Purchasing Shemitah Fruits Grown by Non-Jews

Is it permissible to purchase Shemitah fruits from crops grown by non-Jews, such as fields owned by Arabs?

Crops grown by non-Jews do not have the prohibition of sefichin, but there is a dispute whether they have the sanctity of Shemitah. Therefore, in some places, vegetables grown by non-Jews are sold in the usual manner because they believe that Shemitah sanctity does not apply to non-Jewish produce [this is the practice in many vegetable stores in Jerusalem]. However, according to the opinion of those who hold that they do have Shemitah sanctity (such is the opinion of Maran the Chazon Ish, who believes that everyone should be stringent in this matter), one can only purchase through the appointment of agency, and therefore a special form must be signed, which is available in vegetable stores that observe Shemitah sanctity [this is the practice in stores in Bnei Brak and many places across the country, as well as in a significant portion of stores in Jerusalem]. The explanation of the agency form is detailed in "Azmera Leshimcha" (Shemitah, Issue 2, end of the bulletin). However, one must be careful and thoroughly verify that the vegetables were indeed grown on land owned by a non-Jew, as there are many errors where it turns out that the land is not truly non-Jewish land according to halachic definitions, or that the vegetables were not grown attached to the non-Jew's land but were brought in boxes from elsewhere. It is also necessary to verify if an agreement was made with the non-Jew to grow the vegetables for Israel.

Chazon Ish (Shevi'it, Siman 1); Azmera Leshimcha (Shemitah, Issue 2).