Abstract
The fungal pathogen Diaporthe citri is a major cause of diseases in citrus. One common disease is melanose, responsible for large economic losses to the citrus fruit industry. However, very little is known about the epidemiology and genetic structure of D. citri. In this study, we analyzed 339 isolates from leaves and fruits with melanose symptoms from five provinces in southern China at 14 polymorphic simple sequence repeat (SSR) loci and the mating type idiomorphs. The genetic variations were analyzed at three levels with separate samples: among provinces, among orchards within one county, and among trees within one orchard. The five provincial populations from Fujian, Zhejiang, Jiangxi, Hunan, and Guizhou were significantly differentiated, while limited differences were found among orchards from the same county or among trees from the same orchard. STRUCTURE analysis detected two genetic clusters in the total sample, with different provincial subpopulations showing different frequencies of isolates in these two clusters. Mantel analysis showed significant positive correlation between genetic and geographic distances, consistent with geographic separation as a significant barrier to gene flow in D. citri in China. High levels of genetic diversity were found within individual subpopulations at all three spatial scales of analyses. Interestingly, most subpopulations at all three spatial scales had the two mating types in similar frequencies and with alleles at the 14 SSR loci not significantly different from linkage equilibrium. Indeed, strains with different mating types and different multilocus genotypes were frequently isolated from the same leaves and fruits. The results indicate that sexual reproduction plays an important role in natural populations of D. citri in southern China and that its ascospores likely represent an important contributor to citrus disease.
Highlights
IntroductionDiaporthe citri (anamorph Phomopis citri) is one of the most important pathogenic fungi on citrus
At the spatial scale among the four orchards used for analyzing potential local population structuring and host tree effects on pathogen gene diversity, 13 of the 14 loci were polymorphic, while the DC1 locus was monomorphic among the 102 isolates
Melanose caused by D. citri is an important disease of citrus in southern China and has significantly influenced the marketability of citrus fruits [3]
Summary
Diaporthe citri (anamorph Phomopis citri) is one of the most important pathogenic fungi on citrus. It induces melanose on the leaves, twigs, and fruits; stem-end rot of postharvest fruits; and gummosis and blight of shoots, perennial branches, and trunks of citrus. Among these diseases, melanose brings perhaps the most significant economic loss since it significantly reduces the marketability of citrus fruits [1,2,3,4,5]. All citrus species are susceptible to D. citri infection, with grapefruit and lemons being the most severely affected [5]
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