For this course, unless you are already familiar with another commonly used citation style format, your instructor strongly suggests that you use the citation style that is asked of authors who publish articles in one of the many journals from the so-called "TriSocieties" (American Society of Agronomy / Crop Science Society of America / Soil Science Society of America or ASA-CSSA-SSSA); that style is also referred to as "ASA".
Tri-societies Writing Guide (ASA style)
https://dl.sciencesocieties.org/publications/style/
Gourabathini, P., M.T. Brandi, K.S. Redding, J.H. Gunderson, and S.G. Berk. 2008. Interactions between food-borne pathogens and protozoa isolated from lettuce and spinach. Appl Environ Microb 74(8):2518-2525. doi:10.1128/AEM.02709-07
Gourabathini, P., Brandi, M.T., Redding, K.S., Gunderson, J.H., & Berk, S.G. (2008). Interactions between food-borne pathogens and protozoa isolated from lettuce and spinach. Applied and Environmental Microbiology, 74(8):2518-2525.
Gourabathini, P., Brandi, M.T., Redding, K.S., Gunderson, J.H., Berk, S.G. Interactions between food-borne pathogens and protozoa isolated from lettuce and spinach. Applied and Environmental Microbiology, 74.8 (2008):2518-2525. Print.
If you are looking for a writing guide for either APA or MLA rules, Purdue University has one of the best guides in terms of usability and reliability and I would suggest using their online guide: Purdue OWL: https://owl.english.purdue.edu/owl/