproconazole, tebuconazole, and epoxiconazole have a slightly higher potential for controlling BRRD than mepronil, prochloraz, and cyprodinil + fludioxonil. Because all tested fungicides lacked basipetal translocation, soil drenching should be considered instead of trunk injection for their use in BRRD control.

'Candidatus Paenicardinium endonii', an endosymbiont of the plant-parasitic nematode Heterodera glycines (Nemata: Tylenchida), affiliated to the phylum Bacteroidetes

Bacteria-like endosymbionts of females of the plant-parasitic nematodes Globodera rostochiensis and Heterodera goettingiana and juveniles of Heterodera glycines were first observed during transmission electron microscopy (TEM) studies conducted in the 1970s. These organisms were characterized as being rod-shaped, ranging in size from 0.3 to 0.5 microm in diameter and 1.8 to 3 microm in length and containing structures labelled as striated inclusion bodies or tubular structures. A population of H. glycines was obtained from the soybean field where infected nematodes were first discovered in order to conduct TEM studies of females and males and to determine the phylogenetic position of the H. glycines endosymbiont among bacteria by studying the 16S rRNA and gyrB gene sequences. The bacterium was observed in the pseudocoelom and intestine of juveniles, females and males, in hypodermal chords of juveniles and males, in ovary walls and in oocytes and spermatozoa. The bacterium was polymorphic, measuring 0.4-0.8 x 2.5-4.5 microm, and many specimens contained an array of microfilament-like structures similar to those observed in "Candidatus Cardinium hertigii", the endosymbiont of Encarsia spp. wasps. Phylogenetic analysis of the 16S rRNA and gyrB genes of the H. glycines-infecting bacterium revealed 93 % and 81 % sequence identity, respectively, to the homologous genes in "Candidatus C. hertigii". Thus, the name "Candidatus Paenicardinium endonii" is proposed for the bacterial endosymbiont of the plant-parasitic nematode H. glycines.

Emended description of Pasteuria nishizawae

The description of the Gram-positive, obligately parasitic, mycelial and endospore-forming bacterium, Pasteuria nishizawae, is emended to include additional observations on the life cycle, host specificity and endospore morphology. The nucleotide sequence of the 16S rRNA gene is also provided.

A genetic linkage map of the soybean cyst nematode Heterodera glycines

A genetic linkage map of the soybean cyst nematode (SCN) Heterodera glycines was constructed using a population of F2 individuals obtained from matings between two highly inbred SCN lines, TN16 and TN20. The AFLP fingerprinting technique was used to genotype 63 F2 progeny with two restriction enzyme combinations (EcoRI/MseI and PstI/TaqI) and 38 primer combinations. The same F2 population was also genotyped for Hg-cm-1 (H. glycines chorismate mutase-1), a putative virulence gene, using real-time quantitative PCR. Some of the markers were found to be distributed non-randomly. Even so, of the 230 markers analyzed, 131 could be mapped onto ten linkage groups at a minimum LOD of 3.0, for a total map distance of 539 cM. The Hg-cm-1 locus mapped to linkage group III together with 16 other markers. The size of the H. glycines genome was estimated to be in the range of 630-743 cM, indicating that the current map represents 73-86% of the genome, with a marker density of one per 4.5 cM, and a physical/genetic distance ratio of between 124 kb/cM and 147 kb/cM. This genetic map will be of great assistance in mapping H. glycines markers to genes of interest, such as nematode virulence genes and genes that control aspects of nematode parasitism.

Life Cycle, Ultrastructure, and Host Specificity of the North American Isolate of Pasteuria that Parasitizes the Soybean Cyst Nematode, Heterodera glycines

Light and transmission electron microscopy were used to investigate the life cycle and ultrastructure of an undescribed isolate of Pasteuria that parasitizes the soybean cyst nematode, Heterodera glycines. Studies also were conducted to determine the host specificity of Pasteuria. The endospores that attached to the cuticle of second-stage juveniles (J2) of H. glycines in soil did not germinate until the encumbered nematodes invaded soybean roots. Thereafter, the bacterium developed and completed its life cycle only in females. The stages of endosporogenesis were typical of Pasteuria spp. The mature endospore, like that of P. nishizawae, the only other Pasteuria known to infect H. glycines, produces an epicortical layer that completely surrounds the cortex, an outer spore coat that tapers progressively from the top to the base of the central body, and a double basal adhesion layer. However, subtle differences exist between the Pasteuria from North America and P. nishizawae with regard to size of the central body, nature and function of the mesosomes observed in the earlier stages of endosporogenesis, and appearance of the fibers lining the basal adhesion layer and the exosporium of the mature endospore. Endospores of the North American Pasteuria attached to J2 of H. schachtii, H. trifolii, and H. lespedezae but not to Meloidogyne arenaria race 1, Tylenchorhynchus nudus, and Labronema sp. Results from this study indicate that the North American Pasteuria is more similar to P. nishizawae than to any other known member of the genus. Additional evidence from comparative analysis of 16S rDNA sequences is needed to clarify whether these two Pasteuria belong to the same species.

A Simple Method for the Extraction, PCR-amplification, Cloning, and Sequencing of Pasteuria 16S rDNA from Small Numbers of Endospores

For many years the taxonomy of the genus Pasteuria has been marred with confusion because the bacterium could not be cultured in vitro and, therefore, descriptions were based solely on morphological, developmental, and pathological characteristics. The current study sought to devise a simple method for PCR-amplification, cloning, and sequencing of Pasteuria 16S rDNA from small numbers of endospores, with no need for prior DNA purification. Results show that DNA extracts from plain glass bead-beating of crude suspensions containing 10,000 endospores at 0.2 x 10 endospores ml(-1) were sufficient for PCR-amplification of Pasteuria 16S rDNA, when used in conjunction with specific primers. These results imply that for P. penetrans and P. nishizawae only one parasitized female of Meloidogyne spp. and Heterodera glycines, respectively, should be sufficient, and as few as eight cadavers of Belonolaimus longicaudatus with an average number of 1,250 endospores of "Candidatus Pasteuria usgae" are needed for PCR-amplification of Pasteuria 16S rDNA. The method described in this paper should facilitate the sequencing of the 16S rDNA of the many Pasteuria isolates that have been reported on nematodes and, consequently, expedite the classification of those isolates through comparative sequence analysis.

Phylogenetic position of the North American isolate of Pasteuria that parasitizes the soybean cyst nematode, Heterodera glycines, as inferred from 16S rDNA sequence analysis

A 1341 bp sequence of the 16S rDNA of an undescribed species of Pasteuria that parasitizes the soybean cyst nematode, Heterodera glycines, was determined and then compared with a homologous sequence of Pasteuria ramosa, a parasite of cladoceran water fleas of the family Daphnidae. The two Pasteuria sequences, which diverged from each other by a dissimilarity index of 7%, also were compared with the 16S rDNA sequences of 30 other bacterial species to determine the phylogenetic position of the genus Pasteuria among the Gram-positive eubacteria. Phylogenetic analyses using maximum-likelihood, maximum-parsimony and neighbour-joining methods showed that the Heterodera glycines-infecting Pasteuria and its sister species, P. ramosa, form a distinct line of descent within the Alicyclobacillus group of the Bacillaceae. These results are consistent with the view that the genus Pasteuria is a deeply rooted member of the Clostridium-Bacillus-Streptococcus branch of the Gram-positive eubacteria, neither related to the actinomycetes nor closely related to true endospore-forming bacteria.

Influence of Inoculum Density of Verticillium dahliae on Root Discoloration of Horseradish

The relationship between inoculum density (number of microsclerotia per gram of air-dried soil) of Verticillium dahliae at the time of planting and the severity and incidence of root discoloration of horseradish at harvest was investigated in a 2-year study conducted in the greenhouse, microplots, and commercial production fields. The objective of the study was to develop a disease-forecast system that would assist growers in assessing the risk of the disease before planting horseradish in a particular field. Significant correlations were observed between inoculum density and severity and incidence of root discoloration in the greenhouse and microplots, although the form of the relationship varied with trials from linear to quadratic and negative exponential. No correlation was found between inoculum density of V. dahliae and severity and incidence of root discoloration in commercial production fields. In some fields with low inoculum densities, high ratings of severity and incidence of root discoloration were observed even with the partially resistant cultivar 769A. Conversely, in other fields with high inoculum densities, low ratings of severity and incidence of discolored roots were observed even with the susceptible cultivar 647A. These results suggest that a disease-forecast system based solely on inoculum densities of V. dahliae would be unreliable under field conditions when the other factors affecting the inoculum density-disease relationships cannot be controlled. Knowing the amount of initial inoculum may, however, save growers from planting horseradish in highly infested fields, but it would not guarantee a disease-free crop in fields with low levels of infestation.

Population Changes in Heterodera glycines and Its Bacterial Parasite Pasteuria sp. in Naturally Infested Soil

A two-year soil sampling study was conducted on four microplots naturally infested with Heterodera glycines and an undescfibed species of Pasteuria. The objectives of the study were to investigate the population dynamics of both organisms and to assess the potential of Pasteuria sp. as a biological control agent of H. glycines. Seasonal fluctuations were observed in numbers of cysts, eggs per cyst, second-stage juveniles (J2) of H. glycines, number of Pasteuria endospores attached per J2, and percentages of endospore-encumbered J2. Percentages of endospore-encumbered J2, Y, increased with the mean numbers of endospores per J2, X, according to the equation Y = 87.0(1 - e(-0.53X)). In contrast, numbers of J2 per 250 cm(3) soil, Y, decreased with the numbers of endospores per J2, X, according to the exponential decay model Y= 67.4 + 220.1e(-1.2X). The equilibrium J2 density (67.4 +/- 3.3) derived from this function was consistent with the predictions of the Lotka-Volterra model of population dynamics based on the equation 0.0195ln(y) - 0.000336y = 0.000049x - 0.00285ln(x) + 0.06589, where x and y represent the biweekly means of J2 densities and the percentages of endospore-encumbered J2, respectively. In all cases, predicted equilibrium densities of J2 were below the damage threshold reported from field studies. These results indicate that, given sufficient time following introduction into a field, Pasteuria may increase to levels that would be effective as one component in an integrated pest management proglmn to control H. glycines.

Verticillium dahliae Resistance in Horseradish Germ Plasm from the University of Illinois Collection

A total of 113 horseradish cultivars from the University of Illinois germ plasm collection at Urbana were evaluated for their reaction to Verticillium dahliae in the greenhouse following a root-dip inoculation. Root discoloration was rated 2 months after inoculation on a scale of 0 to 3 as follows: 0 = no symptoms; 1 = trace to less than 10% of the root cross-section with vascular discoloration; 2 = 10 to 50% of the root cross-section with vascular discoloration; and 3 = more than 50% of the root cross-section with vascular discoloration. The cultivars exhibited a large amount of variation in response to V. dahliae infection, with mean root discoloration ratings ranging from 0.2 to 2.6. The frequency distribution of responses of the 113 cultivars was normal, with a mean and a standard deviation of 1.2 and 0.4, respectively. Six cultivars, 635A, 1236A, 769A, 125A, 761A, and 28A, were identified as resistant to V. dahliae. The existence of resistance to V. dahliae in horseradish germ plasm from the University of Illinois collection is a great resource for the breeding of improved horseradish cultivars that will combine resistance to V. dahliae with other desirable characters.

Evaluation of Inoculation Methods for Screening Horseradish Cultivars for Resistance to Verticillium dahliae

Four inoculation methods-colonized oatseed, root dip, infested soil, and set dip-were tested in the greenhouse for their effectiveness in identifying horseradish cultivars that are resistant to Verticillium wilt of horseradish. Examination of the inoculum density-disease relationships derived with each of these methods on susceptible (647A) and resistant (769A) cultivars showed that all were effective, though at varying degrees, in differentiating between susceptible and resistant reactions. Results were more consistent with the root dip method as it produced the largest least-squares means difference in wilt index between the two cultivars, the highest r ², the lowest coefficient of variation, the shortest incubation periods, and the highest incidence of foliar symptoms. Overall, inoculum concentrations accounted for only a small amount of the total variation in wilt index (0.14 ≤ r ² ≤ 0.73). This observation, in accord with previous reports on other hosts of Verticillium dahliae, would suggest that inoculum densities may not be a good predictor of the severity of Verticillium wilt of horseradish.