Evaluation of Host Suitability in Prunus for Criconemella xenoplax

Methods were developed for screening Prunus selections for host suitability to Criconemella xenoplax. The relative host suitability of selections was based upon a doubling accumulation value (β) that was defined as the number of degree-days (base 9 C) required for doubling of an increment of the initial nematode population. The β value characteristic for C. xenoplax (139 ± 8 degree-days) on suitable hosts was similar to the average β value determined for several peach rootstocks known to be suitable hosts. The β values were 144 ± 21 for Halford, 141 ± 16 for Lovell, and 138 ± 10 for Nemaguard. A higher value for β could indicate poorer host suitability or resistance of a selection to C. xenoplax. All of 369 Prunus accessions tested, including eight accessions that had survived well on a field site infested with C. xenoplax, were suitable hosts. Apparently, resistance to C. xenoplax was not a factor in survival of the accessions planted in the field. Seedlings from P. besseyi, P. pumila ''Mando'', and two interspecific hybrids, Redcoat and Sapalta IR 549-1, failed to support nematode population increase in 44-81% of tests conducted, but all selections supported population increase in some tests. These accessions may have resistance mechanisms that are active only under specific conditions.     

Pasteuria sp. Parasitizing Trophonema okamotoi in Florida

Two populations of Trophonema okamotoi parasitized by Pasteuria sp. were found on Liquidambar styraciflua (sweetgum) and on an unidentified tropical grass in north-central Florida. Endospores of this Pasteuria sp. attached to motile vermiform second-stage juveniles (J2) and males of T. okamotoi, but not to other developmental stages. Sporangia and new endospores were produced only inside the bodies of swollen and sedentary third- and fourth-stage juveniles and females that developed in the host roots. No egg masses were produced by infected T. okamotoi females. The endospore diameter from the tropical grass population was 4.93 μm and the central core diameter was 1.97 μm; measurements of endospores from the sweetgum populations were similar. Endospores that were collected from T. okamotoi and added to uninfected T. okamotoi and other plant-parasitic nematodes attached/to J2 of T. okamotoi but did not attach to juveniles and adults of Helicotylenchus pseudorotrustus, Pratylenchus brachyurus, or to J2 of either Meloidogyne arenaria race 1, M. incognita race 1, M. javanica, or Tylenchulus semipenetrans. Pasteuria sp. from T. okamotoi differed from the described Pasteuria species in endospore size, host preference, and rate of attachment.     

Effects of Rapeseed and Vetch as Green Manure Crops and Fallow on Nematodes and Soil-borne Pathogens

In a rapeseed-squash cropping system, Meloidogyne incognita race 1 and M. javanica did not enter, feed, or reproduce in roots of seven rapeseed cultivars. Both nematode species reproduced at low levels on roots of the third crop of rapeseed. Reproduction of M. incognita and M. javanica was high on squash following rapeseed, hairy vetch, and fallow. The application of fenamiphos suppressed (P = 0.05) root-gall indices on squash following rapeseed, hairy vetch, and fallow; and on Dwarf Essex and Cascade rapeseed, but not Bridger and Humus rapeseed in 1987. The incorporation of 30-61 mt/ha green biomass of rapeseed into the soil 6 months after planting did not affect the population densities of Criconemella ornata, M. incognita, M. javanica, Pythium spp., Rhizoctonia solani AG-4; nor did it consistently increase yield of squash. Hairy vetch supported larger numbers of M. incognita and M. javanica than rapeseed cultivars or fallow. Meloidogyne incognita and M. javanica survived in fallow plots in the absence of a host from October to May each year at a level sufficient to warrant the use of a nematicide to manage nematodes on the following susceptible crop.     

Lectin Binding to Radopholus citrophilus and R. similis Proteins

Lectin-binding glycoproteins in seven populations of two burrowing nematode sibling species were probed with five different biotinylated lectins on Western blots, and differences were correlated with nematode ability to parasitize citrus and to overcome citrus rootstock resistance. Banding patterns of molecular weight standards were fit best by an exponential decay function, and a predictive equation was used to estimate molecular weights (r² = 0.999). A band (131 kDa) that labeled with the lectin Concanavalin A (Con A) occurred in extracts from cuticles and egg shells of populations of Radopholus citrophilus that parasitize citrus. Wheat germ agglutin labeled a band (58 kDa) in aqueous homogenates of populations that reproduce in roots of citrus rootstock normally resistant to burrowing nematodes. The two sibling species R. citrophilus and R. similis were distinguished by a high molecular weight Con A-labeled band (608 kDa) from cuticle and egg shells. Probing blots with the lectin Limulus polyphemus agglutinin indicated that each population contained a band (12-16 kDa) specifically inhibited by the addition of 25 mM neuraminic acid, suggesting that glycoproteins with sialic acid moieties are present in burrowing nematodes.     

Comparison of Reproduction by Meloidogyne graminicola and M. incognita on Trifolium Species

The reproductive potential of Meloidogyne graminicola was compared with that of M. incognita on Trifolium species in greenhouse studies. Twenty-five Trifolium plant introductions, cultivars, or populations representing 23 species were evaluated for nematode reproduction and root galling 45 days after inoculation with 3,000 eggs of M. graminicola or M. incognita. Root galling and egg production by the two root-knot nematode species was similar on most of the Trifolium species. In a separate study, the effect of initial population densities (Pi) of M. graminicola and M. incognita on the growth of white clover (T. repens) was determined. Reproductive and pathogenic capabilities of M. graminicola and M. incognita on Trifolium spp. were similar. Pi levels of both root-knot nematode species as low as 125 eggs per 10-cm-d pots severely galled white clover plants after 90 days. Meloidogyne graminicola has the potential to be a major pest of Trifolium species in the southeastern United States.     

Modification of the Oligidic Medium for Axenic Culture of Aphelenchoides rutgersi

Aphelenchoides rutgersi was axenically cultured in modified Soytone, yeast extract, lyophilized chick embryo extract medium (3% ST:2% YE:20% CEE-L, w/v:w/v:v/v). Earlier formulations used 10% CEE, v/v, before the manufacturer changed the preparation. After reestablishing A. rutgersi in medium that permitted continuous subcultivadon and reproduction, a second medium was tested that contained 0.5% sucrose and 0.5% Lipid Concentrate. The commercially available Lipid Concentrate made it possible to incorporate nonaqueous soluble chemicals into the medium. In addition, 0.1% Fast Green #3 was added to both media to visually demonstrate active ingestion of nutriment.     

Molecular Analysis of Plant Defense Responses to Plant Pathogens

A number of inducible plant responses are believed to contribute to disease resistance. These responses include the hypersensitive reaction, phytoalexin synthesis, and the production of chitinase, glucanase, and hydroxyproline-rich glycoproteins. Because of the coordinate induction of these responses, it has been difficult to determine whether they are functional defense responses, and if they are, how they specifically contribute to disease resistance. Recent developments in molecular biology have provided experimental techniques that will reveal the specific contribution of each response to disease resistance. In this paper, we describe a strategy to determine if the hypersensitive reaction is a functional plant defense mechanism.     

Dynamics of Belonolaimus longicaudatus Parasitism on a Susceptible St. Augustinegrass Host

St. Augustinegrass (Stenotaphrum secundatum) cv FX-313 was used as a model laboratory host for monitoring population growth of the sting nematode, Belonolaimus longicaudatus, and for quantifying the effects of sting nematode parasitism on host performance in two samples of autoclaved native Margate fine sand with contrasting amounts of organic matter (OM = 7.9% and 3.8%). Following inoculation with 50 Belonolaimus longicaudatus per pot, nematodes peaked at a mean of 2,139 nematodes per pot 84 days after inoculation, remained stable through 168 days at 2,064 nematodes per pot, and declined at 210 days. The relative numbers of juveniles and adults demonstrated senescence after 84 days. Root dry weight of nematode-inoculated plants increased briefly to an apparent equilibrium 84 days after inoculation, whereas root weights of uninoculated controls continued to increase, exceeding those of inoculated plants from 84 to 210 days (P < 0.01). At 210 days, uninoculated plants had 227% the root dry weight of inoculated plants. Transpiration of FX-313 was reduced by nematodes (P < 0.0001) at 84 and 126 days after inoculation; reduction was first observed at 42 days and last observed 168 days after inoculation (P < 0.05). OM content affected all plant performance variables at multiple dates, and generally there were no inoculation x OM content interactions. OM content had no effect on nematode numbers per pot, although there was a slight (P < 0.05) increase in the number of nematodes per gram root dry weight in the low-OM soil compared with the high-OM soil.     

Survival and Infectivity of Bursaphelenchus xylophilus in Wood Chip-Soil Mixtures

To determine the effect of soil environment on the life stages and total numbers of Bursaphelenchus xylophilus, nematode-infested wood chips alone and mixed with soil were incubated at 12 and 20 C. Nematodes were extracted at 2-week intervals for 12 weeks. Numbers of nematodes and percentage of third-stage dispersal larvae were greater at 12 C and in chips without soil. Percentage of juveniles of the propagative cycle was greater at 20 C and in chips with soil. Although B. xylophilus survived in chips with soil for 12 weeks, nematode numbers and life stage percentages changed little over time. To determine if B. xylophilus was capable of infecting wounded roots, infested and uninfested chips were mixed with soil in pots with white and Scots pine seedlings. Trees were maintained at 20 and 30 C and harvested at mortality or after 12 weeks. Only seedlings treated with infested chips contained nematodes. In field experiments, planted seedlings were mulched with infested chips to determine if nematodes would invade basal stem wounds. Among these trees, Scots pine was more susceptible than white or red pines to infection and mortality.     

Sex Determination in Caenorhabditis elegans

In Caenorhabditis elegans, the decision to develop as a hermaphrodite or male is controlled by a cascade of regulatory genes. These genes and other tissue-specific regulatory genes also control sexual fate in the hermaphrodite germline, which makes sperm first and then oocytes. In this review, we summarize the genetic and molecular characterization of these genes and speculate how they mutually interact to specify sexual fate.