PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
Dothistroma research
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See also needle diseases of radiata pine
Improved and cheaper control of dothistroma needle blight
From Forest Health News No. 175, July 2007
The New Zealand forest industry spends around $2.5 million per
year on Dothistroma control by aerial application of copper in a
mixture of oil and water. There have been many advances in copper
product formulation and spray technology over the past 15 years,
offering an opportunity to improve application methods and
efficacy, and to reduce application costs.
The success of copper application to control Dothistroma needle
blight depends on:
• Ensuring efficient spray deposition on pine needles after
release from the aircraft. Key factors are droplet size,
evaporative properties of the spray, and meteorological
conditions.
• Durability of the copper. After deposition the copper is exposed
to rainfall, sunlight, and abrasion. The longer copper deposits
remain on the leaf surface, the longer the protection provided.
In 2006, a research project was started to investigate methods for
improving operational effectiveness of aerial spraying to control
Dothistroma needle blight. The research was funded through the
Dothistroma Control Committee, Forest Industry Development
Agenda (FIDA), and the Forest Biosecurity Research Council
(FBRC).
The project involved:
• reviewing copper and oil products on the market;
• characterising properties of the spray formulation, particularly
in terms of viscosity, stability, and ability to spread after
deposition on the pine needle surface;
• defining the potential for improving spray coverage through
modifying the copper spray mix formulation and by reducing
droplet size;
• testing the ability to lower costs by reducing the amount of
formulation sprayed per hectare;
• determining the persistence of copper on the foliage over time
using formulations developed in earlier stages of the project.
A total of six copper fungicide and eight spray oil combinations
were tested for viscosity and stability at three different oil:water
ratios. Some combinations of fungicide and oil could not be
sprayed (Fig 1). Those combinations involved two fungicides,
which were not considered in further work.
An unsprayable paste made from metallic equivalent of 860 g a.i./ha
mixed with 3 litres of water and 2 litres of winter spray oil
Cuprous oxide produced uniform suspensions with all of the test
oils and, at reduced rates of active ingredient, copper hydroxide
mixed well with the test oils and formed stable suspensions.
Copper oxychloride was very difficult to re-suspend, taking 6 to
8 times longer and requiring much more rigorous agitation than
mixtures containing the other three fungicides.
Relative spread of droplets from one mineral oil and one Dothi-
Spray oil was significantly larger than from the other six oils tested.
This result was repeated with either cuprous oxide or copper
oxychloride in the spray mixture. However, cuprous oxide
consistently produced comparatively larger spread areas than
copper oxychloride when mixed with the same oil. As the oil:water
ratio increased from 40:60 to 50:50 and then to 100:0 the surface
tension of the spray mixture reduced, and spread over the leaf
surface increased. The oil content of the mixtures had much greater
effect on droplet spread than the fungicide or oil used. It should
be recognised that greater spread may not result in greater
fungicidal efficacy because firstly, the copper may not be carried
evenly in the droplet as it spreads, and secondly, distribution of
copper over the entire needle surface may not necessarily improve
control.
For the copper persistence tests, cuprous oxide was mixed with
three spray oils (Dothi-Spray oil, mineral oil, and vegetable oil)
and applied in formulations with three different oil concentrations
(40%, 43%, and 86%), equating to two application volumes
(5l/ha and 3l/ha), giving a total of nine treatments. Each of the
treatments was applied to 60 radiata pine seedlings under standard
conditions in the Ensis track-spraying facility. For the 40% and
43% oil mixtures, the volume median diameter (VMD) of the
applied droplet spectrum was approximately 65 ?m. However,
due to a higher viscosity the VMD of the 86% oil mixture was
approximately 80 ?m. The current aim is production of a 65 ?m
droplet.
After spraying, 10 randomly selected trees from each treatment
were grouped and placed outside where they were exposed to
natural weathering conditions. This procedure was repeated until
six blocks of 90 trees were established. At 0, 4, 8, 10, 13, and 17
weeks after treatment individual blocks of 90 seedlings were
randomly selected and harvested, and copper content was
determined.
There was a rapid reduction in copper over the first 8 weeks, after
which 25–40% of the initial dose remained on the plants. At the
end of the experiment copper still persisted. This finding is
promising compared with earlier work by Gilmour in which copper
was almost completely eroded after 3 months.
Copper persistence with all oils was similar, and the choice of oil
for operational spraying should be dictated by other factors such
as cost, handling properties, and environmental considerations.
Persistence of copper for three oil concentrations expressed as a
percentage of initial deposition
This project has provided some important findings:
1. Copper persistence and droplet spread can be maximised by
increasing the concentration of oil in the spray mix.
2. Copper persistence with high oil concentrations is improved
by reducing total spray volumes from 5 l/ha to 3 l/ha.
3. It may be feasible to reduce spray application rates to 3 l/ha
by spraying with copper and oil alone.
4. With the exception of two fungicides, there was little
difference in performance of fungicides and oils.
These findings suggest that there is potential for reducing total
spray volumes for Dothistroma control because, as water content
is reduced and oil concentration increases, overall copper
persistence and droplet spread increase. This is an important result
because it suggests that reducing total spray volumes from 5 l/ha
to 3 l/ha is worth evaluating in field studies. Such a reduction in
total spray volumes would produce significant cost savings, and
may result in increased efficacy.
Reports from these trials have been posted on the FBRC website
(www.fbrc.org.nz).
Stefan Gous
