The vinifera species is very variable in its natural state, and accidental discovery of choice seedlings in the wild is the origin of most important grape varieties. Many varieties have also been established by man's breeding programs (Olmo, 1939).

The berries develop and their seeds contain embryo plants that have characteristic of both parent plants. These new plants are hybrids and will be unlike either parent; in a thousand or more a few might be superior to either parent. The production of desirable new grape varieties depends largely on the choice of proper parents and the growing of a sufficiently large number of hybrids plants. One must also know which varieties best transmit superior qualities, since some very vigorous varieties yield sick and weakly offspring. Thus knowledge about the breeding behavior of many varieties is required in a successful grape breeding program.

Growing The Seedlings

At maturity the bagged clusters are collected and the berries of each cluster counted. The seeds are extracted from the pulp, washed, dried, and then filed in packets until planting time (Olmo, 1939). In December or January the seeds are planted in boxes containing a mixture of peat moss, sand, loam, and manure. The compost must first be sterilized with steam, or soil organisms may kill the tender seedlings. The boxes are stored out of doors for eight to nine weeks. This chilling is very beneficial and breaks the seed rest, resulting in a high and uniform germination. Early in March the boxes are moved to a warm greenhouse and within three weeks the seeds germinate. Three weeks later, after the danger of frost is over, the young seedlings are transplanted from the boxes into cloth covered frames out of doors. Here young vines are spaced at appropriate intervals to allow them to grow as much as possible during their first year. By November, shoots are 2 ft (0.6 m) or more in length, and the vines are dug, bundled, and stored in moist sand until time for planting in the vineyard the second year.

In the vineyard, the vines are set 2 ft apart in the row to get the vines to the fruiting stage in as little space as possible. At the beginning of the third year all brush is removed except the strongest cane, which is cut back to 3 or 4 in. in length. During the spring all shoots are removed except the strongest one which is trained to a stake. At the end of the growing season there is often a 5 or 6 ft (1.5 to 1.8 m) trunk.

In the fourth growing season some vines (about 30 percent) have a partial crop; in the fifth, all vines bear a crop and the selection process can then procede (Olmo, 1939). A very few promising vines are grafted to healthy rootstocks at commercial spacings in 5 or 10 vine lots. Another evaluation and selection is made from these trial blocks. Vines that pass the trial blocks selections are given to various growers for trials and, after they had been shown to be entirely satisfactory, are then released for commercial use.

If seeds are planted immediately after harvest, the breeding process can be shortened by one year (Olmo, private communication). After the seeds are removed from the clusters, they are planted in beds of soil on green house benches in December. The greenhouse heat is turned off so the seeds will stratify in the cold. In February the heat is turned on and the seeds germinate and begin to grow. By May they have developed a good root system and shoots are often 8 to 10 in. (20.3 to 25.4 cm) long. In early June the plants are transplanted into vineyard rows at intervals of 2 ft (0.61m), planted in broad furrows, and carefully irrigated. By the fourth season all vines bear a good crop and the selection process can begin.

Wine Grapes. The procedure for breeding new wine grapes is the same as for table grapes except that wines must be made and their quality judged.

Clonal Selection

In clonal selection the best vines of a variety are selected from the best material or vineyards available. In many countries, such as Germany and Australia, there are large research programs in clonal selection. The better strains collected may merely be those that are virusfree, although there may also be genetic changes, known as mutations, that occur in plants.

Degeneration of Varieties

Historically each grape variety arose as a seedling from a single seed. When a single plant is propagated continually by cuttings or buds, the resulting progeny in called a clone, and presumably each daughter plant contains the same genes or inherited units (Olmo, 1951). When the vine has been propagated for many generations, however, the grape clone can change because of virus infection or mutations within the plant cells.

Viruses are minute particles that reproduce within the host and parasitize the plant. The vine can be weakened or even killed by a virus. When cuttings and buds are used for propagation the virus may be carried indefinitely. When a rootstock is infected with a virus, it infects the scion portion of the vine, although the rootstock plant itself may show no visible effects of the virus it carries.

Mutations arise when the heredity units of the cells radically change their function. They arise at random and usually affect the plant adversely. Mutations are also commonly known as bud sports, and these continue in their new state about as stable as the type from which they arose. A positive selection process takes advantage of the rarely occurring benefical changes. Varieties should be maintained and improved whenever possible by a selection of the best individual plants.

Production of Virusfree Wood

With the high incidence of virus disease in vines, it is important to provide growers with clean wood (free of virus). In California this is done by the non profit Grapevine Registration and certification program. This program is cooperative effort of the United States Department of Agriculture, the University of California, the California Department of Food and Agriculture, and nurserymen.
The research organization use transmission tests, known as indexing or electron microscopy (Table 4-1) to identify the individual diseases or disease organisms. For each virus disease, indexing involves the use of a variety called an indicator that is especially susceptible and shows the symptoms clearly within a definite time period. Some varieties will show one virus disease clearly but not others, and rootstock behave in a similar fashion. Rupestris du Lot (St. George) shows the symptoms of fanleaf and other minor diseases very well; Mission and LN-33 illustrate leafroll; and LN-33, those of corky bark.