Resistance 1 1 



RESISTANCE A3 A FUNCTION OF TOLExTANCE LEVELS 

 W. B. Mountain 



The breeding of resistant varieties has been of tremendous help in our 

 efforts to control plant disease. A great deal of progress has been 

 made in breeding for resistance to nematode-induced diseases. However, 

 in some crops, at least, breeders are being forced to use a mechanism 

 of tolerance, rather than one of true resistance. A mechanism of tol- 

 erance is being used in Canada in the tobacco breeding program for 

 resistance to Pratylenchus . As this undoubtedly occurs in other crops, 

 and breeders may have to use it quite frequently, I thought we might 

 look at some of the problems which arise when a nematologist is asked 

 to establish the degree or level of tolerance a plant possesses in rela- 

 tion to a particular nematode. 



In Canada, several lines of tobacco have been developed which possess 

 apparent resistance to the root lesion nematode. These lines were 

 derived from selections based on plant size and without any idea of 

 what nematode was involved. It was found later that, in certain areas 

 following certain crop rotations, the resistance was not nearly so 

 marked. A few years ago, we foimd that a susceptible tobacco variety 

 is one which is severely stionted when a sufficiently high population 

 of Pratylenchus invades the roots. In contrast, a so-called resistant 

 variety is one in which no measurable amount of stunting occurs, 

 although the roots contain as high a population of Pratylenchus as did 

 the roots of the susceptible variety. So, obiously, the mechanism 

 which the breeders have been using is tolerance. Our interest became 

 aroused when we found that this tolerance was quite relative and could 

 be upset rather easily by increasing the number of nematodes within 

 the roots or, as might be expected, by substituting another species of 

 Pratylenchus . It was then apparent to us that the reaction which our 

 breeders were using in their search for resistance would be very diffi- 

 cult to define, since it was relative to the inoculum level (i.e. the 

 number of nematodes within the roots) and to the species of Pratylenchus 

 involved. Therefore, it will be very difficult for the breeder to base 

 his selections on differences in plant size unless the nematologist can 

 tell him whether or not these actually represent true differences in 

 the tolerance level. This will not be as easy as it may appear. 



At present, I am trying to find a technique whereby I might indicate 



■ the plant breeder the precise tolerance level of his material by 

 indicating the number of Pratylenchus per gram of root of a certain 

 species required to reduce the growth of that plant by some arbitrary 

 ajnount. This figure would be accompanied by a standard deviation and 

 would give him a precise comparable meastirement of the tolerance of his 

 breeding material. 



As a basis for this technique, we are making use of two relationships 

 we fmmd durinp the earlier work with brown root rot of tobacco. First, 



