Widespread Occurrence of Cotton leaf curl virus on Radish in Pakistan

Abstract

The current epidemic of cotton leaf curl disease (CLCuD) in Pakistan started in 1988 with the natural host range limited to a few plant species in the family Malvaceae. However, we have observed expansion in the host range of the virus, and several non-Malvaceous plants were found to be infected with the virus. Characteristic symptoms of CLCuD such as leaf curl and enations have been observed on radish plants, primarily in kitchen gardens. However, in 1999, levels of infection of 10 to 90% were observed both in commercial fields and kitchen gardens in the Punjab province of Pakistan. Both symptomatic and nonsymptomatic samples were collected from five different locations. Total DNA was isolated, dot-blotted on nylon membrane, and a full-length clone corresponding to DNA A of cotton leaf curl virus was labeled with 32P dCTP and used as a probe for the detection of a begomovirus. Strong signals were observed in symptomatic plants while no signals were observed in nonsymptomatic plants. Infection with a begomovirus was further confirmed by polymerase chain reaction (PCR) using degenerate primers for DNA A (1). Primers specific for the two distinct begomoviruses associated with CLCuD were also used in PCR reactions (2), and products of the expected size were obtained from all symptomatic samples, confirming infection with begomoviruses similar to those associated with CLCuD. A full-length probe of a nanovirus-like molecule associated with cotton leaf disease (3), called DNA 1 was labeled with 32P dCTP and detected the virus only in symptomatic plants. Similarly, primers specific for DNA 1 (3) amplified a product of expected size when used in PCR. On the basis of symptomatology and the detection of specific viral components associated with the disease, we confirmed that radish plants are infected with Cotton leaf curl virus (CLCuV). Since radish is a short duration crop, infection of CLCuV in radish may not serve as a direct source of infection for the next cotton crop. However, it is a potential threat to tomato crops which overlap with radish in the Punjab province. The detection of CLCuD in radish is another example of the mobilization of begomoviruses to previously unknown hosts.