By: Bonnie L. Grant, Certified Urban Agriculturist
Stem blight of blueberry is especially dangerous on 1 to 2 year plants, but it affects mature bushes as well. Blueberries with stem blight experience cane death, which can result in the fatality of the plant if it is widespread. Failure to start blueberry stem blight treatment in a timely manner could mean more than the loss of the sweet berries; the loss of the entire plant is possible too. Knowing what to do when stem blight of blueberry occurs on your bushes can help you can save your crop.
Blueberry Stem Blight Info
Blueberry stem blight starts insidiously with just a few dead leaves in a single part of the plant. Over time it spreads and soon stems are exhibiting signs of the disease as well. The disease is most common in areas with poor soil or where excess growth has occurred. It is a fungal disease that lives in soil and discarded plant debris as well as several wild hosts.
Stem blight is the result of the fungus Botryosphaeria dothidea. It occurs in both high bush and rabbit eye varieties of blueberry. The disease enters through wounds in the plant and seems to be most prevalent in the early season, although infection can occur at any time. The disease will also infect host plants such as willow, blackberry, alder, wax myrtle, and holly.
Rain and wind carry the infectious spores from plant to plant. Once stems receive injury from insects, mechanical means, or even freeze damage, it travels into the vascular tissue of the plant. From the stems it travels into the foliage. Infected stems will rapidly wilt and then die.
Symptoms on Blueberries with Stem Blight
The first thing you might notice is browning or reddening of the leaves. This is actually a later stage of infection, as most fungal bodies enter the stems. The leaves do not drop but remain attached at the petiole. The infection can be traced to some sort of injury in the branch.
The fungus causes the stem to become reddish brown on the side of the injury. The stem will turn nearly black over time. Fungal spores are produced just under the surface of the stem which spread to neighboring plants. Spores are released all year except winter but the majority of infection occurs in early summer.
Blueberry Stem Blight Treatment
You can read all the blueberry stem blight info around and you still won’t find a cure. Good cultural care and pruning seem to be the only control measures.
Remove infected stems to below the area of infection. Clean pruners between cuts to avoid spreading the disease. Discard diseased stems.
Avoid fertilizing after midsummer, which would produce new shoots that can get cold frozen and invite infection. Do not over-prune young plants, which are most prone to infection.
Clear the area of nesting sites that termites might use. The majority of the insect damage that causes infection is through termite tunneling.
With good cultural care, plants that are caught early enough can survive and will recover the next year. In areas prone to the spread of the disease, plant resistant cultivars if available.
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Stem Blight of Blueberry Fruit Disease Information
Blueberry stem blight, caused by the fungus Botryosphaeria dothidea, is the primary disease limiting establishment of blueberry plantings in southeastern North Carolina. Both highbush and rabbiteye cultivars are susceptible to this disease, which enters the plant through wounds and causes rapid death of individual canes and entire bushes. The disease is especially severe on 1- and 2-year-old plantings of susceptible cultivars.
"Flagging," a symptom of stem blight of blueberry, caused by the fungus Botryosphaeria dothidea.
"Flagging," a symptom of stem blight of blueberry, caused by the fungus Botryosphaeria dothidea.
The most persistent symptom of shoestring disease is narrow, reddish streaks on the young stems of the bush. Some leaves may be misshapen, twisted or strap-like. The disease is spread by the blueberry aphid, which can be controlled with manual removal or aphicides.
The stem blight fungus causes a rapid wilt and browning or reddening of leaves or individual branches, then quickly spreads to the base of the plant. A plant will not drop its dead leaves, creating an easy-to-spot symptom. The blight is more likely to affect young or damaged bushes, so growers should avoid wounding plants. No fungicide is effective, so pruning or cultural methods are necessary to save an individual plant.
- With red leaf disease, terminal leaves on some bushes turn a reddish color mid-summer.
- The blight is more likely to affect young or damaged bushes, so growers should avoid wounding plants.
Fungal diseases below ground
Armillaria root rot
Armillaria root rot of blueberry is caused by several species, including Armillaria mellea and Armillaria gallica. These are soilborne fungi that infect through the roots and have the ability to attack many different plant hosts. They are most commonly found on land that has been recently cleared of native vegetation, so these sites are usually best avoided for new blueberry plantings. The pathogen primarily spreads by producing rhizomorphs, which are black, cord-like structures that grow from plant to plant through the soil.
Symptoms of the disease begin with stunting and leaf discoloration and progress to leaf wilting and cane death. Diagnostic white mycelial fans are produced underneath the bark of affected plants (scrape off bark of dying canes to observe) and occasionally clusters of mushrooms are found in the fall.
Control is best achieved by avoidance. Do not plant on sites recently cleared of native forest vegetation. If a recently cleared site must be used, remove as much woody vegetation as possible, including roots, although it's best to leave the land fallow for several years before planting as buried woody debris serves as a source of inoculum. Soil fumigation can also help reduce inoculum, but all large woody roots need to be removed from the soil beforehand for best efficacy. If the disease develops in already established plantings, infected plants should be uprooted and burned. Remove as much of the root system as possible and do not replant in locations where infected plants were found.
Phytophthora root rot
Phytophthora root rot is caused by the soilborne pathogen Phytophthora cinnamomi. This is a warm weather pathogen that does not survive well where soils freeze deeply in winter.
It has a wide host range that can infect many tree and shrub species. Phytophthora cinnamomi is spread in contaminated water, soil, and on infected nursery stock. Once in the soil, it can survive for many years. During periods of high soil moisture, spores produced swim towards nearby roots, where infection occurs. Rot can then progress from the fine root system up into the larger roots and, in severe cases, up into the stems.
Symptoms are usually most severe on young plants and include stunting, chlorosis and scorch. When soil moisture is abundant and temperatures are warm, wilting and cane death may occur. However, older plants with larger root reserves can survive the disease for years.
Once the pathogen is in a field, it is almost impossible to eradicate. Inspect all incoming nursery stock for infection and reject any plants that appear diseased. Because Phytophthora is mostly a problem of wet sites, select fields with good drainage, avoid heavy soils with high clay content, use sprinkler or microspray irrigation systems instead of drip, and do not overwater. Raised beds, drainage tiles, and compost amendment can also improve soil aeration.
Systemic fungicides help protect new plantings from infection when the pathogen is present. However, once application stops, the pathogen can resume growth and cause damage. In organic production systems, gypsum soil amendment at 1-10 tons/acre can reduce infection but not to the same extent as fungicide application.
Cultivars with some resistance to the pathogen are also available. Mulching with composted sawdust or bark mulch instead of black weed mat may reduce infection by keeping the soils cooler. Black weed mat maintains higher soil temperatures that are favorable to the pathogen. Finally, inspect fields periodically and dig out and destroy plants with more than 50% mortality.
Diagnose Blueberry Diseases
Diagnose mummy berry disease by looking for browning, wilting and dying new growth on the blueberry plant, as well as light tan or salmon-colored blueberries in midsummer that don’t ripen and instead shrivel up and drop. The dropped berries mummify during the winter and perpetuate the fungus the following spring.
Look for red lesions on young shoots to diagnose stem canker in your blueberry plants. The best time to see the lesions is in fall or winter, after the leaves fall off.
Diagnose the fungal disease twig or stem blight by inspecting your blueberry plant for infected, dead twigs that rapidly die back up to 6 inches from the tip. Botrytis blight is a fungus that also attacks the shoots, but it also infects the blossoms and causes them to turn brown or become covered with gray, fuzzy mold.
- Identify the cranberry rootworm (Rhabdopterus picipes) by looking for small white grubs with brown heads and adults that are deep-brown and ¼-inch long.
- Botrytis blight is a fungus that also attacks the shoots, but it also infects the blossoms and causes them to turn brown or become covered with gray, fuzzy mold.
Watch for leaf spots developing in midsummer, causing irregular discolored spots on the blueberry plant’s leaves. This fungal disease causes the leaves to drop from the bottom of the bush upwards.
Identify the blueberry budworm (Abagrotis anchoceliodes) by looking for cutworms in the weeds beneath the blueberry bushes and larvae feeding on the fruit buds. Rake the soil around the blueberry plants thoroughly in early spring, right before bud-break. Rake up and remove all dried fruits from the previous growing season to reduce the chance of mummy berry disease.
Avoid overfertilizing your blueberry plants with nitrogen, because this will make your blueberry plant more susceptible to Botrytis blight.