Breeding Tropical Vegetable Corns

Breeding of tropical corn (maize, Zea mays L.) as a vegetable assumes many genotypes and products. In temperate agriculture, the familiar types are sweet and supersweet corns involving genes sugary-1, shrunken-2, and sugary extender. Most familiar in the tropics are field corns and waxy-1 corns, harvested immature (green), and corns for baby corn, milk, and ice cream. The sweets and supersweets are limited to a few market areas. Temperate sweet and supersweet corns fail almost universally in the tropics. This is shown to be due largely to their susceptibility to a wide range of diseases, pests, and ecological factors, notably short daylengths. The genotypes chosen by tropical breeders uniquely include genes brittle-1 and waxy-1. Their advantages include stress tolerance facilitating year-round production. Vegetable corns play a significant role in the tropics as a source of energy, not primarily as a carnival item. Genetic improvements have been made primarily in the tropical and subtropical ecosystems represented by the authors and Taweesak Pulam (Hawaii, Queensland, and Thailand). Breeding began in Hawaii with Mangelsdorf's 'Hawaiian Sugar', released in 1947 and now underlying almost all tropical sweets. Tropical breeding was initially focused on open-pollinated cultivars that could be grown by small tropical farmers without excessive seed costs. Dominating among these was 'Hawaiian Supersweet #9' (brittle-1 gene) and waxies such as 'Kao Neo'. Conversions to the sweeter shrunken-2 gene and inbreeding for hybrid production closely followed. Diseases and pests and their diverse and constantly evolving races abound on corn in the tropics. Most significant are the families of fusarium, rust, and blight diseases basically found universally in the tropics. The fusariums can cause germination failure, seedling death, stalk rot, and ear and kernel rots. The breeder in the tropics must also cope with short days, limited incident light, high temperatures, and a wide variety of pests (largely insects). Present adoption of tropical vegetable corns is largely limited to hybrids, both public and private. Large-scale production for canneries occurs in Thailand, with outputs equaling or exceeding those of the United States. Tropical production and processing presents some unique challenges, including ear type and unusual products that include sweet corn milk, sweet corn ice cream, and waxy (mochi) food products. Access to markets is often limited, and seasonal variations can greatly limit year-round fresh corn production. The earliest hybrid development depended on open-pedigree inbreds. As the industry develops, however, commercial production may come to rely heavily on closed-pedigree parents as it does in temperate regions, thus limiting hybrids to wealthy tropical estates. With a billion people in the tropics who go to bed hungry and often suffer from vitamin A deficiency, this must not happen. Easily grown and attractive as food world-round, the continued development of improved tropical corn as a vegetable must have public support and that of the major international research centers. © 2015 by Wiley-Blackwell. All rights reserved. All rights reserved