Raspberry and blackberry growers are challenged both by disease and insect concerns. The primary insect of concern for bramble growers (but not the only one) is spotted wing drosophila (SWD), particularly for those in the Northeast. This insect is problematic in summer- and autumn-bearing fruits, but raspberries and blackberries bear the brunt of the damage. Autumn scouting for a primary disease of concern for black and purple raspberries – orange rust – is highly recommended due to the fungal pathogen’s complicated lifecycle and systemic nature.
Also prevalent in brambles, late leaf rust is not systemic. It can cause severe damage, particularly to the autumn raspberry crop, causing yield loss. It also impacts grapes and strawberries as well as summer raspberries.
Both issues were addressed at the University of Maryland Extension 2020 Bay Area Fruit School. Mengjun Hu, Ph.D., and UMD assistant professor of pathology, addressed bramble disease. Maggie Lewis, Ph.D., of the Hamby Lab, UMD Department of Entomology, discussed SWD control.
Orange rust doesn’t affect red raspberries, but it is devastating for their susceptible black and purple cousins. The two fungal pathogens which cause this disease have complex lifecycles with several spore-producing stages. The pathogens are difficult to control unless caught very early. Systemically-infected plants need to be removed from the field promptly to prevent widespread losses.
Chemical control using a variety of fungicides from FRAC groups 3 and 11 can be helpful, but only if the disease is not already established, Hu said. Spraying should only be done if there are signs of early orange rust disease, but not preventatively, as resistance development is highly probable. Control requires scouting and action in early spring at true leaf emergence, as well as during a three- to six-week period in autumn, where reapplication every 14 to 20 days is typically necessary for effective control.
The disease grows best at 70º – 75º F, when there has been a period of six or more hours of moisture.
“Scouting is very important,” Hu said, as the disease has both spring and autumn periods where it proliferates rapidly before ultimately becoming systemic, at which time the plant must be immediately removed from the field to prevent spread.
In spring, waxy spots on the undersides of infected leaves on infected plants soon turn orange. These contain aeciospores, which will be released into the air. Wind and splashing water allow them to infect the leaves of other plants. The underside of infected mature leaves on newly infected plants will then develop brown or black spots which contain teliospores.
Some of these teliospores will germinate and release basidiospores in late spring or early summer, infecting plants via buds on rooting cane. The fungus overwinters in plants’ canes and roots. Others will overwinter in this spore stage and germinate in spring, infecting new shoots at the crown, also causing systemic infection. Defoliated canes and yellow leaves, along with clusters of new shoots emerging from the crown, indicate infection and are seen in spring. Orange rust spots on the cane, crown and new shoots can occur too.
Scouting for early signs of orange rust in spring and autumn allows growers to find new infections before they become systemic, and to utilize fungicidal sprays if signs are found. Systemically-infected brambles cannot be effectively treated and serve as a source of inoculum, so they must be removed immediately.
Chemical spray control involves targeting the waxy leaves before blisters appear in spring. In autumn, applications targeting the base of the canes, to protect developing buds, is needed to prevent systemic infections.
Unlike orange rust, white rust does not become systemic, so plants do not need to be removed and destroyed. This rust causes cane defoliation, which also impacts fruit, rendering it unmarketable. Treatment includes increasing airflow into the canopy and avoiding planting near white spruce, which is an alternative host for the pathogen. Chemical control is available, but should be used cautiously. Copper fungicide applications can help with resistance management issues.
“This fungal pathogen has tremendous ability to develop resistance to different active ingredients,” Hu said. “Chemical control is typically not needed.”
SWD is an invasive vinegar fly which primarily affects autumn raspberries and blackberries. Females are capable of depositing eggs directly into fruit by cutting through the skin, which other types of drosophila cannot do.
The resulting larvae develop inside of fruit tissue within one to two days, and grow rapidly through several life stages, becoming adults within a 10- to 15-day span. They are believed to pupate both in the fruit as well as emerging and dropping to the ground to become adults. This rapid lifecycle allows multiple generations to occur during the growing season, and for populations of SWD to increase dramatically during that time – “particularly if you’re not managing for them,” Lewis said.
The growth of the larvae causes the soft tissues of the fruits to collapse. This, along with the injury caused by the adult female depositing her eggs into the fruit, means that infections by secondary pathogens are likely to occur.
Broad-spectrum insecticides are effective. However, they only target the adult stage of SWD and must be applied every one to two weeks. Organophosphates, carbamates, spinosyns and pyrethroids can all be effective. Issues with application include dense canopy coverage, which causes areas to be missed during spray application, as well as spray being applied unevenly to high, mid and low heights of the canopy.
Ongoing laboratory and field trials at UMD demonstrate that the consistency of spray application throughout the crop is often low, with outer and higher leaves in the canopy getting more effective coverage than inner and lower leaves. Because SWD tends to live in lower canopy levels and on the inner areas of the canopy, sprays often miss much of the population. Resistance may be a concern if insects are exposed to non-lethal levels of insecticides.
Lewis recommends growers evaluate their spray applications using spray cards to measure and calibrate effective coverage at different areas in the canopy. Managing the canopy to promote open, well-spaced canes and adjusting sprayers to achieve more uniform spray patterns can all increase the effectiveness of SWD management.
Autumn bramble season is upon us, but preparing for autumn harvest – and ensuring a healthy crop next season too – requires vigilance throughout the growing season.