Publications

3558

IntroductionHuanglonbing (HLB) is the most serious disease of citrus in the world, associated with three non-cultivable phloem-restricted bacteria Candidatus Liberibacter asiaticus (CLas), Ca L. africanus (CLaf) and Ca L. americanus (CLam). CLas is transmitted by the Asian citrus psyllid Diaphorina citri, and has spread to several countries. The African psyllid Trioza erytreae, the vector of CLaf occurs in Africa and neighbouring islands. Only two major citrus-growing regions - Australia/New Zealand and the Mediterranean Basin - are still HLB-free in the world. However, T. erytreae has recently been introduced into continental Europe (Portugal and Spain) and has become a potential threat to citrus production. The transmission of CLas by T. erytreae had been postulated but never tested. To evaluate the risk of T. erytreae transmitting CLas, comparative transmissions of CLas by T. erytreae and D. citri were assessed.MethodsTransmission tests were performed on excised leaves and seedlings of Citrus volkameriana with different inoculation access periods (in series) for both insect species. Quantifications of bacterial titers were made in excised leaves, seedlings three and six months after inoculation and on individual insects.ResultsOur results showed that T. erytreae was able to efficiently acquire CLas. Furthermore, T. erytreae carried significantly higher bacterial titers than D. citri, and was able to efficiently transmit the bacteria to seedlings at a similar rate that D. citri highlighting the high risk of spread of the most aggressive variant of HLB (CLas) by T. erytreae in Europe.DiscussionThus, extreme precautions to prevent any entry of CLas into Europe should be adopted.

The bacterium ‘Candidatus Liberibacter solanacearum’ is associated with zebra chip disease (ZC), a threat to potato production in North America and New Zealand. It is vectored by potato psyllids. Previous studies observed that ‘Ca. L. solanacearum’ infection causes potato tubers to undergo ZC-symptom-associated shifts in physiology, such as increased levels of amino acids, sugars, and phenolics. However, little is known about how ‘Ca. L. solanacearum’ infections caused by psyllid vector feeding may affect metabolism in potato foliage and stems. This study compared metabolism in potato plants fed upon by ‘Ca. L. solanacearum’-positive psyllids with potato plants not exposed to psyllids. Foliar levels of asparagine, aspartic acid, glutamine, fructose, glucose, sucrose, a ferulic acid derivative, and quinic acid were lower in ‘Ca. L. solanacearum’-inoculated than noninfected plants. However, foliar levels of proline, serine, four phenolic compounds, and most terpenoids were greater in ‘Ca. L. solanacearum’-inoculated than noninfected plants. Upper stem levels of asparagine and aspartic acid, upper and lower stem levels of ellagitannins and most monoterpenoids, and lower stem level of sesquiterpenoids were greater in ‘Ca. L. solanacearum’-inoculated than noninfected plants. These results suggest that many defense-related terpenoid compounds might increase in plants which had psyllids inoculate ‘Ca. L. solanacearum’. This could impact progression and spread of ZC.