July is the prime time for harvesting and enjoying blackberries. The fruit is ripe when the drupelets are uniformly black. However, sometimes individual or multiple drupelets on a blackberry are off-colored. White, tan, red, or brown drupelet discoloration can be caused by various factors during the growing season.
White drupelet disorder on blackberry often occurs during hot, dry summers. Although drupelets enlarge during the growing season, they fail to turn red (Figure 1). These white or tan drupelets can be interspersed individually among dark-colored ones or in groups. In the past, white drupelet disorder was attributed to stinkbug feeding. However, white drupelets are caused by ultraviolet radiation and high temperatures. In studies conducted on red raspberry, unpigmented or white drupelets developed when fruit was exposed to temperatures of 107°F or higher with four or more hours of ultraviolet radiation. In another study, researchers found that the use of 30% shade cloth during the growing season reduced white drupelet disorder by 63%, but the total soluble solids (i.e., sugars) concentration of shaded fruit was 1% lower than non-shaded fruit, which slightly reduced blackberry sweetness. Some of the older blackberry cultivars, such as Kiowa and Apache, are more prone to developing this disorder than others, but several are susceptible. While white drupelets on blackberries may not be aesthetically pleasing, affected fruit are edible.
Interspersed red drupelets on ripe blackberry fruit can develop before or after harvest. Excessive rainfall before harvest has been associated with red drupelets that are soft and never turn black. In 2020, red drupelets were observed on the floricane crop Prime-Ark 45 blackberries grown in the field and in high tunnels in North Carolina. Also, a tiny eriophyid mite (Acalitus essigi) is known to cause "redberry" fruit on blackberry. Late-maturing blackberry cultivars are particularly prone to redberry mite infestations. These mites feed on the fruit core and at the base of berry drupelets. However, these mites are not common in the eastern United States.
Reversion is the most common cause of red drupelets on blackberry fruit after harvest. With this disorder, some of the black drupelets at harvest change to a red color (Figure 2). Reversion occurs on blackberries that are damaged by bruising or fruit compression during harvest or shipping. Also, blackberries that have a core temperature above 73°F at harvest tend to have a higher incidence of red drupelet than cooler fruit, especially during the early part of the season. Thus, a step-cooling process to lower fruit temperature is used to reduce the incidence of this disorder.
Anthracnose is a fungal disease that produces brown, shrunken drupelets on infected blackberries (Figure 3). Infection occurs in the spring during warm, wet conditions. Pruning to enhance air circulation among plants and removal of old fruiting canes in the dormant season reduces the amount of overwintering inoculum in the planting. Also, weed control improves air flow through the planting during the growing season and helps reduce disease infection. An application of liquid lime sulfur (Sulforix) can also be applied to dormant blackberry buds just before they begin to produce new growth will control anthracnose. Blackberries infected with this disease are off-flavored and are unfit for sale.
REVISED: July 8, 2020