First Report of Alternaria alternata Causing Leaf Spot on Rumex dentatus in Pakistan.

Abstract

Rumex dentatus L., commonly known as toothed dock, is an annual and biennial weed in the Polygonaceae. It is reported to be native to southern and eastern Asia and is now established in North America (southern Arizona and Sonora and Baja California) and Europe (Russia, Bulgaria, Greece, and Romania) (1,3). In Pakistan, R. dentatus is one of the most common and problematic weeds in wheat fields (2). In surveys of wheat-growing areas in Punjab Province conducted from 1999 to 2002, leaf spots were found to be very common on R. dentatus. The disease led to 10 to 70% mortality of R. dentatus in various fields in Lahore and Kasur districts. Symptoms initially were small, light brown, circular spots with most becoming irregular, dark brown lesions, while a few remained circular with concentric rings. On severely diseased leaves, several spots coalesced to form large necrotic areas. Leaf spots varied from 2 to 24 mm in diameter. The disease first appeared in mid-February, was widespread from the end of February through March when temperatures ranged from 25 to 30°C, and ended in April. Diseased leaves were cut into small fragments, surface disinfested with 1% sodium hypochlorite, rinsed thrice with sterile water, and cultured on potato dextrose agar in a growth incubator at 25 ± 1°C. The isolated fungal species was transferred to agar slants to maintain pure cultures. The fungal colony surface was grayish white, and then darkened, becoming greenish black or olive-brown with a light border. The fungus produced abundant, branched, septate, brownish hyphae with simple, olive-brown, septate conidiophores that were variable in length. Conidia were terminal, solitary, or in short chains, mostly ovoid with a short conical or cylindrical apical beaks not exceeding one-third of the conidial length, and smooth walled or vemuculose. On the basis of morphological characteristics, the fungus was identified as Alternaria alternata by the Fungal Culture Bank of Pakistan (FCBP). A specimen of the fungal culture was deposited in FCBP. For the pathogenicity test, R. dentatus plants grown outdoors from seed to three- to four-leaf stages were transplanted into 12-cm-diameter plastic pots, two plants per pot. Plants were sprayed with a suspension of the putative pathogen containing 1 × 107 conidia per ml. Control plants were sprayed with sterile water. Each treatment was replicated three times. Plants were covered for 24 h with plastic bags to maintain 100% relative humidity. The bags were removed and plants returned outdoors. All of the inoculated plants showed the same symptoms as observed on diseased plants in the field. The lesions first appeared after 7 days. The pathogen was consistently reisolated from the lesions. The pathogenicity tests were repeated three times. Given the effects of A. alternaria on R. dentatus observed in the field, there is potential for using the fungus as a biological control of the weed.