Effects of supplementary UV-B radiation on development of damping-off in spinach caused by the soil-borne fungusFusarium oxysporum

The effects of UV-B radiation (290–320 nm) on development of damping-off of spinach (Spinacia oleracea) caused by the fungusFusarium oxysporum were examined in a growth cabinet. The incidence of disease greatly increased when experimental plants were grown in visible radiation with supplementary UV-B radiation. This increase was suppressed by increasing the irradiation of visible radiation.Fusarium oxysporum was isolated from the roots of all damping-off plants and the roots of some unwilted plants, indicating that spinach infected with the pathogen did not necessarily suffer from damping-off in 15d. Supplementary UV-B radiation suppressed the increase in growth components such as the number of leaves, the plant height and the fresh weight of aboveground plant parts, but did not affect the fresh weight of roots. The ratio of the number of plants infected with pathogen to the total number of plants was over 80% irrespective of light conditions. It was suggested that the defense response of spinach to this pathogen was greatly influenced by the physiological state of aboveground plant parts resulting from supplementary UV-B radiation.     

Growth and yield of groundnut (Arachis hypogaea L.) plants in soil inoculated with Aspergillus niger Van Tieghem

Aspergillus niger, a soil-borne fungus is a causative agent of hypocotyl malformations in infected groundnut (Arachis hypogaea L.) plants, but its effect on yield is unknown. This study sought to determine its effect on growth and yield. Seeds of Chinese and JL45 varieties were sown in soil inoculated with A. niger. Fresh and dry weights of the shoots and roots were taken at 10-day intervals. Nodule count was done at 30 days after emergence and subsequently at 10-day intervals. Pods of 20 plants each from inoculated and uninoculated soils were harvested. Growth was suppressed in plants grown on A. niger inoculated soil. Eight-day old plants grown in inoculated soil developed curvatures on their hypocotyls. Nodulation was suppressed (p < 0.05) in plants grown in inoculated soil. Although growth was suppressed in plants grown on inoculated soil, yield of both varieties of groundnut was not affected.     

Eradication of Polymyxa betae by Thermal and Anaerobic Conditions and in the Presence of Compost Leachate

The abiotic conditions required for eradication of Polymyxa betae, the vector of Beet necrotic yellow vein virus in sugar beet, were investigated. Survival of resting spores of P. betae was determined under aerobic (30 min, 4 days and 21 days) and anaerobic (4 days) conditions under several temperature regimes in a water suspension and in leachate extracted from an aerobic compost heap. In water under aerobic conditions the lethal temperature was 60, 55 and 40°C for exposure times of 30 min, 4 days and 21 days, respectively. The effect of compost leachate and/or anaerobic conditions on survival of P. betae depended on temperature. After incubation for 4 days at 20°C, no significant effects of anaerobic conditions or leachate on the survival of P. betae were found. However, at 40°C for 4 days under anaerobic conditions, survival of P. betae was significantly lower than survival under aerobic conditions in water as well as in leachate. In leachate taken from an aerobic compost heap, aerobically incubated at 40°C for 4 days, survival of P. betae was significantly lower than survival in water at the same temperature. As anaerobic spots are prevalent in aerobic compost heaps, especially during the thermophilic phase, actual inactivation temperatures under composting conditions are likely to be lower than the temperatures we found for eradication in water under aerobic conditions.     

Complete Genome Sequences of Two Chinese Beet soil-borne virus Isolates Provide Evidence that the Genome is Highly Conserved

The complete nucleotide sequences of two Chinese isolates of Beet soil-borne virus (BSBV) from the Inner Mongolia and Xinjiang provinces (designated BSBV-IM and BSBV-XJ, respectively) were found to share around 99% sequence identity with that of a previously reported German isolate (BSBV-G). The genome organization of the three isolates was identical. A diversity index (Pi) analysis indicated that the overall nucleotide variability of all RNAs among the three isolates was <7%, only for the 5' part of the first triple gene block gene on RNA3 was it >6%. Although the 3' end of BSBV RNA 3 was previously reported to be highly variable, the results of this study show that the total BSBV genomes are considerably conserved, especially RNAs 1 and 2.     

Allelopathic Bacteria and Their Impact on Higher Plants

The impact of allelopathic, nonpathogenic bacteria on plant growth in natural and agricultural ecosystems is discussed. In some natural ecosystems, evidence supports the view that in the vicinity of some allelopathically active perennials (e.g., Adenostoma fasciculatum, California), in addition to allelochemicals leached from the shrub's canopy, accumulation of phytotoxic bacteria or other allelopathic microorganisms amplify retardation of annuals. In agricultural ecosystems allelopathic bacteria may evolve in areas where a single crop is grown successively, and the resulting yield decline cannot be restored by application of minerals. Transfer of soils from areas where crop suppression had been recorded into an unaffected area induced crop retardation without readily apparent symptoms of plant disease. Susceptibility of higher plants to deleterious rhizobacteria is often manifested in sandy or so-called skeletal soils. Evaluation of phytotoxic activity under controlled conditions, as well as ways to apply allelopathic bacteria in the field, is approached. The allelopathic effect may occur directly through the release of allelochemicals by a bacterium that affects susceptible plant(s) or indirectly through the suppression of an essential symbiont. The process is affected by nutritional and other environmental conditions, some may control bacterial density and the rate of production of allelochemicals. Allelopathic nonpathogenic bacteria include a wide range of genera and secrete a diverse group of plant growth-mediating allelochemicals. Although a limited number of plant growth-promoting bacterial allelochemicals have been identified, a considerable number of highly diversified growth-inhibiting allelochemicals have been isolated and characterized. Some species may produce more than one allelochemical; for example, three different phyotoxins, geldanamycin, nigericin, and hydanthocidin, were isolated from Streptomyces hygroscopicus. Efforts to introduce naturally produced allelochemicals as plant growth-regulating agents in agriculture have yielded two commercial herbicides, phosphinothricin, a product of Streptomyces viridochromogenes, and bialaphos from S. hygroscopicus. Many species of allelopathic bacteria that affect growth of higher plants are not plant specific, but some do exhibit specificity; for example, dicotyledonous plants were more susceptible to Pseudomonas putida than were monocotyledons. Differential susceptibility of higher plants to allelopathic bacteria was noted also in much lower taxonomical categories, at the subspecies level, in different cultivars of wheat, or of lettuce. Therefore, when test plants are employed to evaluate bacterial allelopathy, final evaluation must include those species that are assumed to be suppressed in nature. The release of allelochemicals from plant residues in plots of ‘continuous crop cultivation’ or from allelopathic living plants may induce the development of specific allelopathic bacteria. Both the rate by which a bacterium gains from its allelopathic activity through utilizing plant excretions, and the reasons for the developing of allelopathic bacteria in such habitats, are important goals for further research.     

Efficient dsRNA-mediated transgenic resistance to Beet necrotic yellow vein virus in sugar beets is not affected by other soilborne and aphid-transmitted viruses

Rhizomania caused by Beet necrotic yellow vein virus (BNYVV) is one of the most devastating sugar beet diseases. Sugar beet plants engineered to express a 0.4 kb inverted repeat construct based on the BNYVV replicase gene accumulated the transgene mRNA to similar levels in leaves and roots, whereas accumulation of the transgene-homologous siRNA was more pronounced in roots. The roots expressed high levels of resistance to BNYVV transmitted by the vector, Polymyxa betae. Resistance to BNYVV was not decreased following co-infection of the plants with Beet soil borne virus and Beet virus Q that share the same vector with BNYVV. Similarly, co-infection with the aphid-transmitted Beet mild yellowing virus, Beet yellows virus (BYV), or with all of the aforementioned viruses did not affect the resistance to BNYVV, while they accumulated in roots. These viruses are common in most of the sugar beet growing areas in Europe and world wide. However, there was a competitive interaction between BYV and BMYV in sugar beet leaves, as infection with BYV decreased the titres of BMYV. Other interactions between the viruses studied were not observed. The results suggest that the engineered resistance to BNYVV expressed in the sugar beets of this study is efficient in roots and not readily compromised following infection of the plants with heterologous viruses.