AUTOGENOUS NECROSIS IN TOMATOES IMMUNE FROM CLADOSPORIUM FULVUM COOKE

Abstract

The complication of a tomato breeding project by the occurrence, in lines carrying immunity from C. fulvum physiol. races 1 to 4, of a spontaneous and destructive necrosis, chiefly of the foliage, is descr. The autogenous nature of this necrosis was established thoroughly and the simple Mendelian nature of its inheritance detd. Immune plants possessing the factor Cf p1, are potentially necrotic if they are homozygous with respect to the factor ne, found in Lycopersicon esculentum, but remain non-necrotic if homozygous or heterozygous with respect to its dominant allele, ne, found in L. pimpinellifolium. Plants susceptible to C. fulvum are invariably non-necrotic. The visible expression of the gene combination Cf p1 ne was shown to vary widely according to the environmental conditions, seasonal variations in the severity of necrosis being particularly striking. The factor ne was located on Chromosome I by 3-point expts. Necrosis appeared in derivatives of crosses between L. pimpinellifolium and all the vars. of L. esculentum tested. The data indicate the presence, in L. pimpinellifolium, of modifying factors that reduce the severity of necrosis occurring in the presence of the gene combination Cf p1 ne. Evidence is also presented that one or more factors on Chromosome VII, closely associated with the factor H (smooth stem), which occurs in L. pimpinellifolium, may prevent the development of necrosis in immune plants of genotype ne/ne, but the analysis of this phase of the problem is incomplete. Necrosis is considered to be the visible expression of an incompatibility between Cf p1 and a chromosome complex derived very largely from L. esculentum. Speculations concerning the possible evolution, in L. pimpinellifolium, of immunity from C. fulvum, physiol. races 1 to 4, and of freedom from necrosis are outlined. The bearing of the results of these studies upon projects for the breeding of tomatoes resistant to C. fulvum is discussed, particularly in the light of the discovery of races of this fungus that are capable of infecting such vars. as Vetomold, which is immune from races 1 to 4.