Pyracantha, often called Firethorn, is one of the most attractive shrubs in autumn with its clusters of brightly colored red, orange or yellow berries. A major disadvantage was its susceptibility to scab, a disease that causes the berries to turn black, and fire blight, another destructive disease.

Introduced in 1970, a variety named 'Mohave,' hybridized by Dr. Donald F. Egolf, U.S. National Arboretum research horticulturist, with considerable resistance to both of these diseases, quickly became popular in many areas of the world. It received the award of merit from the Royal Horticultural Society in London.

'Mohave' has orange-red berries that ripen in mid-August, changing to orange red as the season advances and persisting until midwinter, and foliage that is heavy, dark-green and semi-evergreen to evergreen.

Now two new varieties of pyracantha, also hybridized by Dr. Egolf with the same high standard of excellence, have been introduced: 'Navaho' and 'Teton.'

'Navaho' is a second-generation cross of Pyracantha augustifolia and Pyracantha watereri, made in 1963. Among the first-generation seedling from it was a plant with short internodes and compact growth habit. It has had introduction in 1973.

'Navaho' is a low, dense-branched plant, which in 12 years has grown to a height of only 2 meters (about 6 feet) and a breadth of 2.5 meters (about 8 feet). It has semi-evergreen to evergreen dark green leaves, abundant white flowers in May followed by a spectacular display of orange to orange-red. 'Navaho' has been evergreen in Washington, and has with-stood temperatures of 18 degrees Celsius. Because of its dense growth habit, 'Navaho' is a choice plant for use as a specimen, a foundation planting, a mass barrier for roadsides or a low hedge, Dr. Egolf says.

'Teton' resulted from the hybridization in 1963 of 'Orange Glow' (probably coccinea and fortuneana and rogersiana 'Flava'). The original 12-year old. plant has grown to a height of 4 meters and a width of 3 meters.

The dense, fine-textured, semipersistent leaves are medium green, the profusion of flowers in May is followed by light yellow berries in mid-October which mature to a medium yellow-orange and persist until January in Washington. In cooler areas, such as the Northwest, the fruit color is a more intense orange and the foliage is: more lush.

'Teton' is the first pyracantha with a distinct vertical growth habit that produces a narrow hedge or screen without pruning, Dr. Egolf says.

When a new variety considered to have superior qualities is developed at the National Arboretum, it is distributed to various institutions and universities around the United States for evaluation. If the results are favorable, the stock is increased through propagation, the plant is named and then released by the Department of Agriculture for propagation and sale to the public. The Arboretum does not have plants for sale.