Farm Forestry New Zealand

PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Simply boron

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From Forest Health News No. 69, November 1997.

For the last four or five years I have noted an increase in the occurrence of symptoms consistent with those of boron deficiency. In particular, large areas of Hawke's Bay and more recently Wairarapa and second rotation sites in coastal Horowhenua and Manawatu. The symptoms include tip dieback, cellular collapse, poor stability and form.

Two seasons ago I inspected three adjacent forests. The first block was planted in the early 1970s and has been rigorously maintained to produce a good stand. Block 2, planted in the 80s, is also a good stand despite establishment problems. The third block  had  similar establishment problems to that of the second block. It was planted in 1992 with GF28. Of the three blocks, only the third has ever exhibited symptoms of boron deficiency. Trees in this stand also exhibit large cankers of up to 5 cm long and 2 cm deep. All things being equal, I suspect that there is a link between boron and genotype  and  that  it  warrants  further investigation.

Last season I visited a number of sites in the Wairarapa exhibiting boron deficiency. Again, the highest frequency and level of severity were associated with high GF rated seedlings. Also prevalent in these areas were large cankers that were often infected by the wound parasites Sclerophoma and Sphaeropsis (Diplodia). It has also been suggested that boron may be implicated in the formation of resin pockets, that root growth is better in boron-treated trees, and that tree health is improved (lower disease levels) where boron has been applied.

(Brent Rogan, MoF - abridged)

A number of questions are posed by Brent's observations and I have canvassed several people for their opinions, (editor)

The photograph of the split shoot of Pinus radiata looks very like several other samples that have been presented in the last year or so. At this point we do not really know what causes the problem. One hypothesis that I have put forward is that initial damage occurs to the new shoot from something like an insect, wind, or frost. Subsequently the rapid growth of the stem diameter is faster than strong callus can form. This results in the stem opening up at that point; a sort of hernia. When any organism, plant or animal, has been bred to grow excessively fast there are often mechanical problems due to tissues not maturing quickly enough to stand the extra stress and weight. There are many factors: animal, vegetable, and nutritional, that need to be checked out before we have an answer. Boron deficiency, or uptake problems, may well be involved, but I doubt that the answer is as straightforward as a single factor.

(Roger Crabtree, FRI)

The postulated association between boron and resin pockets is anecdotal and there is no evidence available for the improved root growth in boron-treated trees that has been suggested. The effect of boron on tree health is not known, but I have heard of Dothistroma being less of a problem in forests in the ACT, Australia, where boron has been applied. I think this maybe related to issues of cell wall resistance to fungal invasion!

The status of our research with boron is: FRI has established trials in young stands of radiata pine (in the 1980's) in Canterbury, Nelson and the central North Island examining long-term boron supply from boron minerals (colemanite and ulexite) and from soluble borax. More recently the New Zealand Forest Site Management Cooperative has sanctioned the installation of 4 trials, nationwide over 4 years, to examine boron nutrition in relation to rate of boron application, and the effect of variable boron nutrition on aspects of wood quality including resin pockets.

(Malcolm Skinner, FRI)

 

From Forest Health News No. 70, December 1997 - January 1998.

SIMPLY BORON: FURTHER COMMENT 1

I found it disturbing to read 'Simply Boron' (FHNews 69: 1-2) and its conclusion that symptoms of boron deficiency can be linked to high GF ranked material based on (in my view), scant anecdotal evidence. While open-minded to the view that a link may exist (after all faster growing trees are likely to have higher nutrient demands), the conclusion drawn does not automatically  follow  from the  evidence presented. The questions I would ask are:

1. While the blocks are adjacent are they in fact the same soil type and was the previous land use consistent?
2. How do the climatic records compare for the three blocks at an equivalent age?
3. Are there any records to indicate what the oldest block looked like at the same age as the 'boron' affected block? While acknowledging that anecdotal evidence does have some part to play in science I would have thought that there is little evidence for the conclusion drawn. Unfortunately this adds to the list of factors (increased toppling and lower density) that an uninformed public now erroneously associate with higher GF ranked planting stock.

(Paul Smale, Rayonier NZ, Auckland)

SIMPLY BORON: FURTHER COMMENT 2

Or, alternatively: Corky pith and putative boron deficiency in Pinus radiata (Bannister, M.H.; Burdon, R.D. Silviculture Report No. 109, Forest Research Institute, 1968).

I think that Bannister and Burdon have put it rather well - of course it is not 'simply' boron. Their exposition shows that the condition Brent Rogan described some 30 years after this report was written was not altogether unrecognised in the '60s. In fact, rather a lot of work went into it -involved were Graham Will (Soils), Dave Etheridge (a Canadian forest pathologist who worked here for the whole of 1967) and Martin Bannister and Rowland Burdon (GTI). The answer is that boron deficiency is very likely to be the primary, but by no means the only, cause.

It is worth quoting parts of the Summary of the Bannister & Burdon report: The first symptoms appear in the pith, which in large, rapidly expanding shoots becomes ruptured and reacts by producing abnormal, corky tissue and often contains brown, necrotic patches. . . . Other symptoms are superficial lesions in the soft cortex of the same shoots .... after the cambium has produced a certain amount of xylem, growth stresses lead to a progressive mechanical failure; ultimately a longitudinal, radial fissure exposes the pith to the outer atmosphere. The usual fate of a shoot in this condition is infection by Diplodia pinea and die-back. With mild symptoms, usually only the leader is seriously affected ..... A more severe expression .. . has been seen . . .the same symptoms occurred in the pith but they were accompanied by many lesions, necrotic patches and sometimes resinosis in the cortex; there were abnormalities in the apical meristems and die-back, commonly affecting the lateral shoots as well as the leaders . . . Progeny and provenance trials show that the species [Pinus radiata] and its varieties have a marked genetic variation in the threshold beyond which the symptoms develop.

A bewildering array of symptoms seem to be associated with boron deficiency but they can be interpreted in the light of probable interactions. These involve the following variables: (1) the time of year when a period of [boron] deficiency begins and the duration and severity of the deficiency, (2) available soil moisture, (3) supplies of other nutrients, especially nitrogen, (4) the influence of microclimate on the rate of pathogenic infection, (5) the diameter of the pith, (6) frost, probably and (7) genotype.'

Enough said.  

(Peter Gadgil, FRI)

 

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