The Hebrew caption on this poster reads, “Buy Hebrew Watermelons.” A seemingly innocuous fruit advertisement, the poster is evidence that even during the yishuv (Hebrew: community) period, approximately 1918 to 1948, friction was building in Palestine between the newly arrived Jewish immigrants and the indigenous Palestinians.
The caption for this poster currently at the website of the Israeli Ministry of Foreign Affairs (MFA) reads as follows:

Farm products of the Jewish settlements in the Land of Israel were pitted in a stubborn struggle against fresh produce from local Arabs and neighboring countries.The very existence of the Jewish settlements’ agriculture and industry was at stake. Watermelons were one of the fruits that needed protection against foreign products.

Ironically, the word “foreign” is used to describe the produce of the indigenous Palestinian farmers. Indeed, for centuries prior to the arrival of Zionist halutzim (Hebrew: pioneers), Palestinians had been growing vegetables, fruit, grains, and legumes and all manner of crops in what is present-day Israel.

Source: “Images of A State in the Making,” Israeli Ministry of Foreign Affairs website