Papers Presented in a Symposium on “Pests of Economic and/or Quarantine Importance in the Arab World”* at the Seventh Arab Congress of Plant Protection held in Amman, Jordan, October 22-26, 2000 Witches’ Broom Disease of Lime

Bove, J. M. and M. Garnier. 2000. Witches’ Broom Disease of Lime. Arab J. Pl. Prot. 18: 148-152. Witches’ broom disease of lime (Citrus aurantifolia) (WBDL) is caused by Candidatus Phytoplasma aurantifolia. The WBDL phytoplasma is closely related to the phytoplasmas of alfalfa, sesame, and sunhemp phyllodies. WBDL was first described in the Sultanate of Oman where thousands of lime trees were killed since the 1980s. It was seen in the United Arab Emirates (UAE) in 1989, and by 1993 all regions were affected. As predicted, the disease was reported next in Southeast Iran in1997, but must have been there, many years before. Finally, witches’ broom was reported as a new disease of lime in the Nagpur region of India in 1999. Monoclonal antibodies and PCR primers for 16S rDNA amplification have been obtained for the Omani phytoplasma. These Omani-specific reagents detect equally well the WBDLphytoplasma from the Emirates and Iran. Wether the Nagpur phytoplasma also reacts with these reagents could not yet be studied. Natural spread of WBDL is very fast. A putative leafhopper vector, Hishimonus phycitis, multiplying actively on lime trees, was identified in 1991 in Oman and found to be also present in the UAE in1993 and in Iran in 1997. The leafhopper is well known in India. Even though H. phycitis appears as the most likely vector of the WBDL agent, experimental transmissions have so far been unsuccessful. The most severely and widely affected species is small fruited acid lime (C. aurantifolia). However, in the UAE, sweet limes (Citrus limetta, Citrus limettiodes) showed severe symptoms. In the field, sweet orange, mandarin and grapefruit trees have never shown symptoms and were free of the WBDL agent. In the greenhouse, these species did not become infected following graft inoculation with infected lime shoots. Control is by prevention. Affected trees are easily spotted on the basis of their characteristic witches’ brooms. Specific reagents are available to confirm visual diagnosis. Symptomatic trees must be removed. Finally, the origin of WBDL, a relatively new disease of citrus, will be discussed. * This symposium was sponsored by the FAO Near East Regional Office, Cairo, Egypt. Introduction Small-fruited acid limes (Citrus aurantifolia (L.) Swingle) have been produced in the Sultanate of Oman along the northern coastal plain, or Batinah, for many generations. In the late 1970s, lime growers witnessed a severe decline of lime trees. Affected trees were characterized by the presence of conspicuous witches’brooms, hence the name “witches’broom disease of lime” (WBDL) (2, 3, 5). Witches’brooms are easily detected on the trees by their compactness and their small, pale-green leaves. The smaller the leaves, the more severe the disease. In the early stages of the disease, the trees show only one, then a few, witches’ brooms, with the other parts of the trees remaining symptomless, except for absence of new shoots. These early witches’brooms are soon followed by many others in various parts of the trees. In the advanced stages of the disease, the leaves of the older witches’brooms become dry and die, but remain attached (9). Eventually, the dead leaves drop, leaving naked twigs and shoots as the only evidence of former witches’brooms. After the first witches’brooms have appeared, the trees decline rapidly and, within a few years, they die: WBDL is a lethal disease. As will be summarized below, the disease is due to a plant mollicute (mycoplasma) and more precisely, a phytoplasma.When we first described the disease from 1986 on (2, 3, 5), the phytoplasmas were not yet recognized as genuine mollicutes, and they were given the name “Mycoplasma-Like Organisms” or MLOs. Here, we will use throughout the name phytoplasma which was officially adopted in 1992 (16). In addition to the phytoplasmas, plants can be infected by members of a second group of mollicutes: the spiroplasmas. The spiroplasmas are helical and motile, and Spiroplasma citri, the causal agent of citrus stubborn disease, was cultured as early as 1971 (13). In comparison, the phytoplasmas have no characteristic morphology, they are not motile, and they have never been cultured so far. The mollicutes, including the spiroplasmas and the phytoplasmas, are true bacteria, but they lack a cell-wall. They are phylogenetically related to Gram-positive bacteria with low guanine (G) and cytosine (C) in the DNA. While S. citri was the first mollicute of plant origin to have been obtained in culture, the WBDL was the first recognized phytoplasma disease of citrus. The phytoplasmal etiology of WBDL is based on several lines of evidence and will be summarized hereafter. These studies were conducted with plant material collected in 1986 in the Sultanate of Oman or derived from this material in our glasshouses in Bordeaux.