1996-1997 New England Apple Pest Management Guide


 STORAGE SCALD CONTROL

 AVAILABLE MATERIALS

 Diphenylamine (DPA). Two formulations of DPA are available in the U.S. They are "No Scald DPA EC-283" (31% active ingredient) from Decco Division of Pennwalt Corporation, and "Shield Liquid DPA Super-Refined" (15% active ingredient) from Shield Brite Corporation. Each of these materials is a distinctive product, formulated with certain solvents, emulsifiers, and other ingredients. Thus, their properties as well as their DPA concentrations vary. Labels should be examined carefully and use should be in strict accordance.

POSTHARVEST TREATMENTS FOR SCALD CONTROL

 When to apply. Apples should be treated immediately after harvest (before storage). The induction period for scald is during the first 30-40 days of storage for at least some cultivars. After that time, treatment may have no benefit.

 

Table 26 - Recommended Concentration for DPA Solution

Cultivar

DPA (ppm)

Baldwin

1,111

Cortland

2,000

Delicious

2,000

Golden Delicious

Not Recommended

Idared

2,000

McIntosh

1,000-1,500

R.I. Greening

2,000

Rome Beauty

1,000

Stayman

1,500-2,000

 

Replenishment. The concentration of chemical in the treatment solution will diminish with time. Approximate concentration of scald-inhibiting chemical in the solution can be measured with test kits that are commercially available: Shield Brite DPA Field Test Kit 121, Decco DPA Calorimetric Test Kit, and Decco DPA Titration Kit all measure DPA concentration. Daily measurement with one of these test kits is strongly recommended.

After careful measurement of the current concentration in part per million (ppm), use the following formula to determine how much No Scald DPA EC-283, or Shield Liquid DPA Super Refined, to add to the tank in order to increase the concentration to the level you desire:

 

Pints of DPA product to add per 100 gallons =

(Desired ppm - Current ppm) x 8

% DPA in product x 100

 

EXAMPLE: You measure the DPA solution and find that it is down to 900 ppm. You want to restore it to 1500 ppm. You need to know how much Shield Brite Liquid DPA to add per 100 gallons of dip tank solution. You see on the label that this product contains 15% DPA.

Pints of DPA product to add per 100 gallons =

(1500-900) x 8 =

600 x 8 =

4800 =

3.2 pints

 

15 x 100

1500

1500

 
         

 

After adding the DPA, add water to bring the level up to a mark on the tank that is exactly the volume desired. Agitate the mix thoroughly before using. It may be desirable to again measure the concentration with a test kit to be sure that replenishment has occurred correctly.

When the suspension becomes dirty, replace it with fresh solution. It is usually necessary to replace the solution after dipping 30 bins in 100 gallons of solution.

Disposal. Do not discharge scald inhibitors into lakes, streams, or rivers. DPA is toxic to fish. Dispose of the material in a manner prescribed by the manufacturer. Methods commonly suggested are: (1) spraying the dilute solution on the orchard floor, not to exceed 1,200 gallons/acre; (2) constructing a plastic-lined evaporation pond; and (3) disposal by a commercial waste treatment firm.

Considerations. If exporting is a possibility, then before treating the fruit, determine whether importing countries will permit sale of treated fruit. In the U.S., all postharvest treatments must be indicated on the cartons. The statement, "Treated with diphenylamine for preventing of spoilage" is required. Do not use treated fruit for livestock feed.

SCALD COMMENTS ON CULTIVARS

McIntosh. McIntosh placed in CA storage will not usually scald much before April. This statement is based on the assumption that the fruit is picked at a flesh firmness of 15 to 17 pounds (pressure tester with a 7/16 inch head), moved from the orchard to storage within 24 hours, and cooled at 32oF promptly. CA storage may not always control scald on this cultivar indefinitely, but will delay the time of its appearance.

Consequently, scald may develop in April, May or June on some lots of CA McIntosh which were entirely free from it earlier. It is suggested that DPA be applied to those lots of CA McIntosh held until April or later, and where experience has proven that scald is a problem. However, as a precaution against scald, most growers use DPA on all McIntosh scheduled for CA storage.

Scald on regular cold storage McIntosh is generally not a problem until after January, at which time the bulk of this fruit should have been marketed. DPA has not been outstanding in controlling scald on McIntosh held beyond January in regular cold storage.

Cortland. In contrast to McIntosh, CA storage often increases the severity of scald in Cortland in comparison to regular cold storage. Chemical treatment with DPA is necessary if Cortland are to be stored in CA or beyond January 1 in regular cold storage. In years when mean temperatures are well above average for six weeks or so before harvest, adequate control of this disorder may not be possible.

Empire. Follow recommendations for McIntosh.

Delicious. Fruit placed in CA, or stored beyond January 1 in regular cold storage, should be treated with scald inhibitor to control scald.

Rome Beauty. CA storage alone has often provided commercial scald control on mature Romes. Early harvested Romes are most susceptible to scald. DPA may cause fruit injury to Rome Beauty.

PREHARVEST CHILLING HOURS

Ongoing research indicates that scald susceptibility of Cortland and Delicious are reduced by preharvest exposure to over 165 hours of temperatures below 50oF. Limited data indicate that the same may be true for McIntosh.

If DPA is used for scald control, a reduced rate is likely to be sufficient for fruit of these cultivars exposed to 165 or more "chilling hours" in the weeks before harvest. For Cortland, 500 ppm is probably as effective as 2000 ppm. For Delicious, 1000 ppm is probably as effective as 2000 ppm.

If 200 or more "chilling hours" have occurred before harvest, then probably no more than 500 ppm DPA is required for scald control on these cultivars.

 

 


1996-1997 New England Apple Pest Management Guide