In a 2 × 2 × 2 factorial experiment, plants of one genotype of tall fescue (Festuca arundinacea) were grown in soil in pots in a glasshouse on two occasions. The treatments were either with (E+) or without (E–) infection by the fungal endophyte Neotyphodium coenophialum; with (N+) or without (N–) inoculation by the root‐knot nematode Meloidogyne marylandi; and with or without water deficit stress. In the first experiment, nematode numbers after 5 weeks had increased 10 times in the E–/N– watered pots and 22 times in the E–/N+ stressed plants. Root dry weight was decreased in all E–/N+ pots, compared with E–/N–. In contrast, nematode numbers in all E+/N+ pots decreased to nearly zero and root dry weight was unaffected. Osmotic adjustment in the growing zone of stressed plants was –0·35 MPa in E–/N– pots, but only –0·10 MPa in E–/N+ pots. Osmotic adjustment was greatest (–0·64 MPa) in E+ pots of both nematode treatments and almost no nematodes survived in the N+ pots. In the second experiment, there was complete nematode mortality in the E+ pots. Plant water relations were unaffected by treatments, however. It is concluded that endophyte‐enhanced persistence of tall fescue in M. marylandi‐infested soils that are prone to drought may be explained at least partly by endophyte protection of roots from nematode damage. Nematode inhibition by the endophyte may operate in addition to direct influences of the endophyte on enhancing drought tolerance of the host.
Sudden death syndrome (SDS) of soybean (Glycine max [L.] Merr.) is a soilborne disease of increasing importance in high‐yield environments. This study was conducted to determine which combinations of soil fertility parameters and soybean cyst nematode (Heterodera glycines Ichinoe) (SCN) second stage juvenile (J2) population levels were associated with SDS disease severity. Also, the effect of SDS disease severity on leaf nutrient content and on soybean yield and seed germination were determined. These studies were conducted at the Pine Tree Station, in Colt, on a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualfs) over 3 yr with the soybean cultivar Lee 74. Soil factors associated with SDS were increased levels of soil available P, electrical conductivity (soluble salts), organic matter, and exchangeable Na, Ca, and Mg. Decreased levels of leaf N, Mg, Cu, and Mn and increased levels of leaf Ca were related to increased levels of SDS severity. Sudden death syndrome severity was negatively correlated with yield in 2 of 3 yr and with seed quality in 1 of 2 yr. Soybean cyst nematode second stage juvenile population density decreased as SDS severity increased in the study relating soil and leaf nutrient levels fo SDS. This may reflect a decrease in the ability of a diseased root system to support nematode reproduction. In the yield studies, however, SCN was negatively correlated to yield and positively correlated to SDS in 1 of 2 yr. Soybeans grown in high production environments of increased soil fertility and irrigation appear to be more susceptible to SDS. Sudden death syndrome affects yields by a reduction in seed size and number and may, under the proper environmental conditions, affect seed germination. The role of SCN in SDS is still unclear. Research Question Sudden death syndrome (SDS) is an important soilborne disease of soybean in parts of the South and Midwest. The disease is associated with high‐yield production'environments. This study compared the effects of soil and plant chemical factors on SDS severity and the relationship between SDS severity on yield and seed germination. Literature Summary Sudden death syndrome usually appears after flowering in vigorously growing, well‐watered soybeans, and is often associated with the soybean cyst nematode (SCN). Losses due to this disease vary greatly, but can be severe. Like other soilborne diseases, SDS distribution usually is aggregated in the field. Disease aggregation as well as inoculum distribution is strongly influenced by the soil environment. Since SDS is associated with high‐yield production environments, soil chemical factors may have a strong effect on SDS severity as they do with other soil‐borne diseases. While SDS appears to affect yield, the relationship of disease severity and yield has not been demonstrated nor has the effect of SDS severity on seed germination been determined. Study Description The study was conducted at the Pine Tree Station, Colt, AR, in a field with a history of SDS. Soil and plant samples were collected,...
Recently, reniform nematode (Rotylenchulus reniformis Linford and Oliveira) has been recognized to be a major problem on soybean [Glycine max (L.) Merr.] in the southern USA. Planting reniform nematode–resistant soybean cultivars is the most cost effective control method. With restriction fragment length polymorphism (RFLP) markers, Pioneer Hi‐Bred International, Inc. had previously identified quantitative trait loci (QTL) on linkage group B1 (LG‐B1) and another on LG‐L associated with reniform nematode resistance in a cross between ‘BSR101’ and PI 437654. In this study, we refined the QTL locations and identified additional QTL conditioning resistance to reniform nematode in a population of 228 recombinant inbred lines (RILs) from a cross of BSR101 × PI 437654. A QTL (R2 = 21%) conditioning reniform reproductive index (RI) was found on LG‐L and was flanked by Sat_184 and Satt513. Two other QTL were identified, one on LG‐B1 (R2 = 16%) and the other on LG‐G (R2 = 8%). These two QTL acted in an epistatic manner with lines homozygous for PI 437654 alleles at both QTL providing the lowest reniform RI. The 31 RILs from BSR101 × PI 437654 that were homozygous for the PI 437654 alleles at Satt513, Satt359, and Sat_168 averaged 1077 reniform eggs and juveniles (RI = 0.63), while the 33 RILs homozygous for the BSR101 alleles averaged 10606 (RI = 6.24). By screening the RILs population of ‘Prichard’ × ‘Anand’ we confirmed that the QTL on LG‐G and‐B1 conditioned reniform RI.
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