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Cooperative Extension Fact Sheet FS1363

Scald in Cranberry Fruit: Part 1 Understanding Causes

  • Peter Oudemans, Extension Specialist in Blueberry and Cranberry Pathology
  • Giverson Mupambi, Extension Assistant Professor, Cranberry Physiology, University of Massachusetts Amherst

Introduction

Cranberry scald is a physiological disorder caused by overheating of the fruit and is sometimes misdiagnosed as fruit rot (Fig. 1). Scald was first recognized on cranberry beds in New Jersey in 1995 but since 2015 it has become a significant concern to commercial growers throughout the Northeast as well as other cranberry-growing regions. Environmental factors such as high ambient temperature, intense solar radiation, and low relative humidity contribute to fruit overheating in cranberry. New hybrid varieties tend to be more vulnerable since increased yields result in more berries being crowded into the upper canopy, where they are exposed to the sun. Fruit surface temperature in the upper canopy can exceed ambient temperature by more than 30°F during periods of high solar radiation.

Damage to cranberry fruit occurs when the internal temperature increases to the point that fruit tissues are irreparably damaged. Scald affects both the fruit surface as well as the internal tissues and symptoms appear within 24 hours after exposure to high temperatures (Croft, 1995). Initially, the fruit softens, and the underlying tissues discolor and become watery (Fig. 1). Typically, there is a firm margin between damaged and sound tissues unlike fruit rot, where the margin is indistinct. The heat-damaged fruit may eventually develop into fungal fruit rot within a few days if the fruit is already infected with pathogens.

Figure 1.

Figure 1. Cranberry fruit showing symptoms of scald. Note the distinct margin where overheating occurs. Damaged tissue can develop fungal rots as shown by the browning where fungal growth is beginning..

Causes of Scald

High Ambient Temperature

Surface temperatures in cranberry fruit increase with increasing exposure to solar radiation. Fruit, especially those on top of the canopy, experience high levels of sun exposure. Direct solar radiation on exposed fruit surfaces causes scalding in two ways: (1) Photochemical reactions can occur under excess solar radiation leading to cellular damage in fruit. (2) Direct solar radiation acts as a heat source through its effect on radiant heating. Data collected in New Jersey (2015-2020) has shown a close relationship between solar radiation and internal berry temperatures.

The developmental changes of fruit can also affect the vulnerability to scald with vulnerability increasing as the fruit ripens. Antioxidants and pigments such as chlorophyll on the surface of the fruit can act as a defense against radiant heating by utilizing light energy to drive chemical reactions such as photosynthesis. Chlorophyll is the green pigment in plants that converts solar radiation into carbohydrates and is abundant in young fruit. As fruit matures, chlorophyll is replaced by red pigments (anthocyanins) and the solar radiation is now converted to heat. Thus, as fruit matures, the vulnerability to overheating from solar radiation increases.

External fruit surface temperature in cranberry can exceed ambient temperature by up to 30°F depending on factors such as solar radiation, wind speed, and relative humidity. The temperature of leaves in the canopy, the shaded ambient temperature, and fruit surface temperature can all be quite different and therefore use of traditional temperature monitoring methods does not tell the full story (Fig. 2).

Figure 2.

Figure 2. Thermal image showing temperature differences in a cranberry canopy between fruit and leaves. Bright yellow represents higher temperatures while darker purple represents lower temperatures.

To understand the timing of scald, it is necessary to know the critical temperature where damage occurs. The temperature threshold is the temperature at which the metabolic processes in the cranberry fruit are interrupted and the tissues die. Previous research found the threshold to occur after fruit reach 108 °F for 2 hours.

Relative Humidity

Relative humidity influences the initiation of scald by interacting with direct factors such as high temperature and excessive solar radiation. Low relative humidity increases stress under conditions which result in high evapotranspiration rates. Research has shown that dry air with relative humidity around 35 to 45% in combination with high ambient temperature and solar radiation increases the risk of scald development in cranberry.

Measuring Microclimate and Detecting Overheating Events

A simple weather station equipped with six sensors can provide enough data to measure and identify overheating events across a relatively large area (Fig. 3). The sensors should include a shaded temperature and relative humidity sensor as well as a leaf wetness sensor, a solar radiation sensor and a temperature probe (thermistor type) that can be inserted into an artificial berry (Shapeways.com). This simple weather station can be deployed to the field and record conditions leading to an overheating event.

Figure 3.

Figure 3. Components of a simple weather station that can be used by cranberry growers to monitor overheating conditions on their bogs.

For example, in August 2018, a particularly bad year for scald , nine overheating events were recorded in New Jersey(Fig. 4). These events were recorded using two temperature sensors, one inside the artificial berry and the other shaded. A threshold of 108°F (artificial berry temperature) was used to identify overheating events. It is important to note that the fruit becomes more vulnerable later in the season, so overheating events in early August coincide with a lower vulnerability, while events in late August and September are most critical.

Figure 4.

Figure 4. Internal berry temperature and air temperature on a cranberry bog during August 2018. Red arrows indicate the possibility of an overheating event.

Scald Development Scenarios

It is difficult to provide a predictive model for scald since cloud cover, wind speed, ambient temperature, and solar radiation all interact to create the conditions that damage cranberry fruit. The motivation to predict is to provide proper and effective protection. Irrigation for evaporative cooling is critical but requires precision.

In Fig. 5 below, different scenarios are presented, indicating whether overheating may or may not occur leading to scald. In Fig. 5a the internal berry temperature was above the threshold of 108°F from approximately 10 a.m. until 3:30 p.m. During this period, unshaded fruits were very likely damaged by overheating. The ambient temperature was around 100°F, and there was very little cloud cover. The dew point was below 80°F, and it is expected that cooling by irrigation would be very effective under these conditions. In the next two examples, Fig. 5B and C, overheating is unlikely because of significant cloud cover (B) and low ambient temperature (5C). In Fig. 5D the berry temperature could remain above the 108°F threshold if there was less cloud cover. In this case, however, overheating did not occur because solar radiation was blocked by frequent cloud cover.

Take home points: Critical factors for risk of scald:

  1. Ambient temperature above 90°F
  2. Low relative humidity
  3. Clear skies with minimal cloud cover
  4. High solar radiation
  5. Internal berry temperatures exceeding 108°F
Figure 5.

Figure 5. Scenarios for the development of overheating on a cranberry bog. (See text for detailed description).

Information Gaps

As the incidence of overheating in cranberry continues to increase with increasing temperatures, there is a need for additional research to assist cranberry growers with information that can be used for decision-making to protect their crops.

Further Reading Material

Acknowledgment

The authors would like to acknowledge the Cranberry Institute for funding.

Mention or display of a trademark, proprietary product, or firm in text or figures does not constitute an endorsement by Rutgers Cooperative Extension and does not imply approval to the exclusion of other suitable products or firms.

February 2025