Archivio Journal of Plant Pathology

Abstract:

During summer 2016, fruit rot symptoms were observed on tomato (Solanum lycopersicum L.) plants in the field. Symptoms appeared as small circular water-soaked lesions and enlarged in size with time. Lesions destroyed sub-epidermal tissue and white mycelium emerged from lesions, leading to collapse of the fruits. Symptomatic fruit portions were surface sterilized with 1% NaOCl solution and placed on Corn meal agar (CMA) plates at 18±2°C. White depressed colonies appeared on plates; oogonia were globose, 19.5 µm in diameter. Fungal isolate produced globose intercalary and terminal sporangia 21.5 µm in diameter. Crook necked antheridia were found on oogonium, mostly emerging very close to the oogonial stalk. Main hyphae were up to 7-8 µm wide and oospore was single, globose aplerotic, 18-22 µm in diameter. Fungal DNA was extracted and used as a template for PCR with ITS1 and ITS4 primers (White et al., 1990). BLASTn analysis of the ITS sequences (GenBank accession No. LT670911) showed 100% homology with Pythium ultimum. The fungal isolate was confirmed as P. ultimum on its morphological and molecular basis. A pathogenicity test was performed by using cork borer prick method on twenty fruits and the experiment was repeated three times. After inoculation, fruits were kept in a plastic bag and incubated at 18±2°C in a growth chamber. All inoculated fruits exhibited symptoms similar to those recorded in the field and control inoculated fruits did not show infection. The pathogen was re-isolated and confirmed as P. ultimum, thus fullfing Koch’s postulates. According to our knowledge, P. ultimum causing symptoms of fruit rot in tomato is the first record in Pakistan.