Effects of crop rotation and nonfumigant nematicides on peanut and corn yields in fields infested with criconemella species.

The effects of nematicide treatments and corn-peanut cropping sequences on the population development of Criconemella ornata, and C. sphaerocephala and the related impact on crop yields were investigated at two North Carolina locations. Criconemella ornata and C. sphaerocephala were present at the Norman Perry farm, Bertie County (BERTIE); however, only C. ornata was found at the Central Crops Research Station, Johnston County (CCRS). An untreated control was compared to aldicarb 15G, carbofuran 15G, ethoprop 10G, and terbufos 15G granular formulations applied at a rate of 2.2 kg a.i./ha. The cropping sequences were monocuhured corn (C-C-C); monocultured peanut (P-P-P); and two corn-peanut (C-P-C; P-C-P) rotations. Nematicides were inconsistent in controlling C. sphaerocephala and C. ornata. Nematicide treatments enhanced corn yields in the monoculture-cropping cycle in the final year of the experiment at CCRS. Peanut yields were greater in the rotated cropping sequence than under monoculture at BERTIE, but rotation had less effect on peanut yields at CCRS. Declining yields were correlated with an increase in numbers of nematodes. Corn was an intermediate host for C. sphaerocephala and a moderate to poor host for C. ornata. Peanut was an excellent host for C. ornata and a poor host for C. sphaerocephala.

Growth and yield responses of soybean to aldicarb.

A series of greenhouse, phytotron, field, and microplot experiments evaluated factors that influenced plant-growth.stimulation associated with the use of the pesticide aldicarb. A phytotron experiment showed.that aldicarb increased growth, of Ransom soybean at all temperatures but was somewhat phytotoxic to Coker 156 soybean at 30 C. Soybean gave the greatest response to this nematicide at 22 C in a commercially available medium, Metromix 220. Soybean cultivars Ransom and Coker 156. exhibited increased growth in response to aldicarb or, to a lesser extent aldicarb sulfone treatments under greenhouse and microplot conditions. Enhanced soybean growth, however, did not always result in significantly greater soybean seed yield. Soil type affected soybean sensitivity to aldicarb, with. the greatest growth and yield increases generally occurring in fine-textured soils or those with high.organic matter. Plant-growth stimulation by aldicarb occurs in the absence of pests but is dependent upon concentration and edaphic and other environmental factors.

Monoclonal Antibodies to Glycoprotein Antigens of a Fungal Plant Pathogen, Phytophthora megasperma f. sp. glycinea.

Genetic studies of plants and their pathogens indicate that dominant alleles for resistance in hosts are complemented by corresponding dominant alleles for avirulence in pathogens. Products of these genes have not yet been identified. We have produced murine monoclonal antibodies (mAbs) to extracellular antigens of the fungal soybean pathogen Phytophthora megasperma f. sp. glycinea (Pmg, race 1) as part of a larger effort to identify antigenic determinants associated with particular avirulence genes. Thirty-six independent mAbs have been characterized by binding to Western blots of Pmg extracellular glycoproteins and by enzyme-linked immunosorbent assay with glycoproteins modified by treatment with periodate, alpha-mannosidase, and endo-beta-N-acetylglucosaminidase H. The mAbs are predominantly carbohydrate-specific and can be placed in six groups based on interactions with Pmg glycoproteins. Binding patterns of various mAbs to Western blots indicate that Pmg proteins may have single or multiple types of attached carbohydrate antigens. Races of Pmg with differing avirulence genes exhibit more characteristic differences by Western blot analysis than by protein staining of glycoprotein profiles. Several of the mAbs show much higher reaction levels to glycoproteins from race 1 than from two other races. All of the glycoprotein-specific mAbs cross-react with purified mycelial walls.

Extracellular Polysaccharide of Erwinia amylovora: a Correlation with Virulence.

The extracellular polysaccharides produced as slime or capsule layers by bacterial pathogens of animals and plants have been often implicated as factors essential to pathogenesis. In the present study, virulence of the plant pathogen Erwinia amylovora was correlated with the ability to produce extracellular polysaccharide (EPS). EPS production by a series of field isolates and bacterio-phage-resistant mutants differing in the extent to which they cause symptoms in host tissues was examined by quantitation with a modified Laurell rocket immunoelectrophoresis assay. The amount of EPS produced as an easily removed capsular layer or slime on solid nutrient agar approximated the capacity to exhibit symptoms in host inoculation tests. Features common to the virulent isolates are mucoid colony morphology, sensitivity to EPS-specific bacteriophages [Sphi3 and PEal(h)], and ability to produce a characteristic EPS. Mutants selected for resistance to Sphi3 or nonmucoid colony morphology are deficient in EPS production and have lost the ability to multiply in host tissue and cause symptoms. We conclude that EPS may be directly involved in symptom expression and provide a function essential to the growth of the pathogen in host tissues.

Cellobiose oxidase, purification and partial characterization of a hemoprotein from Sporotrichum pulverulentum.

An extracellular enzyme which utilizes molecular oxygen to oxidize cellodextrins to the corresponding aldonic acids has been isolated from culture filtrates of the white-rot fungus Sporotrichum pulverulentum. This enzyme, tentatively named cellobiose oxidase, has been highly purified by classical techniques and has been demonstrated to be a glycoprotein with a molecular weight of approximately 93000. Ultraviolet spectra of the enzyme in the presence and absence of substrate are characteristic of a hemoprotein. Acidic hydrolyses of the enzyme followed by a spectrofluorimetric investigation of the hydrolysate has demonstrated the presence of approximately one flavin component per enzyme molecule. The possible role of this complex enzyme in cellulose degradation is discussed.

Host-Pathogen Interactions: IX. Quantitative Assays of Elicitor Activity and Characterization of the Elicitor Present in the Extracellular Medium of Cultures of Phytophthora megasperma var. sojae.

Resistance of soybean (Glycine max L.) seedlings to Phytophthora megasperma var. sojae (Pms) is in part due to the accumulation in infected tissue of a compound which is toxic to Pms. The accumulation of this compound, a phytoalexin called glyceollin, is triggered by infection, but it can also be triggered by molecules, "elicitors," present in cultures of Pms. The ability of the Pms elicitor to stimulate phytoalexin accumulation in soybean tissues has been used as the basis for biological assays of elicitor activity. Two bioassays were developed and characterized in this study of the Pms elicitor. These bioassays use the cotyledons and the hypocotyls of soybean seedlings. The cotyledon assay was used to characterize the extracellular Pms elicitor. This elicitor was isolated from Pms cultures and purified by ion exchange and molecular sieving chromatography. The extracellular Pms elicitor was determined to be a predominantly 3-linked glucan, which is similar in composition and structure to a polysaccharide component of Pms mycelial walls.

Host-Pathogen Interactions: X. Fractionation and Biological Activity of an Elicitor Isolated from the Mycelial Walls of Phytophthora megasperma var. sojae.

An elicitor of phytoalexin production in soybean (Glycine max L.) tissues was isolated from purified Phytophthora megasperma var. sojae mycelial walls by a heat treatment similar to that used to solubilize the surface antigens from the cell walls of Saccharomyces cerevisiae. The wall-released elicitor is a discrete, minor portion of the P. megasperma var. sojae mycelial walls. The elicitor released from the mycelial walls was divided by diethylaminoethylcellulose and concanavalin A-Sepharose chromatography into four fractions, each having different chemical characteristics. The four fractions were obtained from each of the three races of P. megasperma var. sojae. The corresponding fractions from each of the three races are very similar in composition and elicitor activity. The results suggest that the elicitor activity of each fraction resides in the glucan component of the fraction. Evidence is presented to demonstrate that the elicitors are not race-specific and that the accumulation of glyceollin is not sufficient to account for race-specific resistance.

Host-Pathogen Interactions: XI. Composition and Structure of Wall-released Elicitor Fractions.

The structures of the four wall-released elicitor fractions isolated from the Phytophthora megasperma var. sojae mycelial walls have been examined. The results demonstrate that fraction I is primarily composed of a branched beta-1,3-glucan, similar in structure to the extracellular elicitors described previously (Ayers, A., J. Ebel, F. Finelli, N. Burger, and P. Albersheim. 1976. Plant Physiol. 57: 751-759). Fractions II and IV are primarily composed of a highly branched mannan-containing glycoprotein, with fraction IV richer in protein than fraction II. Fraction III contains, attached to protein, a mixture of the two polysaccharide types found in fraction I and in fractions II and IV. The structural data presented here, in concert with the biological data presented in the previous two papers (Ayers et al. 1976. Plant Physiol. 57: 751-759; 760-765), demonstrate that the only compound produced by P. megasperma var. sojae which contains elicitor activity is the glucan. Evidence is presented that the terminal glycosyl residues of the glucan are required for elicitor activity. In addition, it is demonstrated that 90% of the glucan can be removed enzymically without any loss of biological activity. The active residue of the enzymic digestion is a highly branched 3- and 3,6-linked glucan containing about 4% mannosyl residues. The results presented suggest that the mannosyl residues of the glucan, which represent only about 1% of the undegraded glucan, are likely to participate in the active site of this molecule. The role of elicitors and phytoalexins in host-pathogen interactions is discussed. Evidence for the existence of and possible identity of another factor, which determines race specificity of host-pathogen interactions, is summarized.

Host-Pathogen Interactions: XII. Response of Suspension-cultured Soybean Cells to the Elicitor Isolated from Phytophthora megasperma var. sojae, a Fungal Pathogen of Soybeans.

The glucan elicitor isolated from the mycelial walls of Phytophthora megasperma var. sojae, the fungus which causes stem and root rot in soybeans, stimulates the activity of phenylalanine ammonia-lyase and the accumulation of glyceollin in suspension-cultured soybean cells. Nigeran, a commercially available fungal wall glucan, was the only other compound tested which has any activity in this system. Glyceollin is a phenylpropanoid-derived phytoalexin which is toxic to P. megasperma var. sojae. Evidence is presented to support the hypothesis that the action of elicitors in stimulating phytoalexin synthesis is not species or variety specific but, rather, is part of a general defensive response of plants.