Maria Herrero

Identification of Pythium aphanidermatum using the RAPD technique.

The suitability of random amplified polymorphic DNA for identification of Pythium aphanidermatum was investigated. Three oligoprimers were selected after testing three isolates of P. aphanidermatum and one isolate of P. ultimum with a total of 40 primers. The selected 10-mer primers were used with 20 isolates of P. aphanidermatum , four isolates of P. deliense , two isolates of P. ultimum , two isolates of P. irregulars and one isolate of P. paroecandrum . Most of the P. aphanidermatum isolates (13 of 20), were obtained from samples of Cucumis sativus , or water from cucumber greenhouses. The three selected primers gave identical fingerprints for 18 of the 20 P. aphanidermatum isolates, including all the isolates from cucumbers. Two of the primers gave fingerprints that could be used to differentiate between isolates of the Pythium species studied. The banding pattern of P. aphanidermatum given by the third primer could not easily be distinguished from the fingerprint of P. deliense . However, when used in conjunction with the other two primers, the third primer can be used to verify the identity of P. aphanidermatum .

Pythium polare, a new heterothallic oomycete causing brown discolouration of Sanionia uncinata in the Arctic and Antarctic

Pythium polare sp. nov. is a new heterothallic oomycete species isolated from fresh water and moss from various locations in both the Arctic and Antarctic. This water mould is able to infect stems and leaves of Sanionia moss (Sanionia uncinata). Pythium polare causes brown discolouration in in vitro inoculation tests at 5 °C after 5 weeks of inoculation. It is characterized by globose sporangia with various lengths of discharge tubes releasing zoospores and aplerotic oospores with usually one to five antheridia. The sexual structures are only produced in a dual culture of antheridial and oogonial isolates. Phylogenetic analysis, based on ITS sequencing, places all isolated strains of P. polare in a unique new clade, hence it is considered a novel species. Pythium canariense and Pythium violae are the most closely related species of P. polare based both on morphology and the phylogenetic analysis.

Occurrence of Pythium ultimum var. ultimum in a greenhouse on Spitsbergen Island, Svalbard.

Pythium ultimum var. ultimum was isolated from carrot seedlings with damping off and from soil used for growing the plant in a greenhouse on Spitsbergen Island, Svalbard. The fungus caused severe damping off of carrot, cucumber and tomato seedlings after artificial inoculation. The rDNA internal transcribed spacer sequences of the Svalbard isolate were identical to those of Canadian and Japanese isolates of P. ultimum var. ultimum. The results suggest that the pathogen in the greenhouse on Svalbard was probably introduced from temperate regions through contaminated plants and/or soil imported to the island. This is the first record of P. ultimum var. ultimum within the Arctic zone.

The effect of tar spot pathogen on host plant carbon balance and its possible consequences on a tundra ecosystem

In Arctic tundra, plant pathogens have substantial effects on the growth and survival of hosts, and impacts on the carbon balance at the scale of ecological systems. To understand these effects on carbon dynamics across different scales including plant organ, individual, population and ecosystem, we focused on two primary factors: host productivity reduction and carbon consumption by the pathogen. We measured the effect of the pathogen on photosynthetic and respiratory activity in the host. We also measured respiration and the amount of carbon in the pathogen. We constructed a model based on these two factors, and calculated pathogenic effects on the carbon balance at different organismal and ecological scales. We found that carbon was reduced in infected leaves by 118% compared with healthy leaves; the major factor causing this loss was pathogenic carbon consumption. The carbon balance at the population and ecosystem levels decreased by 35% and 20%, respectively, at an infection rate of 30%. This case study provides the first evidence that a host plant can lose more carbon through pathogenic carbon consumption than through a reduction in productivity. Such a pathogenic effect could greatly change ecosystem carbon cycling without decreasing annual productivity.