Endophytic fungi promote plant growth and mitigate the adverse effects of stem rot: an example of Penicillium citrinum and Aspergillus terreus

Disease resistance is a highly desirable crop trait in the sustainable agricultural industry. Endophytic fungi with gibberellins-secreting potential are now widely known for their ability to stimulate plant growth, but their role in promoting disease resistance in plants has rarely been reported. We have studied the role of Penicillium citrinum LWL4 and Aspergillus terreus LWL5 in time-dependent manner on sunflower (Helianthus annuus L.) growth, disease resistance and their capacity for the regulation of hormone signaling networks involved in plant defense against the stem rot caused by Sclerotium rolfsii for 3, 6 and 12 days after treatment (DAT). Our results show that plant growth characteristics (i.e. shoot length, shoot diameter, shoot fresh/dry weight, transpiration, stomatal conductance, photosynthesis and chlorophyll content) were promoted in fungi-treated plants with or without the disease caused by Sclerotium rolfsii as compared to their respective controls in 3, 6 and 12 DAT. The negative impacts of stem rot in endophyte-treated diseased plants were greatly reduced in comparison to control diseased plants shown by low disease severity in 3, 6 and 12 DAT. Similarly, fungal endophytes in diseased plants relieved the biotic stress in time-dependent manner (3, 6 and 12 DAT) as shown by low level of endogenous salicylic acid and jasmonic acid contents and were significantly higher in control diseased plants. Furthermore, we observed that the Penicillium citrinum LWL4 association had a greater positive effect on sunflower plants than Aspergillus terreus LWL5. It was concluded that inoculation with fungal endophytes reprogramed plant growth during disease incidence by regulating responses associated with host plant defense. Management strategies involving endophytic symbiosis can help achieve sustainability in agriculture in an eco-friendly manner by reducing excessive fungicide use.     

Phytophthora cryptogea root rot of tomato in rockwool nutrient culture. I. Analysis of root infection

The spatial and temporal colonisation of tomato plants by Phytophthora cryptogea was studied in rockwool nutrient culture. Root growth and the distribution and progress of infection were measured on dissected root fragments obtained from a detailed destructive sampling of the substrate. Dissected fragments of root or roots and fibres were plated on BNPRA agar, a selective Phytophthora medium. Roots colonised all parts of the rockwool substrate 60 days after planting with the exception of surface marginal and central areas of the slab which had a lower solution content. Most root biomass occurred in and immediately beneath the original growing block. The distribution of P. cryptogea closely followed the pattern of root colonisation. An alternative, novel method for root analysis involved the dissolution of the mineral fibre and its formate bonding resin by digestion in 1 m H₃PO₄ for 45 min. Comparative recoveries of P. cryptogea from plated fragments of dissected root and fibre or comminuted samples of acid-released or dissected root showed that the acid treatment initially reduced the number of Phytophthora colonies in block and slab roots by 67% and 61% respectively. After 28 days, colony recovery from acid-released roots in the rockwool slab increased and was between 4% and 13% lower than from other plating methods. Since 1 m H₃PO₄ was lethal to zoospores and surface sporangia, the colonies recovered were interpreted as originating exclusively from root lesions. Root fresh weight of healthy and inoculated plants declined during the initial period of fruit formation. P. cryptogea infection led to a progressive reduction in root weight in the growing block and main slab and 28 days after inoculation, was approximately 50% of the controls. The acid digestion of rockwool fibre is proposed as a new approach to the problem of root and pathogen analysis in this substrate.     

An overview of the virulence factors and the biocontrol potential of Sarocladium oryzae

Sarocladium oryzae is a filamentous fungus, commonly related to sheath rot, a disease until recently considered of low relevance but whose frequency has increased worldwide in rice cultivation. Few research groups have studied this microorganism, and consequently, the knowledge concerning biochemical and genetic factors that differentiate isolates within populations in terms of virulence (virulence factors/phytotoxin production) is limited. Some works have demonstrated that avirulent isolates of S. oryzae may act as biological control agents (BCAs), primarily due to their high potential for production of the secondary metabolite cerulenin, a potent antifungal. For these reasons, the goal of this paper is to review what is known about the virulence factors of S. oryzae, to highlight the main secondary metabolites produced by the fungus and their role in sheath rot development, and to try to establish a relationship between virulent, avirulent and potential BCA strains of S. oryzae.     

Patterns of colonization and spread in the fungal spruce pathogen Onnia tomentosa

The basidiomycetous fungus Onnia tomentosa is one of the most widespread root rot pathogens in North America. Although the disease is more severe on spruce and pine trees, this pathogen can infect several coniferous species. To study the population structure of O. tomentosa , we harvested 180 basidiocarps in a 45-year-old white spruce plantation in western Quebec in autumn 1997 and extracted DNA directly from individual basidiocarps. Using a combination of spatial coordinates and molecular data based on the analysis of two mitochondrial and three nuclear loci, we measured the average genet size and molecular diversity and assessed the relative contribution of basidiospores and vegetative growth to the stand colonization. Most of the sampled basidiocarps that clustered spatially belonged to the same genet. A total of 37 discrete multilocus genets of an average size of 3.42 m were obtained. The genet size distribution was skewed towards smaller genets (<3 m) that displayed higher diversity than the larger genets (>3 m). The nuclear loci were in Hardy–Weinberg equilibrium in the larger genets, but not in the smaller genets, which displayed a deficiency of heterozygotes. This suggests a Wahlund effect, whereby different colonization events resulted in expected heterozygosity higher than observed heterozygosity. Using an estimate of the growth rate of the fungus, only a few of the largest genets were approximately the age of the plantation. These observations are consistent with the colonization by basidiospores subsequent to site preparation and tree planting followed by secondary colonization events and vegetative spread.     

Biological control of fruit-rot of jackfruit by rhizobacteria and food grade lactic acid bacteria

Fungal species causing fruit rot of jackfruit have been isolated from seven different locations of Birbhum and Burdwan districts of West Bengal, India. Each isolate showed more or less similar microscopic characteristics. A representative strain VBAM1, isolated from a severely infected jackfruit was identified as Rhizopus stolonifer by 28S rDNA sequence homology. Increased reducing sugar content in pectin broth indicates pectinase production by the pathogen. The pathogen was not inhibited by ⩾500 μg/ml of Mancozeb and Bavistin. Copper oxychloride, Blytone 50% a.i. showed antifungal activity at comparatively lower concentration (200 μg/ml). Two rhizospheric bacterial strains, Burkholderia cenocepacia VBC7 and Pseudomonas poae VBK1, and three different strains of Lactococcus lactis subsp. lactis can produce significant zones of inhibition against the pathogen in dual culture overlay plates. They induced mycelia breakage of pathogen as evidenced from scanning electron micrographs. When applied to whole plants, the strains reduced or prevented disease and when applied postharvest to Rhizopus inoculated fruit delayed and/reduced disease incidence. These agents were also re-isolated from the applied surfaces and survived for long time when mixed with suitable carrier base indicating stability in a formulation over time.