Phytophthora species are potent pathogens that can devastate terrestrial plants, causing billions of dollars of damage yearly to agricultural crops and harming fragile ecosystems worldwide. Yet, virtually nothing is known about the distribution and pathogenicity of their marine relatives. This is surprising, as marine plants form vital habitats in coastal zones worldwide (i.e. mangrove forests, salt marshes, seagrass beds), and disease may be an important bottleneck for the conservation and restoration of these rapidly declining ecosystems. We are the first to report on widespread infection of Phytophthora and Halophytophthora species on a common seagrass species, Zostera marina (eelgrass), across the northern Atlantic and Mediterranean. In addition, we tested the effects of Halophytophthora sp. Zostera and Phytophthora gemini on Z. marina seed germination in a full-factorial laboratory experiment under various environmental conditions. Results suggest that Phytophthora species are widespread as we found these oomycetes in eelgrass beds in six countries across the North Atlantic and Mediterranean. Infection by Halophytophthora sp . Zostera, P. gemini , or both, strongly affected sexual reproduction by reducing seed germination sixfold. Our findings have important implications for seagrass ecology, because these putative pathogens probably negatively affect ecosystem functioning, as well as current restoration and conservation efforts.

Fusarium oxysporum f. sp. lactucae, the causal agent of fusarium wilt of lettuce (Lactuca sativa), occurs in most countries in which lettuce is grown and causes serious economic losses. Three races (1, 2 and 3) of the pathogen have previously been identified on the basis of their ability to cause disease on differential lettuce cultivars, as well as by means of molecular tools developed to characterize different races of this pathogen. Only race 1 has been detected in Europe so far. In this study, two isolates of F. oxysporum, obtained from lettuce plants grown in the Netherlands showing symptoms of wilt, have been characterized by combining the study of pathogenicity with differential cultivars of lettuce and molecular assays to determine whether the isolates are different from the known races of F. oxysporum f. sp. lactucae. This study reports the presence of F. oxysporum f. sp. lactucae for the first time in the Netherlands. The causal pathogen has been identified, using the IRAP‐SCAR technique, as a new race of F. oxysporum f. sp. lactucae. Specific primers have been designed to identify this new race.

Restoration is increasingly considered an essential tool to halt and reverse the rapid decline of vital coastal ecosystems dominated by habitat-forming foundation species such as seagrasses. However, two recently discovered pathogens of marine plants, Phytophthora gemini and Halophytophthora sp. Zostera, can seriously hamper restoration efforts by dramatically reducing seed germination. Here, we report on a novel method that strongly reduces Phytophthora and Halophytophthora infection of eelgrass (Zostera marina) seeds. Seeds were stored in seawater with three different copper sulphate concentrations (0.0, 0.2, 2.0 ppm) crossed with three salinities (0.5, 10.0, 25.0 ppt). Next to reducing seed germination, infection significantly affected cotyledon colour: 90% of the germinated infected seeds displayed a brown cotyledon upon germination that did not continue development into the seedling stage, in contrast to only 13% of the germinated non-infected seeds. Copper successfully reduced infection up to 86% and the 0.2 ppm copper sulphate treatment was just as successful as the 2.0 ppm treatment. Infection was completely eliminated at low salinities, but green seed germination was also dramatically lowered by 10 times. We conclude that copper sulphate treatment is a suitable treatment for disinfecting Phytophthora or Halophytophthora infected eelgrass seeds, thereby potentially enhancing seed-based restoration success.

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