Mungbean phyllody disease in Pakistan: symptomatology, transmission, varietal response and effects on yield characteristics

Abstract

Mungbean plants showing typical symptoms of infection by a phytoplasma that causes phyllody disease (transformation of floral parts into green, leaf-like structures) were examined in field crops in both spring and summer seasons. The most common symptoms were floral virescence, phyllody, reduction in leaf size, extensive proliferation of branches and stunting of plants. Sometimes these symptoms were accompanied by longitudinal splitting of green pods followed by germination of green (physiologically mature) seeds, producing small plants. Phyllody disease pathogen was successfully transmitted from diseased to healthy mungbean plants using either grafting and the leafhopper Orosius albicinctus under laboratory conditions. Transmission via seeds and by insects that included leafhopper Amrasca devastans, pea aphid Acyrthosiphon pisum, beetles Brumoides suturalis, Hippodamia variegata, Menochilus sexmaculatus, and the whitefly Bemisia tabaci failed to produce disease under laboratory conditions. During the spring and summer seasons of 2009, a field trial of 162 exotic and indigenous mungbean genotypes obtained from 8 different geographical regions was conducted, aiming to shed light on the causes of susceptibility to phyllody disease. Sixty-one genotypes showed infection, ranging from 1.00% to 4.35%. Significant variation in all yield and morphological characteristics due to phyllody disease on mungbean plants depending on growth stages was also recorded. Comparison between healthy and diseased plants showed significant percentage reduction at all growth stages. Maximum reduction in all the tested charcateristics was recorded when plants were infected before flowering followed by infection at the flowering and pod-setting stages. The most severely affected plant variables at all infection stages were the number of seeds per plant, the number of normal seeds per plant and yield per plant. Severely infected plants produced no seeds but some partially infected plants produced very small, sickle-shaped, upright and leathery pods with degenerate seeds.

Keywords:

• Orosius albicinctus
• Pakistan
• phyllody
• phytoplasma
• Vigna radiata

Acknowledgements

The authors are grateful to the Asian Vegetable Research and Development Centre (AVRDC), Thailand and RCA/IAEA project for the provision of mungbean germplasm used in this study. The authors also acknowledge the Pakistan Atomic Energy Commission for providing the financial support.