Biological control and growth promotion in Solanum spp.

Sammanfattning: Potato (Solanum tuberosum) is Sweden’s most treated crop in terms of chemical disease control, primarily to prevent potato late blight caused by Phytophthora infestans. Another important disease is potato early blight caused by Alternaria solani. In this thesis biological disease control was explored as a supplement or an alternative to chemical disease control. The idea was to isolate, identify and apply bacteria on potato plants and examine their effects on growth and disease control. Two bacterial strains were isolated from the potato relative bittersweet nightshade(Solanum dulcamare), assuming that these bacteria would also colonize potato roots. Screening for suitable bacteria used tissue prints and biofilm forming capability. Identification of the bacterial species was based on partial gene sequencing of the 16S rRNA gene and several housekeeping genes as well as biotests. Initial gene sequencing results did not completely match any bacteria in the NCBI database, but indicated relatedness to the genus Stenotrophomonas. Since certain Stenotrophomonas strains are opportunistic human pathogens, time was re-allocated to further investigate the identity before further applied work with the bacteria. Many Stenotrophomonas strains are plant-associated and candidates for growth promotion and biological control according to literature. The growth promoting effect of the two bacteria was weak but partly statistically significant in greenhouse tests. The in vitro experiments with disease control of P. infestans and A. solani were difficult to evaluate. Pathogen inoculation of potato leaves previously treated with the bacterial strains, indicated a control effect in certain bacteria/pathogen combinations. In tubers however, pathogen inoculation resulted in no disease symptoms neither in the control group nor in the group pre-treated with the bacterial strains. These experiments should be regarded as pilot studies and given published studies, additional experiments should be conducted with our isolates. Growth promotion and biological control based on beneficial plant-microbe interaction will play an important role in future crop production and pest management, either as a complement or as a substitute to chemicals. Challenges are to increase efficacy of these treatments in complex biological environments and assure successful transfer of in vitro effects to field conditions.