Nice looking young grapefruit trees on sour orange. Microjet irrigation with intensive fertigation.

A number of people are going back to Sour Orange as a rootstock choice.  The rationale is that Sour is the most adaptable rootstock for our area, and that psyllid control programs will also control brown citrus aphid and other tristeza vectors.   There’s a lot of truth there, and it just might work.

There is a note of caution, though — beware of Diaprepes!  Most of the trees on Sour that we’ve  lost lately were due to Diaprepes rather than Tristeza.  Sour is resistant to the regular strain of Phytophthora, but is very sensitive to the one that invades Diaprepes feeding sites on the roots — Phytophthora Palmivora.  That strain really works on Sour Orange hard, and where Diaprepes has become established, trees on Sour declined dramatically, and most of them died.  Drainage, soil type, and other factors have some bearing on this, of course, but as a general rule Sour gets hit harder than trees on other rootstocks.  So, if Sour Orange is under consideration, one of the decision points may be whether Diaprepes is established in the area.

When we first planted Rock Bottom and the trees started really growing fast, the Diaprepes exploded.  There were adults on every new shoot.  We’ve knocked them down by injecting nematodes through the drip system, but when we pull up a tree there’s always some Diaprepes damage visible on the roots.  We don’t have Sour in the planting, so I can’t say how it would react compared to the other rootstocks.

The point is, when you plant new trees and start growing them fast, if there are any Diaprepes in the area, they’re going to come to the grove.  If that’s the case, rootstocks such as Cleo, X639, or US897 — all good performers where Diaprepes is present — or even other rootstocks may be better choices than Sour.   X639 and US897 both look good in Rock Bottom under OHS, so they’re viable alternatives.  Pete