'NuMex Arthur' is a low pungency, intermediate-day, yellow onion cultivar that matures from July 23 to Aug 1 when spring seeded in Las Cruces, NM. Suggested planting dates at Las Cruces are February 1-10 to achieve maturity in late July. 'NuMex Arthur' can mature in early August if planted in late February. From transplants, 'NuMex Arthur' matures earlier, about July 15-20. 'NuMex Arthur' has excellent yield, pink root resistance, and late maturity. Bulbs are very mild, firm, large, and nearly round. 'NuMex Arthur' is the only low pungency onion cultivar in its maturity class and is recommended for spring seeding or transplanting to provide a harvest of low pungency ("sweet") onions during late July. 'NuMex Arthur' originates from an intercross between 'NuMex BR1' and sweet Spanish type entries in the breeding program. 'NuMex Arthur' has sterile cytoplasm from the 'NuMex BR1' maternal parent. In 1983, ten bulbs of each of the following parents were planted for intercrossing and seed production: 'NuMex BR1', 'Peckham Yellow Sweet Spanish', PI249538 from Greece, a progeny developed from 'El Capitan' x PI249538, 'Buffalo', 'Ben Shemen', and a progeny developed from crossing 'Ben Shemen' with 'Yellow Granex' and several sweet Spanish types. This latter progeny was in 'Ben Shemen' cytoplasm and the various sweet Spanish parents included: 'Tucker Yellow Sweet Spanish', 'Utah Sweet Spanish', 'El Capitan', 'Ring King', 'Inca', 'Peckham Yellow Sweet Spanish', and 'Colorado #6 Yellow Sweet Spanish'. Seed was collected from the 'NuMex BR1' parent in the crossing cage and planted for seed to seed production.

In 1986, bulb selections were made within the population for late maturity, pink root resistance, large bulb size, and round bulb shape. The main index for onion maturity is collapse of the tissues in the neck, resulting in lodging of the foliage. Selections were made for maturity in late July in Las Cruces, N.M. when spring seeded. Selection for pink root resistance was accomplished by growing onion populations in soil severely infested with the pink root fungus. Bulbs with less than 20% pink roots were selected. Selected bulbs were intercrossed to produce seed designated as 87-1017. Seed of 87-1017 was planted in spring 1989 for bulb production. In fall 1989, bulbs of 87- 1017 and 'NuMex Starlite' were planted in an isolation cage for intercrossing and seed production. Seed of the intercross between 87-1017 and 'NuMex Starlite' was harvested from the 87-1017 maternal parent in 1990, and in spring 1991, the seed was planted for bulb production. The population was selected for maturity, pink root resistance, and bulb size and shape. Selected bulbs were planted for intercrossing and seed production. In 1993, another cycle of recurrent selection was made for late maturity, bulb size, shape, and firmness. The selected bulbs were intercrossed and seed was produced.

In 1995, the population was selected for bulb shape, size, and firmness, late-July maturity, pink root resistance, single centers, and low pungency. Pungency was measured in the laboratory using the pyruvic acid (PA) technique. Low PA production is correlated with low sensory perception of pungency. Single centers were evaluated by cutting each bulb transversely at the equatorial plane. Any bulb with one or two growing points within the center diameter of 2.5 cm was considered to be single-centered. The top portions of bulbs were analyzed for PA, and the bottom portions were saved for selected bulbs. Only bulbs with pungency levels less than 3.0 µ mol PA/ml juice were selected. The mean pungency of selected bulbs was 1.6 µ mol PA/ml juice. These bulbs were intercrossed in an isolation cage, and the seed was collected from each maternal plant in 1996.

In 1997, seed of the maternal, half-sib lines was planted for bulb production, and another selection cycle was made in the field and laboratory, similar to 1995. Selection was made among and within half-sib lines. Selected bulbs had a mean pungency of 1.0 µ mol PA/ml juice and were intercrossed to produce 98-33 seed. In 1999, bulbs of the 98-33 population were selected for late- July maturity, large size, round shape, and single centers. These selected bulbs (137) were planted and intercrossed for production of foundation seed of 'NuMex Arthur' in 2000.

[Note: µ mol PA/ml = micromole of Pyruvic Acid per milliliter]