I have heard rumors that some growers, again, are planting sour orange rootstock. As was my intent in the article on Macrophylla (Alemow) rootstock in my last edition of this newsletter, the purpose of this article is simply to caution those that may have heard that sour orange is the new rootstock of choice for the San Joaquin Valley. You should know the pros and considerable cons before planting sour orange in Kern County.

Citrus Tristeza Virus (CTV) also called Quick Decline disease, killed approximately 3 million citrus trees on sour orange rootstock in Southern California during the 1940s and 1950s. CTV, like the common cold that plagues humanity, is a virus that comes in many strain of varying strength. A strain that is mild in one variety may be quite severe on another. The effect on the tree of infection by several strains may be quite different than the effect of any single strain. The severity of the disease also varies with rootstock. CTV‐infected grapefruit, mandarins and oranges, which may rapidly decline on sour orange rootstock, are tolerant of most California CTV strains if grown on commonly used citrange (i.e. Troyer, Carrizo, C‐35, etc), trifoliate and lemon‐type rootstocks.

The Citrus Tristeza Agency was formed in California in 1963, to eradicate CTV. While CTV is still with us, the activities of the Citrus Tristeza Agency have resulted in a very low level of CTV in Kern County, thanks in large part to the continued financial support of this agency by the citrus industry (i.e. citrus growers) in this county. All of which leads to the point of this article.

Sour orange rootstock could be a good rootstock for Kern County. We have our share of calcareous, salty, poorly drained soils and our share of cold winter temperatures, and sour orange rootstock is fairly tolerant of these compared to other available rootstocks. Sour orange also produces a good tasting piece of fruit. However, its susceptibility to CTV should make growers especially cautious about planting this rootstock. So in light of the previously described citrus death and destruction, why may some growers be planting sour orange again? While I am just guessing, perhaps the rumors are untrue. Perhaps growers are using sour orange as a lemon rootstock. Perhaps someone has discovered that certain varieties of mandarin are not as highly susceptible to CTV on sour orange as most other mandarins, oranges or grapefruit. Perhaps prospective growers are counting on a continued low population of CTV‐infected trees in the county and that the brown citrus aphid won’t show up in the San Joaquin Valley to spread CTV more effectively. One advantage of sour orange rootstock with many existing mandarin, orange and grapefruit varieties is that you won’t have to wonder for long if you orchard has a high level of CTV infection. The quickly collapsing trees will let you know. The bad news is that while an infected tree dies suddenly once it reaches a ‘threshold’ value of infection, it is still a source of infection until that point is reached.

The recent research showing an interaction between dry rot and CTV should not be surprising (see Citrograph article, Current knowledge on Fusarium dry rot of citrus, Nov/Dec. 2011). Dry rot is thought of by most plant pathologists working on a variety of California crops as a secondary pest that enters after a primary pest has provided a wound or weakened the tree. Dry rot is not the real culprit in recent outbreaks in old stands of citrus on sour orange rootstock. Even if dry rot goes away, CTV won’t. The primary agent attacking the trees were various strains of CTV, now endemic in areas of the citrus belt in the San Joaquin Valley, and although relative mild compared to much more severe strains, are eventually deadly to many varieties on sour orange rootstock. A weakened tree is always much more susceptible to dry rot organisms, such as Fusarium solani, which is a common soil and root-inhabiting fungus. This organism is associated with the roots, stem and bark of healthy as well as diseased citrus trees. The ability of the fungus to become a pathogen is associated with stress factors, such as root damage caused by Phytophthora, over‐watering, poor soil drainage, excess fertilizer, heat stress (especially under dark‐colored trunk wraps on young citrus trees), freeze damage (especially to lemons) and root injury due to gophers, meadow mice, plowing, or herbicides. The recent outbreak of CTV in sour orange trees is most likely the result of the slow spread of CTV by the cotton aphid, and not anything new in the Fusarium solani organism.