Chance escape

Many plant parasites, and particularly the soil-borne parasites, exhibit an irregular dispersal known as a 'patchy distribution'. Plants in the centre of a concentration of the parasite appear to be susceptible, even thought they may be quite resistant. And plants that are entirely free of parasites appear to be resistant, even though they may be very susceptible. This accidental freedom from parasites is called 'chance escape'.

Accurate assessments of horizontal resistance can be made only if there is a uniform distribution of the parasite. Unfortunately, a uniform distribution does not often occur. At the very least, there are usually parasite gradients, with a gradual change of parasite population density from one part of a field to another.

There are two approaches to solving this problem of patchy distribution. The first is inoculation of the screening population with populations of the various species of parasite. There are many different techniques, which include spraying the screening population with suspensions of spores, inoculating the soil of seedlings to be transplanted, and the planting of susceptible surrounds or separator rows (see 7.4.2). If the 'one pathotype technique' (see 7.5) is being employed, the designated pathotype of each parasite with a gene-for-gene relationship must obviously be used.

The second approach is to use a grid screening. The entire screening population is divided into a grid of appropriately sized squares, and the least parasitised individuals in each square are selected, regardless of the level of parasitism in that square. Any parasite-free individual, and any square that lacks parasites entirely, is rejected. With a soil-born parasite that occurs in large patches, the screening should be conducted only within those patches.

0 0

Post a comment