Rootstocks for citrus

J C Owen-Turner, formerly Department of Primary Industries and Fisheries.

The rootstock

Early in the life of the industry, citrus trees were grown as seedlings. Unfortunately seedlings have a very long juvenile period, that is, they grow for years before producing any crop. The only exception to this is West Indian Limes which are grown successfully as seed of from cuttings. In addition, many seedling varieties are susceptible to most of our soil borne diseases and pests such as Phytophthora and nematodes. Some varieties may not grow true to type, producing anything but a commercial type fruit.

Since these problems were diagnosed the industry has been based on orchards of trees which consist of two parts, the rootstock which goes in the ground and the scion (pronounced cy-on) which is budded or grafted on to it. The selection of the combinations made looking at a whole host of factors, the desirable characteristics of the tree.

Rootstock selection is a major consideration when planning any citrus orchard. Historically, citrus has been regarded as a long term tree crop, thirty years and better. Modern methods, techniques, situations and selections have changed the thinking a little with a fifteen plus year life a viable consideration. Either period is a long time for any living organism to persist in the ground under current day horticultural production conditions.

Choice of rootstock

Choosing the right rootstock is fundamental to the success of the orchard. The rootstock has several major responsibilities:

• it anchors the tree in the ground and supports the upright growth
• the root system must absorb water and nutrients for tree growth and fruit production
• the root system is one part of the tree which stores energy as carbohydrate
• the rootstock has the potential to provide a level of tolerance of soil borne diseases
• the rootstock allows the scion to adapt to a variety of soil conditions
• some rootstocks are more while others are less tolerant to salty water
• some rootstocks are susceptible to citrus virus and viroid diseases. Trifoliata and its hybrids Troyer, Carrizo, Swingle and Benton must be budded with virus free material.

In addition to these responsibilities, each variety and species has the following characteristics to varying degrees: vigour, disease tolerance, nematode tolerance, compatibility with scion, drought resistance, cold hardiness, tolerance to salinity, effect on scion, fruit size and quality.

The idea is to find a rootstock with the greatest number of desirable characteristics to meet the set of orchard conditions where the tree is to be grown. For any one set of conditions there won't be a perfect rootstock even though some do come close. It should probably be said here that a recommendation made in California, Florida, Southern states of Australia or in any other citrus growing country may not necessarily be the best combination for mandarins in Queensland. The reasons why they sometimes don't work may be found somewhere in this article. For every one of the combinations not recommended here, there will be an exception in someone's experience. The reasons and situation for this occurrence should be closely examined before repeating or using that combination.

A phrase often used is `in a replant situation'. This is where a citrus tree has been removed because it is old or for whatever other reason and citrus is put back in the same spot. Nematodes and Phytophthora are the two principal diseases which knock out plants on other than tolerant rootstocks. A large amount of replanting is taking place and more marginal soils are being used.

Queensland's citrus industry is based on mandarins (60%) with oranges a remote second, followed closely now by lemons. It would appear that lemons will increase in planted area over the next five years, and for new combinations we need more experience and research. For oranges and the rootstocks used to date there have not been any real problems.

It is our wide range of mandarin varieties and rootstocks where we have more of a problem in making a choice. In recent years, of particular importance has been the decline of Imperial mandarin trees on Troyer citrange rootstock.

Brief notes on combinations

Lemons

All lemons do well on Rough lemon, but on new and deep soils only. For replant and shallow soil situations, Villa and Lisbon may be budded to Troyer BUT the budwood must be virus free. Lemons don't like Swingle, there is an incompatibility there. Try Meyer on Troyer with virus free budwood, for replant.

Eureka may be budded to Rough lemon for new soil. For replanting and shallow soil Benton citrange has been the only compatible rootstock. Two other rootstocks have recently been recommended by NSW agriculture for Eureka. These are Fraser and Cox. Whilst we have no experience in Queensland with these, they are a fair bet and may be tried.

Eureka is incompatible with Tri and all its hybrids except Benton. The incompatibility shows up as a 'yellow ring' at the bud union, which seems to choke the tree.

Cleopatra mandarin and Sweet orange have been used under lemons but do not appear to provide any advantage over Rough lemon. There is very little historical data or experience to support the use of these two stocks.

Oranges

All our oranges will do well on all rootstocks but there are the usual restrictions for replant and shallow soil situations. Oranges on Cleopatra mandarin does not have a great deal of experience to recommend it.

Washington Navel on Swingle may be a shy bearer. The vigour from Swingle results in a large tree very quickly. Results for the first 10 years of a trial indicated that it produces much less fruit for the same size tree on Troyer. In addition, the quality of fruit on Swingle is only as good as Rough lemon. Swingle appears to be well suited to grapefruit varieties.

Mandarins

Best rootstocks for mandarins are Cleopatra mandarin and Sweet orange for new soils. For replant situations we do not have many choices. Troyer is used for most of the mandarins in replant, but one of the less desirable scions combinations is Ellendale. Fortunately there do not seem to be any Ellendales being planted now. There is an urgent need for mandarin rootstock trial work to help sort out many of our problems.

The 'incompatibility' of Imperial on Troyer has been fairly well discussed in the Queensland Citrus Bulletin. This problem shows up as a severe overgrowth, benching, of the rootstock up and around the scion. This action virtually chokes the tree in a slow death. Tree decline can commence as early as 5 years. The more the tree is stressed the earlier the problem will start, and stress may be in many different forms. Once the overgrowth commences there does not appear to be any method to either stop or to slow the effect. This does not mean that Troyer should not continue to be used. Good management and a planned replanting program will help since Troyer does offer some advantages.

Swingle has been looked on as a replacement for Troyer under Imperial, and growers are using it. However, there does appear to be a larger bench forming. There has not been any trial work or any long-term experience with Swingle under Imperial, over a range of conditions and in commercial plantings for a recommendation to be made. No doubt over the next ten years current plantings of this combination will give results. The Department does not recommend this combination.

Swingle under Murcott mandarin has no trial backup for a recommendation, but should be a fair bet as a suitable combination. There is less benching, so far, and the orange parentage in Murcott should aid in this combination. The Department does not recommend this combination.

Table 1 Rootstock selection

ROOTSTOCKS
Scion	Soil Situation	Rough lemon	Cleopatra mandarin	Bentonv citrange	Troyerv citrange	Poncirusv trifoliata	SweetT orange	SwingleV citrumelo
Imperial and other mandarins	A B		* *		* *I		*
Ellendale	A B				* *	*	*
Murcott	A B	*	*		* *		*
Oranges	A B	*			* *	*	*	* *
Villa Franca and Lisbon lemon	A B#	*			* *
Eureka lemon	A B+	*		* *
Meyer lemon	A B#	*
Grapefruit	A B	*			*			* *
Lime	A B	*			* *	*

Notes on rootstock selection table

* Suitable combinations for different situations
A = Soil greater than 75 cm deep which has not previously grown tree crops
B = Replant soil or soil less than 75 cm deep
+ = Benton citrange is recommended in this situation. Eureka is not compatible with Troyer, other citranges or Swingle.
# = No proven replant rootstock has been researched. The use of Rough lemon or Cleopatra mandarin and soil fumigation may be best.
v = To avoid reduced tree life and productivity, use only propagation material certified free from virus and viroid diseases.
T = With the occurrence of the severe sweet orange strain of Tristeza in Queensland use of this stock could reduce tree life and productivity.
I = Rootstock overgrowth of the scion can cause reduced tree life and productivity under poor soil and management conditions.

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Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

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Last updated 18 July 2005