Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 326

Clonal cultivars may become infected by pathogens, some of which may be systemic (e.g. viruses). Two general approaches may be used to purify a cultivar to restore it to its disease‐free original genetic purity.

1 Screening for disease‐free materialPlant materials may be visually inspected for the presence of pathogens. However, because some pathogens may be latent, a variety of serological and histological techniques are used to detect the presence of specific pathogens. Called indexing, these techniques can detect latent viruses (viral indexing) as well as other pathogens. A negative test may not always be proof of the absence of pathogens. It could be that the particular assay is not effective. The clean clonal material is then used as starting material for multiplication for propagation.

2 Elimination of pathogenA positive test from indexing indicates the presence of a pathogen. Should this be the only source of planting material, the breeder has no choice but to eliminate the pathogen from the plant tissue by one of several methods.Tissue cultureEven when the pathogen is systemic, it is known that tissue from the terminal growing points is often pathogen‐free. Tissue from these points may be ascetically removed and cultured under tissue culture conditions to produce disease‐free plantlets. Through micropropagation, numerous disease‐free plants can be obtained.Heat treatmentThis may be short‐ or long‐duration. Short‐duration heat treatment is administered to the plant material for about 30 minutes to 4 hours at 43–57 °C. This could be in the form of hot air treatment or by soaking the material in hot water. This works well for fungal, bacterial, and nematode infections. For viruses, a longer treatment of about several weeks (two to four weeks) is used. Potted plants are held at 37 °C in a controlled environment for the duration of the treatment. Cuttings from the treated plants may be used as scions in grafts, or rooted into a seedling.Chemical treatmentThis surface sterilization treatment is suitable for elimination of pathogens that are external to the plant material (e.g. in tubers).Use of apomictic seedViral infections are generally not transmitted through seed in cultivars that are capable of apomixis (e.g. citrus).