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Parasitism of Barley by Ustilago Hordei (Pers.) Lagerh. : some quantitative aspects of Disease expression Groth, James Vernon


Quantitative studies were made of disease reactions in the barley-Ustilago hordei system. Inoculation studies involving a compatible combination of host and parasite, in which one of the mating-types was greatly diluted in relation to the other, indicated that the partial-vacuum inoculation method results in at least 100 sporidia per seed being available to form dikaryons which smut the plant. Resistance, which can be influenced by environemnt, accounts for the inoculated plants which are nonsmutted. Two aspects of disease exist: within-plant reaction and between-plant reaction. Both being genetically determined, the two were closely correlated in all disease-producing combinations of 12 barley cultivars and 21 U. hordei dikaryons. Within-cultivar and within-dikaryon correlations were also found. In over 500 plants, using a virulent pathogen dikaryon, studies were made of within-plant smutting patterns and the effects on plant growth of inoculation and smutting. Distribution of smutted heads within smutted plants was not random. Nodal tiller families tended to be either entirely smutted or entirely healthy. When families were differentially smutted, the older members most often were nonsmutted. Entire older families most often remained nonsmutted. The principal culm did not fit this age-frequency pattern. Regardless of the occurrence of smut, inoculation of plants caused reduction of tillering and changed the pattern of tillering so that inoculated plants produced lateral tillers at higher nodes of the principal culm.

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