Durable Resistance in Plant Breeding
It refers to long lasting resistance, Examples of durable insect resistance are: in Wheat to Hessian fly, stem sawfly and green bugs, in cotton to Jassids, in raspberry to aphid, in apple to wolly aphid, in rice to stem borers and leaf hoppers. In maize to European corn borer and ear worm, and in alfalfa to spotted aphid and pea aphid.
The durability of resistance depends mainly on four factors viz, 1) Formation of new races/ biotype, 2) Genetics of resistance, 3) Morphological features of host plant, and 4) Biochemical substances associated with resistance.
1. New Biotypes:
The formation of new biotypes of insect affects the durability of resistance. The formation of new races of fungal and bacterial pathogens is very common. New races are frequently observed for cereal rusts and mildew. New physiological races are formed through spontaneous mutations and hybridization. The formation of new insect biotypes is much lower than physiological races of pathogens. Moreover, new biotypes appear more frequently in some insects than others. For example, appearance of new biotypes is higher in nematodes and Hemiptera, particularly aphids, than in moths and butterflies beetles and weevils and sawflies.
2. Genetics of Resistance:
The mode of inheritance of resistance also affects the durability of resistance. In general, monogenic resistance is lesser durable than oligogenic and polygenic resistances. However, in some cases, monogenic resistance is very durable. For example, resistance to stem eelworm, controlled by single dominant gene, is very durable. Another example of monogenic durable resistance is Jassids resistance in cotton. On the other hand, in some cases, polygenic resistance is overcome by new biotypes. For example, polygenic resistance of rapeseed varieties to aphid was overcome by new resistance breaking biotypes of cabbage aphid in New Zealand and England. The polygenic resistance involves several resistance mechanisms which lead to longer durability.
3. Morphological Characters:
Insect resistance associated with morphological and anatomical features of host plant is more durable than other kinds of resistance. Jassid resistance ( associated with hairiness) in cotton and stem sawfly ( associated with solid stem) in wheat are good examples of durable resistance associated with morphological characters. These morphological features act as non preference as well as antibiotics for the insect. Cases of durable disease resistance associated with morphological features of host plant are less known.
4. Biochemical Factors:
In some cases, durable resistance is associated with biochemical substances present in the host plant. For example, durable resistance to European corn borer in maize is associated with high DIMBOA content to stem borer in rice with high silica content, and to spotted and pea aphid in alfalfa with high saponin content.