THE PARASITISM OF CLADOSPORIUM FULVUM COOKE AND THE GENETICS OF RESISTANCE TO IT

Abstract

Four physiologic forms of the imperfect fungus Cladosporium fulvum Cooke, causal agent of the leaf mold disease of tomatoes, have been differentiated by differences in pathogenicity. Cultural studies likewise have shown that this species is a composite of physiologic forms.Saltant strains of differing degrees of stability have been isolated repeatedly from cultures arising from single one-celled spores. Since each cell of a conidium contains a single nucleus, these saltant strains are considered to arise as a result of mutations, in the broad sense of the term.Four main classes of reaction to C. fulvum have been defined: complete susceptibility, two types of partial resistance, and immunity.The reaction between pure lines of host and parasite is plastic. Environmentally conditioned variations in each of the four reaction types have been described. Of such variations the seasonal fluctuations in the reaction of Lyco-persicum esculentum var. Stirling Castle to Form 1 are outstanding. It has been shown that the failure of the expression of the inherent resistance of this variety during midwinter at Toronto is due largely to the reduced light experience of plants grown at this time, while the failure of such plants to support sporulation is caused by the low relative humidity then prevalent in the greenhouses.The genetics of the three types of resistance was fully analyzed. The Red Currant tomato, L. pimpinellifolium, carries, in addition to the dominant factor for immunity, an independently segregating dominant factor which, in the absence of the immunity factor, governs resistance to all four forms of C. fulvum. The resistance of Stirling Castle to Forms 1 and 3 has been shown to be due to another dominant factor.Conspicuous among the genetic factors in the host which modify the main reaction types is the recessive lutescence factor in the homozygous condition. Its most striking effect is the production, on genetically immune individuals, of small inconspicuous infection spots whose increase in size is arrested very soon after symptoms appear.As a result of linkage studies the three resistance factors have been located in MacArthur's (12) chromosome maps of the tomato.The conflicting reports concerning the resistance of esculentum tomato varieties to C. fulvum are discussed in the light of physiologic specialization and of a plastic host reaction.