As befits a volume in the Advanced Series in Agricultural Sciences, this book was written with problems of practical agriculture in mind. One of the ways of controlling plant disease is by using resistant cultivars; and from the wide literature of genetics and biochemistry in plant pathology I have emphasized what seems to bear most closely on breeding for disease resistance. This has a double advantage, for it happens all to the good that this emphasis is also an emphasis on primary causes of disease, as distinct from subsequent processes of symptom expression and other secondary effects....