Vertifolia Effect and Boom and Bust Cycle in Plant Breeding
Vertifolia Effect:
Van der Plank introduced the term Vertifolia effect derived from the German potato variety ‘Vertifolia’ having the late blight resistance genes R3 and R4. The variety became susceptible when the appropriate Pathotypes P(3,4) evolved in the pathogen leading to a nearly complete failure of the crop; such a total failure of vertical resistance leading to a disease epidemic is known as Vertifolia effect. Thus Vertifolia effect refers to an epidemic development in a variety carrying vertical resistance genes and a low level of HR, leading to heavy economic losses. The failure occurs because of the following two reasons: 1) The level of horizontal resistance in the varieties carrying oligogenes for resistance is usually low, and 2) The pathogen is able to evolve the virulent Pathotypes specific to the oligogenes for vertical resistance.
Boom and Bust Cycle:
It may be expected that when a new variety carrying VR is developed, it is resistant to the prevalent Pathotypes of the pathogen in question. Therefore, it becomes popular rapidly and occupies a large proportion of the growing area under the concerned crop. This period of boom (high popularity and cultivation in a large proportion of area) continues till the Pathotypes specific to the VR gene (s) employed in the variety evolves and /or increases in frequency to cause an epidemic in the field occupied by the variety. The now susceptible variety loses popularity and area under it declines sharply thus the variety is effectively busted by the virulent Pathotypes. This has been called ‘boom and bust’ cycle by Priestey (1970). When a variety goes bust, breeders develop new variety with a new VR gene, which makes it resistant to the predominant Pathotypes. This variety also undergoes the ‘boom and bust’ cycle, and the breeders put forth a new variety and so on. The period for which such a variety may last is , on an average, -5 years in the case of air-borne fungal pathogens. But in many cases, this period is much shorter, in extreme cases, the variety go bust even before it reaches farmers fields.
The reason for boom and bust cycle line in the genetic system of the pathogen and the selection pressure exerted by the host on the pathogen Pathotypes. When a resistant variety becomes popular it is cultivated on large areas. The plants of this variety act as strong selection agents for the Pathotypes specific to the new variety since only this Pathotypes can cause disease in the plants of this variety. Therefore, the specific Pathotypes increases rapidly in frequency in the pathogen population. This specific Pathotypes may be newly evolved or may have already been present in the pathogen population, albeit, in a very low frequency. Soon the frequency of the virulent Pathotypes reaches a stage at which it causes an epidermic in the thus far resistance variety. Usually, the epidemic would occur after a couple of or more years following the first detection of the virulent Pathotypes. The popularity of and the area under the variety will define rapidly following the epidemic. This will reduce or eliminate the selection pressure in favour of the Pathotypes virulent on the variety. As a result, the frequency of this Pathotypes in pathogen population will also decline.