 Hi welcome back to the course on IPM. Now I am going to deal with one of the important components of IPM that is host plant resistance. See the insects have started colonizing on the plants millions of years ago and like we for the plants the insects are also a major pest. So they due to the damage that they are going to cause on these plants the insects I mean the plants will suffer a lot. So during the course of evolution the plants have also started developing a mechanism in order to either avoid or to escape or to resist or to tolerate the damage caused by the plant. So in this class we are going to look at what are the mechanisms and the types of the host plant resistance. Now what do you mean by the host plant resistance? There is a beautiful definition given by a great scientist called R.H. Painter in 1951 who has worked a lot on this host plant resistance. So according to him the host plant resistance are those characters so that enable a plant either to avoid, tolerate or recover from the attacks of the insects under certain conditions that would have otherwise caused a greater injury to the other plants of the same species. I mean to say that certain plant species or the certain individuals plants so which have a capacity to either to withstand or tolerate or escape the damage by the plants otherwise it would have caused a greater damage to the other individuals. Similarly so another scientist like Maxwell in 1972 has defined the host plant resistance as those heritable characteristics which are possessed by the plant which influence the ultimate degree of damage done by the insect. Now let us look into the history of the host plant resistance some of the important milestones in it. In fact the human being or man has started observing this phenomenon quite a long time. So if you look at in the third century BC itself, so Theopharthus has recorded the variation in the susceptibility of the crop against the disease. Then in 1782 the variety called underhill in wheat was reported to be a resistant to one of the major pests that is hacian phy in USA. Similarly in 1817 some sorghum crop varieties have reported to be resistant to the grasshoppers. Similarly in 1831 so winter majateen a variety on apple was reported to be resistant to the woolly aphid in USA. So this historical development will clearly indicates that man is constantly observing the variations exhibited by the plant against the pest attack and trying to utilize those characters and incorporate in developing a new varieties or the hybrids in order to maximize the production. Now what are the resistance and how this can be assessed? So the resistance in the crop can be assessed by four important factors. So the characters which are heritable and controlled by one or more gene in the sense like the character which is exhibited by the plant against a pest attack should be controlled by the gene so that it is heritable and transferred to the next generation. Then the character which is relative and measured only by comparison with a susceptible cultivar of the same plant species in the sense. So some plants or the groups will exhibit certain characters of tolerance or a resistance than the individuals of the same species and it should be a relative term and the character should be measurable that means it can be measured qualitatively is determined by the analysis of the standard scoring system or quantitatively by the insect establishment. And also it can be assessed and the assessment will be variable and it will be likely to modify by various abiotic and the biotic environment. Now let us look into the type of resistance exhibited by the different plants. So first and foremost type we can classify it as the ecological resistance or it is also referred as a pseudo resistance because this type of resistance is not an inbuilt or inherent character of the plant but by certain variation in the ecological means the plant will somehow will avoid the pest attack. Among that so we do have three types as a host aversion in the sense the host will pass through the most susceptible stage quickly or at a time when the insect numbers are reduced. For example, if we look at certain characters like the plant has got an early maturity or if there is like a late planting of the plant then the plant will escape from the insect attack and or the time when it matures when the plant I mean insect population is quite low. The second one is an induced resistance which is temporarily increased resistance resulting from certain variation in the plant environment. For example, like if you look at an irrigation given to a plant or the fertilizer application which boosts the the vigor of the plant so thereby it is going to be quite tolerant or resistant to the insect attack. The third way is that it escapes that is a lack of infestation due to the inadequate pest load. So these are the means of the resistance exhibited by the plant due to the ecological factors. Now the genetic resistance so based on the genetic resistance we can classify the resistance into based on the number of genes as a monogenic, oligogenic or polygenic. Monogenic which is mainly controlled by a single gene and this type of resistance is very easy to be developed but at the same time there is a risk of breakage of the resistance quite quickly. Oligogenic is controlled by few genes and polygenic which is controlled by many genes in this case the resistance character is going to be quite permanent. Based on the type of genes which are going to govern the resistance we can also classify as a major or the minor genes as a major gene resistance or the minor gene resistance. Then based on the biotype reaction so we have either a vertical resistance or the horizontal resistance. In the sense the plants which are effective against certain specific biotypes we also call it as a specific resistance and the horizontal resistance is the one where they are quite resistant to all the known biotypes. Based on the plant population or on the line concept we can classify it as the pure line resistance or the multi-line resistance. Pure line resistance is exhibited by those plant lines which are phenotypically and genotypically are similar. In case of multi-line resistance it is exhibited by those plants which are phenotypically similar but genotypically dissimilar. Then based on multi-trophic interactions we have either cross resistance or the multiple resistance. A cross resistance means those varieties which are resistance is been incorporated basically against the primary pest but that character will also impart resistance to the another insect. In the multiple resistance they are quite robust and quite resistant showing that kind of a resistance against variety of environmental stresses which might be due to insects, diseases, nematode, etc. Then based on the evolutionary concept so we can classify the resistance as the simpatric resistance or the allopatric resistance. In case of simpatric resistance there is a co-evolution of plant and insects in order to which is governed by the major genes. That means the insect will attack on the plant for which the plant will develop certain defense mechanism and for this plant defense mechanism the insect will develop another mechanism to overcome that and this type of arms race will continue in the simpatric resistance. In allopatric resistance it is not by a co-evolution of the plant and insect which is in fact governed by many other genes. Now how do you measure the resistance? So these are absolute or the relative. In both way we can actually measure the resistance. The absolute scale will be developed in measuring the resistance but most often we look at the relative resistance factor. The relative scale is we will see that whether the plant is highly resistant, resistant or moderately resistant, susceptible or highly susceptible by comparing with the other individuals of the same species. So in this class we are dealt with the definition of host plant resistance and the types of resistance. In the next class let us look into the mechanisms of resistance. Thank you.