FINDINGS OF A STUDY IN THE EGVV ON THE USE OF SHADE NETTING TO REDUCE SUNBURN IN APPLE

Do shade nets ease the burn or do they burn a hole through your pocket?

FINDINGS OF A STUDY IN THE EGVV ON THE USE OF SHADE NETTING TO REDUCE SUNBURN IN APPLE Daan Brink1,2 Willie Kotze1,3 Wiehann Steyn1 HORTGRO Science 2Two-a-Day Technical Dept. 3Dutoit Agri

1

Why did we do the work?

in mind that radiant heating may increase

What did we do?

Sunburn is a major defect of apples pro-

the fruit surface temperature by up to 16 ºC

Full-bearing ‘Granny Smith’, ‘Cripps' Red’,

duced for the fresh markets under the warm

above air temperature. Shade nets decrease

‘Cripps' Pink’ and ‘Fuji’ orchards were se-

growing conditions in South Africa. Affected

sunburn by decreasing the light exposure

lected in the EGVV area. Alternating double

fruits are characterized by bleached or golden

and thereby also the radiant heating of the

rows of ‘Granny Smith’ on MM109, ‘Cripps

brown discoloured areas or, in its most severe

fruit peel. This makes shade netting the most

Red’ on M25 and ‘Cripps' Pink’ on M25 were

form, necrosis of the peel. Since the visual ap-

effective means to reduce sunburn on ap-

covered in an orchard of producer (6), while

pearance of the fruit plays an important part

ples.

alternating double rows of ‘Granny Smith’

in the consumer preference, affected fruit are

In 2007, Two-a-Day Pty Ltd and Vegtech

and ‘Fuji’ on M793 were covered in an or-

downgraded. In ‘Granny Smith’, which is very

initiated a project to evaluate the production

chard of producer (66). ‘Fuji’ trees on a M25

susceptible to sunburn and where even the

of different apple cultivars under shade nets

were also covered at producer (41). An area

slightest bleaching is visible, the pack-out may

in the Elgin, Grabouw, Vyeboom, Villiers-

of approximately 0.5 ha was covered with

be as low as 40% (personal communication:

dorp (EGVV) area. Funding for the project

horizontal nets. Adjacent uncovered trees

Two-a-Day technical team).

was obtained from HORTGRO Science. Dif-

served as control. There were at least 5 repli-

Sunburn is caused by high light levels

ferent coloured nets were used in the trials;

cates for each treatment. Data were collected

and high fruit peel temperatures. Sunburn

however, we here only present the effects of

only from central trees under each strip.

browning occurs at a peel temperature of 48

the netted strips (nets) to uncovered (con-

Vigour control was adjusted under the

ºC while 10 minutes above 50 ºC is enough

trol) strips due to unsuitable trial design and

nets, but irrigation, nutrition and all other

to kill peel cells resulting in necrosis. These

inconsistent effects obtained for the different

orchard practices were managed the same

seem like very high temperatures, but bear

colour nets.

as the control. This is not ideal since netting

An example of sunburned apples produced without shade netting and apples that were protected by netting.

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inci-

average yields of ‘Cripps' Red’ and ‘Cripps'

dence of sunburn in

Pink’ to increase under the nets although

‘Granny Smith’. On

these increases were not statistically signifi-

average, sunburn at

cant (Fig. 3). The apparent increase in yield

the ‘Granny Smith’

can be ascribed to an increase in tree bear-

(6) site was reduced

ing area under the nets due to an increase

by 25% per annum

in shoot growth. The effect on ‘Fuji’ yield

over a 4 year pe-

and pack out was variable over the differ-

duced

Fig 1: Sunburn as percentage of all the fruit on the tree. (#) indicates the site number. * Number of * indicates the numbers of years of data for each orchard that were available for data analysis.

the

riod. At the Granny

ent seasons and alternate bearing in the last

affects irrigation and nutritional demands

Smith (66) site, sun-

season resulted in yield losses under the nets.

and also affects fruit set, therefore requiring

burn was reduced to lesser extent due to in-

Although not assessed, the decrease in yield

adjustments in thinning programmes. Pearl,

termittent net coverage. The decrease in sun-

was possibly due to reduced flower bud de-

white, yellow and red nets were supplied by

burn was directly reflected in a considerable

velopment. Previous research has shown that

Vegtech while Knittex supplied a blue net.

increase and decrease in the percentage class

‘Fuji’ shows a very strong growth response to

The nets were all classified as 20% shade net,

1 and 3 fruit, respectively (Fig. 2). The aver-

netting (Smit, 2007) and this may have a det-

which means that 20% of the area covered

age yield for netted ‘Granny Smith’ was not

rimental effect on both fruit red colour and

by the net consists of the net material; the

affected compared to the uncovered strips

yield regularity.

amount of light absorbed by the nets differs

(Fig. 3).

In ‘Cripps' Pink’, the slight decrease in sunburn (± 6 %) did not compensate for the

based on the absorbance characteristics of the net material, i.e. a 20% black net absorbs

‘Cripps' Red’, ‘Cripps' Pink’ and ‘Fuji’

loss in red colour (± 20 %) despite the slight

more light than a 20% white net. Please refer

Netting almost completely eliminated sun-

increase in yield observed in some years.

to the suppliers for specifications on the dif-

burn in ‘Cripps' Red’ and ‘Cripps' Pink’ and

More class 2 and 3 and less class 1 were pro-

ferent nets.

reduced sunburn by ±10% in ‘Fuji’ on aver-

duced (Fig. 2). In ‘Cripps' Red’, the decrease

age (Fig.1). On the downside though, the

in fruit with adequate red colour was evened

What did we find?

nets also decreased fruit red colour resulting

out by the reduction in sunburn. Hence, due

‘Granny Smith’

in a large increase in the percentage of poorly

to the slight increase in yield, nets increased

Fig. 1 shows that the nets substantially re-

coloured fruit (Fig. 4). There was a trend for

the class 1 fruit produced.

Fig 2: The class distribution of the different cultivars as affected by the treatments. (#) Indicates the site number. * Number of * indicates the numbers of years of data for each orchard that were available for data analysis.

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Fig 3: The effect of netting on average yield of the different cultivars. (#) Indicates the site number. * Number of * indicates the numbers of years of data for each orchard that were available for data analysis.

Fig 4: Fruit with inadequate red colour as percentage of all the fruit on the tree. (#) Indicates the site number. * Number of * indicates the numbers of years of data for each orchard that were available for data analysis.

So what’s the net result?

• It’s not a good idea to cover orchards on

the effect of coloured nets on both the light

• It makes economic sense to cover new

vigorous rootstocks under nets except maybe

spectrum and quantity of light that is allowed

‘Granny Smith’ plantings under nets. The

if the trees for some reason are stunted or

through.

reduction in sunburn alone justifies the con-

if you like pruning very much and have a

siderable cost of netting.

mountain of growth retardants in the chemi-

• Older ‘Granny Smith’ orchards may also benefit from a reduction in sunburn under nets. Nets may also invigorate worn out ‘Granny Smith’ orchards, resulting in a potential yield increase.

• The reduction in sunburn in less sunburnsensitive and in red and blushed cultivars does not justify the cost of netting. The economics may improve for orchards on dwarfing rootstocks like M9 or G222 and with better coloured strains of blushed cultivars.

cal store. Whereas the growth response under nets may be a benefit in the case of trees on dwarfing rootstocks, on vigorous rootstocks it results in poor colour in blushed cultivars, decrease fruit quality in general and may also negatively affect total yield and the regularity of cropping.

Further net benefits not assessed in this study, but that should be part of the equation: • It falls outside the scope of this study, but the economics of netting improves dramatically in hail-prone regions, especially for the most lucrative cultivars. This is a risk decision the producer takes together with his in-

• The project did not answer the question of whether coloured nets are better than the durable and most commonly used black nets.

However, such research is difficult due to

surer and bank manager.

• Apart from the obvious reduction in sunlight levels under nets, netting also increases humidity and lowers

CONTINUED ON PAGE 86

Fig 5: Airial view of the ‘Granny Smith’, ‘Cripps' Red’ and ‘Cripps' Pink’

Fig 6: Airial view of the ‘Fuji’ net trail at site 41. Note the volume,

net trial at site 6. Note that the control trees are fairly sparse.

indicative of vigour, of the control trees.

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Historic Pests Elise-Marie Steenkamp In the corner of a laboratory under a pile of books peeked an old maroon leather bound scrap book. The front read Misch & Co’s – Christmas and New Year Cards 1907-1908. On the inside it states: Office of the Eastern Province Entomologist, Grahamstown 1908.

T

he thin yellow pages tell their own sto-

nated to learn that they battled the

ries. Some still ring true today and makes

same problems we have to today

you wonder how much have we actually ac-

and it was clear that arsenic was

complished, said Dr Antoinette Malan, en-

the treatment choice of the day.”

tomologist from the US Department of Conservation Ecology and Entomology, and

“Discovery” of Codlin Moth

‘discoverer’ of the old book.

On the first page a newspaper cutting dated

“I first saw the book a while ago, but took no notice of it. Somehow I picked it up

7th March 1905 reads: Fruit Pests – The Codlin Moth.

and saw that it contained newspaper clip-

The report is about a certain Mr C.W.

pings about insects and pests dating back

Mally, Government Entomologist, who gave

more than a hundred years ago. I was fasci-

a lecture to the Cradock Farmers’Association

on Codlin Moth and asked fruit growers to be “on watch” for the pest.

“Recently Mr James Butler sent a specimen to the office at Grahamstown, which proved to be the genuine Codlin Moth.

The question then arose whether this was a chance infection or whether it was an estab-

Do shade nets ease the burn or do they burn a hole through your pocket?

CONTINUED FROM PAGE 81

wind speeds. Hence, netting may decrease irrigation needs, allow spraying for

Research under local conditions is needed so that informed decisions that consider all potential benefits and drawbacks of netting can be made.

pests and diseases when conditions outside are unsuitable and allow better drying of spray chemicals. Full enclosure of orchards– like the Oak Valley Orchard of the Future – may keep out some insect pests. A net covering provides some of the same physical benefits of a mulch layer, such as buffering soil temperature and decreasing evaporation of irrigation water from the soil.

Dr Simon Middleton of the Queensland Department of

• The increase in growth in response to netting may be an aid when planting pre-

tralia, did some great work on netting. Interested readers

cocious dwarfing rootstocks.

Agriculture, Fisheries and Forestry at Stanthorpe, Ausare referred to a summary of his work published in the Compact Fruit Tree as Middleton, S. & McWaters, A. 2002.

Hail netting of apple orchards – Australian experience. The Compact Fruit Tree 35 (2): 51-55. The article can be accessed on the web at: http://www.virtualorchard.net/idfta/ cft/2002/april/page51.pdf Those who believe that local is lekker and are not afraid of thick books can request the MSc thesis of Armand Smit from the authors of this article. Armand’s study was conducted under the supervision of Prof Stephanie Midgley at Stellenbosch University. Smit, A. 2007. Apple tree and fruit responses to shade netting. MScAgric, Stellenbosch University, Stellenbosch.

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