Cochliobolus carbonum. [Descriptions of Fungi and Bacteria].

A description is provided for Cochliobolus carbonum. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Zea mays. DISEASE: Leaf spot of Zea mays. Leaf symptoms differ according to the physiologic race. Race 1 gives oval to circular, straw-coloured, dry and papery lesions which become zonate with light to purplish-brown margins; they are abundant, up to 1.5 × 2.5 cm and often coalescent. Race 2 gives oval, chocolate-brown spots, sometimes irregular, up to 0.5 × 2.5 cm and less abundant than those of race 1. Both races cause ear rot, a black, felt-like growth over the kernels giving a charred appearance (23: 293). GEOGRAPHICAL DISTRIBUTION: Central and southern Africa, S.E. Europe, India (E.), Cambodia, China, Australia (NSW, Qd); New Caledonia, Canada (Ontario), USA (E.), Argentina and Colombia (CMI Map 380, ed. 2, 1969). Additional records not mapped are: Costa Rica, Guatemala, Salvador. TRANSMISSION: Seed infection is often deep seated (47, 3058); also air-dispersed through the conidia.

Gene-for-gene relationships in the Avena: Puccinia graminis host–parasite system in Canada

The results of 48 years of oat stem rust physiologic race surveys are interpreted in terms of the prevailing host genotype. Changes in the frequencies of genes that govern virulence in the pathogen can be explained only in part by changes in the resistance genes carried by the host population. Genes for virulence on newly released types of resistance have spread very quickly through the rust population, after initial 'breakdown' of the resistance. The most successful physiologic races carry genes for virulence in excess of those required for successful parasitism in North America. Many races carry genes for virulence on types of resistance that have never been used on this continent.

Spontaneous variability of single isolates of Phytophthora infestans. II. Pathogenic variation

The variation in two aspects of pathogenicity, aggressiveness and virulence (physiologic race), among single zoospore cultures from three wild isolates has been examined. Two components of aggressiveness were measured, rate of growth on tubers and generation time on detached leaflets. Virulence was assessed from the pattern of compatible and incompatible reactions to members of the R gene differential series. Extensive variation in aggressiveness was detected in samples of zoospore cultures from all three isolates. The level of aggressiveness ranged from a high, equivalent to that of the parental wild isolates, to a complete lack of pathogenicity; as much as 45% was of the latter type. No instance of a change in virulence was found among 104 cultures tested.The significance of the observed pathogenic variation is discussed. It is concluded that zoospore variation is not important as a source of new physiologic races or strains adapted to particular, horizontally resistant varieties. Comparison of populations of zoospore cultures with samples of wild isolates suggests that directional selection for high growth rate and high and rapid sporulation operates in nature. The correlations between a number of cultural and pathogenic characters are examined, and an association between abundant sporulation in culture and high aggressiveness demonstrated.