Parasite interference

The statistical methodology used for analysing field trials is excellent for investigating various agronomic variables, such as the effects of fertilisers, the spacing between the plants, or the yields of different cultivars. But it has been a source of major error when it comes to assessing the importance and control of crop pests and diseases. This was first recognised by Vanderplank (1963) who called it the 'cryptic error' in field trials. The error occurred because crop parasites are mobile. They can move from one field plot to another, and this phenomenon is now called inter-plot interference, or parasite interference (Fig. 7.1).

When measuring horizontal resistance, parasite interference can easily increase the levels of parasitism in test plots by a hundred-fold, and sometimes by as much as a thousand-fold. This happens because the control plots, included for

Figure 7.2 Parasite interference

These rectangles represent small field plots in which parasites migrating from one plot interfere with the level of parasitism in another plot. Red represents a very high level of parasitism, yellow represents a moderately high level, and green represents freedom from parasitism.

Plots A, C, and F have vertical resistance gene 1, and they have been matched. They also have very low levels of horizontal resistance, and their levels of parasitism are consequently very high (red).

Plot B also has vertical resistance gene 1, and it too is matched. It also has a very high level of horizontal resistance but this is completely obscured by the intense interference of parasites migrating into it from plots A and C. Consequently, it looks terrible (yellow).

Plot E, on the other hand, has vertical resistance gene 2, and it is unmatched. It accordingly has no parasitism whatever. However, it has a very low level of horizontal resistance, but this defect is obscured by the functioning vertical resistance. Consequently, this plot looks ideal, even though its vertical resistance is ephemeral, and its horizontal resistance is negligible. purposes of comparison, contain plants that are highly susceptible, and highly parasitised. These parasites then move into neighbouring plots in large numbers.

Perhaps the most dramatic example of parasite interference is seen in the small plots used by wheat breeders working with vertical resistance. These family-selection (i.e., ear-to-row) plots consist of a single row of only a few plants taken from the seeds of one head of wheat. The available rust spores cannot match the vertically resistant wheat. They can only produce the minute hypersensitive flecks that result from non-matching allo-infections. But these flecks may occur in millions. There can be so many of them that the resistant wheat appears diseased, and the wheat breeders warn that this phenomenon must not be mistaken for true disease. This level of allo-infection from outside indicates just how misleading parasite interference can be.

Parasite interference is responsible for three different kinds of error. The first error concerns the use of crop protection chemicals. If test plots sprayed with a pesticide suffer parasite interference, they may need more pesticide than if there were no interference. Recommendations to farmers, concerning pesticide use, may be based on erroneous field trials. This error occurred so commonly during the 1950s and 1960s that no one can be quite sure how excessive our use of crop protection chemicals was during that period. Indeed, no one is quite sure how excessive our current use of crop protection chemicals may be, because of this error in field trials.

The second error concerns vertical resistance. It will be observed that parasites moving from one field plot to another are allo-infecting the new plot. If the receiving plot has an unmatched, and functioning, vertical resistance, the interference will have no effect at all, other than the hypersensitive flecks mentioned above. The function of vertical resistance, after all, is to control allo-infection. Consequently, under the conditions of maximum interference, vertical resistance looks good, because there is no parasitism. As we have just seen, these conditions occur typically in pedigree breeders' small screening plots. But this excellence is an illusion, because neither the temporary nature of the vertical resistance, nor a low level of horizontal resistance when it fails, is apparent. This illusion has been deceiving members of the Mendelian school of plant breeders for most of the twentieth century.

The third error concerns horizontal resistance. This kind of resistance can be seen and measured only after vertical resistance has been matched. If the matched plot in question has the level of its parasitism increased by, perhaps, one hundred-fold, or even one thousand-fold, because of parasite interference, the horizontal resistance will be totally obscured. Under these circumstances, pedigree breeders can hardly be blamed if they conclude that horizontal resistance is useless or, even, that it does not exist. Far more important is the fact that this level of horizontal resistance may be entirely adequate to control the parasite completely, when it is employed in farmers' fields that are free from interference.

Parasite interference is also important in recurrent mass selection. One resistant plant might be surrounded by relatively susceptible plants. The resistant plant will then appear more susceptible than it really is, because of parasite interference. This is why the screening must use relative assessments. Only the best plants are kept, even though they may be quite severely parasitised.

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