Carole Martinez

Antagonist microorganisms with the ability to control Pythium damping-off of tomato seeds in rockwool.

A total of 237 microorganisms were isolated from five different greenhouse tomato growing media. Of those, 40 microorganisms reduced the in vitro mycelial growth of both Pythiumaphanidermatum and Pythiumultimum. The ability of these microorganisms to control damping-off was then tested in rockwool. As a result, Pseudomonas corrugata strains 1 and 3, Pseudomonas fluorescens subgroup F and G strains 1, 2, 3, 4 and 5, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, Pseudomonas syringae strain 1 and Pseudomonas viridiflava significantly reduced damping-off caused by P. ultimum or P. aphanidermatum. Pseudomonas marginalis was the only microorganism that significantly reduced damping-off caused by both pathogens.

Identification of Pythium species associated with cavity-spot lesions on carrots in eastern Quebec

In this study, 248 isolates of Pythium spp. were recovered from cavity-spot lesions of carrot and identified by DNA sequencing. The isolates came from carrots harvested in different fields in the area of the city of Québec. The majority of the isolates belonged to Pythium sulcatum and P. sylvaticum, which represented 93% of the isolates collected. Pythium macrosporum, P. debaryanum, P. dissotocum, P. attrantheridium, P. rostratum, and P. acanthicum were also found to be associated with cavity-spot lesions. Among these species, only P. acanthicum failed to cause cavity-spot lesions when inoculated on carrots. Based on mycelial growth, 3%, 40%, 80%, and 100% of P. sylvaticum, P. sulcatum, P. macrosporum, and P. dissotocum isolates, respectively, were tolerant to metalaxyl while all isolates of P. attrantheridium, P. debaryanum, P. rostratum, and P. acanthicum were sensitive. In the area of the city of Québec, P. sulcatum and P. sylvaticum were by far the two most important species associated with the disease. This reinforces the concept that Pythium spp. causing cavity spots in carrot are diverse and may vary according to geographical areas.

Identification of soils suppressive against Helminthosporium solani, the causal agent of potato silver scurf

Abstract Silver scurf, caused by the fungus Helminthosporium solani, is an important disease affecting potato tubers. Control of the disease has been hampered by the development of H. solani strains resistant to thiabendazole. Currently, there is no alternative to thiabendazole for the efficient control of the disease. In this study, 45 selected soils from the province of Quebec were tested in greenhouse assays for their effect on silver scurf development. The results showed that soil influenced significantly silver scurf development and that specific soils displayed an interesting level of suppressiveness against potato silver scurf. Investigations into the cause(s) of soil suppressiveness revealed on the one hand, significant negative correlations between silver scurf severity and soil N-NO3 and available Fe contents, and on the other hand, absence of significant correlations between silver scurf severity and the different soil microbial populations. Investigations also revealed the presence of microorganisms antagonistic to the pathogen in the most suppressive soils. The antagonist microorganisms isolated in those soils were Bacillus cereus, Cellulomonas fimi, Kocuria varians, Pseudomonas putida, Rhodococcus erythropolis and Rhodococcus globerulus. Considering that few microorganisms were previously reported to display antagonism against H. solani, these results open the way to new avenues of investigation towards achieving biocontrol of silver scurf.

Control of greenhouse tomato root rot [Pythium ultimum] in hydroponic systems, using plant-growth-promoting microorganisms

Twenty-eight microorganisms showing in vitro antagonistic activity against Pythium ultimum were tested for their ability to reduce root rot [P. ultimum] on mature tomato plants grown in a greenhouse under hydroponic conditions. Of those, Penicillium brevicompactum, Penicillium solitum strain 1, Pseudomonas fluorescens subgroup G strain 2, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, Pseudomonas syringae strain 1, and Trichoderma atroviride were shown to strongly reduce root rot severity, to improve the anchorage, and to increase the marketable yields of the plants grown in rockwool infested with P. ultimum. Experiments conducted in an organic medium containing peat, pine sawdust, and compost (v / v / v ; 60:30:10) also revealed the capability of most of these microorganisms to reduce root rot severity and to improve the anchorage of the tomato plants. However, Pseudomonas marginalis was the only microorganism that significantly improved fruit production of infected plants grown in organic medium. Moreover, Penicillium brevicompactum, Penicillium solitum strain 1, Pseudomonas fluorescens subgroup G strain 2, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, and T. atroviride were shown to stimulate the growth of healthy tomato seedlings, suggesting that they act as PGPR (plant-growth-promoting rhizobacteria) or PGPF (plant-growth-promoting fungi). This study led to the selection of potential biocontrol agents against root rot of tomato caused by P. ultimum in hydroponic systems. This may open the way for new alternatives for the biological control of Pythium diseases in hydroponic systems that not only protect the crop but also have a beneficial effect on the plant growth and development in the absence of pathogens.

Effect of chlorine atmospheres on the development of rhizopus rot [Rhizopus stolonifer] and gray mold [Botrytis cinerea] on stored strawberry fruits

Gray mold and rhizopus rot, caused by Botrytis cinerea and Rhizopus stolonifer, respectively, are major diseases of strawberry (Fragaria ×ananassa) fruits, causing significant economic losses during storage. In this study, the effect of chlorine atmospheres on spore viability and mycelial growth of B. cinerea and R. stolonifer and on the development of gray mold and rhizopus rot on stored strawberries was evaluated. The results showed that spore mortality in R. stolonifer and B. cinerea was complete following a 6 h exposure to a chlorine atmosphere of 10 mg per litre of moist air, while mycelial growth of R. stolonifer and B. cinerea was completely inhibited at concentrations of 5 and 20 mg/L, respectively. At concentrations as low as 10 mg/L, chlorine provided a marked reduction in incidence of rhizopus rot and gray mold on strawberries after a 9 h exposure period. For both diseases, a chlorine atmosphere of 20 mg/L reduced the incidence by a minimum of 60% without affecting strawberry color or firmness. The results of this study suggest that exposure of strawberries to chlorine atmospheres represents a potential method for controlling postharvest gray mold and rhizopus rot.

Integrated management of potato silver scurf (Helminthosporium solani).

Abstract Silver scurf is a surface blemish disease of potato (Solanum tuberosum L.) tubers, caused by Helminthosporium solani Durieu & Mont., which has gained increasing economic importance in recent years. The disease develops initially in the field but fully expands in the warehouse under storage conditions favourable to the spread of the pathogen. Control of the disease, once provided by the fungicide thiabendazole, is now difficult due to the appearance of thiabendazole-resistant strains and the lack of potato cultivars with high levels of resistance to silver scurf. An integrated disease management programme including appropriate cultural methods and storage conditions along with the use at planting and/or at harvest of synthetic chemical fungicides is recommended to reduce the incidence and the severity of silver scurf. Recent studies put forth the possibility that ‘generally recognized as safe’ (GRAS) compounds and microbial antagonists could eventually be integrated into silver scurf management strategies.