Q: My blackberries are awesome, but last year, about half the crop looked like they’re half albino. Any idea what causes this?
Laurel Roberds, Elk Grove
According to UC master gardener Rachel Tooker, two issues that might be causing your situation are “white drupelet disorder” and redberry mite. From the photo you submitted, it is most likely that your blackberries have the former condition.
It’s important for you to carefully review the descriptions below before embarking on any course of treatment, as the management approaches are quite different.
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White drupelet disorder is an abiotic issue in the plant, meaning that it is caused by environmental stresses on the plant, as opposed to a pest or pathogen. White drupelet is a tan-to-white discoloration of one to many drupelets on the fruit. Drupelets are the many individual fleshy structures that form each blackberry.
Most often, white drupelets will appear when there has been an abrupt increase in temperature accompanied by a drop in humidity; it is especially pronounced when there is wind.
While white drupelets may seem to be directly caused by weather, they are actually caused by ultraviolet radiation. Weather conditions modulate the effect of penetrating UV rays on the fruit. Cool, humid air scatters and absorbs UV radiation, while hot, dry air allows more direct UV rays to reach the fruit.
Wind increases the movement of humidity away from the plant canopy and therefore allows more direct UV rays to reach the fruit. Because the interior of the plant is less acclimatized to UV rays, it is even more susceptible when dry, windy conditions allow UV rays to penetrate further into the plant.
Some growers use overhead irrigation early in the day to wet the canopy and maintain cool temperatures for as long as possible. However, this can be a wasteful use of water in drought conditions and can also promote the growth of plant pathogens and pests.
Another possible approach would be to provide a shade cloth over areas of the canopy that receive significant afternoon sun. While some varieties – such as Apache blackberry, Kiowa blackberry and Caroline red raspberry – tend to get white drupelets more frequently than others, almost all caneberry varieties are susceptible to white drupelet to some degree.
Depending on the weather conditions each year, your plants will be more or less affected by this disorder.
Redberry mite is another issue. This tiny pest is a big problem for blackberries, both wild and cultivated. The adults look like tiny translucent worms.
Berries infected with these mites do not develop properly colored drupelets; instead, the drupelets remain hard (instead of juicy) and stay green or turn bright red.
To combat the mites, prune out and destroy infested fruit. Both mature and immature mites remain inactive during the winter, hiding under bud scales, and migrate to flowers in the spring to attack developing fruit.
A delayed-dormant application of sulfur after budbreak in addition to an application at bloom or when shoots are 2 to 6 inches long may provide control. This treatment also helps control powdery mildew.
Several applications of horticultural oil spaced two to three weeks apart after green fruit stage may also provide control. Don’t apply oil within one month of a sulfur application. Try to plant the least susceptible varieties; “Himalaya” (the large blackberries that grow wild along many local waterways) and “Evergreen” are the most susceptible varieties to redberry mites.
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