Cylindrocladium black rot (CBR) is a destructive root disease of peanut caused by Cylindrocladium crotalariae. New and quantitative approaches were developed to estimate the inoculum potential for this soil-borne pathogen under soil-temperature tank conditions favorable for CBR development. Using cultural plating methods, numerous Q. crotalariae infections (1 to > 1,000 per plant) were observed on asymptomatic root systems of peanut plants, susceptible and resistant to CBR, grown in naturally infested soils at 25 C. Regression-line slope values of log10-log10 plots for microsclerotial inoculum density versus the number of observed root infections per plant and per unit root length [0.98 (R2 =0.94) and 0.99 (R2 = 0.94), respectively] indicated direct proportionality between the respective variables. Efficiency of inoculum for observed infection (percent of germinating microsclerotia that infect roots) estimates were high (near 100%), while efficiency of observed infection for necrosis (percent of infections that develop into necroses) estimates were low (0.27 to 0.28%). Observed infection observed infections per m root per day per rates, I 0 r (the number of microsclerotium per g of soil) were significantly lower (P = 0.001) for CBR-resistant 'Spancross• than for CBR-susceptible 'Florigiant•. The majority of observed infections did not appear to be restricted to outer cortical root tissues. A portion of observea infections near root surfaces resulted in subsequent colonization of the inner cortex and stele of plants for each cultivar. A significantly greater (P = 0.05) number of segments from steles of CBR-susceptible Florigiant taproots were colon~zed by Q. crotalariae than those from CBR-resistant Spancross. Using naturally infested peanut field soils, a dose of 7.4 )-lg NaN3 (sodium azide)/g soil was effective in reducing initial microsclerotial populations by 50%, based upon a highly significant dosage-response curve (R2 = 0.96, P = 0.0001). CBR development was reduced significantly (P = 0.05) for Florigiant plants grown in Q. crotalariae-infested soils treated with 7.5 fl' NaN3/g soil or higher, compared to plants grown in untreated soils. Sublethal doses of NaN3 combined with biological control agents may be useful in the development of integrated CBR control measures. / Ph. D.

Higher percentages of peanuts than soybeans or cowpeas become infected when these crops are growing equal distances from a source of peanut mottle virus (PMV). The total number of aphids trapped in these crops are about equal and the reason for this differential percentage infection has not been demonstrated. Known vectors of PMV such as Aphis craccivora (Koch) and Myzus persicae (Sulzer) comprised 31% of the aphid population in peanuts compared to 14% in soybeans and 17% in cowpeas and could be responsible for the higher number of peanut infections. In addition, trapping of live aphids in peanut fields showed that viruliferoup Rhopalosiphum maidis (Fitch) were present. Laboratory studies confirmed that R. maidis could transmit PMV from peanut to peanut. This is the first report of R. maidis as a vector of PMV. Virus transmission tests using 5 aphids per peanut seedling were conducted in the greenhouse. Myzus persicae (Sulzer), Aphis craccivora (Koch), and Rhopalosiphum maidis (Fitch) were able to vector peanut mottle virus from peanut to peanut 30, 14, and 4%, respectively. In subsequent tests, where only one aphid was used per test seedling, M. persicae, A. craccivora, and R. maidis transmitted PMV at 9, 4, and 2%, respectively. The retention abilities of the three aphid species tested for PMV was low when compared to other experiments (20). Switching varieties from Florigiant to Florunner did not appear to significantly increase virus transmission rates of any of the aphid species tested in comparison to tests done with Florigiant. This is the first report of transmission testing with these three aphid species and PMV V745-473, a strain of the virus isolated from soybean in Virginia and with the Florigiant variety of peanuts. In feeding preference tests, all three aphid species tested showed a preference for settling onto a host, regardless of the host species used, to wandering at random in the feeding arena and not feeding on any host plant. A. craccivora showed the greatest propensity to wander, with only 33% of those aphids tested choosing to settle on a host. In contrast, 58% of R. maidis tested chose to settle on a host. In comparisons of feeding preferences between combinations of two host plants, M. persicae preferred chinese cabbage in 4 comparisons and soybean in 2, A. craccivora preferred both soybean and chinese cabbage in 2 comparisons each, and R. maidis preferred chinese cabbage in 4 comparisons and sorghum in 3. Chinese cabbage, soybeans, cowpeas and sorghum are preferred by these three aphid species over peanuts. Based on feeding preference it would be impossible to separate the PMV vectoring ability of these three aphid species. The three aphid species tested were able to reproduce well on the plants on which they were reared, but only A. craccivora was able to reproduce on peanuts. / M.S.

Development of early leafspot, caused by Cercospora arachidicola Hori, was monitored on' Florigiant' peanut (Arachis hypogaea L.) at two field sites in Suffolk, Virginia. In one study, plants in 27-cm-diameter plots were inoculated with 20,000 conidia and inoculation dates were replicated in five randomized complete blocks. At location one in 1985 and 1986, lesions/leaf at two weeks after inoculation correlated significantly (P ≤ 0.05) with infection indices (IND) developed by the Virginia leafs pot advisory and hours of relative humidity (RH) ≥ 95%. At location two, correlations between lesions/leaf and IND as well as hours of RH ≥ 95% were significant in 1986, but not in 1985. Certain site specific factors were believed to have altered plant susceptibility to leafspot at this site in 1985. In another study, pots with greenhouse-grown peanut were placed between unsprayed rows of field plants, heavily colonized by C. arachidicola. Plants were removed after 3, 5, and 7 days of field exposure for six consecutive weeks in 1986 and returned to the greenhouse. Lesions/leaf at two weeks after initial exposure were correlated with IND values computed by five versions of the leaf spot advisory. Significant correlations were found between lesions/leaf on plants with field exposures of 5 and 7 days and cumulative IND values and hours of RH ≥ 90% and 95%. The low incidence of lesions resulting with field exposures of only 3 days coupled with a lack of significant correlations between disease and cumulative IND values for 3 days after inoculation in both studies suggests that infection processes require several days, and that fungicides may be applied to achieve disease control during this time. / M.S.

The effects of herbicides on axenic growth of Cylindrocladium crotalariae and on the development of CBR in the greenhouse and in the field was studied. In laboratory tests, dinitramine at 1, 5, and 10 ug/ml increased axenic growth of two isolates of C. Crotalariae in herbicide-amended potato-dextrose broth (PDB). Dinoseb and dinoseb + naptalam at the same rates also increased growth, whereas alachlor, benefin, diphenamid, vernolate and 2,4-DB were found to have no consistent effect. Treatment of infested soil with dinitramine at 1, 5, 10, 50, and 100 ug/g soil had no significant effects on populations of C. crotalariae microsclerotia (ms) in a Ruston or a Woodstown loamy fine sand. Dinoseb reduced ms populations significantly (P=0.5) in Woodstown soil at 5, 10, 50, and 100 ug/g soil and in Ruston soil at rates of 50 and 100 ug/g soil. In greenhouse tests, soil treatment with dinitramine at 0.56 kg/ha but not at 0.84 kg/ha increased significantly (P=0.05) the severity of CBR in Ruston and in Woodstown soil. In Ruston soil, dinitramine increased the severity of CBR only at an inoculum density of 5 ms/g soil, but in CBR only at an inoculum density of 5 ms/g soil, but in Woodstown soil, the herbicide resulted in disease increase only at 50 ms/g soil. Additional tests showed that disease severity was increased when peanut seedlings were pretreated with dinitramine and transplanted to herbicide-free infested soil. Dinitramine treatment was postulated to alter growth and development processes in ‘Florigiant’ peanut in a way to result in increased susceptibility to CBR. Dinitramine treatment of soil did not increase disease severity in ‘NC3033’ peanut, a CBR-resistant cultivar. Dinoseb at 1.68 kg/ha increased significantly (P=0.05) the severity of CBR in Woodstown soil, but did not in Ruston soil. Higher rates of dinoseb (3.36 and 6.72 kg/ha) were found to have no effect. Field tests were conducted in microplots (77-cm diameter) to determine the effects of herbicides on CBR. The severity of pod rot caused C. crotalariae was increased significantly (P = 0.05) by pre-plant soil treatment with dinitramine at 0.56 kg/ha, but not at 0.84 kg/ha in 1979. Although not significant (P=0.05), substantial increases in root rot and top symptom severity occurred in 1979 in response to this treatment. Pre-plant soil treatments with dinoseb at 1.68, 3.36, and 6.72 kg/ha did not affect disease development in microplots in 1979. In 1980, dinitramine (0.56 kg/ha) increased the severity of both root and pod rot significantly (P=0.05) in two separate microplot tests. Dinoseb at 1.68, but not at 3.36 or 6.72 kg/ha also increased significantly the severity of CBR in these tests.. Soil assay results indicated that herbicide treatments had no consistent effect on populations of C. crotalariae ms, .Macroposthonia ornatum or Meloidogyne hapla in microplots. These results provide evidence that certain herbicides can affect the development and severity of CBR in peanut. Furthermore, the present study emphasizes that knowledge of the non-target effects of pesticides on the biosphere can be significant and must be considered in tbs development of effective disease control programs. / Ph. D.

Sclerotinia minor Jagger myceliogenic sclerotial germination, growth, infection, and colonization of peanut (Arachis hypogaea L.) tissue was optimum at 20-25 C. Ninety-five to 100% relative humidity (RH) for more than 12 hours was necessary for germination. There was no difference in infection and colonization of main stem versus lateral branch tissue but younger plants were more susceptible than older plants. Correlation and regression analyses were conducted on data from field experiments with artificially and naturally infected plants. Important independent variables in regression models for lesion length (LL), weekly change in lesion length, or disease severity index (DSI) were: number of days with temperature ≤16.7 C (DA17), the interaction of DA17 with precipitation (DA17*P), RH, maximum temperature (TMAX), and plant height for the week prior to disease measurement, and TMAX, P, and soil moisture (SM) at 0 to 5-cm for the period two weeks prior to disease measurement. were studied in field Infection and disease development plots with modified canopy. / Ph. D.

Sclerotinia blight, caused by <i>Sclerotinia minor</i>, is a severe disease of peanut in Virginia. Vinclozolin (V), iprodione (I), dicloran (D), and pentachloronitrobenzene (PCNB) were evaluated for their fungitoxicity to <i>S. minor</i>. The mean ED₅₈ values for five isolates were found to be 0.07, 0.11, 0.91, and 1.27 μg/ml, for V, I, D, and PCNB, respectively, on fungicide-amended glucose yeast-extract agar (GYEA). Fungicide-resistant growth sectors developed on media amended with I or V. Nine such strains occurred; they were capable of growth on GYEA amended with up to 1000 μg/ml of I or V, and were cross-resistant to D or PCNB. Resistance was maintained in all but two strains after repeated culture in the absence of fungicide for 3 yr. In field microplots, two resistant strains were pathogenic to peanut and survived as well as a fungicide-sensitive field isolate. D, I and V were applied to peanuts in the microplots for 3 yr at total annual rates of 8.41, 3.36, and 2.52 kg/ha, respectively. Disease severity caused by the resistant strains was suppressed 19, 33, and 87% by D, I, and V, respectively, as compared to 15, 24, and 76% for the sensitive isolate. Isolates recovered from tissue biopsies still grew on fungicide-amended GYEA indicating that <i>in vitro</i> and <i>in vivo</i> resistance are not equivalent in this case. Fungicide treatments reduced sclerotial populations of all strains, and reduced the viability of sclerotia from sensitive but not resistant strains. Fungicide-resistant strains were capable of surviving and competing pathogenically in microplots infested with equal numbers of sclerotia from a sensitive and a resistant strain; this trend was enhanced by fungicide applications. A survey of 763 isolates from fields treated with these fungicides failed to detect resistant strains. One fungicide-resistant isolate was recovered from an iprodione-treated microplot originally infested with a sensitive field isolate. A technique utilizing excised peanut stems was devised to evaluate isolate pathogenicity, cultivar resistance to the disease, susceptibility of different age peanut tissues, and fungicide persistence on peanut stems in the field. The method was also used to screen fungicides; results verified previous findings which indicated that <i>in vitro</i> resistance is not equivalent to <i>in vivo</i> resistance. Resistance to these fungicides may eventually become a field problem, but with correct management they should provide years of disease control. / Ph. D. / incomplete_metadata

The immigration, population dynamics and intra-field movement patterns of the tobacco thrips, Frankliniel/a fusca (Hinds), in peanuts were studied in 1990 and 1991 in the City of Suffolk, Virginia. The long-term goal was to improve our understanding of the role of tobacco thrips in the epidemiology of tomato spotted wilt virus in peanuts; the short-term goal was to develop quantitative descriptions of thrips development and movement for inclusion in a systems model intra-field movement patterns were assessed by establishing an isolated population of thrips in the center of a (67 m2) peanut field and monitoring the subsequent spread of thrips throughout the field. Thrips did not redistribute step-wise, as expected. Instead, thrips dispersed evenly throughout the field. However, failure to observe step-wise movement was probably due at least in part to the failure of insecticides to control the thrips in the corner areas of the experimental fields. Immigration and inter-field movement of adults were monitored with sticky traps. Thrips were caught on these traps only during the immigration period, which ended approximately two weeks after plant emergence. The spatial pattern of thrips immigration indicated that their movement was strongly influenced by wind direction and also by field borders. Proximity to fields planted the previous year in peanut also apparently influenced immigration. A computer model was developed based on previous and recent laboratory studies to simulate the phenology of the tobacco thrips as a function of temperature. The model was validated using field data from the 1991 season and used to help explain apparent anomalies in the 1990 data. The model confirmed that thrips damage to peanut was caused by two generations (F1 and F2) during June and July. Model results also suggested that rainfall was a significant mortality factor in thrips population dynamics. A second model was developed to simulate thrips movement in relation to wind speed and direction. This model will be used in future studies to test hypotheses about the ability of thrips to move within and between fields. / Master of Science

Experiments were conducted at Blacksburg from 1975 to 1978 on soybean, Glycine max (L.) Merrill. The objectives were: (1) to study the inheritance of reaction to peanut mottle virus (PMV); (2) to determine the allelic relationships between genes for resistance from various germplasm sources; and (3) to screen a part of the soybean Plant Introduction germplasm collection and identify PMV-resistant strains. Soybean cultivars, Plant Introduction (PI) strains, and advanced generation progenies derived from selected crosses were artificially inoculated with PMV-S/V74S (a Virginia isolate) and evaluated for their reaction to PMV in the field and greenhouse. Two cultivars, 'Virginia' and 'Pine Dell Perfection', that were previously reported as resistant to a mild PMV strain were found to be susceptible to PMV-S/V74S. Crosses between resistant and susceptible lines and among resistant lines were made in the field in 1976 and 1977. The F₁, F₂, and F₃generation seedlings derived from selected crosses were tested for reaction to PMV in the greenhouse. It was shown that resistance in the cultivar 'Peking' is conditioned by a single recessive gene designated rpv₂. Evidence based on segregation in F₂ populations was also presented that indicates the presence of three other dominant genes for resistance to PMV-S/V74S. The three dominant genes are designated Rpv₁ (first reported by workers in Georgia), Rpv₃, and Rpv₄. Preliminary F₁ and F₂ data were obtained from crosses between 15 resistant PI strains and two resistant "testers," 'York' and PI 89,784. The F₂ data obtained from five crosses indicate the possibility of still other genes for resistance to PMV. The two susceptible lines used in the study, Virginia and PI 229,315, differed markedly in their reactions to PMV. The presence of different alleles or modifying genes controlling the susceptible reactions in the two lines is suggested. A total of 2161 FC and PI strains in Maturity Groups II, III, and IV were inoculated with PMV-S/V74S in the field during 1976 and 1977. Three hundred sixty-six strains that showed 10% or less virus infection were identified. These "resistant" strains provide a pool from which other genes for PMV resistance perhaps can be isolated. Differences in PMV disease reactions of plants from the same strain were noted when plants were tested in both the field and greenhouse. The differences were attributed to the following three factors: (1) differences in stage of plant growth at time of inoculation (field grown plants were generally larger at time of inoculation than plants inoculated in the greenhouse); (2) differences in environmental conditions between the field and greenhouse; and (3) the artist's airbrush inoculation technique was used in the field while the rub inoculation technique (mortar and pestle) was used in the greenhouse. / Ph. D.

Two isolates of peanut stripe virus (PStV), stripe and blotch, were compared ultrastructurally in peanut (Arachis hypogaea L. 'Florigiant') at several stages of leaf expansion. Ultrathin sections of young leaves infected with either isolate of PStV revealed pinwheel inclusions attached to the cell wall near plasmodesmata. The cytoplasm of infected cells were highly vesiculated. Virus particles amassed in crystalline arrays were observed in blotch infected cells. Virus particles were observed along the arms of pinwheel inclusions. Scroll inclusions appeared in PStV infected cells at a later stage of leaf expansion. In more mature leaves, pinwheel and scroll inclusions occurred in the cytoplasm in association with mitochondria. Virus particles were observed free in the cytoplasm as well as concentrated in linear arrays along the inner surface of the tonoplast. Membrane and organelle degradation was evident in cells infected with either isolate of the virus. Numerous cytoplasmic inclusions and virus particles were observed in cells from light green areas of the leaf. Cells from dark green areas did not contain cytoplasmic inclusions and contained few if any virus particles. Particle measurements show stripe and blotch isolates to have a mean length of 753 nm and 747 nm for leaf dip preparations and 746 nm and 745 nm for partially purified preparations, respectively. Both isolates had a modal length of 750 nm, regardless of the extraction procedure. The relative virus titer of each isolate was determined in peanut leaves at five stages of leaf expansion and in dark green and light green areas of infected leaves. Virus titer increased significantly from the closed to the fully expanded stage, at which time the virus titer peaked and then decreased slightly. Virus titer was consistently higher in leaves infected with the blotch isolate at all expansion stages. Virus titer was also higher in cells from light green areas of the leaf than from dark green areas of the leaf, regardless of isolate. / M.S.

This study assessed the mycological and aflatoxin contamination of peanuts collected from Kinshasa, DRC and Pretoria, South Africa. Forty peanut samples were collected randomly at informal markets in the two cities and analysed for mycoflora and aflatoxins (B1, B2, G1 and G2) using standard methods. The results indicated that 95% and 100% of peanut samples collected from Kinshasa and Pretoria, respectively were contaminated with aflatoxigenic fungi with Kinshasa’s samples being the most contaminated (up to 49, 000 CFU/g). Seventy percent (70 %) of Kinshasa-samples and 35% of Pretoria-samples exceeded the maximum allowable limit of aflatoxin B1 set by JECFA (5 ppb). Statistical evidence showed a significant positive correlation between mycoflora and aflatoxin level for Kinshasa-samples (r = 0.4743, p < 0.005) while Pretoria-samples showed no correlation. The study reveals that high level of contamination in Kinshasa-samples could be due to the tropical nature of the climate and poor storage conditions as compared to Pretoria which is sub-tropical and sanitary regulations are enforced. / Life & Consumer Sciences / M. Sc. (Life Sciences)