Evaluating Citrus Rootstock Tolerance to Biotic and Abiotic Stress

Citrus crops are threatened by a broad assortment of biotic and abiotic stresses.  Since many of these have a major effect on the rootstock of the tree, it is critical to use rootstocks that are resistant or tolerant.  USDA has an active citrus rootstock development program in Florida, and the evaluation of new candidate rootstocks for tolerance to the important threats is critical for success.  Four new rootstocks released by USDA in the past 10 years have attracted large commercial interest, in part, because of resistance or tolerance to several important diseases and abiotic stresses.  In addition to long term field trials at multiple locations where tolerance to stress is assessed in a commercial production setting, short term experiments are conducted in the greenhouse or field to assess tolerance to specific diseases or other stresses.  Greenhouse studies involving controlled inoculation with citrus tristeza virus (CTV) or Candidatus Liberibacter asiaticus (Las), the suspected causal organism of huanglongbing disease, are used to assess rootstock tolerance to these important diseases.  Greenhouse and short term field trials are also used to evaluate rootstock tolerance to Phytophthora nicotianae and P. palmivora.  The recently released rootstocks US-897 and US-942 were observed to have some tolerance to CTV, Las, and Phytophthora species.  Similar testing is being used to evaluate the tolerance of other rootstocks being considered for future release.  Greenhouse studies with high pH soil are also being conducted to identify rootstocks which tolerate conditions of pH 7.5-8.5, which are common in some Florida citrus production areas. Recent results of testing for these biotic and abiotic stresses will be presented.