CLEANING STORAGES AND APPLE BINS TO MINIMIZE POSTHARVEST DECAY PROBLEMS
(Dave Rosenberger dar22@cornell.edu,
Plant Pathology, Highland)
Postharvest decay has emerged as a significant problem in Empire
fruit held in CA storage for more than six months after harvest. Most
of the losses are attributable to blue mold decay caused by fungicide-resistant
strains of Penicillium expansum. Postharvest treatment with
thiabendazole (TBZ) plus diphenylamine (DPA) does not control these strains
of P. expansum. Treated fruit often develop more decay than
non-treated fruit because they become inoculated when postharvest treatments
are applied. Even non-treated fruit can develop decay if they are place
in a room with a high level of airborne inoculum. Losses can approach
15% in severely affected lots of fruit.
Decay problems are currently more severe in some storage operations
than in others. Apparently, not all storages have the fungicide-resistant
strains of P. expansum that are responsible for the most
severe problems. However, the Empire decay problem is becoming more widely
distributed each year. Where problems have occurred in the past, action
must be taken this summer to minimize potential decay problems with the
1999 Empire crop.
Why are decay problems usually limited to Empire? We are still working
to answer that question, but it appears that Empire fruit are uniquely
susceptible to blue mold decay. Experiments conducted during the past
two years have shown that P. expansum can invade Empire
fruit through the stems if spores contact the ends of stems after harvest.
Invasion through stems does not occur in Empire fruit held in air storage,
and the incidence of decayed fruit is CA storage is usually low until
fruit have been stored for at least six months.
The ability of P. expansum to cause infection through
stems had previously been reported for pears but not for apples. We do
not know if other apple varieties can be infected through stems. In other
apple varieties, the postharvest pathogens seem to be limited to fruit
that can be accessed via wounds in the fruit skin. There are relatively
few wounded fruit in a bin, so the prevalence of decay presumably remains
low except for varieties like Empire that can become infected through
the stem. The susceptibility of Empire fruit to infection via stems explains
the prevalence of blue decay in uninjured Empire fruit from CA storages.
Fungicide-resistant strains of P. expansum presumably
developed because of repeated exposure to postharvest fungicides. Spores
of P. expansum can survive a long time on storage floors
and walls and on apple bins. The spores survive very well even on bins
that are stored outdoors under hot and dry conditions. Pathogen strains
that survive on bins are recirculated through postharvest fungicide treatments
year after year, and this pathogen population gradually became resistant
to TBZ and DPA. As the population became resistant to the postharvest
treatment, the incidence of decays increased and bins became a bit more
contaminated each year. As a result, more spores were available for infecting
Empire stems and the incidence of decays increased even more.
There are no quick or easy solutions for preventing postharvest decays
of Empire fruit in storages where the fungicide resistant strains are
rampant. The biocontrols that have been registered for postharvest use
on apples either do not work or are not available. As a result, many storages
have high populations of a pathogen for which there is no effective postharvest
treatment.
At this point, sanitation measures seem to be the only option for
breaking the cycle of increasing inoculum levels and increasing losses
to postharvest decays. Using clean bins and minimizing exposure to inoculum
is probably the only way to reduce Empire decays for the next several
years. Sanitation measures will need to be completed during summer before
bins are taken back out to the orchards.
The following steps should help to reduce inoculum and minimize Empire
decay problems during the coming year.
1. Disinfect packinghouse and storage room floors and walls during
summer. Floors and walls of storage rooms become contaminated with spores
that will be blown into the air by forklift traffic and evaporator fans
when storages are refilled in the fall. Therefore, the storage rooms and
access hallways should be sanitized before rooms are refilled. Quaternary
ammonia compounds are registered for disinfecting storage rooms and can
be purchased from your chemical supply dealer. Follow directions on the
product labels. In addition to eliminating inoculum, cleaning storage
rooms during summer will also eliminate foul odors caused by non-pathogenic
bacteria and fungi (molds) that sometimes develop on storage walls. Storage
odors can persist in packed fruit, so cleaning storage walls and floors
may improve fruit quality at the same time that it reduces the inoculum
for post harvest decays.
2. Do not apply postharvest treatments to Empire fruit in 1999: Recirculating
postharvest solutions accumulate and recirculate spores. In the absence
of effective postharvest fungicides, it will be essential to avoid the
spore contact that is inevitable in postharvest drenchers.
3. Whenever possible, use new bins for long-term storage of Empire.
Fruit that are moved into storage without postharvest treatment can still
develop decay problems if exposed to high levels of airborne inoculum
that can originate with contaminated bins. In an experiment conducted
in fall-winter of 1997—98, we applied different treatments to replicated
bins of fruit from the same orchard block. Some bins received fungicide
treatments and others were moved into storage without any postharvest
treatment. When fruit were removed from CA storage on July 15, the incidence
of decay in the treated and untreated fruit was similar (about 200 decayed
fruit/bin). Non-treated fruit became infected as a result of airborne
inoculum that presumably originated with other contaminated bins in the
same CA room. If other apple varieties are to be stored in the same rooms
with Empire, then bins for those fruit must also be reasonably clean so
as to avoid contaminating air in the CA room.
4. Sanitize contaminated bins: Bins from storage rooms that had an
Empire decay problem will be heavily contaminated with spores of fungicide-resistant
P. expansum. The same is true for bins that came out of
packinghouses while decayed fruit were being removed from packing lines.
We know from spore trapping experiments that packinghouses are full of
airborne spores of P. expansum. These spores land on and
persist on bins unless the bins are disinfected. The spores can be carried
on both plastic and wooden bins.
 Chlorinated
water in the flotation tanks in packinghouses may help to sanitize bins
as they are emptied. However, in many cases the empty bins are recontaminated
before they are removed from the packinghouse. Running bins through chlorinated
water provides no residual protection against recontamination. If bins
coming out of chlorinated water dumps are allowed to dry inside the packing
room, then they will certainly be recontaminated with airborne spores.
Furthermore, treatment with chlorinated water will not eliminate fungal
inoculum that is embedded in smashed or decayed apples remaining in the
bottoms of the bins.
Fruit with blue mold decay usually sink to the bottoms of bins as
the bins are emptied in water flotation tanks. These decayed fruit are
sometimes left in the bottoms of bins after the bins are nested and removed
from the packinghouse. Decayed fruit in the bottoms of bins can provide
millions of spores for infecting the next year’s crop. No sanitizing
treatment (except perhaps steam) will effectively sterilize decayed fruit.
 To
sanitize bins, the bins must be unbundled, all decays must be removed,
visible dirt or spore accumulations on the bin floors and walls must be
scrubbed out, and the bins must then be run through a drencher that contains
a labeled quaternary ammonia compound. Quaternary ammonia is considered
more effective than chlorinated water for disinfecting bins. The same
equipment that is used to apply postharvest treatments can probably be
used to apply quaternary ammonia sanitizers, but the outlets on the drencher
may need to be modified to ensure good coverage of the bins.
Sanitized bins should be kept separate from bins that have not been
sanitized. If sanitized bins are moved back into non-sanitized storage
rooms, or if they are transported through contaminated passageways, then
the bins will become recontaminated with airborne spores that are stirred
up by the forklifts. Bins that appear "clean" and that came
from storage rooms where decays were not a major problem can probably
be used without sanitizing for apple varieties that other than Empire.
However, unsanitized bins that appear "clean" have the potential
for recontaminating sanitized bins if they are stored in the same CA room.
Sanitizing several thousand or tens of thousands of apple bins is
no small job, and the benefits have not been verified in controlled experiments.
However, I see no alternative for handling badly contaminated bins, especially
if those bins will be reused to hold Empire fruit in long-term CA storage.
Without effective fungicides for controlling blue mold, spores from contaminated
bins will contaminate storages and will contribute to continued problems
with postharvest decay in Empire.
Next in this issue: 7/12 Insects
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