Run every source through the CRAAP test below: if you find you cannot answer some of the questions or the answers come up negative, then the source is probably not trustworthy.
When was the source published?
Has the issue changed much since then?
Does the source speak to your topic?
Who is the author?
Why do you trust them? Do they have experience in the field or a degree that qualifies them to speak on the subject?
What kinds of evidence are used to support the source's arguments?
Are citations, research, or statistics used?
Does the source exist to further a biased point of view?
Is advertising present? Or is the source non-profit?
There are a number of good clues that tell you whether a source is worth using:
If all of those are true, you are probably looking at a good source. On the other hand, sources with no citations that offer only opinions are not worth using.
Currency: published February 2013
Relevance: pretty good if you're researching potato viruses
Authority: one author is a professor, the other works for the Department of Agriculture
Accuracy: many references, schematic diagrams
Currency: can't tell when content was written, no publication dates
Relevance: just gives general potato information, nothing specifically on diseases or viruses
Authority: can't tell who writes the content
Accuracy: facts are stated but no citations, can't be verified
Purpose: the site is essentially a pro-potato lobbying organization which would have an interest in concealing negative aspects of potatoes (e.g. diseases or unhealthiness)