Survey and molecular detection of two citrus viroids affecting commercial citrus orchards in the Northern part of Sudan

Abstract

Two extensive surveys and laboratory work were conducted to determine the occurrence of citrus exocortis viroid (CEVd) and hop stunt viroid (HSVd), the causal agent of cachexia (CVd-IIb) disease in the main citrus growing areas in three states in the northern part of Sudan, viz, Northern, River Nile and Khartoum. For CEVd, all, but one of the examined citrus trees failed to show typical exocortis symptoms. The only symptomatic orange tree encountered in the survey displayed the characteristic CEVd symptoms including tree stunting and bark cracking or bark splitting of the rootstock. While for the cachexia disease, most of the commercial citrus trees were found to be symptomless carriers except mandarin. Symptomatic mandarin trees which were grafted on sour orange rootstock displayed heavy gum impregnation (bark gumming), wood staining and deep bark cracking, in addition to severe stem pitting, bark pegging, twig dieback and yellowing of leaves. Pitting and gum development were usually encountered near the budunion but could spread to other sensitive portions of the infected tree. However, using viroid specific primers and reverse transcription polymerase chain reaction (RT-PCR) approach, both viroids were shown to exist in total RNA preparations from symptomatic and asymptomatic citrus species. Bands of 370 bp and 300 bp corresponding to the full length genomes of CEVd and CVd-IIb, respectively, were detected. The test was positive in 54 for CEVd and 20 for CVd-IIb out of 200 random samples tested from orange, grapefruit and mandarin. The results indicated that a considerably high percentage of citrus species in the northern part of Sudan, particularly in Khartoum State, were symptomless carriers of both CEVd and CVd-IIb. Being rapid, sensitive and in most cases reliable, the molecular approach could be considered indispensable for viroid detection and in testing programs to produce certified viroid-free planting materials.

Indexing Virus and Virus-Like Diseases of Citrus

“…Stunting, dwarfing, bark cracking, yellowing of leaves, backward leaf bent, pin holing, yield loss and ultimately tree decline are the common symptoms of citrus viroid diseases [63,71,72]. Citrus viroids alone or with other viruses or prokaryotes in the host contribute considerably in tree decline [73]. Exocortis and cachexia are the major viroids which are widely distributed in citrus orchards.…”

“…Exocortis and cachexia are the major viroids which are widely distributed in citrus orchards. Other citrus viroids have also been detected from citrus orchards in different parts of the world [73]. Unlike viruses, viroids do not have protein coat, therefore, these are very difficult to detect through serological methods.…”