Factors affecting the survival of Trichometasphaeria turcica (Helminthosporium turcicum) on Zea mays

Abstract

Differential survival abilities of three isolates of Trichometasphaeria turcica were evaluated in the parasitic phase on maize by serial inoculations and by natural spread in the field. As related to the concept of stabilizing selection, the inbred line of maize met the criteria for a simple host and one isolate was a simple race and two were complex sensu van der Plank. In one mixture of two isolates, the simple isolate predominated within three generations, whereas the complex isolate predominated in the second mixture after four generations. Increased infection, disease, and sporulation efficiency, and a decreased incubation period were fitness attributes associated with the predominant isolate in each mixture. It is concluded that the genetic simplicity for virulence could not alone attribute to the survival fitness of the simple race. Other biological attributes of the organism, entirely independent of genes for virulence, govern the success or failure of a race in a given racial composition of the pathogen population during its parasitic phase on a given host substrate.Since the sole current rationale for stabilizing selection in non-obligate parasites focuses on fitness to survive saprophytically, these studies suggest that stabilizing selection in the parasitic phase deserves attention. Changes in the racial composition of a pathogen in the parasitic phase is important, not only in determining disease severity during a given year, but also in stabilizing selection and survival in the saprophytic phase. Attributes governing fitness to survive during the parasitic and saprophytic phases may not be correlated.