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Title: Growth, health and Liberibacter asiaticus titer in diverse citrus scions on mandarin vs. trifoliate hybrid rootstocks in a field planting with severe huanglongbing

Author
item Stover, Eddie
item McCollum, Thomas
item Ramos, John
item Shatters, Robert - Bob

Submitted to: Proceedings of Florida State Horticultural Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2015
Publication Date: 6/1/2015
Citation: Stover, E.W., Mccollum, T.G., Ramos, J.E., Shatters, R.G. 2015. Growth, health and Liberibacter asiaticus titer in diverse citrus scions on mandarin vs. trifoliate hybrid rootstocks in a field planting with severe huanglongbing. Proceedings of Florida State Horticultural Society. 127:53-59.

Interpretive Summary: Many recent publications and considerable discussion have focused on root health and the importance of citrus rootstock response on huanglongbing (HLB) development, with special emphasis on bicarbonate intolerance of trifoliate hybrid rootstocks affecting HLB severity. USDA advanced citrus variety selections are routinely planted on both mandarin and trifoliate hybrid rootstocks. The experimental site displayed widespread HLB that is markedly hampering tree health, and has high irrigation water bicarbonate but low soil pH (high soil pH would further increase bicarbonate levels. Tree mortality was 30% on trifoliate hybrid rootstocks and 15% on mandarin rootstocks, which likely is unrelated to HLB. HLB bacterium levels were assessed in roots and leaves and there was no affect of rootstock type, with 30X the level of HLB bacteria in leaves compared to roots. Comparison of leaf and root HLB bacteria levels indicated that leaf levels were more closely associated with HLB symptoms than were root levels. Variety forming the above ground parts of the tree (the scion which produces fruit) had a significant effect on leaf HLB bacteria levels: scions which had trifoliate orange as an ancestor had one third the level of leaf HLB bacteria compared to other scions and also less severe HLB symptoms. However, having trifoliate in the ancestry had no significant effect on root HLB bacteria levels.

Technical Abstract: 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 CaCO3 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 but significantly 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 ~31. 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 Las titer.