I. Mukhtar, First Fungal Culture Bank of the Pakistan (FCBP), Institute of Agricultural Sciences, Lahore, Pakistan;
R. Khurram, Department of Biosciences, COMSATS Institute of Information Technology, Islamabad, Pakistan;
A. Hannan, Department of Plant Pathology, University of Agriculture, Faisalabad, Pakistan; and
Z. Hayat, Department of Biosciences, COMSATS Institute of Information Technology, Islamabad, Pakistan
During July 2011, a severe powdery mildew outbreak was recorded in pumpkin (Cucurbita moschata Duch.) fields in economically poor areas in the Neelum Valley (Leswa, Mir Pura, Jura, Kundal Shahi, and Bela). Disease symptoms included grayish white circular to irregular patches consisting of epiphytic mycelia and conidia on both surfaces of the leaves of infected vines. Fungal mycelia and conidiophores were also visible in white powdery patches on the main stems, leaves, and petioles of affected plants. Leaves became necrotic with age. Powdery mildew symptoms were more severe on pumpkin vines located in the shade, and severely affected vines collapsed. A voucher specimen (IR00027) has been deposited in First Fungal Culture Bank of the Pakistan (FCBP), Institute of Agriculture Sciences, University of the Punjab, Lahore, Pakistan. For morphological observation, a clear tape strip was used to remove fungal tissue from infected leaves. The tape was mounted on microscope slides with water and examined with a light microscope (2). Conidiophores were unbranched, cylindrical, erect and arose singly on hyphal cells, 110 to 200 × 6 to 12 μm, composed of a foot cell and three to eight barrel-shaped conidia formed in chains with a sinuate edge, followed with a basal septum at the branching point or slightly displaced from the mycelium. Foot cells were 50 to 80 × 6 to 12 μm, and slightly swollen with constriction at the base. Conidia were cylindrical to doliiform, 30 to 50 × 14 to 20 μm and produced in chains. The length/breadth ratio of the conidia was 1.8 to 2.6. Fibrosin bodies were absent in both conidiophores and conidia. No cleistothecia were observed. Identification of the causal agent as Golovinomyces cichoracearum (DC.) V.P. Heluta (synonym Erysiphe cichoracearum) was based on morphology (1). Pathogenicity was determined using field-infected plant leaves transported to a greenhouse. Fresh conidia of field isolates of G. cichoracearum were transferred by a sterile brush from the affected leaves to fully expanded four to five healthy leaves of three 40-day-old vines. For the control, three vines were stroked with a clean sterile brush and control plants were placed 100 m away from the inoculated plants. For disease development 20°C and 80% relative humidity was maintained in the greenhouse. A plastic sheet was placed around each vine for a week and removed later. Inoculated vines developed visible white spots of powdery mildew on the leaves after 10 days in the greenhouse, whereas control plants remained asymptomatic. Fungal colony and conidial morphology on the leaves of inoculated plants were as described above. Previously, G. cichoracearum has been reported on other cucurbits (3) in the plains of Pakistan. To our knowledge, this is the first report of occurrence of G. cichoracearum as a causal agent of powdery mildew on pumpkin in Neelum Valley, Azad Kashmir.
References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) J. C. Correll et al. Plant Dis. 71:248, 1987. (3) A. Wahid et al. Pak. J. Agric. Res. 9:209, 1988.