Identification of Alternaria spp. in orange trees of Golestan and Mazandaran provinces and investigating their virulence on orange cultivars

Alternaria brown spot and postharvest black rot of fruit are serious diseases of citrustrees. These diseases are caused by Alternaria spp. and can be economically destructive on susceptible citrus trees. The aim of this study was to identify the pathogenic species of this fungus and to introduce orange cultivars resistant to this disease. For this purpose, samples suspected to Alternaria infection were collected from orchards of Golestan and Mazandaran provinces and were cultured on PDA and PCA media in the laboratory. Then, the fungi were identified according to the morphological key to Alternaria spp. In order to verify the morphological, molecular identification was done using universal primers ITS1, ITS4 and PCR product sequencing. From 45 isolates collected, the identified species were A. alternata (28 isolates), A. dumosa (12 isolates) and A. atra (5 isolates). For investigating pathogenicity of the isolates, 4 orange cultivars including Thomson Navel, Jaffa Sweet, Sanguinelli Blood, and Valencia were used. The pathogenicity test was done by using spore suspension with 105 spores per ml on the leaf, fruit and, seedling. The results revealed that the isolates of these three species showed serious symptoms on the Thomson, Jaffa, and Valencia cultivars. However, the Sanguinelli was relatively resistant in three pathogenicity tests on the leaf, fruit, and the seedling and the mentioned species were not able to create disease symptoms at this cultivar. Thus, Sanguinelli was introduced as a resistant cultivar against Alternaria spp. in orange.