Molybdenum (Mo) is the minor of the minor nutrients and usually doesn't show up in my strawberry and caneberry tissue samples at a concentration higher than 1 or 2 ppm.  Molybdenum exists in the soil as molybdate ion (MoO₄ ^-2 ) and in most soils one won't find more than an ounce per acre.

Low as the concentrations in the soil and plant tissue may be, molybdenum is more important than you might think. 

Mo functions in nearly all nitrogen transformations in the plant which result in proteins.  If you recall from your high school biology, one functional group of the amino acids is the amine, which of course contains nitrogen.  In short, the plant takes up nitrogen as nitrate, which is then converted to ammonium, then to the amine containing amino acids followed by their combination to form proteins. Absent molybdenum, nitrate is simply taken up by the plant, not converted to protein and results in a visibly weaker plant.

Molybdenum become substantially more soluble (ie available) as the pH of soil rises, and as such the first approach of the grower or agricultural practitioner to addressing the admittedly rare event of Mo deficiency would be to raise the soil pH. 

An additional problem concerning Mo can crop up from the use of sulfur containing fertilizers and soil amendments (for example, think ammonium sulfate [(NH₄)₂SO₄] and gypsum [CaSO₄^-2]).  Sulfate ion (SO₄^-2 ) and molybdate ion (MoO₄ ^-2) are chemically very similar to one another and as such compete with one another for absorption by the plant roots.  So lots of sulfate in a soil (as can be common in a soil which is acidic), equals more sulfate taken up by the plant and proportionally less molybdate.

The concluding statement I am going to make here is that while molybdenum deficiency it is rarely an issue (I've never seen it personally) in Central Coast grown strawberries and caneberries, I still feel knowledge of its purpose and causes of deficiency are important for growers and agricultural practitioners of this area.

Much of the information in this post comes from the article "Molybdenum in Vegetable Production" in the periodical "Vegetables West" August 2005 (pp 14-15) by Dr. Tom Ruehr from CalPoly.