Negative screening

Negative screening removes the most susceptible individuals in order that the remaining, more resistant population may be parasite-free.

This concept is derived from parasite interference in field trials. Consider a heterogeneous tree crop in which each individual tree approximates to a single plot in a disease resistance field trial. The most susceptible trees will be the equivalent of susceptible control plots, and they will generate parasite interference (see 7.16.2) in neighbouring trees. The entire plantation will then suffer considerably increased levels of disease. If the most susceptible trees are removed, this interference will stop, and the remaining trees may well have sufficient horizontal resistance to produce population immunity (see 17.16.3). There are not many crops in which this negative screening is possible but, in those in which it can be used, it is valuable.

The advantage of negative screening is that it saves the existing population. Positive screening results in a new population, which necessitates replanting. This can be an expensive business with a tree crop.

Possibly the most promising example of negative screening is with cocoa and witch's broom disease (Crinipellis perniciosa). After the most susceptible trees have been removed, the brooms must be eradicated from all the remaining trees. The best approach is to remove the most susceptible 1% of trees. If this is inadequate, the process should be repeated until no further elimination is required. This negative screening emphasises the importance of population immunity.

Other examples include plantation forest trees, seedling (as opposed to clonal) tea, and any tree crop grown from genetically segregating seeds.

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