Emmalea Ernest, Extension Vegetable & Fruit Specialist; emmalea@udel.edu
We typically recommend that blueberry producers in Delaware reference the Commercial Blueberry Pest Control Recommendations for New Jersey for disease control recommendations. However, New Jersey blueberry growers face more disease pressure than the typical Delaware grower and not all the disease control measures recommended for New Jersey are warranted here. The blueberry diseases that have been reported in Delaware are: Phomopsis twig blight, Botrytis, Phytophthora root rot, various viruses, Exobasidium leaf and fruit spot and mummy berry. Management practices to control many of these diseases occur in the dormant or early growth stages (February to April).
Phomopsis Twig Blight [Phomopsis vaccinii (Diaporthe vaccinii)] was the most frequently identified disease among samples submitted to UD’s Plant Diagnostic Clinic. Symptoms of Phomopsis twig blight first appear in early spring when flower buds are in the green tip stage. The flower buds turn brown and die and the necrosis spreads into the stem tissue. This fungal pathogen overwinters on twigs infected the previous year. In late winter, spores are released from blighted twigs and spread by wind and rain. Flower buds are the point of infection and older wood is not susceptible. The fungus remains active through the growing season and new infections to fruit-bearing twigs can occur throughout the summer. If unmanaged, the disease will progress to Phomopsis Canker, causing entire branches to turn brown and die. The pathogen can also cause leaf lesions and fruit infections. Plant stress, mechanical damage and freeze injury can exacerbate the disease.
To control Phomopsis Twig Blight and Phomopsis Canker, maintain vigorous plants by managing soil pH, irrigation, weeds and fertilizer applications. Avoid promoting vigorous growth in the fall which will make plants more susceptible to freeze damage. Prune out old canes annually and remove pruned wood from the field. Some varieties have reported tolerance (Legacy, New Hanover, O’Neal) but there are no resistant varieties. Apply lime sulfur or sulfur spray when plants are dormant. If the disease is present, apply a recommended fungicide in the budbreak to prebloom period.
Botrytis Blight [Botrytis cinerea or other Botrytis spp.] is another frequently identified blueberry disease, and it can be difficult to differentiate from Phomopsis twig blight since symptoms appear on similar plant parts in the early part of the season. Botrytis symptoms can occur on leaves, twigs, blossoms or berries. Necrotic lesions form on the affected plant part, and grayish fungal growth with spores may be visible under humid conditions. Botrytis spores are airborne and it has a wide host range, so inoculum may come from within or outside of the planting. Wet, foggy conditions with temperatures of 61-68 °F are most favorable for disease development. Frost damaged tissue is susceptible to infection. Blossoms are very susceptible and slow pollination or extended flowering increases the risk of infection.
To control Botrytis Blight, maintain vigorous plants by managing soil pH, irrigation, weeds and fertilizer applications. Prune out old canes annually and remove pruned wood from the field. Site plantings in areas with air drainage to avoid late frosts and long periods of fog/humidity and avoid dense plant spacing. Scout blossoms for botrytis infection starting at early flowering. If disease is present or conditions are favorable, apply a recommended fungicide at mid-bloom. If disease is present, continue fungicide applications every 7-10 days until petal fall.
Phytophthora Crown and Root Rot [Phytophthora cinnamomi | Phytophthora sp./spp.] has been identified in a few Delaware blueberry plantings. Because this disease affects root function the aboveground symptoms are general poor plant health, leaf discoloration affecting most of the plant and slow growth. Roots are rotten, rather than white and healthy and stems may also darken and rot at the crown of the plant. Symptoms are often first and worst in poorly drained sections of the field. P. cinnamomi is a pathogen of ~5,000 woody plant species and it can survive in the environment on decaying plant tissue and as long lived chlamydospores. The disease can enter a field on plants, mulch material, irrigation water or through flooding. Once in a field P. cinnamomi can spread through swimming zoospores, which travel in flowing or ponded water. Saturated soils provide ideal conditions for infection.
To prevent Phytophthora Crown and Root Rot, obtain plants from a reputable nursery. Inspect plants at planting and discard questionable ones with poor root health. Avoid planting in poorly drained areas and make sure that water does not pond in the field. If draining problems are likely, plant on raised beds. Avoid excessive irrigation. Avoid using mulch materials that are likely to contain diseased woody plant material. If Phytophthora Crown and Root Rot infection is suspected submit a sample to the Plant Diagnostic Clinic to confirm the diagnosis. If Phytophthora is confirmed, remove infected plants (including roots) that have lost more than 50% of their shoot volume. Apply recommended fungicides during the two periods of rapid root growth: budbreak-prebloom and fruit maturation. Some varieties are reported tolerant of Phytophthora: Aurora, Legacy, Liberty, Reka, Overtime and Clockwork.
Viruses have been identified in Delaware blueberry plantings only rarely. The two viruses that have been confirmed are Tobacco Ringspot Virus (TRSV) and Blueberry Red Ringspot Virus (BRRV).
TRSV is transmitted by dagger nematodes which move the virus from one plant to another when they feed on roots. Many horticultural crops are hosts for TRSV (bean, cowpea, cucumber, grape, peach, potato, raspberry, strawberry, tobacco and tomato) as well as some broadleaf weeds (chickweed, dandelion and plantain). Perennial crop and weed hosts can serve as reservoirs for the virus.
BRRV has a narrow host range (blueberry and cranberry) and may be transmitted by blueberry mealybugs which feed on the plant roots.
To prevent Viruses in blueberry plantings, use only virus-tested plants from a reputable nursery and isolate the planting from wild blueberry plants. Test the soil for plant-pathogenic nematodes before planting. If dagger nematodes are present, fumigation or biofumigation can be used to reduce populations before planting. Manage dicot weeds in and near the planting. If virus infected plants are identified, remove and destroy them. Monitor nearby plants for symptoms.
Exobasidium Leaf and Fruit Spot [Exobasidium maculosum] has been diagnosed in home garden blueberries in Delaware. This fungal disease produces clearly defined light green spots on blueberry leaves and spots on fruit that fail to ripen. The fungus overwinters on the surface of blueberry stems and infection occurs in the early spring as the first leaves and flowers emerge. To prevent Exobasidium Leaf and Fruit Spot, obtain plants from a reputable nursery, avoid dense plant spacing, prune out old canes annually and remove pruned wood from the field. If present, the disease can be managed with an application of lime sulfur at the late dormant stage, approximately 2 weeks before bud break.
Mummy Berry [Monolinia vaccinii-corymbosi] has also been diagnosed in home garden blueberries in Delaware. This fungus overwinters in infected fruit “mummies” that remain in the blueberry planting. Early in the spring, the mummies grow mushroom like structures that release spores. The spores infect newly emerged blueberry leaves which produce spores that are carried to flowers by pollinators. Infected flowers develop into infected fruit, continuing the disease cycle. The source of initial infection for this disease could be infected plants from the nursery or nearby wild blueberries. If the disease is present, cultural controls are important for management. Collecting and destroying infected fruit mummies in the fall using a leaf blower and bagging mower or by cultivation can reduce inoculum. Applying 3-4 inches of mulch in the fall or winter can prevent the growth of spore releasing structures in the spring. Also, some varieties are reported tolerant (Legacy, O’Neal, Duke and Elliot). If the disease is present, fungicide applications are recommended in the budbreak, prebloom and bloom periods.
Recommended Factsheets for Additional Information
Phomopsis
https://content.ces.ncsu.edu/twig-blight-of-blueberry
Botrytis
https://njaes.rutgers.edu/fs512/
Phytophthora
https://plantpathology.mgcafe.uky.edu/files/ppfs-fr-s-19.pdf
Viruses
https://www.canr.msu.edu/blueberries/uploads/files/E3048.pdf
Exobasidium
https://content.ces.ncsu.edu/exobasidium-leaf-and-fruit-spot-of-blueberry
Mummy Berry
https://content.ces.ncsu.edu/mummy-berry-disease-of-blueberry
Commercial Blueberry Pest Control Recommendations for New Jersey
https://njaes.rutgers.edu/pubs/publication.php?pid=e265