No, you cannot. Someone has already provided a good explanation for this in a post on a previous sentence, but the gist of it was that in Romanian we use the plural because when you make a salad or juice you use more than one vegetable or fruit. For example, we say "salată de fructe" not "salată de fruct" ("fruit salad") or "suc de mere" not "suc de măr" ("apple juice") because one would typically use more than one apple to make juice.
You're welcome. Well, it all made sense to me until someone reminded me that a popular Romanian dish is "salată de boeuf." However, I dismissed it as a logical exception since "boeuf" is a French word. And then for some bizarre reason "salată de conopidă" came to mind where "conopidă" is the singular form of "cauliflower."
So I took a look at the Salads section in my Romanian cookbook and found several other exceptions: "varză" (cabbage), "sfeclă" (beets), "praz" (leeks), "fasole boabe" (beans), "fasole verde" (green beans), "sparanghel" (asparagus), "țelină" (celery root), and "spanac" (spinach).
I'm not sure whether there is any logical explanation for those exceptions (e.g., uncountable because there are too many to count or because you could use half of one to make a salad) or whether one just needs to learn these word combinations as they are.