GENETIC CONTROL OF LOW VIRULENCE IN USTILAGO

Abstract

Introduction Covercd smut is a discase of cultivated barley (Hordez~nz uulgrre L.) that is caused by a fungal pathogcn (Ustilago hordei (Pers.) Lagcrh.). Following inoculation of barley with the pathogen, and depending on the conlbination of barley variety and physiologic race that is chosen, the proportion of plants that develop the disease may fall anywhere in thc range from zero to 100 per cent. Taplte (1937), in his itudies of physiologic races of U. hordei, introduced an arbitrary classification, based on infection percentages, which recognized threc kinds of host rcaction: resistant (0 5 per cent); intermediate (6 35 per cent); and susceptible (36 100 per cent). Implied in this classification also is the recognition, based on thc same percentage classes, of three levels of virulencc on the part of the parasitc. These may be referred to as avirulent (0 5 per cent), intermediate (6 35 per cent), and virulent (36 100 per cent). Classifications such as this are also used in other parasitic systems and inuch genetic work on uarasitism has been carried out wi;hin this kind of emuirical U framework: reporked segregations arc usually clear-cut, with thc progeny falling 1