Golden nematode

See: Globodera rostokiensis. Gooseberry

See: Ribes grossularia. Gossypium spp.

Cotton. This genus has about thirty species that are divided into linted and non-linted species. The four linted species in cultivation are divided into Old World and New World cottons. The two Old World cottons are diploid and are Gossypium arboreum and G. herbaceum. The two New World cottons are tetraploid and are G. barbadense and G. hirsutum. The last of these is Upland cotton and is responsible for about 95% of world production. The long staple Sea Island and Egyptian cottons are G. barbadese and account for about 5% of world production.

Cotton is naturally cross-pollinated, but it is tolerant of inbreeding and inbred cultivar can be maintained. population breeding presents no difficulties.

Since the first use of DDT, cotton has suffered a major vertifolia effect with respect to its insect pests. The increased susceptibility has been aggravated by the boom and bust cycle of insecticide production, and the tendency for politicians and bankers to interfere in the cultivation of the crop. Population breeding for horizontal resistance in cotton is likely to produce unexpectedly promising results. However, cotton breeding is somewhat technical, particularly in assessing fibre yield and quality, and it should perhaps be undertaken by university breeding clubs. Gourds

See: Cucurbitaceae. Gradient

See: Parasite gradient. Grafting

The technique in which a scion is biologically joined to a stock. The stock is usually a horizontally resistant rootstock, and this provides a means of controlling root and trunk diseases. The scion is usually a high quality but susceptible cultivar. The classic example of this control method was the grafting of classic wine grapes on to American rootstocks in order to control Phylloxera. Occasionally, a double graft is used, as with a susceptible rubber trunk being grafted to both resistant rootstocks and leaf blight-resistant crowns Other uses of grafting include the grafting of potato parents on to tomatoes to produce a vine with many inflorescences for use in true seed production.

There are two general techniques of grafting. A bud graft involves inserting a bud of the scion under the bark of the stock, and is the usual method for tree crops. A wedge graft involves inserting a wedge of the scion into a V-slit cut into the stem of a decapitated stock, and is the usual method for herbaceous plants. A third technique is the 'approach' graft in which the cut surfaces of the stems of two separately rooted plants are bound together, but it is rarely used.

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