Population Changes of Heterodera glycines and Soybean Yields Resulting from Soil Treatment with Alachlor,Fenamiphos, and Ethoprop

The population dynamics of Heterodera glycines as influenced by alachlor, fenamiphos, and ethoprop alone and in herbicide-nematicide combinations were studied in the field. Numbers of H. glycines juveniles and eggs were higher at midseason and harvest where nematicides were applied. Fenamiphos alone or in combination with alachlor provided better control of H. glycines and greater seed yields than treatments with ethoprop. Numbers of H. glycines eggs at harvest in 1980 were positively correlated with numbers of juveniles at planting in 1981 and negatively related to seed yield in 1981.     

The Sex Ratio of Heterodera glycines at Low Population Densities

The sex ratio of the Arkansas 1 isolate of Heterodera glycines was determined in experiments in which ''Lee'' soybean was inoculated with either one or two larvae. A 3:1 male to female sex ratio was established for this isolate under the test conditions used. No influence of one nematode on the penetration and development to adult of another nematode in the same root was detected in double larval inoculations.     

Nonhost Root Penetration by Soybean Cyst Nematode

A total of 66 plants in 50 species were inoculated with eggs and juveniles of soybean cyst nematode, Heterodera glycines. Roots were stained and observed for penetration and development of the nematode. Twenty-six plants were not penetrated; twenty-three were penetrated, but there was no development of the nematode; eight were penetrated with some nematode development; two were penetrated and had considerable nematode development, but few nematodes, if any, matured; and seven were penetrated with many nematodes maturing. The penetration of nonhosts may imply some susceptibility and that populations eventually would build up on the penetrated plants. Plants not penetrated may be useful as rotation plants because no reproduction would occur.     

Resting spore formation ofLeptocylindrus danicus (Bacillariophyceae) during short time-scale upwelling and its significance as predicted by a simple model

Resting spore formation during short time-scale upwelling and its significance were investigated in the field and by a simple theoretical model. Field observations of spore formation ofLeptocylindrus danicus were made off Izu Peninsula, Japan. A rapid increase in ratio of resting spore to vegetative cell numbers indicated thatL. danicus formed resting spores quickly as a response to nutrient depletion in the upwelled water, although only a very low number of resting spores was found in the upwelling. A simple model was constructed to investigate the possible advantages of spore formation during short time-scale upwelling. This showed that there is a critical time-scale for resting spore formation to be advantageous. The nutrient depletion period of the upwelling off Izu was shorter than the critical time-scale determined by the model. Rapid-sinking of resting spores may increase further the critical time-scale, unless spores return with upwelling water. For short time-scale upwelling, the vegetative cell may be better suited than the resting spore for enduring a short period of nutrient depletion. Contribution from Shimoda Marine Research Center, University of Tsukuba, No. 475.     

Gene expression in nematode-infected plant roots

Summary A major pathogen of potato plants (Solanum tuberosum) is the potato cyst nematode (Globodera spp.), which induces localized redifferentiation of a limited number of host cells to form a specialized feeding-site termed the syncytium. A novel strategy utilizing the polymerase chain reaction (PCR) was employed to construct a cDNA library from dissected potato roots highly enriched in syncytial material. The library was differentially screened with cDNA probes derived from the infected root tissue from a compatible interaction and from healthy root tissue. Characterization of one gene identified by the library screen indicated an expression pattern that correlated with events in the immediate vicinity of the pathogen after syncytial establishment. The strategy for library construction and screening could be applicable to the study of gene expression in any plant-pathogen interaction in which the limited supply of cells at the interface of the two organisms precludes a more traditional approach.     

Improving the Accuracy of Sampling Field Plots for Plant-Parasitic Nematodes

The validity of nematode data from field experiments depends largely on how well samples represent the nematode population. Data from an intensive sampling of three field plots before and after spring cultivation were used to compare eight simulated sampling schemes. Average deviation from the plot mean ranged from 10% to 34% before cultivation and from 7% to 16% after cultivation. Samples taken from only the plant row erred most before cultivation but were comparable to other schemes after cultivation. Several schemes achieved a 25% deviation or less in 90% of the sample simulations. Sampling a nematode population usually involves subsampling a composite bulk sample, however, and this increases error by an estimable amount. A random sample with 35 cores and four random subsamples estimated mean plot densities within 25% with probabilities ranging from 0.77 to 0.85. The probability of a sample-subsample combination coming within a specified percent error of the true mean can be extended cautiously to any field mean and variance more-or-less independent of species and area using formulae presented herein. The most economical method of increasing sample accuracy was to increase the number of soil cores.     

MORPHOLOGICAL VARIABILITY IN POPULATIONS OF CHRYSOPHYCEAN RESTING CYSTS. I. GENETIC (INTERCLONAL) AND ENCYSTMENT TEMPERATURE EFFECTS ON MORPHOLOGY1

Many chrysophycean species produce resting cysts (statospores) with purportedly species-specific morphology. I investigated variation in the cyst morphology of a single species that may result from genetic differences among the vegetative clones involved and from variation in the temperature of the environment during cyst development. Populations of Dinobryon cylindricum Imhof cysts were produced under defined conditions in vitro and then sampled for morphological analysis based on SEM micrographs. Morphological data is presented and then used in a multivariate discriminant analysis to determine the utility of each morphological character in distinguishing the six populations studied. Results suggest that some features of cyst morphology (i.e. cyst diameter) are invariant among the populations, while other features show distinctive variation. The density of spines covering the cyst body as well as the morphology of those spines appear correlated to the specific clones involved, and thus may represent useful phenotypic genetic markers. The length and definition of both the spines and the cyst collar, on the other hand, are markedly influenced by encystment temperature. The implications of these findings for paleoecological studies is discussed.     

Influence of Planting Date on Damage to Soybean Caused by Heterodera glycines

Bragg soybeans were planted in nematicide-treated and nontreated plots on 15 May, 15 June, 1 July, and 15 July in 1980 and 1981 to determine the influence of planting date on damage caused by H. glycines. Although earlier studies showed the nematode was sensitive to high soil temperatures (> 34 C), late planting did not reduce damage caused by the nematode. Yields from plots treated with 1, 2-dibromo-3-chloropropane (57.5 kg a.i./ha) were 48, 118, 395, and 403% higher than yields from nontreated plots with planting dates of 15 May, 15 June, 1 July, and 15 July, respectively, when data were averaged over the 2 years. Increase in both seed size and number accounted for the yield increases in treated plots. Soil temperatures were highest during July in 1980, averaging 8.9 and 6.5 hours per day above 34 C at 10- and 20-cm depths, respectively. Larvae populations of H. glycines were reduced by the nematicide but not by late planting. These results indicate that damage caused by H. glycines may actually increase with later planting and that nematicides may be more beneficial when soybeans are planted late in a double-cropped production system.     

Interactions Between Calonectria crotalariae and Heterodera glycines on Soybean

The interactions of Heterodera glycines at four egg inoculum levels (0, 100, 1,000, and 10,000 per pot) and three cyst levels (0, 100, and 200 per pot) and Calonectria crotalariae at 500, 5,000, and 50,000 microsclerotia per pot were evaluated on soybean. At the two lowest nematode egg levels, the presence of C. crotalariae did not affect nematode reproduction. At 10,000 eggs per pot, however, nematode reproduction was increased significantly at each microsclerotial level. The increase in nematode reproduction was stepwise at 500 and 5,000 microsclerotia per pot but declined at 50,000 microsclerotia per pot. Similar results were obtained when cysts rather than eggs were used as nematode inoculum. The nematode x fungus interaction significantly affected 60-day plant growth parameters of both Lee 74 and Centennial soybean. The nematode x fungus interaction was antagonistic to plant roots and significantly influenced root injury ratings. The presence of C. crotalariae in tissues of stock plants or plants used as race differentials did not alter the analysis of this population as race 3.     

Control of Globodera tabacum solanacearum by Alternating Host Resistance and Nematicide

The feasibility of alternating use of resistant vs. susceptible flue-cured tobacco cultivars to improve control of Globodera tabacum subsp, solanacearum (TCN) was investigated at two Virginia locations in 1984-86. Post-harvest TCN population densities were reduced in each year of the study when fenamiphos was used with a TCN-resistant cultivar (NC 567), relative to susceptible cultivars (K 326 or Mc 944). Using NC 567 with fenamipbos also reduced preplant TCN population densities in the next growing season. Egg population densities before planting in 1986 were significantly lower in plots planted with NC 567 in 1984, even when a susceptible cultivar had been planted in 1985. Use of fenamiphos with NC 567 in 1984 and 1985 further reduced preplant egg population densities in 1986. Economic returns were significantly greater in 1984 when NC 567 was used with fenamiphos, rather than a susceptible cultivar. Treatments involving fenamiphos and (or) NC 567 in 1984 and 1985 resulted in higher economic returns in 1986 than did treatments using a susceptible cultivar without fenamiphos in both previous years. Economic returns were highest in 1986 when fenamiphos and NC 567 were used in 1984 and 1985 and a susceptible cultivar was planted in 1986.