In 1985, H. Schneider and J. Pehrson published an article documenting a decline of Frost Nucellar navel oranges on a number of trifoliate rootstocks (H. Schneider and J.E. Pehrson, Jr. 1985. Decline of navel orange trees with trifoliate rootstocks. California Agriculture. Sept. – Oct. 1985 p. 13-16)
http://ucce.ucdavis.edu/files/repositoryfiles/ca3909p13-62889.pdf
In this decline, which began in the 1970s, trees began demonstrating symptoms when they were 15 to 20 years-old. Affected trees showed leaf discoloration, some defoliation, twig dieback and subnormal growth. They describe how some declining orchards were removed and, in others, individual trees were removed and replanted. Schneider and Pehrson concluded the following: “disorganized phloem and cambial tissues at the budunion proliferate into a tongue like wedge that protrudes from the inner side of the bark. Affected tissue acts as a girdle and is presumed to be responsible for the decline of the trees.” In this article, Schneider and Pehrson provide excellent micrographs illustrating what was occurring at the budunion. However, the actual cause of this aberrant growth pattern was not described.
If we fast-forward to 2012, citrus growers in Kern and Tulare County, and presumably in other counties of the San Joaquin Valley, are experiencing similar tree symptoms to those described and pictured by Schneider and Pearson. The problem has been observed with blood oranges; navel oranges, including Fukumoto, Earli-Beck, Newhall, Atwood, and Powell; and on Satsuma and Page mandarins; on trifoliate and citrange rootstocks such as C-35 and Carrizo. This decline has not been reported, to our knowledge, in California outside of the Central Valley. There are similar reports of bud union disorders in Florida. We are observing symptoms much earlier in orchards than did Schneider and Pehrson. Decline is present in one two-year old blood orange orchard and in several navel orange orchards that are 7 years-old or less. This decline is not common, but can be devastating in a particular orchard, with most trees within an affected orchard showing decline or evidence of the disorder of the graft union. In some orchards only a few trees may initially demonstrate symptoms.
Normally, the scion of a navel orange tree grafted onto trifoliate or citrange rootstock will grow more slowly than the rootstock and a ‘bench’ will form at the graft union. This bench begins to form when a tree is six or seven years old. Conversely, the growth of the scion and rootstock are more similar in affected trees when young and the scion will usually show a slight overgrowth of the rootstock. In Fukumoto, the graft union is an area of intense suckering, and the graft union can become much distorted. As described by Schneider and Pehrson, a groove containing a light brown gum is apparent at the graft union of affected trees. In young trees only staining may be present at the union. The groove does not always traverse the entire circumference, especially in the early stages. This groove is associated with the decline and death of trees.
The cause of the decline is not known. No pathogen has been identified, consistently, in affected trees. If the decline is a result of incompatibility between the scion and rootstock, there must be an additional stimulus, as the decline is not common and trifoliate and citrange rootstocks are the preferred rootstocks in this citrus growing area. In some young affected orchards, most trees show the groove at the graft union, and it seems unlikely that this uniformity was the result of tree-to-tree transmission of a pathogen. Currently, we have no suggestions on how to prevent this problem or alleviate the symptoms once found. Causes of the problem are being investigated.