1. Introduction
Onion (Allium cepa L.) is one of the most important vegetable crops grown in Korea, with 20,965 ha of production producing 1.20 million ton (Korean Statistical Information Service (KOSIS),2012), and consumption has been increasing, due to awareness of the health benefits of onions. Intermediate-day onions plantedin the fall have been introduced to temperate environments inthe southern parts of Korea since the 1950s. The Korean onionhas been producing more than 50 t ha−1of bulb yield since 1989,and obtained the highest productivity all over the world (Foodand Agriculture Organization (FAO), 2012). Onion productivity hasbeen sustained by intensified cultural practices, including the useof transplants, high-density planting, polyethylene film mulches,and increased use of fertilizer, compost and synthetic chemicals