Inhibition of Fusarium moniliforme var. subglutinans, the causal agent of pine pitch canker, by the soil bacterium Arthrobacter sp

Pine Pitch Canker (PPC): Pathways of Pathogen Spread and Preventive Measures

Fusarium circinatum (Nirenberg and O’ Donnell) is the causal agent of pine pitch canker (PPC) disease, one of the most devastating forest diseases worldwide. Long-distance spread occurs mainly through the movement of infected seeds whereas at regional level, the movement of seedlings, substrates, or containers may play an important role in fungal dispersal. Invasion of nurseries takes place via infected seeds and further spread can occur by planting contaminated seedlings, especially due to the possibility of infected plants remaining symptomless. Once established, F. circinatum spreads by rain, wind, and insects. The natural spread of the pathogen is limited due to the short dispersal distances of the spores and the fairly short flight distances of disseminating insects. In this review, we summarize the currently known dispersal pathways of the pathogen, discussing both natural and human-assisted processes. With the purpose of understanding how to best intervene in the disease’s development in nurseries and forests, we outline the epidemiology of the pathogen describing the key factors influencing its spread. Preventive measures to control the spread of F. circinatum locally and globally are described with special emphasis on the challenges in implementing them.

Potentiality of Bacillus subtilis as biocontrol agent for management of anthracnose disease of chilli caused by Colletotrichum gloeosporioides OGC1

A soil bacterium, Bacillus subtilis, isolated from the rhizosphere of Chilli, showed high antagonistic activity against Colletotrichum gloeosporioides OGC1. A clear inhibition zone of 0.5–1 cm was observed in dual plate assay. Microscopic observations showed a clear hyphal lysis and degradation of fungal cell wall. In dual liquid cultures, the B. subtilis strain inhibited the C. gloeosporioides up to 100 % in terms of dry weight. This strain also produced a clear halo region on chitin agar medium plates containing 0.5 % colloidal chitin, indicating that it excretes chitinase. The strain also produced other mycolytic enzymes—glucanase and cellulase, demonstrated by a clear zone of hydrolysis of yeast cell wall glucan (YCW 0.1 % v/v) and carboxymethylcellulose (CMC 0.1 % v/v). In liquid cultures, the strain showed appreciable levels of chitinase, glucanase and cellulase activities and hydrolytic activity with C. gloeosporioides OGC1 mycelia as the substrate. The role of the B. subtilis strain in suppressing the fungal growth in vitro was studied in comparison with a UV mutant of that strain, which lacked both antagonistic and hydrolytic activity. The mycolytic enzyme mediated antagonism of B. subtilis was further demonstrated by heat inactivation (70–100 °C), treatment with trypsin and TCA of the crude enzyme extract which lacked antifungal property also. Treatment of the chilli seeds with Bacillus sp. culture showed 100 % germination index similar to the untreated seeds. The treatment of the seed with co-inoculation of the pathogen with Bacillus sp. culture showed 65 % reduction in disease incidence by the treatment as compared to the seed treated with pathogen alone (77.5 %).

Expression of the chitinase gene from Trichoderma aureoviride in Saccharomyces cerevisiae

Chitinase gene ech42 was obtained from Trichoderma aureoviride M and amplified by PCR. The isolated DNA of ech42 was then sequenced. The results showed that the open reading frame of ech42 was 1,447 bp long, encoding 421 amino acids. Three introns were found in the sequence. The cloning vector pMD18-T and an E. coli DH5alpha host were used to yield clones as E. coli DH5alpha/ech42. The ech42 gene was integrated into the genomic DNA of pYES2 by insertion into a single site for recombination, yielding the recombinant pYES2/ech42. Chitinase expressed by pYES2/ech42 was induced by galactose (maximal activity 0.50 units ml(-1)) and was produced in fermentation liquid cultured for 36 h.

Growth of chitinolytic dune soil beta-subclass Proteobacteria in response to invading fungal hyphae

It has frequently been reported that chitinolytic soil bacteria, in particular biocontrol strains, can lyse living fungal hyphae, thereby releasing potential growth substrate. However, the conditions used in such assays (high bacterial density, rich media, fragmented hyphae) make it difficult to determine whether mycolytic activity is actually of importance for the growth and survival of chitinolytic bacteria in soils. An unidentified group of beta-subclass Proteobacteria (CbetaPs) was most dominant among the culturable nonfilamentous chitinolytic bacteria isolated from Dutch sand dune soils. Here we demonstrate that the CbetaPs grew at the expense of extending fungal mycelium of three dune soil fungi (Chaetomium globosum, Fusarium culmorum, and Mucor hiemalis) under nutrient-limiting, soil-like conditions. Aggregates of CbetaPs were also often found attached to fungal hyphae. The growth of a control group of dominant nonchitinolytic dune soil bacteria (beta- and gamma-subclass Proteobacteria) was not stimulated in the mycelial zone, indicating that growth-supporting materials were not independently released in appreciable amounts by the extending hyphae. Therefore, mycolytic activities of CbetaPs have apparently been involved in allowing them to grow after exposure to living hyphae. The chitinase inhibitor allosamidin did not, in the case of Mucor, or only partially, in the cases of Chaetomium and Fusarium, repress mycolytic growth of the CbetaPs, indicating that chitinase activity alone could not explain the extent of bacterial proliferation. Chitinolytic Stenotrophomonas-like and Cytophaga-like bacteria, isolated from the same dune soils, were only slightly stimulated by exposure to fungal hyphae.