Heterobasidion is a conifer pathogen that causes tremendous economic loss across the northern hemisphere. It is a root rot pathogen and Heterobasidion infected wood can no longer be used as timber. On spruce the decay is usually observed only in the final cutting, on pines it is visible earlier as the tree dies. It is a worldwide pathogen that is quite common in Europe, Asia and north America. Scientists have yet to find an effective way to treat this disease. The use of viruses against Heterobasidion as a biocontrol method has shown new promise to cure it. In nature, Heterobasidion partitivirus stays inside their host fungus and often shows no symptoms. It was believed before that each type of partitivirus has its own host Heterobasidion species and does not transfer to other Heterobasidion species. However, previous studies have shown that partitivirus is able to transfer to new Heterobasidion host in laboratory and in field, and surprisingly can cause growth rate decrease in its new host. Researchers have previously studied the effects of many partitivirus species on Heterobasidion, but they have focused on analyzing the effects caused by only one virus strain at a time. This thesis focuses on studying the transmission of multiple partitiviruses to a new Heterobasidion host, and on understanding how mixed partitivirus infections affect the growth rate of their new host. To study the transmission of many viruses, two different fungus species both hosting five different partitiviruses were selected as donor fungus strains, and ten different virus-free Heterobasidion fungus strains were selected as recipient fungus strains. Then growth rate experiment was conducted to figure out whether and how multiple viruses infection affect the growth rate of Heterobasidion. The results of this thesis show: 1) It is possible for multiple partitiviruses to transfer to new Heterobasidion hosts. Among 40 transmission experiments, in 13 experiments the recipients were successfully infected by two or more partitiviruses and in 9 experiments the recipients were successfully infected by three or more partitiviruses. 2) Partitiviruses transmission from H. parviporum to H. annosum is considerably less effective than from H. parviporum to H. parviporum. This enlightens us that partitiviruses are easier to be transmitted within species border than across species border. 3) Growth rate experiments showed that multiple virus infection has diverse effects on the growth rate of Heterobasidion hosts. It can be debilitating or beneficial, or sometimes there is no significant change. The growth rate experiments also showed that infection by multiple viruses does not mean more debilitating effects on the growth rate. In conclusion, it is possible to infect Heterobasidion isolates with multiple partitiviruses to generate new virus-host combinations to be tested as putative biocontrol strains. However, more experiments need to be done regarding more virus compositions and more recipients. In the future, it will be interesting to compare the influence of single virus and multiple virus infections.

One of the human- and animal-pathogenic species in genus Yersinia is Yersinia enterocolitica, a food-borne zoonotic pathogen that causes enteric infections, mesenteric lymphadenitis, and sometimes sequelae such as reactive arthritis and erythema nodosum. Y. enterocolitica is able to proliferate at 4 degrees C, making it dangerous if contaminated food products are stored under refrigeration. The most common source of Y. enterocolitica is raw pork meat. Microbiological detection of the bacteria from food products is hampered by its slow growth rate as other bacteria overgrow it. Bacteriophages can be exploited in several ways to increase food safety with regards to contamination by Y. enterocolitica. For example, Yersinia phages could be useful in keeping the contamination of food products under control, or, alternatively, the specificity of the phages could be exploited in developing rapid and sensitive diagnostic tools for the identification of the bacteria in food products. In this review, we will discuss the present state of the research on these topics.