Susceptibility of Ontario Grown Apples to Scald in C.A. Storage

Agdex#:	211/66
Publication Date:	02/00
Order#:	00-029
Last Reviewed:	08/09
History:	Original Factsheet
Written by:	Dr. C. L. George Chu - University of Guelph; Ken Wilson - Apple Specialist/OMAF

Table of Contents

1. Introduction
2. Using Diphenylamine (DPA)
3. Precautions

Introduction

Scald is a physiological disorder that develops on some apple cultivars and makes the apple unfit for market. It is superficial in nature, characterized by a browning of the skin, especially on the green face of the apple (Figure 1). In severe cases a portion of the skin may be dead and brown, and the underlying flesh may also be affected. Not only does it look unappealing but the dead skin can be easily damaged during handling.

The development of scald does not appear to be related to nutrient status of the fruit, nor to insect or disease sources. It appears to be more related to the characteristics of the cultivar itself, and to the length and heat of the growing season, and maturity of the fruit at time of harvest.

Sensitive cultivars grown in hot weather, where in the 6 weeks before harvest there are few hours with temperatures below 10oC, are prone to scald. Fruit that is picked immature is more prone to scald than that picked at advanced maturity.

The incidence of scald is reduced if apples are stored in controlled atmosphere (C.A.). The faster apples are cooled and the sooner the modified atmosphere is applied, the better.

Diphenylamine (DPA) reduces the susceptibility of fruit to scald. Different concentrations are used for different cultivars (Table 1).

In a really bad scald year no treatment may be 100% effective.

Figure 1. Typical symptoms of moderate scald on Red Delicious.

Using Diphenylamine (DPA)

For best results, apply immediately after harvest. You may receive some effect up to 2 weeks after harvest. Fruit temperature should be between 16°C and 27°C. If the temperature is too low, effectiveness is lost; if too high, there may be injury. For similar reasons the temperature of the treatment solution should be between 10°C and 32°C.

There are many different methods of applying DPA solutions. It is important to keep the solution in the holding tank well agitated. Prepare fresh material after treating 2,000 bushels (100 bins) per 4,500 L (100 gallons) and when it becomes too dirty. Fit the system with a filter to prevent recirculation of dirt that might cause surface discolouration of the apples. Cover all fruit thoroughly. Drain off any excess material. Because diphenylamine is in suspension, the active ingredient may be removed on the fruit without the corresponding amount of water. The strength of the suspension may decrease with time, and even adding premixed material may not restore the original concentration.

There are commercial kits available to test DPA concentration in dip solutions.

Precautions

• Read and understand the directions on the label of the DPA concentrate before beginning treatment. There is more than one strength of DPA, each requiring different mixing rates for the various suppliers. The Food and Drug Directorate have set residue tolerance, and misapplication may result in residue above legal tolerances.
• In other countries, tolerances may vary. If you are planning export, check with the Canadian Horticultural Council 310-1101 Prince of Wales Drive, Ottawa, Ontario K2C 3W7, (613) 226-4187
• Do not feed Pomace from apples treated with diphenylamine to animals, as this may lead to residue above the tolerance for meat and milk. There is no legal requirement in Ontario for labeling apples treated after harvest.
• When working with DPA, wear rubber gloves.
• Be careful when disposing of spent DPA drenching solution. Don't let this material get into watercourses and pollute streams or wells. DPA is extremely toxic to fish. Follow the disposal procedure suggested by the Ministry of the Environment -spread the solution on the orchard floor keeping clear of streams, drainage ditches and wells. Don't dump in the same place twice, and don't pasture livestock on the grass.

Table 1. Potential Severity of Scald Symptoms Developing in Storage

* ppm is parts per million