Factors associated with fusarium crown and root rot of asparagus outbreaks in quebec

ABSTRACT The Fusarium spp. causing Fusarium crown and root rot (FCRR) are ubiquitous and abundant in soils, but in contrast, disease expression is localized and sporadic. Previous studies have related FCRR infection to phenolic acids released by asparagus, to the repression of Mn-reducers in soil, and to various soil physicochemical conditions. Fifty commercial asparagus plantations were surveyed using an exploratory approach in order to pinpoint the ecological conditions associated with FCRR development. Twenty-eight variables were used to describe the soil environments of the asparagus crops as well as the influence of crop management practices used locally. The data set was analyzed both as a whole and parsed by main cultivars (Jersey Giant and Guelph Millenium). Both field conditions and percentage of field area affected by FCRR varied widely between asparagus plantations. Planting depth was positively correlated with percentage of field area affected by FCRR and, hence, deep planting may favor FCRR infection. Plantation age was positively correlated with percentage of field area affected by FCRR, while soil available Mn was inversely correlated. Most importantly, soil Mn availability decreased with increasing plantation age, supporting the hypothesis of an asparagusmediated negative impact on Mn-reducing bacteria and of the involvement of reduced Mn availability in FCRR development. Improving the availability of Mn could provide a solution to the problem of FCRR in asparagus plantations.     

Impact of 12-year field treatments with organic and inorganic fertilizers on crop productivity and mycorrhizal community structure

The effect of manure and mineral fertilization on the arbuscular mycorrhizal (AM) fungal community structure of sunflower (Helianthus annuus L.) plants was studied. Soils were collected from a field experiment treated for 12 years with equivalent nitrogen (N) doses of inorganic N, dairy manure slurry, or without N fertilization. Fresh roots of tall fescue (Festuca arundinacea Schreb.) grass collected from the field plots without N fertilization and unfumigated field soils were used as native microbial inoculum sources. Sunflower plants were sown in pots containing these soils, and three different means of manipulating the microbial community were set: unfumigated soil with fresh grass roots, fumigated soil with fresh grass roots, or fumigated soil with sterilized grass roots. Assessing the implications with respect to plant productivity and mycorrhizal community structure was investigated. Twelve AM fungal OTUs were identified from root or soil samples as different taxa of Acaulospora, Claroideoglomus, Funneliformis, Rhizophagus, and uncultured Glomus, using PCR-DGGE and sequencing of an 18S rRNA gene fragment. Sunflower plants grown in manure-fertilized soils had a distinct AMF community structure from plants either fertilized with mineral N or unfertilized, with an abundance of Rhizophagus intraradices-like (B2). The results also showed that AM inoculation increased P and N contents in inorganic N-fertilized or unfertilized plants, but not in manure-fertilized plants.     

Quantification of Fusarium solani f. sp. phaseoli in Mycorrhizal Bean Plants and Surrounding Mycorrhizosphere Soil Using Real-Time Polymerase Chain Reaction and Direct Isolations on Selective Media

ABSTRACT The capacity of the arbuscular mycorrhizal fungus Glomus intraradices in reducing the presence of Fusarium solani f. sp. phaseoli in bean plants and the surrounding mycorrhizosphere soil was evaluated in a compartmentalized experimental system. Quantification of the pathogen and the symbiont in plant tissues, the soil regions of the mycorrhizosphere (rhizosphere and mycosphere), and the bulk soil was accomplished using specific polymerase chain reaction (PCR) primers in real-time PCR assays, culture-dependant methods, and microscopic determination techniques. Nonmycorrhizal bean plants infected with the pathogen had distinctive Fusarium root rot symptoms, while infected plants previously colonized by G. intraradices remained healthy. The amount of F. solani f. sp. phaseoli genomic DNA was significantly reduced in mycorrhizal bean plants and in each mycorrhizosphere soil compartment. The presence of G. intraradices in the mycorrhizosphere was not significantly modified, although the mycorrhizal colonization of roots was slightly increased in the presence of the pathogen. The results suggest that the reduced presence of Fusarium as well as root rot symptoms are caused by biotic and/or abiotic modifications of the mycorrhizosphere as a result of colonization with G. intraradices.     

Mycorrhizal colonization with Glomus intraradices and development stage of transformed tomato roots significantly modify the chemotactic response of zoospores of the pathogen Phytophthora nicotianae

The chemotaxic response of zoospores of the plant pathogen, Phytophthora nicotianae, towards exudates from mycorrhizal and non-mycorrhizal transformed tomato roots was studied. A bi-compartmental in vitro system was used to grow Ri T-DNA-transformed tomato roots colonized or non-colonized with the arbuscular mycorrhizal fungus, Glomus intraradices, and to collect root and mycorrhizal exudates. The root and mycorrhizal growth dynamics were first characterized in order to determine two times of exudate sampling. Exudates collected from 16-wk-old mycorrhizal roots were significantly more attractive for P. nicotianae zoospores than exudates from non-inoculated roots. On the contrary, concentrated exudates harvested from 24-wk-old mycorrhizal roots were repulsive to zoospores compared to exudates from non-colonized roots and the water control. In exudates of G. intraradices-inoculated roots, HPLC–MS analyses revealed significantly higher concentrations of proline and isocitrate after 24 wk of growth, while after 16 wk of growth, proline concentration did not differ between exudate types, and the isocitrate concentration was lower in mycorrhizal root exudates. Mycorrhizal inoculation had no effect on the amounts of other amino acids and organic acids and on the sugars quantified within exudates. Our results suggest that modification in exudate composition of mature roots by mycorrhizal colonization may provoke the repulsion of P. nicotianae, and that their capacity to infect host roots may in this manner be reduced.     

Effectiveness of Protective Layers in Reducing Metal Fluxes from Flooded Pre-Oxidized Mine Tailings

A problem in implementing water covers over existing tailingimpoundments is the dissolution of minerals produced throughoxidation and the subsequent flux of metals into the water cover.One possible solution is to place a protective layer of non-reactive soil at the tailings/water interface to inhibit metaltransport. A laboratory evaluation of different water coversystems was performed employing columns packed with tailingssubmerged beneath 1 m of water. A ten-centimeter layer of sand orpeat was placed at the tailings/water interface. The experimentswere kept stagnant for 183 days, and then flushed with water at asteady rate for 468 days. Both protective covers prevented degradation of water cover qualityfor the duration of the experiments, as pH exceeded 5.5; however,the quality of the tailings pore water remained poor and evendeclined slightly from pH > 4 to pH < 3. Leaching of iron andsulphate from the tailings with a sand protective layer ceasedduring the experiments. Conversely, in the columns with a peatlayer, substantial leaching of metals and sulphate from thetailings continued to the end of the experiments. It ispostulated that the peat is a source of chelating agents, such asorganic acids, which are known to accelerate the dissolution ofcertain minerals formed through weathering. The sensitivity ofmetal transfer rates to the thickness and type of protectivecover above the tailings were modelled. A 10-cm peat layer waspredicted to prevent substantial degradation of the water coverfor at least ten years.