Agriculture and Agri-Food Canada (AAFC) researchers in Kentville, N.S., are seeking remedies for disorders that continue to plague Honeycrisp.
Two of the disorders are soft scald on the skin and an internal condition known as internal breakdown or low-temperature breakdown, explains Dr. Robert Prange.
Dr. Prange, a specialist in post-harvest physiology at the AAFC’s Atlantic Food and Horticulture Research Centre (AFHRC), says internal breakdown is a condition that can’t be detected on a packing line because it has no external indications.
Previous research by AFHRC researchers on fruit storage has determined that internal breakdown and soft scald are problems related to delayed cooling. They have demonstrated that the two conditions can be successfully controlled by delaying cooling of the apples for six days after picking, keeping the fruit at 20 C. After the six days, the Honeycrisp can be placed in controlled atmosphere (CA) storage.
Conversely, if the apples are cooled in conditions of less than 20 degrees for less than six days, the door is opened to problems, notes Dr. Prange.
He says AFHRC researchers have shown if they pre-cool at 25 or even 30 C, the pre-cooling storage period can be shortened to as little as just one day at 30 C.
This, however, creates “a logistical issue,” concedes Dr. Prange, because storage operators need to find a source of heat, “and storages are not made to be heated.”
There was also some concern that at 30 C there might be moisture loss from the fruit, but Dr. Prange notes very little moisture is lost in just one day.
They have learned, however, that delayed cooling will not stop either bitter pit or senescent breakdown (late storage complex), he says.
Senescent breakdown appears as a subcutaneous, differential browning of the apple cortex, while bitter pit is a calcium deficiency well known in Honeycrisp grown in other regions, Dr. Prange says.
Senescent breakdown was particularly noticeable in the Annapolis Valley during the 2009 crop storage season, “but we feel it has been there in other harvest years and other Honeycrisp growing areas,” he says.
Bitter pit appeared during the first four weeks of the harvest, was quiescent during the fifth week, and after the fifth week it was replaced by senescent breakdown, Dr. Prange says.
In the 2008 and 2009 harvest seasons, AFHRC researchers ran a 10-week sampling program, he says, noting the term “senescent” implies the fruit is older and over-mature and may be related to late picking. Dr. Prange says fruit from orchards in all weeks of the sampling program was stored for three months without delayed cooling and they found a substantial increase in senescent breakdown in late-picked Honeycrisp.
These observations suggested that there was an optimum picking period when these two disorders were least likely to occur, states Dr. Prange.
“Therefore, in order to reduce the occurrence of bitter pit and senescent breakdown we have to get more precise data on what is the optimum picking window.”
According to the 10-week harvest samples from 2008 and 2009, Honeycrisp fruit from the fifth harvest, which was closest to the optimum harvest time, had fruit internal ethylene content of about 2.88 ppm, a 5.37 starch index and red colour development of about 68 per cent, says Dr. Prange.
With fruit held in a refrigerated air (RA) storage of 3.5 C, bitter pit appeared in early harvested fruit from week one to week five, he says. In fruit harvested later – week five to week 10 – senescent breakdown occurred.
As a result, Dr. Prange believes the period from week five to seven is “the optimum harvest window. We seem to be getting a handle on the optimum harvest week.”
The optimal picking date in 2010 – averaged over eight orchards – was Sept. 27, several weeks early than 2009, says Dr. Prange.
In 2010, weekly samples were again taken and this time he included measurements from a new instrument called a DA meter, which Dr. Prange acquired from its Italian manufacturer. It is being evaluated for its ability to predict optimum harvest by measuring how much green or chlorophyll remains and is active in the fruit skin, says Dr. Prange, adding it is a non-destructive test.
Preliminary results from the 2010 weekly sampling showed that the DA values declined steadily as the fruit matured. As such, Dr. Prange believes a range of DA meter values can be identified that pegs the optimum harvest window and it can be customized for any given orchard.
Dr. Prange also examined the possibility of calcium deficiency since bitter pit and senescent breakdown in many apple cultivars is known to be reduced by increased calcium inputs, as calcium goes where water goes in the tree systems, he says, observing that Honeyrcrisp has very low water permeance, which means that Honeycrisp’s skin tends to retain water inside the apple more than any other apple cultivar.
“That is why it is crispy.”
Poor summer weather may influence calcium deficiency in Honeycrisp as early as July, when fruit cells should be growing rapidly and the cell walls need calcium, Dr. Prange says.
To reduce bitter pit and senescent breakdown, he recommends growers use calcium spray and harvest at the appropriate maturity. They should also clip Honeycrisp stems during harvest to control skin punctures and decay.
To avoid soft scald and low temperature breakdown, he advises growers to delay cooling and “don’t drench. We are not seeing what we want to see with calcium spraying.
“We have trees sprayed with calcium, yet they are showing bitter pit if harvested too early and senescent breakdown if harvested too late. Calcium doesn’t seem to torque it one way or another. One tree may get calcium and have problems. Another may not and be quite happy.”
Fortunately, he notes, even in trees with fruit that developed bitter pit (early picked) or senescent breakdown (late picked), the fruit harvested in the optimum one to 1.5 week harvest window had very little of these two disorders.
As for storage times, Dr. Prange recommends less than four months for refrigerated storages, while apples can be stored for more than four months in controlled atmosphere (CA) storages. CA storages also prevent greasiness on apples.
They can also have a low oxygen level of between 0.5 and 0.8 per cent, he says, while CO2 levels in the first four weeks of CA storage should be kept below one per cent, after which they can rise up to one per cent.
“We are testing several compounds as foliar sprays to reduce disorders and improve flavour,” says Dr. Prange, emphasizing the tests will have to be repeated over several seasons.
He notes this research is funded by the National Research Council, the federal IRAP program and the Nova Scotia government.
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