Effect of soil water potential on the survival ofCapsicum annuum foot rot fungus in soil

Summary In drier soils,Sclerotium rolfsii the prime pathogen responsible for the foot rot ofCapsicum annuum caused severe crop losses. Although sclerotia of the pathogen remain dormant at high water potentials they actively germinated in soil of low water potentials.     

Bacterial antagonists of Sunflower (Helianthus annuus L.) fungal pathogens

Bacteria isolated on nutrient agar and King's medium B from sunflower leaves, crown and roots inhibited in vitro growth of the leaf spot and wilt pathogens Alternaria helianthi, and Sclerotium rolfsii, respectively, and also the root rot pathogensRhizoctonia solani and Macrophomina phaseolina. Antagonistic bacteria from leaves were mainly actinomycetes and pigmented Gram-positive bacteria, while those from roots and crowns were identified asPseudomonas fluorescens-putida, P. maltophilia, P. cepacia, Flavobacterium odoratum andBacillus sp. In soil bioassays, when used as seed inoculum in the presence ofS. rolfsii, P. cepacia strain N24 increased significantly the percentage of seedling emergence. Bacterial strains which exhibited broad spectrum in vitro antagonistic activity were tested for colonisation of sunflower roots, when used as a seed inoculum. Good colonisers (10⁴ to 10⁶ bacteria/g root) were consistent in their ability to reduce disease and fungal wilt. A seedling having a primary root length < 5 cm with fewer lateral roots, necrosed cotyledons or crown and a wilted shoot indicated its diseased status. On an average, only 30% of seedlings were diseased when treated with the antagonistic strains, in the presence of the pathogen, while 60% of the seedlings were diseased in the presence of the pathogen alone. In microplots treated with strain N24, only 1 to 3% of the seedlings were wilted, while 14% of the seedlings were wilted in the presence of the pathogen alone. The results obtained show that bacterial antagonists of sclerotial fungi can be used as seed inocula to improve plant growth through disease suppression     

Effects of some carbon and nitrogen sources on spore germination,production of biomass and antifungal metabolites by species of Trichoderma and Gliocladium virens antagonistic to Sclerotium cepivorum

Conidia of Trichoderma pseudokoningii (IMI 322662) and T. viride (IMI 322659) were incubated in 1% bacteriological peptone at 25° C for 20 h and more than 95% of the spores germinated. In the same medium, only 35% of the conidia of Gliocladium virens (G20) and T. viride (IMI 322663) germinated but when 1% glucose was added, germination was increased to 70%. In the presence of glucose as a carbon source, maximal biomass production of G. virens (G20) after seven days at 25°C was obtained with either potassium nitrate or L‐alanine as the nitrogen source, whereas the Trichoderma isolates needed an organic nitrogen source. With L‐alanine as a nitrogen source, glucose, galactose and sucrose were readily utilized for biomass production by all fungal isolates. Maltose utilization by G. virens (G20) and T. pseudokoningii was incomplete after 21 days incubation, whereas glucose utilization was complete by this time. G. virens, T. pseudokoningii and T. viride (IMI 322663) produced antifungal metabolites which were effective at reducing radial growth of Rhizoctonia solani, Botrytis cinerea as well as S. cepivorum. The metabolites produced by G. virens were very active against all three pathogens but the metabolites produced by T. pseudokoningii and T. viride (IMI 322663) were less active. T. viride (IMI 322659) was a very poor antifungal metabolite producer.     

Additive Effects of Meloidogyne arenaria and Sclerotinin rolfsii on Peanut

Field observations have suggested that infection of peanut by Meloidogyne arenaria increases the incidence of southern blight caused by Sclerotium rolfsii. Three factorial experiments in microplots were conducted to determine if interactions between M. arenaria and S. rolfsii influenced final nematode population densities, incidence of southern blight, or pod yield. Treatments included four or five initial population densities of M. arenaria and three inoculum rates of S. rolfsii. Final nematode population densities were affected by initial nematode densities in all experiments (P = 0.01) and by S. rolfsii in one of three experiments (P = 0.01). Incidence of southern blight increased with increasing inoculum rates of S. rolfsii in all experiments and by the presence of the nematodes in one experiment (P = 0.01). Pod yield decreased with inoculation with S. rolfsii in all experiments (P = 0.05) and by M. arenaria in two of three experiments (P = 0.05). In no experiment was the interaction among treatments significant with respect to final nematode population densities, incidence of southern blight, or pod yield (P = 0.05). The apparent disease complex between M. arenaria and S. rolfsii on peanut is due to additive effects of the two pathogens.     

Biological Control Agents in Combination with Fertilization or Fumigation to Reduce Sclerotial Viability of Sclerotium rolfsii and Disease of Snap Beans in the Greenhouse

The use of biological control agents in combination with fertilization or fumigation to reduce sclerotial viability of Sclerotium rolfsii and the disease it causes on snap bean was investigated in the greenhouse. The fertilizers ammonium sulphate [(NH₄)₂SO₄], ammonium nitrate (NH₄NO₃), diammonium phosphate [(NH₄)₂HPO₄], or urea applied to soil at a field rate of 135 kg/ha, 15 cm deep of nitrogen (N) (0.09 mg of N/g) or Gliocladium virens (Gl-3) biomass at a rate of 7.5 kg/ha, 15 cm deep (0.05 mg/g) did not reduce the viability of sclerotia of S. rolfsii (Sr-1) when each was applied alone. However, treatment with fertilizer together with the low rate of Gl-3 biomass significantly reduced the sclerotial viability. The treatments that were effective in reducing the viability by more than 75% were the application of (NH₄)₂SO₄ or (NH₄)₂PO₄ and the low rate of Gl-3 biomass. Application of the high rate (0.25 mg/g) of Gl-3 biomass alone only reduced the sclerotial viability by 25%. The addition of any of the fertilizers with the low rate of biomass generally resulted in bean seed germination in the pathogen-infested soil that was higher than that achieved with each individual component. The disease severity (DSI) on beans was appreciable (<3.0) in pathogen-infested soil treated with or without the fertilizer (NH₄)₂SO₄ and in pathogen-infested soil without fertilizer but with a low rate of Gl-3. However, in pathogen-infested soil treated with the fertilizer and the low rate of Gl-3 biomass together, the disease was reduced to a DSI value of less than 1.0. In fumigation studies with metham sodium (Vapam), a dose-response study to investigate the viability of sclerotia of S. rolfsii (Sr-3) indicated that fumigant rates of less than 23.3 μ g/g of soil were sublethal. It was also shown that 5.4 μ g/g of metham sodium was inhibitory to Gl-3 biomass but not to conidia. Consequently, the conidia of isolates Gl-3, Thm-4 of Trichoderma hamatum, and Tv-1 of Trichoderma viride were used together with metham sodium at 17.1 μ g/g of soil. Conidia that were applied to the soil 2 days prior to metham sodium reduced the viability of sclerotia more than each individual component. The results of this study suggest the feasibility of effective disease reduction with an approach utilizing biological control in combination with fertilization or fumigation.     

Elimination of broad-host range plasmid vectors in Escherichia coli by curring agents

A comparative study was made of the susceptibility of broad-host range vector plasmids belonging to Inc P1 and Q groups in Escherichia coli to various curing agents. Plumbagin and SDS eliminated RP4 (Inc P1 group) plasmid whereas pKT231 (Inc Q) and pRK2013 (having ColE1 replicon) were eliminated by hexamine ruthenium (III) chloride, alpha-santonin, coumermycin A1 and cis-dichloro diamine platinum (II). The curing activity of these agents was specific.     

Evidences of biological control capacities of Streptomyces spp. against Sclerotium rolfsii responsible for damping-off disease in sugar beet (Beta vulgaris L.)

Ten antibiotic-producing Streptomyces spp. isolated from Moroccan soils were evaluated for their ability to inhibit in vitro Sclerotium rolfsii development. Four isolates having the greatest pathogen inhibitory capabilities were subsequently tested for their ability to inhibit sclerotial germination in sterile soil. This test was carried out by using biomass inoculum, culture filtrate, and spore suspension of the isolates as treatment. Treatment with biomass inoculum and culture filtrate gave the highest inhibition of sclerotia. Biological control tests against Sclerotium rolfsii damping-off of sugar beet seeds showed that the selected Streptomyces isolates reduced significantly the disease severity, the J-2 isolate being the more potent. In addition, treatment with the isolate J-2 resulted in a significant increase (P ≤ 0.05) in seedling development compared to the control. All antagonistic Streptomyces selected here were able to grow in the rhizosphere soil from infected sugar beet culture.     

Two mannanases from Sclerotium rolfsii in total chlorine free bleaching of softwood kraft pulp

The plant pathogenic basidiomycete Sclerotium rolfsii produces a wide range of extracellular hemicellulolytic enzymes. To study the effect of β-mannanases in total chlorine free bleaching of softwood pulp, two purified β -mannanases from S. rolfsii, with molecular masses of 42 and 61 kDa, a xylanase preparation from S. rolfsii and combinations of these were tested in a O(QX)P bleaching sequence (O = oxygen delignification, X = treatment with enzymes, Q = chelation of metals, P = treatment with hydrogen peroxide in alkaline solution). A brightness increase of 1.6 and 1.9% ISO was obtained with the 42 and 61 kDa mannanase and a combination of each of these enzymes with xylanases gave a brightness increase of 2.5 and 2.8% ISO, respectively. The effect of mannanases and xylanases was nearly additive. Both mannanases alone caused a lower decrease of the kappa number as compared to xylanases. The mannanases differed in their ability to release oligosaccharides from different mannans. The 61 kDa mannanase liberated larger fragments and caused rapid depolymerisation of mannans, which seems to promote the bleaching of pulp.     

Evaluation of ash from some tropical plants of Nigeria for the control of Sclerotium rolfsii Sacc. on wheat ( Triticum aestivum L.)

Eleven ash samples, from organs of nine tropical plants, were screened for their abilities to inhibit mycelial growth and sclerotial germination of a Nigerian isolate of Sclerotium rolfsii on agar and in the soil. Ten ash samples showed some activity against mycelial growth of S. rolfsii in vitro. Ash samples from Delonix regia stem wood, Mangifera indica leaf and Vernonia amygdalina leaf were most effective as each totally inhibited mycelial growth of S. rolfsii in vitro. Ocimum gratissimum leaf ash, D. regia wood ash and Musa paradisiaca flower bract ash inhibited sclerotial germination on agar. Nine ash samples protected seeds against pre-emergence rot. Ash from M. indica leaf, V. amygdalina leaf and Azadirachta indica leaf protected seedlings against post-emergence infection. Eichornia crassipes ash, which was ineffective in vitro, offered some protection to seeds in soil against pre-emergence rot. The study demonstrates potentials of ash samples from tropical plants in control of S. rolfsii on wheat. This revised version was published online in June 2006 with corrections to the Cover Date.     

蜜环菌侵染后猪苓菌核防御结构的发生及功能

蜜环菌通过猪苓菌核表皮侵染菌核时,菌核表皮下层的菌丝具特异的拮抗反应,如细胞中有结晶颗粒出现,厚壁菌丝形成,部分薄壁菌丝有质壁分离现象。蜜环菌的侵染诱导了菌核防御结构的发生:离侵染点一定距离的部位出现由少量木质化菌丝和厚壁菌丝形成的疏松带状;蜜环菌侵入后,上述菌丝增多并紧密排列形成菌核的初级隔离腔,入侵的蜜环菌和部分菌核被隔离在腔中;在初级隔离腔形成的同时,外围的次级隔离腔开始发育。蜜环菌菌索和皮层菌丝分枝可突破初、次级隔离腔的壁,再以菌索产生的侵染带侵染菌核较外部的最后防线即三级隔离腔。本文较系统的阐述了蜜环菌侵染后菌核各防御结构的发生特点及功能。