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From page 64... ... 52 GENETIC VULNERABILITY OF MAJOR CROPS for biosynthesis of phytoalexins (and other compounds) is released by infection and the intensified host metabolism may reflect this. Read the entire page →
From page 65... ... Contents Genetics and Epidemics GENETIC RESOURCES Sources at Hand Varieties from Abroad Breeders' Stocks Older Varieties Wild Relatives Induced Mutations GENETICS OF RESISTANCE Patterns of Resistance One or a Few Genes; Many Genes; The Cytoplasm Genes and Environment HOW PLANT BREEDERS WORK HOST AND PARASITE INTERACTIONS HOW RESISTANCE IS DEPLOYED Different Aspects of Disease Parasite Variation; Obligate Parasites and Facultative Saprophytes; Facultative Parasites Strategies General Resistance; Specific Resistance; Multilines; General and Specific Resistance Combined Space and Time Geographic Diversity; Resistance Genes Assigned to Re- gions; Variety Rotation Source and Dispersal of Inoculum Source; Simple versus Compound Interest Crops and Cropping Practices PESTICIDE RESISTANCE GENERAL ASSESSMENT 53 64 65 66 68 Read the entire page →
From page 66... ... To deal effectively with disease resistance, the research worker must clearly recognize two genetic systems: (a) That of the host; and (b) Read the entire page →
From page 67... ... Genetics and Epidemics 55 can generate additional genetic variation by treating his material with mutagenic agents. Sources at Hand The first place to seek resistance is among the cur- rent native commercial varieties. Read the entire page →
From page 68... ... 56 GENETIC VULNERABILITY OF MAJOR CROPS many varieties and selections of a crop were grown. This is still true in certain less-developed areas of the world. Read the entire page →
From page 69... ... Genetics and Epidemics 57 most mutations are deleterious, the induction of mutations, however, has not proved to be a very reliable source of new germ plasm. GENETICS OF RESISTANCE Resistance to a parasite or insect can take various forms and is sub- ject to several kinds of genetic control. Read the entire page →
From page 70... ... 58 GENETIC VULNERABILITY OF MAJOR CROPS olism of the host and parasite that is governed by the genetic ma- terial of both. Genetic variation in the parasite is of great importance since a given host may be resistant to some forms of a parasite but completely susceptible to others. Read the entire page →
From page 71... ... Genetics and Epidemics 59 Corn plants with Texas cytoplasm are also more susceptible to yellow leaf blight caused by Phyllosticta maydis. In general, however, the genetic material affecting disease reaction in plants resides in chromosomal genes in the nucleus. Read the entire page →
From page 72... ... 60 GENETIC VULNERABILITY OF MAJOR CROPS the most rapid and efficient methods possible. A number of methods are available, and there is little point in cataloging them here; they are adequately described in plant-breeding texts. Read the entire page →
From page 73... ... Genetics and Epidemics 61 cause of this, many support the view that all disease resistance is but temporary, that variability in most parasites permits the overcormng of resistance, and that breeding for disease resistance must be a con- tinuing program. There are many examples, however, of resistance that has remained stable. Read the entire page →
From page 74... ... 62 GENETIC VULNERABILITY OF MAJOR CROPS HOW RESISTANCE IS DEPLOYED Certain principles that govern the use of specific and general resistance in crops have emerged in recent years. Some are established ; others are speculative; still others are novel and have not been tried. Read the entire page →
From page 75... ... Genetics and Epidemics 63 not common and they are not easy to recognize. When pathogenicity is not essential to survival, the competitive elimination of races patho- genic on hosts with strong resistance genes has been called stabilizing selection. Read the entire page →
From page 76... ... 64 GENETIC VULNERABILITY OF MAJOR CROPS rather than when they are developed and released by the breeder as a series of new varieties, each by the step by step addition of a new gene for resistance. Multilines Multiline varieties are mechanical mixtures of lines in which the components are alike in genes for agronomic and quality characteristics but different in genes for specific resistance. Read the entire page →
From page 77... ... Genetics and Epidemics 65 parasite reproduction for a later variety. Under these conditions the spread of disease between fields, but not within fields, is reduced. Read the entire page →
From page 78... ... 66 GENETIC VULNERABILITY OF MAJOR CROPS types are needed for specific resistance to exist. It follows then that specific resistance will not be effective if the parasite survives con- tinually on plants of the same genotype or apart from these plants only as a resting stage. Read the entire page →
From page 79... ... Genetics and Epidemics 67 Specific resistance delays the onset of an epidemic if races virulent to the resistant host are absent or rare in the parasite population. When the parasite is carried in vegetative propagating material, this advantage is lost since the host is infected at the start of the season. Read the entire page →
From page 80... ... 68 GENETIC VULNERABILITY OF MAJOR CROPS PESTICIDE RESISTANCE Many epidemics are held in check by fungicides. These are chemicals that interfere with the pathogen by preventing its entry into the host, restricting its growth, or reducing its ability to form spores. Read the entire page →
From page 81... ... Genetics and Epidemics 69 the use of specific resistance and has produced a set of useful rules or considerations. These define circumstances under which specific re- sistance is likely to fail and to succeed. Read the entire page →
From page 82... ... Contents CHAPTER 6 Dynamics of Insect Outbreaks FACTORS REGULATING INSECT POPULATIONS Autoregulation Density Vigor Weather HOW INSECTS FIND THEIR HOSTS COMPUTER SIMULATION SIGNIFICANCE OF HOST UNIFORMITY INSECTS AS DISEASE VECTORS INSECT QUARANTINES AND SURVEILLANCE 70 Read the entire page →
From page 83... ... Insect outbreaks have troubled man throughout recorded history: wit- ness the plagues of gnats, flies, and locusts recorded in the Book of Exodus and the mass devastation described by the Prophet Joel. These writers and many that followed concerned themselves with "why" outbreaks occurred. Read the entire page →
From page 84... ... 72 GENETIC VULNERABILITY OF MAJOR CROPS there is usually a range of populations in which the ratio of insect density to crop damage approximates a straight line. Within this range predictions can be made relating numbers of insects to crop potential, which has been defined as the yield that can be produced with the maximum use of available technology. Read the entire page →

From page 64...

... 52 GENETIC VULNERABILITY OF MAJOR CROPS for biosynthesis of phytoalexins (and other compounds) is released by infection and the intensified host metabolism may reflect this.