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Breeding for resistance to diseases and insect pests by Dham Pal Singh

By Dham Pal Singh

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These pores were found to penetrate deeply into the palisade layer providing passage into the houseglass layer. Fungal hyphae were observed to extend into these pores, which thus provide a means of fungal entry without the presence of visible seed coat defects. f) Internal Physical Barriers. The thickness and toughness of the cell walls of the tissues being invaded vary and may sometimes make the advance of the pathogen difficult; for example, the presence of extended areas of sclerenchyma cells may stop the spread of some pathogens like the stem rust of cereals.

1). Hanover (1975), however, postulated four basic plant resistance mechanisms; (1) the morphology and anatomy of host; (2) chemical repellents produced by the host; (3) chemical attractants produced by the host; and (4) the nutritional status of host. Here, the non-preference, antibiosis and tolerance classification of mechanism of resistance by Painter is discussed along with the good points of Hanover's four-point classification. a) Preference and Non-Preference. Non-preference is the insects' response to plants that lack the characteristic to serve as hosts, resulting from negative response or total avoidance during search for food, oviposition sites or shelter or for the combinations of the three.

The functional resistance can be classified into three types. a) Host Evasion. For an insect pest, not only rmding the right host, but also contacting it at an appropriate stage of development are of equal importance. The phenologies of the host and insect must synchronize. Host evasion takes place when the plant growth pattern is modified so as to bring in asynchronies of insect-host phenologies (panda 1979). Early maturity has been used to be a good example in economic entomology. Rice crop planted in late August and thereafter was severely affected by gall midge (Pachydiplosis oryzae) in Orissa, India, because the peak period of gall midge infestation was from the second week of October to the second week of November.

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