Resistance of six papaya varieties and their hybrids to malformed top disease

Y. K. Chan and C. Mak

Abstract 

Six inbred varieties of papaya, i.e. Sunrise Solo, Eksotika, L19, Paris and Subang and 15 of their hydrids (reciprocals mixed), were tested over six envoirements and rated for their resistance to malformed top diasease caused by Cladosporium oxysporum - thrip complex.The combined ANOVA showed that environment, genotype and their interaction had significant difference. Environment differences in occurrence of the disease were related to the cropping history of papaya in the plots. Environments where papaya had been cultivated previously showed high incidence of the disease ranging from 33.8% to 65.7% while the disease was rather negligible in non-papaya cultivated areas. For genotypes, three distinct groups, viz. resistant, susceptible and moderately tolerant, were found in the expression of malformed top disease.The resistant group consisted of Morib, Paris, Subang and their hybrids, the susceptible group consisted of Eksotika, Sunrise Solo, Line 19 and their hybrids while the moderately tolerant was made up of hybrids developed from crosses between resistant and susceptible parents. Parent-offspring regression showed a high heritability estimate (h² = 94.8%). GxE effects and stability of genotypes in expression of the disease over environments were evaluated with a non-parametric ranking method. It was found that the major portion of the GxE was exhibited by moderately tolerant hybrids generated by crosses between the resistant and susceptible parents. On the other hand, most of the susceptible or resistant genotypes showed consistent performance in disease ranking through all the environments tested. From this study, some steps, immediate as well as long-term, to overcome the problem of susceptibility of Eksotika to malformed top disease were discussed.

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