Inoculate with parasites

Simple inoculation techniques must be used to ensure that the high yield is due to resistance, and not to chance escape from infection or infestation. This refers to locally important parasites only, because there is usually little point in screening for resistance to parasites that lack epidemiological competence in the area in question. The inoculation techniques vary with the different kinds of parasite (see also Chance Escape, 7.16.4).

Wind-borne parasites. It is often sufficient to plant spreader-rows, or surround-rows of a susceptible cultivar to act as a source of parasites. However, care must be taken to ensure that these susceptible cultivars do not introduce undesirable pollen into the screening population. Depending on the crop, this can be prevented by subjecting the spreader plants to decapitation, total destruction, or timed planting to ensure an out-of-phase anthesis. Alternatively, various plant pathogens can be suspending in water and sprayed on to the screening population. Insects and the vectors of viruses can be cultured on plants in portable insect cages. At the appropriate time, the cages are carried to the field, and the insects are released to infest the screening population.

Soil-borne_parasites. The major problem with soil-borne parasites is a patchy distribution, which can lead to a frequent escape from parasitism, and a false indication of resistance. The best way to overcome a patchy distribution is to inoculate either the young seedlings that are to be transplanted as the screening population, or to inoculate the seed prior to planting. For example, the seed might be planted into small peat pots which contain soil that has been inoculated with nematodes, wilt fungi, or whatever. Once the seedlings are growing, the entire pot is planted in the screening field. Other techniques are possible, depending on the species of parasite in question. This is an area in which amateur breeders are likely to need advice from specialists.

Seed-borne _parasites. Seed-borne parasites may be carried inside the seed (infected seed) or externally on the seed (contaminated seed). For example, inoculation of cereal or grass seed with a covered smut involves dusting the seed with smut spores prior to sowing. Inoculation with a loose smut necessitates blowing smut spores over the screening population at the time of anthesis. Many bacterial and fungal parasites can be uniformly inoculated by soaking the seed in a spore suspension prior to sowing the screening population.

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