Growth, Health and Liberibacter asiaticus Titer in Diverse Citrus Scions on Mandarin versus Trifoliate Hybrid Rootstocks in a Field Planting with Severe Huanglongbing

摘要

Many recent publications and considerable discussion have focused on root health and the importance of rootstock response on huanglongbing (HLB) development, with special emphasis on bicarbonate intolerance of trifoliate hybrid rootstocks affecting HLB severity. USDA advanced citrus scion selections are routinely planted on multiple rootstocks, to assess potential incompatibility and to enhance field-survival of each selection. Such plantings were established at the Ft. Pierce farm from 2000-2004, with trees of 54 diverse scion pairs planted on both mandarin (Sun Chu Sha, SCS) and trifoliate hybrid (Swingle, Carrizo, US-812, US-942, US-952) rootstocks. This site is Pineda sand, has never been limed, and is exclusively irrigated with shallow well water of pH 7.2 and 240 mg/L CaCO, equivalents. The experimental site displayed widespread HLB that is markedly hampering tree health. HLB was identified at this site in 2006 and has affected virtually the entire grove. Tree height, width, trunk cross-sectional area, fruit number, overall health, and HLB symptoms were assessed in March 2014. Tree mortality was 30% on trifoliate hybrid rootstocks and 15% on SCS. Among surviving trees, HLB symptom severity and all assessments of tree growth were slightly butsignificantly greater on SCS than on trifoliate hybrids. Las titer was assessed on random fibrous root samples and most symptomatic leaves from each tree quadrant. There was no significant difference between SCS and trifoliate hybrid rootstock on Las titer in the foliage or the roots. HLB symptoms were significantly but only slightly less severe in trees on trifoliate hybrid rootstocks than on SCS. Of 84 surviving trees the median tree ratio of leaf to root Candidatus Liberibacter asiaticus (Las) titers was -4.5. Comparison of leaf and root Las titers with other parameters indicated that leaf Las was more closely associated with HLB symptoms than was root Las titer. Scion pedigree had a significant effect on leaf Las titer with scions derived from Poncirus having one third the level of foliar Las compared to other scions and less severe HLB symptoms, but scion type had no significant effect on root Lastiter.