A drought and heat wave in Europe this summer could cause a shortage of potatoes big enough for industrial production of French fries, a German industry group warned on Tuesday. Potato crop yields might be cut by at least 25 percent, the group said. Large size potatoes needed by food processors for production of French fries are even now looking rare. “The result is that a significant tightening of raw material supplies and a reduction in quality is foreseen,” the association said. “The result could be supply problems for the popular potato-processing products such as French fries.” The rain urgently needed by potato crops is not forecast, and the picture is worsening week by week, the association said. Drought has also caused major damage to Germany’s grains harvest. Low water after dry weather also means ships on the Rhine and Danube rivers are unable to sail fully loaded.