South African Avocado Growers’ Association Yearbook 1987. 10:34-36.

Proceedings of the First World Avocado Congress

 

 

Field experiences with clonal rootstocks

 

WH BROKAW

Brokaw Nursery, Inc, PO Box 4818, Saticoy, CA 93004, USA

 

 

This year, 1987, marks the tenth anniversary of significant production of clonally-rooted avocado trees in Cali­fornia. The purpose of this paper is to share some observations on what has been produced over these past 10 years. The presentation will be some­what personal in nature; that is, from author's non-scientific viewpoint as a nurseryman and grower.

 

Just ten years ago Brokaw Nursery delivered 4 000 trees for two new plantings. The trees were Hass, Pinker­ton, and Bacon on Duke 7. Both plantings were on clean soil, but very near orchards infected with Phytoph­thora cinnamomi. Both orchards are intact today.

 

From these beginnings 500 000 clo­nally-stocked trees have been de­livered to date. Rooted clones, as distinct from seedling rootstock trees, make up 95 per cent of current sales.

 

Avocado root rot (P. cinnamomi) is the death threat that first induced the author to raise clonally-stocked trees. Shortly thereafter, there were other reasons for producing them, such as salt and lime tolerance, tree size control and fruit production. Thus far, trees have been selected only for P. cinna­momi or lime tolerance, as well as general tree performance and produc­tion.

 

The comments which follow are based on observations of the field performance of clonal rootstocks cur­rently in use and new, promising candidates. In all cases, reference to 'variety' or 'cultivar' means a rootstock clone. Unless otherwise stated, the rootstock has a Hass scion graft. Trees grafted to Hass are regarded as ideal for testing purposes, since these seem more vulnerable to the ravages of root rot than Zutano, Bacon or Fuerte.

 

DUKE 7 - THE FIRST REAL ADVANCE

When first produced, Duke 7 was regarded with considerable scepticism for a number of reasons, one being cost. Nursery trees on clonal Duke 7 sold for $8,80 in 1977, while seedling rooted trees cost $5,30. Furthermore, the young clones were often planted in adverse conditions with shallow soils, in boggy areas and among older trees in established orchards. The growing procedures at that time also resulted in smaller, weaker trees than the ac­cepted standard. Some used to say, 'Duke 7's don't grow, don't bear, and aren't root-rot resistant'. That evalua­tion has all but disappeared and most growers recognise the variety for what it is - imperfect, but significantly superior to most of its seedling relatives.

 

Here is how it stacks up:

 

Has Duke 7 been root-rot proof?

Absolutely not.

 

Has Duke 7 been root-rot resistant?

Yes.

 

Conditions under which it has been a solid performer are the following:

(a)    In deep and well-drained soil,

(b)    When not planted in swales where water runs and seeps.

(c)    When properly started during the first few months. At this time Ridomil® helps, mounding helps, and frequent light irrigations are requisite.

 

When do Duke 7s perform poorly?

(a)    When planted too close to large, infected trees. They often can not stand continual infection from large, older trees.

(b)    When planted among older in­fected trees and not separately irrigated. Too often the grower tries to stretch his regular irrigation. The young trees then suffer from a too­wet soil (common in P. cinnamomi­ infected orchards) and dryness -in the ball, where all the young roots are.

(c)    When in a swale and fighting continuously against re-infection.

(d)    When planted in soil of greater than average acidity, such as some of the less loamy soils in San Diego county.

 

The experience of the author is that when the conditions are right, the trees well-started and management tuned in to special requirements, trees survive and perform well under P. cinnamomi­ threat. In P. cinnamomi-free conditions, the Duke 7 has been a model per­former. The tree is vigorous, has borne large crops of fine Hass fruit and produces remarkably uniform orchards. It survives well under limey conditions and salt tolerance is better than average.

 

WHAT ABOUT OTHER ROOT-ROT RESISTANT ROOTSTOCKS?

A number of new rootstocks, which show promise of being more tolerant of P. cinnamomi, are now available. The University of California has introduced most of them - a few came from private sources. Leading the field at present are the G755s, recently renamed the Martin Grande. Close behind, and being watched, are Thomas, Barr Duke, Toro Canyon and P1. Below are the author's impressions:

 

G755A, B, and C: No rootstocks have been observed with a greater tolerance for P. cinnamomi. It is definitely more tolerant than Duke 7 and probably also more tolerant than most of the other contenders. As with any special pur­pose stock, though, there have been some troubling occurrences:

 

(a)    It has a history of chlorosis where not expected. Chlorosis has shown up in sand culture and in some of the new young plantings. Hass on G755 is also of a detectably lighter shade of green than other cultivars in the nursery.

(b)    While its trunk girth is probably greater than the other cultivars, when grafted to Hass, the foliage is sometimes rather sparse - not always, but noticeable.

(c)    Some troubling episodes occurred during its first summer in the orchard. One customer, who planted G755 for the first time last year, lost about 5 per cent of his trees because of a peculiar die­back of the Hass graft to the rootstock. It does not occur in all plantings, but has been repeated elsewhere.

 

Against these negatives, there is evidence of superb G755 performance in other infested locations. It also has an evident ability to set fruit well and produce large sizes in P. cinnamomi-­infected soils.

 

Thomas: It is believed that this will be an outstanding cultivar. It has not done well in all locations, but on the whole it appears as consistent as any other cultivar. It may be a bit weak with regard to salt tolerance, but certainly handles P. cinnamomi well.

 

Thomas is a Mexican cultivar, a fact which is reassuring to Californians, who have traditionally preferred Mexican rootstocks. True to its heritage, it has a dark-green foliage and is perhaps the easiest of all to establish in the orchard.

 

Barr Duke: Unimpressed by early observations, the author did not pursue this Mexican variety. However, Dr Michael Coffey of the University of California has recently revealed some outstanding performance by this culti­var.

 

Toro Canyon: A Mexican -Guatemalan hybrid, this cultivar seems to possess an assortment of fine qualities. It has an excellent colour and foliage vigour, appears to be salt-tolerant and has pro­duced good crops of fine fruit.

 

The Toro Canyon is probably not as P. cinnamomi-tolerant as the G755. It has, however, done very well in most locations and attracted a lot of attention without much advertising.

 

P1: This Mexican-Guatemalan hybrid may be a sleeper. Thus far, under severe P. cinnamomi-infected condi­tions, it has not been outperformed in terms of apparent vigour, defined by the author as lush foliage. Caution must be taken, however, for these observations are based on a very limited number of very small plots.

 

The P1 does not appear to be a fast, strong grower, the foliage is vigorous. The author has not seen it bear yet, but knows that its siblings have been pre­cocious and prolific.

 

Borchard: Any review of field per­formance of avocado rootstocks would be incomplete without mentioning the Borchard. Although vulnerable to P. cin­namomi, it has nonetheless earned a place in California for its tolerance to limey conditions. Selected on the basis of its parents' continued verdancy when planted among other chlorotic seedlings, it appears to transmit this trait to a grafted Hass scion. Only a limited number of these rootstocks were produced, as they are needed in only a few locations. Their success has been phenomenal. Customer satis­faction has been greater with this variety than any other ever produced by the author.

 

WHAT IS THE BASIS FOR THESE OPINIONS?

The author relies on his -own obser­vations and reports from growers who are trying out new cultivars. Part of the nursery programme is to search for trees that are performing well under adverse conditions, such as in P. cinna­momi-infested or limey soils. Root­stocks of these trees are cloned and distributed to interested growers along with University-developed ones. Typi­cally the author suggested that 6-20 trees of assorted varieties are planted and then observed. The following are reports on two of these grower plant­ings in the Santa Barbara and Ventura counties:

 

Grant Plot

Mr Campbell Grant's original grove, planted on shallow acid soil, was de­stroyed by P. cinnamomi. His early re­planting was accomplished with Hun­talas*, Duke 7 and assorted seedlings of a survivor tree, which he called Parida. The best of his replants were Huntalas and assorted seedlings of Parida. (*The Huntalas variety is infected with sunblotch. Therefore it is not in common use in California.)

 

In May 1986, he was provided with an assortment of varieties to be planted in place of deceased Duke 7s. The best of these have been G755 and P1. Sur­prisingly, the Thomas has not been out­standing. In the following evaluation, the lower the score, the healthier the foliage:

 

 

 

 

 

 

 

 

Wertz Plot (planted in October 1985) The author was looking for salt to­lerance. Trees of nine cultivars were planted in pairs - two trees per hole in 21 holes where Duke 7 and/or G6 had died. Each tree was periodically com­pared to its partner. The current ranking of the trees is as follows (a high score is favourable in this case):

 

 

 

 

 

 

 

 

 

 

 

This test is, of course, not significant because of the smallness of the sample. It does, however, give some basis on which to make hypotheses with regard to which cultivars are expected to survive salty conditions. An irrigation water analysis at this location is as follows:

 

 

 

 

 

 

 

 

 

 

 

 

 

This water is from a variably salty source. It is characteristically sweet in spring and early summer, increasing to very high levels of saltiness in late summer and fall.

 

In addition to observing the plantings of Brokaw Nursery's customer-growers, some of the clonal rootstock experi­mental plots in southern California were also observed. Most information was gathered by University of California re­searchers Dr George Zentmyer and Dr Michael Coffey, some facts of which are also supported by the Californian Avocado Society, while others are no longer maintained. Here are the results of the author's most recent review of two of these plots:

 

Embaracadero (planted in 1984)

One of Dr Coffey's plots was recently examined in a new way. The Embara­cadero plot in Santa Barbara county comprises some 739 trees of 18 varieties. It is planted on a P. cinna­momi-infested plot where the former orchard was abandoned. Soil condi­tions are considered severe for root rot and trees are treated with moderate dosages of Ridomil®.

 

There are a few difficulties in the published analyses of this plot, for example, the soils are not uniform and the cultivars are not randomly dis­tributed. There is a broad waterway running from northeast to southwest. The soils in this area are regarded to be saltier, with less aeration than other soils in the plot. At the head of the waterway is a swampy area that indi­cates an underground seep.

 

A quick, unscientific survey was con­ducted, leading to some interesting results. The procedure was to select the very worst areas of the plot - those areas which appeared to be most root­rot prone. Trees were then ranked in the selected areas in terms of appa­rently healthy foliage. Results were interesting and are tabulated below. The lower the score, the healthier the foliage:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Goland Plot (planted in 1982)

This was one of Dr Coffey's early P. cinnamomi experimental plots. Twenty four Duke 7, 34 G755 ungrafted cuttings, 10 Toro Canyon and 10 Topa­-Topa seedlings were planted. One year later, deceased trees were replaced with nine G6 cuttings and nine Hass/G6. The G755s were topworked later than Hass. In subsequent years, deceased trees were replaced by citrus. Table 1 summarises the survi­ving trees.

 

G755C may be unfairly represented, because the grafting of it may have caused some of the trees to succumb.

 

What is clear, however, is that in this case a couple of newer Californian cultivars performed much better than Duke 7.

 

WHAT DOES THE FUTURE HOLD?

During the 1930s and '40s, there was a huge proliferation of candidate fruiting scion varieties in California. Fuerte held centre stage, but many rivals were trying to exploit the Fuerte's weak­nesses and particular strengths. It was not until the 1970s that the industry effectively narrowed itself down to five or six fruiting varieties and this elimi­nation processes still continues. New candidates still appear - such as Pinkerton and Gwen - but such new­comers are rare and face tough competition from the currently-reigning Hass.

 

The author feels that the current state of affairs will remain unchanged during the 1 980s and '90s. For eight of the past 10 years, Duke 7 has been king among clonal rootstocks. Those appearing now are in his opinion, only the first of many would-be pretenders. G755 is currently the strongest chal­lenger, but it will be surprising indeed if it is not supplanted by other, perhaps yet undiscovered, varieties.

 

This pursuit of the perfect rootstock requires the engagement of both the scientific researcher and the 'out-in-­the-field' practitioner. In California, there is beneficial co-operation between university, nurserymen and growers. As the search continues and widens to include the use of cloned rootstocks to respond to problems other than di­sease, the sharing of experience between avocado-growing communi­ties around the world will be of im­mense value.