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.     

Die Ultrastruktur der Sklerotienentwicklung von Typhula incarnata

Die Sklerotien von Typhula incarnata sind aus einer einschichtigen pseudoparenchymatischen Rinde und einer plectenchymatischen Medulla aufgebaut. Die Differenzierung verschiedener Zellwandtypen wurde detailliert mit TEM-Untersuchungen verfolgt. Bemerkenswert sind besonders die schrittweise Einlagerung von melaninartiger Substanz in die Cuticula, welche als einzige Sklerotialstruktur von Trichoderma viride nicht abgebaut werden kann. Ferner wurden in den Medullazellen bisher unbekannte linsenförmige Zellwandappositionen gefunden, die wohl als Kohlehydratspeicher für die karpogene Keimung dienen.     

Changes of the cytoplasmic ultrastructure during development of sclerotia in Claviceps purpurea (Fr.) Tul.

The development of sclerotia of Claviceps purpurea was investigated by light and electron microscopy. During the first days after infection sterigma and conidiospores are formed. The spores show a moderately developed vacuolar system, they are thick walled and contain about 20% lipid (related to the cell volume) embedded in glycogen. The sterigma are cylindrical unicellular hyphae with electron dense cytoplasm and isolated strongly contrasted lipid droplets. In maturing sclerotia the hyphae become septated with increasingly thick cell walls and a large lipid content. The lipid forms small droplets in young cells, while in the mature sclerotium it occurs in the form of very large drops, occupying the major part of the cell. Simultaneously the composition of the lipid is changed. The mature cells have several nuclei. They are partially connected by osmiophilic substances, forming a network of intercellular spaces.Abbreviations HEPES N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid - DMSO Dimethylsulfoxide     

In vitro antifungal effects of potassium bicarbonate on <Emphasis Type="Italic">Trichoderma</Emphasis> sp. and <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis>

Bicarbonates are often utilized in the food industry to avoid fermentation and to improve pH, flavor, and texture. In the same manner, bicarbonates have been demonstrated to control postharvest phytopathogens; however, there are no reports describing the effects of these chemical compounds either on soil-borne pathogens such as Sclerotinia sclerotiorum or on antagonist fungi such as Trichoderma species. This study evaluated the antifungal effect of increasing concentrations (0, 2, 4, 6, 8, 10, 25, and 50 mM) of potassium bicarbonate (KHCO₃) on the growth of Trichoderma sp. strain R39 and S. sclerotiorum under in vitro systems. Applications of KHCO₃ greater than 8 mM significantly inhibited (P < 0.001) the growth of both fungi. Concentrations of KHCO₃ lower than 25 mM did not affect the antagonistic effect of Trichoderma on the growth of S. sclerotiorum; however, this fungal interaction was not observed when exposed to 50 mM KHCO₃ because of its strong inhibition of fungal growth. In addition, KHCO₃ concentrations higher than 8 mM caused significant (P < 0.001) reduction of the sclerotium formation of S. sclerotiorum. Sclerotium germination and de novo sclerotium formation were significantly (P < 0.001) inhibited as the concentrations of KHCO₃ increased. Results show the potential benefits of potassium bicarbonate for controlling both growth and development of S. sclerotiorum, although it also exerts negative effects on the Trichoderma strain that is a natural antagonist to S. sclerotiorum.     

How many fungi make sclerotia?

Most fungi produce some type of durable microscopic structure such as a spore that is important for dispersal and/or survival under adverse conditions, but many species also produce dense aggregations of tissue called sclerotia. These structures help fungi to survive challenging conditions such as freezing, desiccation, microbial attack, or the absence of a host. During studies of hypogeous fungi we encountered morphologically distinct sclerotia in nature that were not linked with a known fungus. These observations suggested that many unrelated fungi with diverse trophic modes may form sclerotia, but that these structures have been overlooked. To identify the phylogenetic affiliations and trophic modes of sclerotium-forming fungi, we conducted a literature review and sequenced DNA from fresh sclerotium collections. We found that sclerotium-forming fungi are ecologically diverse and phylogenetically dispersed among 85 genera in 20 orders of Dikarya, suggesting that the ability to form sclerotia probably evolved ≥14 different times in fungi.     

The effects of inoculum density and physical factors on the assessment of disease caused by Sclerotium rolfsii

Greenhouse and laboratory experiments were conducted to determine the effects of various physical factors on the assessment of disease caused by Sclerotium rolfsii using field and artificially infested soils. Lentil(Lens esculenta Moench) seedlings growing in trays or pots with sand were inoculated by surrounding them with a layer of soil infested with the pathogen. The number of dead plants was maximal within a 10-day period following inoculation. Seedling mortality increased with the number of sclerotia in the soil to a maximum that depended on seedling spacing, depth of the soil layer, and soil type.     

Variation inSclerotium rolfsii isolates in Japan

Sixty-seven isolates of the southern blight fungus from Japan were divided into five groups based on ITS-RFLP analysis of nuclear rDNA. Morphological characters of sclerotia varied between groups. Three groups were reidentified asSclerotium rolfsii, and two resembledS. delphinii in RFLP patterns and/or in having large sclerotia and relatively low optimal growth temperature (28°C). Sclerotia of the latter, however, varied in size according to temperature and became indistinguishable from those ofS. rolfsii at high temperatures. Hyphal anastomosis (imperfect fusion) was observed between different ITS-RFLP groups, as well as between different isolates belonging to the same groups. These results indicate that populations of this fungus in Japan consists of several different subgroups, although morphological differences are not always evident.