From a previous discussion in Duo I realized how inconsistent Hebrew is in this respect. In some contexts one of the two is much more natural than the other; and which one depends on the vegetable or fruit, with no rhyme or reason I can discern. In your example, אני הולך לקנות תות/תותים and אני מבשל גזר/גזרים both work. But סלט גזר and definitely not סלט גזרים, whereas סלט עגבניות and definitely not סלט עגבניה. Also, לקנות גזר would be understood as buying any number of carrots, while לקנות תפוח would be understood as buying just one apple. Go figure.
I'm pretty sure it varies. Some bulk nouns, like water (מים) are plural. But others, like fun (כיף) are singular. The same thing happens in English, where fruit is singular but vegetables are plural, and where you might cook chicken (singular) or carrots (plural). Go figure.